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Article

Technical Characterization of a High-Power Diode Laser at 445 nm for Medical Applications: From Continuous Wave Down to Pulse Durations in the µs-Range

by
Zhaoqun Liu
1,
Yunfan Lu
1,
Birgit Lange
2 and
Jörg Meister
3,4,5,*
1
Department of Mathematics and Technology, University of Applied Sciences Koblenz—RheinAhrCampus, 53424 Remagen, Germany
2
Medical Laser Center Lübeck, 23562 Lübeck, Germany
3
Department of Periodontology, Operative and Preventive Dentistry, Bonn University, 53111 Bonn, Germany
4
Center of Applied Medical Laser Research and Biomedical Optics (AMLaReBO), Bonn University, 53111 Bonn, Germany
5
Laser Application in Medical Sciences Research Center, Shahid Beheshti University of Medical Sciences, Tehran 1989934370, Iran
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(3), 1041; https://doi.org/10.3390/app15031041
Submission received: 24 November 2024 / Revised: 11 January 2025 / Accepted: 15 January 2025 / Published: 21 January 2025
(This article belongs to the Special Issue Diode Lasers: Materials, Devices and Application)
Browse Figures
Versions Notes

Abstract

:
The technical development of diode lasers with regard to wavelengths, output powers and pulse durations in the order of microseconds is expanding the range of medical applications. The 445 nm wavelength, in particular, offers a promising approach. Such a laser system is now available, and its technical and optical properties are introduced here. To characterize the diode laser with a wavelength of 445 nm, laser settings were recorded in both CW and pulse modes in the range µs to ms, with duty-cycles of 1% up to 40% (partially also up to 90%). At the same time, the temporal behaviour of the laser pulses was documented. In addition, the influence of different irradiation parameters for pulse durations of 10 µs, 100 µs and 1 ms at duty-cycles of 50%, 25% and 15% on porcine tissue was investigated. With regard to the cutting depth and thermal damage zones, cuts were carried out in the contact mode using an optical fiber with a core diameter of 375 µm and a cut speed of 3 mm/s. The average power was set between 2 W and 8 W. Output powers of up to 12.7 W in CW mode and peak powers > 30 W, with pulse durations of 5 and 10 µs, were achieved. Cuts with a depth of up to 1 mm could be created using 100 µs pulses with different duty-cycles and also in CW mode. The width of the thermal damage zone was between 200 and 350 µm for all settings. The 445 nm wavelength is characterized by a large number of possible advantages in surgical applications. Pulse durations down to the µs-range with peak power approximately three times higher than average power could open up new applications.

1. Introduction

Due to their versatility, diode lasers have been established in different parts of medicine. Application areas can be found in ophthalmology, dentistry and otolaryngology (ear–nose–throat (ENT) surgery), respectively. The wavelengths are available from the visible (VIS) to the near infrared (NIR). Examples of applications include retinal treatments, such as retinal disorders at a wavelength of 514 nm [1,2], adhesion coagulation of retinal defects or retinal detachments, cholesteatoma surgery at 532 nm [3,4,5] and caries diagnostics at 655 nm [6] for the visible spectral range. In periodontology, the NIR wavelengths 810, 940, 980 nm are used [7], and the wavelength 1470 nm is used, for example, in the treatment of glottic carcinoma [8].
In dentistry and otolaryngology, the laser wavelength of 445 nm (VIS, blue) has also become established [9,10,11,12]. In otolaryngology in particular, however, this wavelength is in direct competition with the Q-switched and frequency-doubled neodymium:YAG (Nd:YAG) and neodymium:vanadate lasers (Nd:YVO4) (often erroneously referred to in the literature as “KTP” lasers, both with an emission wavelength of 532 nm) [13,14]. With pulse durations in the lower nanosecond range (ns), high peak-pulse powers can be generated with neodymium lasers, which cause intensive light–tissue interactions and can lead to a low heat input into the tissue due to, e.g., photomechanical ablation.
The basic frequency of these Q-switched and frequency-doubled lasers is in the kHz range. However, in order to ensure a controlled medical application with adapted dosing of the applied energy, pulse trains in the order of milliseconds (ms) are generated with an adapted average power and pulse repetition rates in the lower Hertz range [15,16]. Within the pulse trains (ms), however, the pulses (ns) remain at the basic frequency.
For diode lasers, pulse peak-power levels as high as those of Q-switched solid-state lasers cannot be realized. However, technical developments have made it possible to achieve pulse durations down to the µs-range together with increased peak power.

Operation Modes for Diode Lasers

In diode laser technology, different operating modes are possible. Usually, the continuous wave mode (CW) and the “classic” pulse mode (also called “chopped” or “gate” mode) with pulse durations in the millisecond (ms) range are used. However, the use of diode lasers in the so-called “over-pulsing” mode is unusual. In this case, the standard operating current of the diode is briefly exceeded, i.e., it is operated above its destruction threshold. As a result, pulses are generated whose output power is in the higher watt range (>30 W), with pulse durations in the order of microseconds (µs). Figure 1 illustrates this situation. Initial applications can be found in dentistry at a wavelength of 810 nm [17,18,19], but applications have not been realized for 445 nm diode lasers yet.
This paper presents and characterizes a diode laser system with a wavelength of 445 nm, peak-pulse powers > 30 W, pulse repetition rates in the order of kilohertz and pulse durations of down to 1 µs. Initial laser cuts on porcine tissue were performed in contact mode and histologically analyzed.

2. Materials and Methods

2.1. Laser Source

The radiation source consists of three individual diode laser emitters of type NDB7Y75 from Nichia (Nichia Corporation, Anan, Japan), which are arranged as a diode laser stack (Figure 2).
Each individual diode laser emitter emits an output power of >4.5 W at a center wavelength of 455 nm. This results in 13.5 W full output power in CW mode. The arrangement of emitter stack, driver electronics, optics and fiber coupling was custom-made by A.R.C. Laser (A.R.C. Laser GmbH, Nuremberg, Germany) (Figure 3).
The power regulation of the laser system for the CW and pulse operation modes was carried out by the so-called set point voltage (SPV) (voltage regulation), generated by the use of two Volt/Ampere meters, type MP710530 (MulticompPRO, Chicago, IL, USA).
A function generator model AFG-2025 (Gw Instek, New Taipei, Taiwan) was used to generate the laser pulses. This allows both the pulse repetition rate and the duty-cycle (DC) to be set individually. From the combination of pulse repetition rate f and DC, pulse durations τ in the range from 1 µs up to 900 ms were ultimately generated (Table 1). Mathematically, the relationships can be described as follows:
D C = τ T τ ∙ f
P p e a k = P a v g D C = P a v g τ ∙ f
where T is the pulse period, Pavg is average power and Ppeak denotes peak power.
The pulses generated with the function generator are emitted continuously without interruption if the laser is switched on. To generate defined pulse trains in the millisecond (ms) range, the continuously emitted pulse sequence is now systematically interrupted by the use of an additional custom-made pulse generator (A.R.C. Laser GmbH, Nuremberg, Germany). Figure 4 illustrates this situation.
For light transmission, step-index bare fibers with diameters of 283/300 µm and 375/405 µm (core/cladding) from heracle (heracle GmbH, Jena, Germany) were used. The usable wavelength range of these fibers is between 400 and 1000 nm with a numerical aperture (NA) of 0.22. These fibers were used for the power calibration of the laser system and for the subsequent application.

2.2. Technical Equipment

The power measurement was carried out with a LabMax Top energy and power meter from Coherent (Santa Clara, CA, USA). An associated PM10 power sensor (Coherent, Santa Clara, CA, USA) was used for power measurement. Due to the high pulse repetition rates (up to the kHz range), the output power was recorded over a period of 10 s. The resulting mean value was used as the value for the output power Pavg to be displayed.
The temporal behaviour of the laser pulses (single pulses and pulse trains) was detected using a Thorlabs photodetector (Model PDA36A(-EC), Newton, NJ, USA). The rise time was calculated according to the manufacturer’s specifications as trise = 30 ns and is, therefore, 2 orders of magnitude below the pulse durations of the laser system (µs).
Oscilloscopes from Tektronix Inc. (Type TDS 2024, Beaverton, OR, USA) and Teledyne LeCroy (Type T3DSO1204, Chestnut Ridge, NY, USA) were used to display the temporal pulse profiles.

2.3. Laser Properties/Parameter Selection

In the first step, the properties of the pulsed laser system and the relationships of laser output power with the pulse duration, pulse repetition rate and the duty-cycle (Equations (1) and (2)) were determined. For this purpose, parameters were chosen (Table 1) to characterize the laser system based on the measurement of the maximum achievable laser output power.
The duty-cycle of 10% (0.1) was used as an example from Table 1 to generate defined pulse trains. For each setting from τ = 1 µs to τ = 100 ms, pulse trains with a DCtrain of 50% and 25% were generated for 2, 5 and 10 Hz, respectively, resulting in a total of 66 measurements.

2.4. Sample Preparation and Sample Number

Regarding the utilization and the evaluation of this laser system in the field of otorhinolaryngology (ear, nose and throat (ENT) application), ear samples from freshly slaughtered pigs (German landrace, 8 weeks old, mass 30–40 kg) were used within 2 h postmortem. Approx. 6 tissue blocks of 1.5 × 2 cm were cut from the pig ear in its entire thickness. Until the irradiation was carried out, the samples were covered with gauze saturated with a sterile rinsing solution (0.9% physiological saline solution, 0.01‰ sodium azide, Merck, Darmstadt, Germany) and stored cooled at 4 °C [20].
The number of samples depended on the parameter set of the output power, pulse duration and duty-cycle. The average output power was chosen between 2 and 8 W in increments of 0.5 W. Pulse durations of 10 µs, 100 µs and 1 ms, as well as CW as a control, were selected. In order to achieve 8 W maximum output power (Pavg) for all pulse durations (i.e., photoangiolytic and cutting properties in laryngeal surgery [21]), a duty-cycle of 50% (100% for CW) was needed. This resulted in a total of 52 cuts.
In order to compare the thermal effects on the tissue at different duty-cycles, additional tissue cuts were created at a fixed pulse duration and duty-cycles of 25% and 15%. As it was no longer possible to achieve an average power of 8 watts under these conditions, cuts were made from 2 watts to the maximum achievable power in 1-watt intervals, resulting in an additional 24 cuts (Table 2).
To enable statistical analysis of the results, each irradiation series was performed five times, resulting in a total of 380 cuts.

2.5. Experimental Setup and Method of Irradiation

By standardization of all influenceable variables (mechanical and optical parts) a maximum reproducibility during the irradiation procedure was achieved. This includes the cut direction, cut speed, cut distances between two cuts, fiber distance to sample surface in the contact mode and control of the laser output power. Tissue samples were embedded and fixed in a magnet holder, filled with a plastic silicone mass (Contrast putty soft, VOCO GmbH, Cuxhaven, Germany) [20]. Due to the corrugated surface of the samples, a 3D-printed plastic enclosure including cut slits was used to cover the magnet holder including the tissue sample (Figure 5).
Therefore, the irradiated surface of the samples was nearly at the same level. The magnet holder was attached to a 3-dimensional computer-controlled micropositioner (VT-80, Micos, Eschbach, Germany). During irradiation, the sample was moved constantly at a speed of 3 mm/s. The optical fiber was guided inside a handpiece and also through an additional attached metallic sleeve, whereby the sleeve showed an inclination angle of 30° to the sample surface. Under these conditions, the fiber was located at a fixed position above the tissue sample [20]. The sample movement itself was in a parallel orientation along the cut slits of the plastic enclosure. The applicator used was an optical fiber with 375/405 µm (core/cladding) diameters manufactured by heracle (heracle GmbH, Jena, Germany) with NA = 0.22. Additionally, the fiber distance to sample surface in the contact mode was visually controlled in real time by an Elio-Microscope camera system (Ekler, Paris, France).
To verify the emitted laser output power at the distal end of the optical fiber. a comparison with the aid of the calibration value was performed. A new configuration of the fiber tip became necessary if the output deviations exceeded the 10% limit. After the irradiations, the tissue samples were stored in 4% formaldehyde solution (Merck, Darmstadt, Germany) [20].

2.6. Histological Preparation and Evaluation

For this first investigation, only cuts with a continuously pulsed laser were examined, without using the pulse train technology. Methods for histological preparation of the tissue samples and the evaluation procedure to determine the thermal damage zones have been described in [20]. In addition to the width of the thermal damage zone, the depth of cut was also evaluated.

2.7. Statistical Evaluation

Cut depths and damage zones were recorded in tabular form using Microsoft Excel (Office 2016, Seattle, WA, USA) together with the corresponding laser parameters. Data evaluation was carried out descriptively. Average values and standard deviations were determined using the calculation and graphics program OriginPro 8G (OriginLab Corporation, Northampton, MA, USA). Graphical presentations of the cut depths and damage zones were carried out as well with OriginPro 8G. For visualization of the data behaviours, splines and linear fit functions were used. Furthermore, cut depths were visualized as a function of power, pulse duration and duty-cycle in the form of default bar charts.

3. Results

3.1. Power Characteristics

The laser powers were determined for both continuous wave (CW) and pulsed operation modes at duty-cycles from 1% up to 30%. Figure 6 shows the emitted laser power values as a function of the set point voltage. For pulsed operation mode, the duty-cycle of 15% is shown here as an example.
In the case of measurement of the output power in the continuous wave operation mode, a value of 12.7 W was achieved. Therefore, the real power value is 0.8 W below the manufacturer’s specifications of 13.5 W. Nevertheless, the discrepancy is within the acceptable limit of 20% and, therefore, in accordance with the guidelines of the International Electrotechnical Commission (IEC) 60601-2-22 [22]. Figure 7 shows the achievable power values (peak power) depending on the different pulse durations.
For pulse durations of 5 µs and 10 µs and duty-cycles of 5 and 10%, maximum peak powers of up to 34.5 W were achieved. Figure 8 illustrates the relationship between the set point voltage (SPV), the achievable average power of these two pulse durations and the resulting peak powers. The single pulses in the millisecond range (ms) show a linear increase in the average power with increasing duty-cycle up to the CW limit line. As expected, the resulting peak values according to Equation (2) are above the CW limit line and increase with decreasing pulse duration.

3.2. Temporal Behaviour of the Laser Pulses

The temporal behaviour of the generated laser pulses shows, for all settings, a rectangular form (Figure 9), with rise and fall times in the nanosecond (ns) range. Power drops at higher set point voltages (SPV) can be detected for certain settings via the temporal behaviour of the laser pulse. Figure 10 illustrates this behaviour using the example of a single laser pulse with a duty-cycle of 15% and a pulse duration of 150 ms (see also Figure 6).

3.3. Pulse Trains

An exemplary illustration (Figure 11) shows the expected behaviour of the generated pulse trains at a DCtrain of 50% and 25%. Accordingly, the average power values generated here were 1.65 W and 0.83 W, based on the average output power in normal pulse mode of 3.3 W (see Figure 8 below). However, the pulse trains of 2, 5 and 10 Hz generated within a DCtrain with corresponding pulse durations in the order of milliseconds cannot be distinguished.

3.4. Tissue Irradiation

The cut depths achieved at different average power levels with pulsed and CW irradiation are shown in Figure 12; on the left depending on the DC for different pulse durations, on the right depending on the pulse durations for different DCs. For all pulse durations and a DC of 50%, cut depths of ≥1 mm were achieved. Furthermore, cut depths in the ranges of 500 µm–1 mm and 250–700 µm were achieved for DC = 25% and DC = 15%, respectively. The greatest cut depths were achieved with the pulse duration of 100 µs for all duty-cycles, including CW operation mode. This also can be verified from Table 3. As expected, the overall trend shows increases in cut depth with increasing average output power.
In general, all operating parameters show an increase in thermal damage in accordance with the rise of the laser output power from 2 W to 8 W (Figure 13). Taking into account the limited number of samples and the range of fluctuation, no significant differences can be seen between the various pulsed settings. Towards higher average power, the curve of the width of the thermal damage zone of the CW irradiation shows a steeper increase than that of the pulsed settings. However, the standard deviations of the curves overlap.

4. Discussion

Diode lasers are characterized by their great versatility. Their bandwidths in terms of wavelength (ultraviolet 375 nm up to mid-infrared > 10 µm), emission power (>1 W per emitter) and operating mode (CW or pulsed) prove to be enormously advantageous, especially in the medical field. Both their size and the possibility of connecting optical fibers directly to these laser systems predestine them for practical applications in medicine. However, these laser systems are also subject to continuous technical development to improve and expand their range of applications. This can be seen in the generation of µs-pulses for the wavelength 445 nm which, hitherto, has been technically implemented only for the wavelength 810 nm in dentistry [17,18,19].
Regarding the lifetime of the laser diodes used, no reduction in the laser output power could be observed so far in the over-pulsing operation mode. In fact, two distinct damage mechanisms which can be responsible for the destruction of a laser diode should be taken into consideration: optical as well as electrical overloads. The optical overload describes the destruction of the optical resonator induced by exceeding the resonator threshold based on high-intracavity light intensities (vaporization of the mirror surface), whereas the electrical overload describes the destruction of the p-n-junction caused by a localized overheating effect. Both phenomena can be provoked by an over-voltage or an over-current condition, especially in low-power laser diodes with an output power ≤ 200 mW [23]. According to the manufacturer’s information [oral communication with A.R.C. Laser GmbH], no limitations could be observed after testing the laser diodes used for approx. 10,000 h in an over-current condition of 20% above standard operation. The characterization of the used laser diodes showed only a thermal influence at high set point voltages, resulting in a saturation behaviour of the output power in CW operation mode, as well as for the over-pulsing mode, as illustrated in Figure 6 and Figure 8. Therefore, an efficient cooling system is of importance to guarantee a long lifetime independent of the mode of operation.
The wavelength 445 nm is highly absorbed in different tissue structures, primarily in hemoglobin (µA(HbO2) = 440 cm−1; µA(Hb) = 1526.7 cm−1) and melanin (µA(Skin) = 1033 cm−1) [24,25]. The absorption in water is negligible (µA(H2O) = 2.85 × 10−4 cm−1) [26]. This results in strong interactions between laser radiation and the blood-carrying tissue layers, which can be verified in the effect of tissue cutting (Figure 12 and Table 3).
Hanke et al. [20] compared eight different laser wavelengths (405, 445, 514, 532, 810, 980, 1064 and 1470 nm) for surgical applications in soft tissue. They investigated cutting effects and thermal influences depending on the laser wavelengths. Finally, a new parameter, the so-called “efficiency factor” was defined. This factor enables researchers to make forecasts about the expected tissue interaction processes. In this work, very promising results for soft tissue cutting were achieved for 445, 514 and 532 nm.
Furthermore, a bactericidal effect for 445 nm could be verified for different dental applications, e.g., cavity disinfection within the scope of restorative dentistry [27], and decontamination of implants [28], as well as in the field of endodontics [29]. Additionally, the irradiation of dental enamel with the laser wavelength of 445 nm in the field of caries prevention shows encouraging results [30].
With the achievable pulse peak power, the tissue ablation mechanism is purely thermal (unlike with pulsed, Q-switched solid-state lasers). The energy of a single 10 µs pulse is not sufficient for significant ablation. Therefore, µs-pulses are applied with frequencies in the kHz range. Considering typical thermal relaxation times for different kind of tissues (aorta: 1 ms–550 ms [31], kidney: 1 ms–520 ms [32], calculated on the basis of [31,33]), heat accumulates over several pulses.
For ms-pulses, ablation is possible starting with single pulses. The thermal nature of the ablation with all settings is confirmed by the results of the cutting depth achieved with different pulse parameters or CW radiation. The values for the different pulse and CW settings do not differ significantly, taking into account the limited number of samples and range of fluctuation. In addition, a linear increase in the cutting depth with the average power, which is to be expected for thermal ablation, is confirmed.
With a thermal relaxation time of >200 ms, one would also expect the same thermal damage zone for all experiments. This can actually be seen for the various pulse settings (10 µs/100 µs/1 ms, see Figure 13). However, the linear fit of the CW data shows a steeper increase, indicating that for average power > 4 W, the thermal damage zone might be deeper than for pulsed irradiation. The diameter of the laser spot (~400 µm) is moved across a tissue site within approx. 130 ms at a fiber speed of 3 mm/s; therefore, it could be in a range at which there is higher thermal diffusion into the surrounding tissue. This should then actually be reflected in a reduction in the depth of cut, which is not the case. Considering the limited number of samples and the range of variability in the results for one parameter set, this trend has yet to be verified and needs to be further investigated.
Switching the laser pulses in pulse trains allows further flexibility (Figure 11). The pulse train acts as an external on/off switch for the laser, whereby the basic frequency (kHz with µs-pulses) is maintained within the pulse train. Here also, further investigations should be carried out.

5. Conclusions

The technical development of diode lasers, particularly those with a wavelength of 445 nm, is proving to be a sensible step for medical applications. The realization of µs-pulses with peak powers a factor of approx. three times higher than their average power will allow their use in new application areas where a precise, low-energy input in the µs-range is required. At the same time, a pulse repetition rate up to the kHz range makes it possible to achieve the average power required for thermal ablation during tissue cutting. By switching from µs-pulses with a kHz repetition rate in pulse trains, it is possible to flexibly adapt the temporal course of the energy input.

Author Contributions

Conceptualization, J.M.; methodology, Z.L., Y.L. and J.M.; Formal analysis, Z.L. and Y.L.; investigation, Z.L. and Y.L.; data curation, J.M.; writing—original draft preparation, J.M.; writing—review and editing, J.M. and B.L.; visualization, Z.L., Y.L., J.M. and B.L.; supervision, J.M.; project administration, J.M. and B.L. All authors have read and agreed to the published version of the manuscript.

Funding

A.R.C Laser GmbH and the Medical Laser Center Lübeck were funded by the Federal Ministry of Education and Research Germany (Grant-No.: 13N15848 & 13N15846).

Institutional Review Board Statement

This research does not involve human samples nor par-ticipants nor animals that would necessitate informed consent documents or approval of the local ethics committee.

Informed Consent Statement

Experimental study type.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors would like to thank A.R.C. Laser GmbH for the technical support to realize this scientific investigation.

Conflicts of Interest

The authors declare no conflicts of interest.

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Applsci 15 01041 g001
Figure 1. Illustration of the different operating modes for diode lasers.
Figure 1. Illustration of the different operating modes for diode lasers.
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Figure 2. Arrangement of the diode laser emitter.
Figure 2. Arrangement of the diode laser emitter.
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Figure 3. Setup of the experimental laser head.
Figure 3. Setup of the experimental laser head.
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Figure 4. Illustration of a pulse train (detected with a Thorlabs photodetector (Model PDA36A(-EC), Newton, NJ, USA)) with a pulse duration of 100 ms (left). On the right in the magnification, the beginning and end of the pulse train is shown in a high temporal resolution, where the individual pulses (DC = 10% at 20 kHz) can be recognized within the pulse train. Therefore, a total of 2000 pulses with a pulse duration of 5 µs are generated.
Figure 4. Illustration of a pulse train (detected with a Thorlabs photodetector (Model PDA36A(-EC), Newton, NJ, USA)) with a pulse duration of 100 ms (left). On the right in the magnification, the beginning and end of the pulse train is shown in a high temporal resolution, where the individual pulses (DC = 10% at 20 kHz) can be recognized within the pulse train. Therefore, a total of 2000 pulses with a pulse duration of 5 µs are generated.
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Figure 5. 3D-printed plastic enclosure to keep the irradiated surface of the tissue samples nearly at the same level. The slit width for cutting is 4 mm, and the row spacing between two cuts is in the order of 6 mm to prevent thermal influence from one cut to the next.
Figure 5. 3D-printed plastic enclosure to keep the irradiated surface of the tissue samples nearly at the same level. The slit width for cutting is 4 mm, and the row spacing between two cuts is in the order of 6 mm to prevent thermal influence from one cut to the next.
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Figure 6. Emitted average output powers for CW and pulse operation modes for the example of the 15% duty-cycle and corresponding pulse durations from Table 1.
Figure 6. Emitted average output powers for CW and pulse operation modes for the example of the 15% duty-cycle and corresponding pulse durations from Table 1.
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Figure 7. Graph of the pulse peak power Ppeak as a function of the duty-cycle and the pulse duration (above the CW limit line). Below the CW limit line, the average output power Pavg of the single pulses (1 Hz) is shown as a function of the pulse duration. All curves approach the CW limit line with increasing duty-cycle.
Figure 7. Graph of the pulse peak power Ppeak as a function of the duty-cycle and the pulse duration (above the CW limit line). Below the CW limit line, the average output power Pavg of the single pulses (1 Hz) is shown as a function of the pulse duration. All curves approach the CW limit line with increasing duty-cycle.
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Figure 8. Diagram of the average output powers for the pulse durations 5 µs (top) and 10 µs (bottom) as a function of the various duty-cycles with the peak powers Ppeak achieved.
Figure 8. Diagram of the average output powers for the pulse durations 5 µs (top) and 10 µs (bottom) as a function of the various duty-cycles with the peak powers Ppeak achieved.
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Figure 9. Example of the temporal behaviour of emitted laser pulses with a pulse duration of 5 µs at full-width at half-maximum (FWHM). The pulse shows nearly a rectangular shape with a rise time in the nanosecond range.
Figure 9. Example of the temporal behaviour of emitted laser pulses with a pulse duration of 5 µs at full-width at half-maximum (FWHM). The pulse shows nearly a rectangular shape with a rise time in the nanosecond range.
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Figure 10. Temporal behaviour of the emitted laser pulses immediately before and after exceeding the power maximum (2.1 W at 500 mV SPV → 1.9 W at 520 mV SPV).
Figure 10. Temporal behaviour of the emitted laser pulses immediately before and after exceeding the power maximum (2.1 W at 500 mV SPV → 1.9 W at 520 mV SPV).
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Figure 11. Example of generated pulse trains in the ms-range with duty-cycles of 50% and 25%. The emitted average powers are correspondingly 50% and 25% below the continuously pulsed average output power of 3.3 W, whereby the pulse repetition rates (2, 5 and 10 Hz) emitted in the respective duty-cycles cannot be distinguished within a DCtrain.
Figure 11. Example of generated pulse trains in the ms-range with duty-cycles of 50% and 25%. The emitted average powers are correspondingly 50% and 25% below the continuously pulsed average output power of 3.3 W, whereby the pulse repetition rates (2, 5 and 10 Hz) emitted in the respective duty-cycles cannot be distinguished within a DCtrain.
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Figure 12. Graphs show the cut depths depending on the different duty-cycles for pulse durations of 1 ms, 100 and 10 µs (left side) as well as for the different pulse durations for the corresponding duty-cycles (right side). Additionally, trend curves of the cut depth for continuous wave operation are included in all graphs (for a better overview without error bars), which are also in the same order of magnitude as for the pulsed operation.
Figure 12. Graphs show the cut depths depending on the different duty-cycles for pulse durations of 1 ms, 100 and 10 µs (left side) as well as for the different pulse durations for the corresponding duty-cycles (right side). Additionally, trend curves of the cut depth for continuous wave operation are included in all graphs (for a better overview without error bars), which are also in the same order of magnitude as for the pulsed operation.
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Figure 13. Depth of the thermal damage zones below the incisions. Graphs show the dependency of the different duty-cycles for pulse durations of 10 µs, 100 µs and 1 ms, (left side) as well as on different pulse durations for the corresponding duty-cycles (right side). Additionally, trend curves of the thermal damage zones for continuous wave operation are included in all graphs (for a better overview without error bars), which also are in the same order of magnitude as for the pulsed operation.
Figure 13. Depth of the thermal damage zones below the incisions. Graphs show the dependency of the different duty-cycles for pulse durations of 10 µs, 100 µs and 1 ms, (left side) as well as on different pulse durations for the corresponding duty-cycles (right side). Additionally, trend curves of the thermal damage zones for continuous wave operation are included in all graphs (for a better overview without error bars), which also are in the same order of magnitude as for the pulsed operation.
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Table 1. Parameter selection to determine the maximum achievable output power of the diode laser in the over-pulsing mode depending on the different duty-cycles and pulse durations (τ).
Table 1. Parameter selection to determine the maximum achievable output power of the diode laser in the over-pulsing mode depending on the different duty-cycles and pulse durations (τ).
DC = 1% (0.01)
10 kHz (1 µs)2 kHz (5 µs)1 kHz (10 µs)200 Hz (50 µs)133.3 Hz (75 µs)100 Hz (100 µs)66.7 Hz (150 µs)50 Hz (200 µs)40 Hz (250 µs)25 Hz (400 µs)1 Hz (10 ms)
DC = 5% (0.05)
50 kHz (1 µs)10 kHz (5 µs)5 kHz (10 µs)1 kHz (50 µs)666.7 Hz (75 µs)500 Hz (100 µs)333.3 Hz (150 µs)250 Hz (200 µs)200 Hz (250 µs)125 Hz (400 µs)1 Hz (50 ms)
DC = 10% (0.10)
100 kHz (1 µs)20 kHz (5 µs)10 kHz (10 µs)2 kHz (50 µs)1.33 kHz (75 µs)1 kHz (100 µs)666.7 Hz (150 µs)500 Hz (200 µs)400 Hz (250 µs)250 Hz (400 µs)1 Hz (100 ms)
DC = 15% (0.15)
150 kHz (1 µs)30 kHz (5 µs)15 kHz (10 µs)3 kHz (50 µs)2 kHz (75 µs)1.5 kHz (100 µs)1 kHz (150 µs)750 Hz (200 µs)600 Hz (250 µs)375 Hz (400 µs)1 Hz (150 ms)
DC = 20% (0.20)
200 kHz (1 µs)40 kHz (5 µs)20 kHz (10 µs)4 kHz (50 µs)2.67 kHz (75 µs)2 kHz (100 µs)1.33 kHz (150 µs)1 kHz (200 µs)800 Hz (250 µs)500 Hz (400 µs)1 Hz (200 ms)
DC = 25% (0.25)
250 kHz (1 µs)50 kHz (5 µs)25 kHz (10 µs)5 kHz (50 µs)3.33 kHz (75 µs)2.5 kHz (100 µs)1.67 kHz (150 µs)1.25 kHz (200 µs)1 kHz (250 µs)625 Hz (400 µs)1 Hz (250 ms)
DC = 30% (0.30)
300 kHz (1 µs)60 kHz (5 µs)30 kHz (10 µs)6 kHz (50 µs)4 kHz (75 µs)3 kHz (100 µs)2 kHz (150 µs)1.5 kHz (200 µs)1.2 kHz (250 µs)750 Hz (400 µs)1 Hz (300 ms)
DC = 40% (0.40)
40 kHz (10 µs) 1 Hz (400 ms)
DC = 50% (0.50)
50 kHz (10 µs) 1 Hz (500 ms)
DC = 60% (0.60)
60 kHz (10 µs) 1 Hz (600 ms)
DC = 70% (0.70)
70 kHz (10 µs) 1 Hz (700 ms)
DC = 80% (0.80)
80 kHz (10 µs) 1 Hz (800 ms)
DC = 90% (0.90)
90 kHz (10 µs) 1 Hz (900 ms)
Table 2. Parameter selection for tissue irradiation in the pulsed mode (without using pulse train mode).
Table 2. Parameter selection for tissue irradiation in the pulsed mode (without using pulse train mode).
DC = 50%
Rep. rate
(τ)		Power [W]Rep. Rate
(τ)		Power [W]Rep. rate
(τ)		Power [W]
50 kHz
(10 µs)		2.0–8.0
(increment 0.5 W)		5 kHz
(100 µs)		2.0–8.0
(increment 0.5 W)		500 Hz
(1 ms)		2.0–8.0
(increment 0.5 W)
DC = 25%
25 kHz
(10 µs)		2.0–6.7
(increment 1 W)		2.5 kHz
(100 µs)		2.0–6.0
(increment 1 W)		250 Hz
(1 ms)		2.0–5.0
(increment 1 W)
DC = 15%
15 kHz
(10 µs)		2.0–4.6
(increment 1 W)		1.5 kHz
(100 µs)		2.0–3.3
(increment 1 W)		150 Hz
(1 ms)		2.0–3.0
(increment 1 W)
Table 3. Overview of the histological sections depending on power (left column), pulse duration (upper row) and DC (second row).
Table 3. Overview of the histological sections depending on power (left column), pulse duration (upper row) and DC (second row).
CWτ = 10 µsτ = 100 µsτ = 1 ms
100%15%25%50%15%25%50%15%25%50%
2 WApplsci 15 01041 i001Applsci 15 01041 i002Applsci 15 01041 i003Applsci 15 01041 i004Applsci 15 01041 i005Applsci 15 01041 i006Applsci 15 01041 i007Applsci 15 01041 i008Applsci 15 01041 i009Applsci 15 01041 i010
3 WApplsci 15 01041 i011Applsci 15 01041 i012Applsci 15 01041 i013Applsci 15 01041 i014Applsci 15 01041 i015Applsci 15 01041 i016Applsci 15 01041 i017Applsci 15 01041 i018Applsci 15 01041 i019Applsci 15 01041 i020
4 WApplsci 15 01041 i021Applsci 15 01041 i022Applsci 15 01041 i023Applsci 15 01041 i024no available
power setting		Applsci 15 01041 i025Applsci 15 01041 i026no available
power setting		Applsci 15 01041 i027Applsci 15 01041 i028
5 WApplsci 15 01041 i029no available
power setting		 Applsci 15 01041 i030no available
power setting		Applsci 15 01041 i031Applsci 15 01041 i032no available
power setting		Applsci 15 01041 i033Applsci 15 01041 i034
6 W no available
power setting		Applsci 15 01041 i035Applsci 15 01041 i036no available
power setting		Applsci 15 01041 i037Applsci 15 01041 i038no available
power setting		no available
power setting		Applsci 15 01041 i039
7 WApplsci 15 01041 i040no available
power setting		no available
power setting		Applsci 15 01041 i041no available
power setting		no available
power setting		Applsci 15 01041 i042no available
power setting		no available
power setting		Applsci 15 01041 i043
8 WApplsci 15 01041 i044no available
power setting		no available
power setting		Applsci 15 01041 i045no available
power setting		no available
power setting		Applsci 15 01041 i046no available
power setting		no available
power setting		Applsci 15 01041 i047
Remark: The green bar mark (lower left-hand corner) is in the size of 500 µm. Exceptions are for the images of 7 W and 8 W in CW mode. Here, the magnification is reduced by a factor of 2 (size: 1 mm).
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MDPI and ACS Style

Liu, Z.; Lu, Y.; Lange, B.; Meister, J. Technical Characterization of a High-Power Diode Laser at 445 nm for Medical Applications: From Continuous Wave Down to Pulse Durations in the µs-Range. Appl. Sci. 2025, 15, 1041. https://doi.org/10.3390/app15031041

AMA Style

Liu Z, Lu Y, Lange B, Meister J. Technical Characterization of a High-Power Diode Laser at 445 nm for Medical Applications: From Continuous Wave Down to Pulse Durations in the µs-Range. Applied Sciences. 2025; 15(3):1041. https://doi.org/10.3390/app15031041

Chicago/Turabian Style

Liu, Zhaoqun, Yunfan Lu, Birgit Lange, and Jörg Meister. 2025. "Technical Characterization of a High-Power Diode Laser at 445 nm for Medical Applications: From Continuous Wave Down to Pulse Durations in the µs-Range" Applied Sciences 15, no. 3: 1041. https://doi.org/10.3390/app15031041

APA Style

Liu, Z., Lu, Y., Lange, B., & Meister, J. (2025). Technical Characterization of a High-Power Diode Laser at 445 nm for Medical Applications: From Continuous Wave Down to Pulse Durations in the µs-Range. Applied Sciences, 15(3), 1041. https://doi.org/10.3390/app15031041

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