Single Pulse Mode Research Articles

Micro-welding of non-optical contact sapphire/metal using Burst femtosecond laser

Recently, welding of dissimilar material, such as transparent material to metal material, is widely used in aerospace, microelectronic packaging, medicine, etc. Ultrafast laser welding provides a new method for high-performance dissimilar material welding, in which the non-optical contact dissimilar material welding has become a new research hot spot. In this paper, we report the effective welding of Fe-36Ni with roughness as high as 1.1 µm to sapphire, which, to our knowledge, is the roughest dissimilar material welding achieved in ultrafast laser. A 238 fs, 1035 nm, 200 kHz Burst mode femtosecond laser (4 sub-pulses with a total energy of 132.5μJ) yields a smooth and uniform interface with a welding strength up to 11 MPa. In this context, the welding performance and mechanism of Burst mode and single pulse mode ultrafast laser are systematically analyzed. The relevant research results are expected to provide theoretical guidance for the welding of high-strength and high-stability dissimilar materials.

Spoke-Type Structures in an Ion Diode with Magnetic Insulation of Electrons

The article presents the results concerning the cross-sectional energy density distribution of a pulsed ion beam for two types of diodes with electron open drift: with external magnetic insulation (250 kV, 80 ns, 0.6 T) and with self-magnetic insulation of electrons (250–300 kV, 120 ns, 0.8 T). Anode plasma is formed using a breakdown along the surface of the anode dielectric coating (single-pulse mode) or explosive electron emission (with double opposite-polarity pulses). It was found that, when the energy density of the ion beam exceeds ≈0.4 J/cm2, periodic spoke-type structures with a step of 3–6 cm in the beam cross section are formed. The processes of formation of such a structure—nonuniform density of anode plasma and self-organization of anode and/or cathode plasma in crossed electric and magnetic fields—are analyzed. It is shown that the formation of local plasma regions in the anode–cathode gap of an ion diode can cause the formation of a periodic structure of the cross-sectional energy density distribution.

Laser ignition of hydrogen/air mixtures in a constant-volume combustion chamber using a pulse-burst Nd:YAG/Cr4+:YAG laser spark plug

In this study, characteristics of laser ignition of H2/air mixtures are investigated in a constant-volume combustion chamber using a compact, passively Q-switched Nd:YAG/Cr4+:YAG laser spark plug. Ignition was conducted at a single point, with precise timing achieved through two laser modes of operation: delivering a single laser pulse and operating in pulse-burst mode, emitting trains of up to five laser pulses. Experiments covered a wide range of relative equivalence ratios (λ= 1.5 to 5.0) at 1 bar initial pressure and extended to 9 bar. Under similar pressure conditions, igniting H2/air mixtures with higher λ values (lean mixtures) results in a reduction of maximum pressure and a slower combustion process. Conversely, maintaining the same λ value for H2/air mixtures, while elevating the initial pressure, yields higher maximum pressures, but concurrently decelerates the combustion process. Pulse-burst mode laser ignition expedited combustion, especially for lean mixtures (λ> 4.0), showcasing advantages over single-pulse laser ignition. Additionally, lean ignition limits were determined at different pressures, revealing pressure-dependent differences between single pulse and pulse-burst modes laser ignition. These results provide insights into the features of laser ignition with pulse trains of H2/air mixtures and the use of this ignition technique in hydrogen-powered reciprocating engines.

Эволюция транспупиллярной лазерной коагуляции сетчатки в лечении активной ретинопатии недоношенных: от режима одиночного импульса до навигационного сопровождения

Abstract Evolution of transpupillary laser coagulation of the retina in the treatment of active retinopathy of prematurity: from single pulse mode to navigational support A.V. Tereshchenko, I.G. Trifanenkova, Yu.A. Sidorova, V.V. Firsova, V.V. Shaulov S. Fyodorov Eye Microsurgery Federal State Institution, Kaluga branch, Kaluga, Russian Federation K.E. Tsiolkovski Kaluga State University, Kaluga, Russian Federation Purpose. To present the development stages and results of 18 years of experience in laser technologies application for the treatment of premature infants with active ROP in the Kaluga branch of S. Fyodorov Eye Microsurgery Federal State Institution. Material and methods. Transpupillary laser coagulation of the retina (LCR) was performed in 1583 children (2537 eyes) with active ROP for the period from 2004 to 2021. LCR was carried out with various stationary laser ophthalmocoagulators. Stages 2 and 3 of active ROP with an unfavorable disease course, stage of early clinical manifestations, manifestation of aggressive posterior ROP were indications for laser treatment. Results. Transpupillary LCR began to be performed for children with active ROP with a stationary ophthalmic coagulator for the first time in Kaluga branch of the S. Fyodorov Eye Microsurgery in 2004. The technology of patterned LCR in active ROP was applied for the first time at domestic and international clinical practice, in the Kaluga branch of Eye Microsurgery in 2009. Pattern LCR began to be performed on the Integre Pro Scan device in 2016. Pattern with the most dense and uniform arrangement of laser applications – hexagonal – was identified. In 2021 transpupillary LCR of the avascular zone of the retina was performed in automatic mode with navigation support on the Navilas 577s laser system. It was possible to perform LCR of the avascular zone of the retina up to the dentate line in all cases. Regression of disease was achieved in 100% of cases. Conclusion. Evolution of laser technologies and the experience of their use demonstrate the high efficacy, safety and predictability of the results of laser treatment of active ROP. Modern pattern laser techniques are highly effective, safe, dosed and tissue-saving, which can serve as the basis for standardization of retinal laser coagulation technology in the treatment of active ROP. Key words: retinopathy of prematurity, unfavorable type of course, laser coagulation of the retina, hexagonal pattern, navigation system

An energy efficiency assessment of Yttrium-aluminum-garnet laser in vitro.

To study the effect range of the Nd:YAG laser through various levels of cloudy medium for targets with varying grayscale values in vitro. The coated paper cards with grayscale values of 0, 50, 100, and 150 were used as the laser's targets, which were struck straightly with varying energies using three burst modes (single pulse, double pulse, and triple pulse). Six filters (transmittances of 40, 50, 60, 70, 80, and 90) were applied to simulate various levels of cloudy refractive medium. Image J software was used to measure the diameters and regions of the laser spots. The ranges of the Nd:YAG laser spots increased with energy in the same burst mode (P < 0.05). Under the same amount of energy, the ranges of the Nd:YAG laser spot increased with the grayscale value of the targets(P < 0.05). The greater the transmittance of the filters employed, the larger the range of the Nd: YAG laser spots produced. Assuming that the total pulse energy is identical, the effect ranges of multi-pulse burst modes were significantly larger than those of single-pulse burst mode (P < 0.05). The effect range of a Nd:YAG laser grows with increasing energy and the target's grayscale value. A cloudy refractive medium has a negative impact on the effect range of the Nd: YAG laser. The single pulse mode has the narrowest and safest efficiency range.

Investigation of Laser-Induced Cavity and Plasma Formation in Water Using Double-Pulse LIBS

This paper deals with double-pulse laser-induced breakdown spectroscopy (LIBS) underwater, which is a promising analytical method for elemental analysis in the deep sea up to a water depth of 6000 m. A double-pulse laser with a wavelength of 1064 nm is used, which provides a pulse energy of up to 266 mJ for each laser pulse (in single pulse mode), a pulse width of 5–7 ns and a pulse delay in the range of 0.5 to 20 µs. In the double-pulse LIBS method, the first laser pulse creates a cavity on the material surface, and then the second laser pulse forms the plasma in this cavity. It is expected that the plasma is affected by the cavity’s size and lifetime. For this reason, the influence of focus position, pulse energy and pulse delay on the cavity and plasma formation at shallow water depth has been investigated.

Generation and Dynamics of Multiple Pulses in an Ultrafast Fiber Laser with a Single-Mode Fiber–Graded-Index Multimode Fiber–Single-Mode Fiber-Based Saturable Absorber

In this study, we have investigated the evolution process and dynamic characteristics of a multi-pulse regime in an erbium-doped fiber ring laser based on a single-mode fiber–graded-index multimode fiber–single-mode fiber (SMF-GIMF-SMF) structure as an optical modulator. By utilizing the excellent nonlinear optical absorption of the SMF-GIMF-SMF (SMS) device with a modulation depth of ~8.68%, stable single-pulse mode locking at the frequency of 9.84 MHz can be readily observed at low pump power. In addition, the single-pulse operation can evolve into a multiple-pulse regime on account of the peak-power-clamping effect via suitably raising the pump power and carefully regulating the polarization state. Further, the single-shot temporal evolution of multiple pulses is monitored, indicating that this state shows unique and interesting temporal characteristics with variable pulse separations and inconsistent pulse intensities, which, as far as we know, is the first such observation in ultrafast fiber lasers. Additionally, this study, based on the time-stretch dispersive Fourier transformation method, suggests that these multiple pulses consist of chaotic wave envelopes with erratic intensities and changeable pulse energy. We believe that these findings have profound implications for revealing fascinating nonlinear pulse dynamics in ultrafast fiber optics.

Investigation of the detection limits for zinc and copper in brass alloys at water pressures of up to 600 bar using double-pulse LIBS

Deep-sea mining and deep-sea exploration require methods for material analysis that can be used directly in the deep sea. A promising method is double-pulse laser-induced breakdown spectroscopy (LIBS), which was already used to measure zinc emission lines up to 600 bar. In this paper, the detection limits for zinc and copper in brass alloys were investigated using double-pulse LIBS at up to 600 bar to show up to which water pressure zinc and copper can be detected for the respective alloy. A double-pulse laser with a wavelength of 1064 nm, a pulse duration of 5-7 ns and a pulse energy of up to 266 mJ (in single-pulse mode) for each pulse was used. To optimize the emission spectrum for the respective water pressure, the delay between the laser pulses was varied. It was found that the detection limit for zinc and copper in brass alloys decreases with increasing water pressure.

Formation of two-dimensional laser-induced periodic surface structures on titanium by GHz burst mode femtosecond laser pulses

GHz burst mode femtosecond (fs) laser pulses, which consist of a series of pulse trains with ultra-fast intervals of several hundred picoseconds, have offered distinct features for material processing compared to conventional irradiation of laser pulses (single-pulse mode). We apply GHz burst mode processing to fabricate laser-induced periodic surface structures (LIPSS) on the material surfaces. In our previous work for silicon (Si), we have found that GHz burst mode can create unique two-dimensional (2D) LIPSS composed of both parallel and perpendicular to the laser polarization direction. We proposed that the formation of 2D-LIPSS is attributed to the synergetic contributions of electromagnetic and hydrodynamic mechanisms. To further investigate more detailed formation mechanisms and explore practical applications, we employ titanium (Ti), whose properties are significantly different from Si. We demonstrate that GHz burst mode fs laser pulses (central wavelength: 1,030 nm, intra-pulse width: 230 fs, intra-pulse repetition rate (an intra-pulse interval): 4.88 GHz (205 ps) and burst pulse repetition rate: 10 kHz) can also fabricate 2D-LIPSS on Ti surfaces. We attribute the dominant formation mechanism of 2D-LIPSS to the generation of hot spots with highly enhanced electric fields due to transient change of material properties during GHz burst pulse irradiation. Based on this speculation, properly tailoring the shapes of the burst pulse with an optimum intra-pulse number enables the creation of well-defined 2D-LIPSS. Furthermore, essentially homogeneous 2D-LIPSS can be formed in a large area by laser scanning of a focused fs laser beam with a stage scanning speed of 5 mm/s.

Determination of modes for safe application of selective micro-pulse individual retinal therapy of the macular zone using the modeling based on the theory of activated complex

Purpose: to make modeling of various modes of laser radiation, to assess the degree of damage to the retinal pigment epithelium and adjacent structures, and to determine modes and safety conditions of selective micro-pulse individual retinal therapy based on the theory of activated complex. Materials and methods. The authors have modeled the degree of damage to the chorioretinal complex and adjacent structures using the activated complex theory with the Eyring equation. The following definitions were used to determine the regime quality and safety conditions: micro-pulse mode is a mode with fixed values of micro-pulse duration, duty cycle and the number of pulses in the packet; power is a variable parameter. Efficiency is the ratio of the amount of denatured protein inside the retinal pigment epithelium layer to the total amount of protein in it. Selectivity is the ratio of the amount of denatured protein inside the retinal pigment epithelium layer to the total amount of denatured protein. Relative harmfulness is the ratio of denatured protein volume outside the retinal pigment epithelium to the volume of protein in it. Results and discussion. The quality of micro-pulse mode was found as depending on the selectivity value with efficiency equal to 90 %. With the efficiency value less than 50 %, the mode is non-selective, with more than 67 % – selective. Safety conditions for selective micro-pulse modes have been established, taking into account safety of the neurosensory retina and choroid in terms of relative harmfulness which should not exceed 1. Parameters at which micro-pulse modes have high values (more than 90 – 99) of efficiency and selectivity at the same time have been determined as well. It requires single pulse mode, micro pulse duration of 25 microseconds and power from 6 to 8 W. To expand the combination of parameters, it is necessary to be able to freely change them in increments of 1 pulse, 5 microseconds, 100 mW, with long interval and short duty cycle. These technical capabilities can be realized and implemented into clinical practice by manufacturers of laser devices. Conclusion. For selective micro-pulse individual retinal therapy of the macular zone with existing lasers, the following recommendation are to be observed: micro-pulse modes with pulse duration no more than 200 microseconds, short duty cycle less than 5 % and number of pulses in an envelope no more than 5; relative harmfulness less than one, radiation power selection, personalized patient’s characteristics.

Restoration of saturated outputs from microchannel plate photomultiplier tubes in sub-microsecond single-pulse-current mode

Microchannel plate (MCP) photomultiplier tubes (PMTs) are frequently used in experimental diagnostics, where they are operated in single-pulse current measurement mode. However, considering the significant amplitude fluctuations in the measured signal, the resulting output signal from the MCP-PMT is inevitably distorted by gain saturation. Therefore, understanding the correlation between the MCP-PMT output signal and gain saturation is critical in assessing the extent of output signal distortion and determining the MCP-PMT saturation level. This knowledge allows for a more precise assessment of the input signal's features. In this paper, we present an experimental method for restoring the initial waveform from the saturated MCP-PMT signal. To correct the amplitude-drop caused by gain saturation, our technique involves calibrating the MCP-PMT's relative gain as a function of the accumulated output charge using a square-wave light source. We then applied this approach to restore a ∼500 ns saturated pulse from a double-layer 10 mm diameter MCP-PMT. The restored signal showed a deviation of less than 6% from the reference waveform, which validates the effectiveness of the technique.

Development of a compact high-current repetitive pulsed x-ray system.

Pulsed x-rays are crucial for various applications such as radiography, biological effects, and the system-generated electromagnetic pulse effect. X rays with a higher dose rate and a higher total dose can generate more significant radiation effects and more effective radiography. However, most pulsed sub-100keV x-ray systems with high dose rates operate in the single pulse mode with limited total dose, and most repetitive pulsed x-ray systems have a low dose rate. This paper develops a compact high-current repetitive pulsed x-ray system with a low-impedance diode to generate high dose rate pulsed x-rays with an average energy below 100keV. A diode with a double-ring cathode is designed and tested to produce uniform pulsed x-rays. In order to investigate the x-ray intensity and the pulse number of repetitions for different anode thicknesses, five typical thicknesses are tested. The experimental results show that this system can operate stably at a repetitive rate of 0.2Hz with a peak voltage of about 200kV and a peak current of about 100kA. The dose rate is about 2.4 × 105 Gy(LiF)/s, and the average x-ray energy is about 55keV with the 40 µm thick tantalum anode. The x-ray uniformity is better than 2:1 over the measuring plane.

Effect of dual-optical pulses with temporal energy distribution on laser ablation performance in in vivo zebrafish model.

A Q-switched laser system has been used in a single-pulse mode for skin melasma treatments because of instant heat deposition in the target. Despite the efficient ablation of the melanophores in the skin, the single, high-fluence pulse often causes undesirable damage to the surrounding tissue, leading to high recurrence rates. This study aims to investigate the feasibility of dual-optical pulses with a temporal energy distribution on the melasma treatment in in vivo zebrafish models in comparison to that of the single optical pulse. Based on the optical detection, the dual-optical pulses had a temporal energy distribution ratio of 4:1 and an interval of 61 μs between the two consecutive pulses. According to the histological analysis, the dual pulses removed melanophores and induced a few apoptotic nuclei with minimal recurrence. This study demonstrated that the feasibility of dual-optical pulses (energy ratio = 4:1) could enhance the laser ablation performance in vivo.

Comparative Study of Percussion Drilling in Glasses with a Femtosecond Laser in Single Pulse, MHz-Burst, and GHz-Burst Regimes and Optimization of the Hole Aspect Ratio.

In this contribution, we present a comparative study on top-down drilling in sodalime glass, with a femtosecond laser operating in single-pulse, MHz-burst and GHz-burst modes, respectively. We investigate the hole depth, drilling rate, and hole morphology for these three regimes while keeping the same experimental conditions. We demonstrate that, for both burst regimes, the burst length has to be adapted for optimizing the hole depth. In the GHz-burst regime, the lower the ablation rate the longer the holes. The three drilling regimes lead to different hole morphologies, where the GHz-burst mode results in the best hole quality featuring glossy inner walls and an almost cylindrical morphology. Furthermore, we obtain crack-free holes, the deepest measuring 3.7 mm in length and 25 µm in entrance diameter corresponding to an aspect ratio of 150, which is the highest aspect ratio reported thus far with femtosecond GHz-burst drilling to the best of our knowledge.

Picosecond laser microvia drilling of ABF material using MHz burst mode

We report on a comprehensive study of laser percussion microvia drilling of Ajinomoto build-up film (ABF) material using an ultrashort pulsed laser in MHz burst mode. After laser processing, microvia drilling quality is being evaluated by the fabricated diameter and taper using laser scanning microscopy and metallography. The influences of the incubation effect, heat accumulation and shielding effects as a result of pulse to pulse interactions are being discussed on the ablation threshold, penetration depth and laser microvia drilling quality. We find that an increasing heat accumulation in MHz burst mode processing is responsible for the void formation and delamination of the insulating ABF layer. Therefore, the parameter clearance is introduced to evaluate these effects on the microvia sidewalls. For a comparable clearance, applying 2 intra-burst pulses achieves an average reduced taper of down to 19.5% compared to single pulse mode. At the same time, a reduced laser drilling time of 16.7% per microvia highlights the enormous potential of the MHz burst mode for laser drilling of ABF material in printed circuit board fabrication.

Study on after-effect of electroacupuncture with different time intervals on corticospinal excitability in primary motor cortex.

To compare the effects of electroacupuncture (EA) with different time intervals on corticospinal excitability of the primary motor cortex (M1) and the upper limb motor function in healthy subjects and observe the after-effect rule of acupuncture. Self-comparison before and after intervention design was adopted. Fifteen healthy subjects were included and all of them received three stages of trial observation, namely EA0 group (received one session of EA), EA6h group (received two sessions of EA within 1 day, with an interval of 6 h) and EA48h group (received two sessions of EA within 3 days, with an interval of 48 h). The washout period among stages was 1 week. In each group, the needles were inserted perpendicularly at Hegu (LI 4) on the left side, 23 mm in depth and at a non-acupoint, 0.5 cm nearby to the left side of Hegu (LI 4), separately. Han's acupoint nerve stimulator (HANS-200A) was attached to these two needles, with continuous wave and the frequency of 2 Hz. The stimulation intensity was exerted higher than the exercise threshold (local muscle twitching was visible, and pain was tolerable by healthy subjects, 1-2 mA ). The needles were retained for 30 min. Using the single pulse mode of transcranial magnetic stimulation (TMS) technique, before the first session of EA (T0) and at the moment (T1), in 2 h (T2) and 24 h (T3) after the end of the last session of EA, on the left first dorsal interosseous muscle, the amplitude, latency (LAT), resting motor threshold (rMT) of motor evoked potentials (MEPs) and the completion time of grooved pegboard test (GPT) were detected. Besides, in the EA6h group, TMS was adopted to detect the excitability of M1 (amplitude, LAT and rMT of MEPs) before the last session of EA (T0*). The amplitude of MEPs at T1 and T2 in the EA0 group, at T0* in the EA6h group and at T1, T2 and T3 in the EA48h group was higher when compared with the value at T0 in each group separately (P<0.001). At T1, the amplitude of MEPs in the EA0 group and the EA48h group was higher than that in the EA6h group (P<0.001, P<0.01); at T2, it was higher in the EA0 group when compared with that in the EA6h group (P<0.01); at T3, the amplitude in the EA0 group and the EA6h group was lower than that of the EA48h group (P<0.001). The LAT at T1 was shorter than that at T0 in the three groups (P<0.05), and the changes were not obvious at the rest time points compared with that at T0 (P > 0.05). The GPT completion time of healthy subjects in the EA0 group and the EA48h group at T1, T2 and T3 was reduced in comparison with that at T0 (P<0.001). The completion time at T3 was shorter than that at T0 in the EA6h group (P<0.05); at T2, it was reduced in the EA48h group when compared with that of the EA6h group (P<0.05). There were no significant differences in rMT among the three groups and within each group (P>0.05). Under physiological conditions, EA has obvious after-effect on corticospinal excitability and upper limb motor function. The short-term interval protocol (6 h) blocks the after-effect of EA to a certain extent, while the long-term interval protocol (48 h) prolongs the after-effect of EA.

The electroplastic effect in coarse-grained and ultrafine-grained titanium

One of the well-known features of the external action of the electric current in the process of plastic deformation is the electroplastic effect manifesting in a decrease in flow stresses and an increase in plasticity (deformability). Understanding the nature of the electroplastic effect provides targeted regulation and application of the effect to improve the efficiency of metal working processes or to change the structure and properties of materials. The deformation behavior of commercially pure titanium under the impact of an electric current of critical density from 12 to 400 A/mm2 is considered. The electroplastic effect in coarse-grained (d = 50 μm) and ultrafine-grained (d = 500 nm) VT1-0 titanium has been studied under a combination of tensile deformation and applied current of various modes and regimes, including the single-pulse, multipulse and direct current modes. It is shown that a decrease in the grain size contributes not only to an increase in the strength characteristics, but also to a decrease in the electroplastic effect, the mechanism of which is closely related to the density of mobile dislocations. It has been shown that the manifestation of the electroplastic effect in titanium is controlled by the grain size, and a decrease in the grain size leads to its electroplastic degradation and finally to the complete disappearance in the amorphous state due to a decrease in the density of free dislocations.

Utilization of intense pulsed light for hair removal in pediatric auricular reconstruction using tissue expander: A retrospective cohort study.

This study aimed to investigate the safety and efficacy of depilation with intense pulsed light (IPL) in congenital microtia patients during their reconstruction treatment. The hairy skin was treated with the M22TM system (Lumenis, German) using a filter of 695 to 1200 mm. A contact prob with a window of 15 cm × 35 mm or 8 cm × 15 mm was used at a radiant setting of 14 to 15 J/cm2 in the non-expander group and 13 to 14 J/cm2 in the expander group, both in a single pulse mode. The efficiency index of hair removal was classified based on the percentage of hair density reduction as excellent (>75%), good (50-75%), fair (25-50%), poor (<25%). The depilation effect was compared between the 2 groups, and any adverse effects were evaluated. A total of 159 patients were included, with 93 patients in the expander group and 66 in the non-expander group. The reduction of the hair density in the expander group after 3 treatments was higher than that in the non-expander group [82.98 (73.47-89.09)% vs 77.84 (71.50-85.34)%; P < .05, Wilcoxon rank-sum test], as well as the efficiency [excellent cases 68 (73.12%) vs 37 (56.06%); P < .05, Chi-square test]. Four cases of folliculitis, 3 cases of blisters, and no instance of expander exposure and cartilage absorption were observed in this study. Hair removal with IPL is a safe and effective photo-epilation method during all stages of ear reconstruction using tissue expander. Depilation in the skin expansion period resulted in better outcomes after 3 treatments, although after 5 treatments no difference between the 2 groups was observed.

Switched reluctance motoring and generating operation in single pulse, current chopping and voltage PWM modes

Switched reluctance motoring and generating operation in single pulse, current chopping and voltage PWM modes

Synthesized evaluation of various injection regimens on hydrogen propelled homogeneous charge compression ignition and dual fuel modes for an automotive application

The present work aimed to perform a comparative study between the hydrogen diesel homogeneous charge compression ignition (HDHCCI) and hydrogen diesel dual fuel (HDDF) modes with multiple injection regimens. The results showed that the maximum feasible hydrogen energy shares (HES) were 73.99% and 27.46% for the HDDF and HDHCCI modes in a single pulse injection. The level of efficiency increased while increasing HES in the HDHCCI mode for any injection regimen. Conversely, the level of efficiency decreased on increasing HES for the HDDF mode, however it yields higher efficiency than the HDHCCI mode at a steady HES operation. The extremely low NO emission was attained using single pulse and twin pulse HDHCCI modes as compared with HDDF mode, and vice versa for smoke and HC emissions, since the existence of cool flame due to too early injection timings drastically altered the combustion characteristics of HDHCCI mode.