Med Lasers 2022; 11(4): 229-235  https://doi.org/10.25289/ML.22.034
Development of 250 picosecond Nd:YAG laser for dermatologic applications in Republic of Korea: development report
Hanyoung Ryu, Jiyoung Lee, Youngseok Seo
R&D Center, WONTECH Co., Ltd., Daejeon, Republic of Korea
Correspondence to: Hanyoung Ryu
E-mail: hyryu@wtlaser.com
ORCID: https://orcid.org/0000-0002-5788-878X
Received: July 20, 2022; Accepted: August 30, 2022; Published online: December 30, 2022.
© Korean Society for Laser Medicine and Surgery. All rights reserved.

This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted noncommercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract
Background and Objectives: Picosecond laser therapy is a fast-growing field in cosmetic dermatology. This paper presents the “PICOCARE MAJESTY”, a picosecond laser system with a pulse duration of 250 ps at 1,064 nm, a peak power of 2 GW, and an energy instability of less than ±5% over 30 minutes of operation.
Materials and Methods: The picosecond laser system consists of a master oscillator power amplifier based on a diode-pumped microchip picosecond laser, a beam expander, an optical isolator, a polarizer, a pre & power amplifier, and a highly reflective coated mirror. The articulated arm was used to deliver the treatment beam and a collimating beam to the skin surface. The system is designed to automatically recognize the type of handpiece and laser spot size to improve user convenience.
Results: We have developed “PICOCARE MAJESTY”, a picosecond 1,064 nm/532 nm Nd:YAG laser system that provides pulses of duration 250 ps/190 ps and has a maximum energy of 500 mJ/250 mJ at wavelengths of 1,064 and 532 nm, respectively. In addition, six types of handpieces, Zoom, Micro Lens Array, Collimation, Dye 595 nm and 660 nm, and Diffraction Optical Element were developed to be recognized automatically.
Conclusion: WONTECH developed the first commercialized 250 picosecond Nd:YAG laser in Republic of Korea. The Ministry of Food and Drug Safety (Republic of Korea) and the Food and Drug Administration (U.S.) approved the MAJESTY laser device in July 2021 and January 2022, respectively.
Keywords: Picosecond laser; Nd:YAG laser; Master oscillator; Potassium titanyl phosphate crystal; Articulated arm
INTRODUCTION

In the past decades, lasers with shorter pulses have developed various industries, including medical, defense, semiconductor, and manufacturing. Since the selective photothermolysis, different medical laser devices have been developed to more precisely achieve the challenge of treating pigmented [1]. In dermatology, picosecond lasers treat lesions with 1/3 to 1/2 of the energy used in nanosecond lasers. Compared to nanosecond lasers, picosecond lasers have more photoacoustic effects and fewer photothermal effects [2,3]. Therefore, the risk of collateral damage to surrounding tissue is less. More people are trying to achieve younger and healthier appearing faces themselves. Accordingly, picosecond laser treatment is a rapidly growing field in cosmetic dermatology. A 1,064 nm Nd:YAG picosecond laser device, enlighten® (Cutera, USA), was first approved by U.S. food and drug administration (FDA) in November 2014 [4]. Nd:YAG picosecond laser is used for skin rejuvenation, removing tattoos, scar treatment, and pigmented lesions [5,6]. The Nd:YAG lasers have been widely developed and have potential applications in scientific research, medical treatment, etc. The energy level structure of an Nd3+ ion in Nd:YAG is illustrated in Fig. 1. Nd:YAG laser can be operated at wavelengths from 1,000-1,450 nm [7], in many cases around 1,064 nm with the maximum laser gain. The 1,064 nm transition provides the lowest threshold laser lines in Nd:YAG. To realize ultra-short pulse laser, it has adopted a particular combination of many techniques such as Q-switching, mode-locking, and cavity dumping [8]. Generally, Q-switching can implement high peak power but is limited to nanosecond pulse width. Mode-locking is a technique in optics by which a laser can produce pulses with pico or femtosecond scale, but it is not easy to obtain high output energy.

Figure 1. Schematic diagram of the laser head with double pass amplifier. Laser head consists of main components as follows: master oscillator, beam expander, optical isolator, polarizer, pre & power amplifier, KTP (Potassium titanyl phosphate [KTiOPO4]) crystal.

This paper presents a picosecond laser system, “PICOCARE MAJESTY,” with a pulse duration of 250 ps and peak power of 2 GW at 1,064 nm. The picosecond laser system has an energy instability of less than ±5% over a 30 minutes operation. This laser system was Ministry of Food and Drug Safety (MFDS) (Republic of Korea) & FDA (U.S.)-approved for producing a medical device in July 2021 and January 2022, respectively.

MATERIALS AND METHODS
Ethics statement: This study is a technology development paper and does not require Institutional Review Board approval.

Laser head design

The schematic head diagram of the laser system is shown in Fig. 2. The laser head consists of mainly three parts an oscillator, preamplifier, and power amplifier. We generated a picosecond laser pulse of high peak power. We used a master oscillator power amplifier configuration based on a diode-pumped microchip picosecond laser as a master oscillator. The laser head was composed of a master oscillator, beam expander, optical isolator, polarizer, pre & power amplifier, and high reflective coating mirror, as shown in Fig. 1. One Xenon flash lamp is used as a pre & power amplifier pumping source. A separate water-cooling system was designed and applied to maintain a uniform temperature of two amplifiers and flashlamps.

Figure 2. Energy levels and laser transition of the quasi-four-level Nd:YAG laser 7. Nd:YAG laser is a four-level laser system, which means that the four energy levels are involved in laser action. These lasers operate in both pulsed and continuous mode.

Laser amplifier design

The gain medium of an optical amplifier can achieve only a limited amount of gain. A method for obtaining a much higher gain is geometrically arranging multiple passes through an amplifier, which is then called a multi-pass amplifier. The simplest case is that of a double-pass amplifier, where a beam passes the crystal just two times, usually with exactly or nearly opposite propagation directions. Using a double-pass amplification configuration as shown in Fig. 2, we can obtain a laser light with energy over 500 mJ using only one mJ of oscillator energy. In other words, it means that the total gain of the amplifier has reached over 500 mJ. Represents a reduction in the required oscillator energy, concerning the single-pass method, by two orders of magnitude. To make an optimized laser beam profile, we performed the computer simulation of beam propagation according to beam size into the amplifier using an optical design tool. Fig. 3 shows the designed output characteristics for input beam sizes of 5.4 mm to 7 mm. The gain medium used Nd:YAG rod with a 100 mm length and 8 mm diameter. As shown in Fig. 3, a larger beam diameter has a better flatness profile versus a small beam due to the uniform distribution of gain medium. The beam diameter of the laser oscillator is around 1 mm and too small to amplify in the gain medium. To expand the beam diameter of the oscillator, we used a beam expander, as shown in Fig. 4. The beam expander consists of 2 lenses with concave and convex optics lenses of Galilean type, one with a positive focal length and one with a negative focal length. Adjustment of input beam diameter can be tuned convex lens with negative focal length moves backward and forward along the optical axis by length control of beam expander. The performance of a laser amplifier depends on crucial parameters such as collimation beam, thermal lens, depolarization, and amplification gain. We considered the oscillator beam designed for the collimation beam passing through the gain medium in the double pass amplifier configuration. The optimal beam size has a 6.5 mm diameter with a collimated beam, as shown in Fig. 5. A Faraday rotator used between Nd:YAG rod and back mirror instead of a quarter waveplate with common characteristics to reduce the depolarization loss of the oscillator beam.

Figure 3. The output beam characteristics of the laser amplifier according to the input beam size of 5.4 mm, 6 mm, and 7 mm.

Figure 4. The beam size adjustment of the laser amplifier according to the beam expander control. The beam expander consists of 2 lenses with concave and convex optics lenses of Galilean type, one with a positive focal length and one with a negative focal length.

Figure 5. Design of optimized input beam passing through the pre amplifier when optimal beam size has a 6.5 mm diameter with a collimated beam.

Laser cooling system design

The separate water-cooling system of two amplifiers consists of radiator, water pump, fans, water tank, flow sensor, temperature sensor, and de-ionizing & particle filter in Fig. 6. The de-ionizing water is driven from the water tank and particle filter through the laser head (pre & power amplifier) and the heat exchanger. Before passing the two amplifiers, water runs through the filtering process. The de-ionizing filter in the water tank keeps the water always de-ionized and clean.

Figure 6. Separated water-cooling system design of laser device. The water-cooling system of two amplifiers include radiator, water pump, fans, water tank, flow sensor, temperature sensor, and de-ionizing & particle filter. The heated water (red lines) is driven through the heat exchanger in radiator and flows back to the water tank.

After passing through the laser amplifier, the heated water is driven through the heat exchanger and flows back to the water tank. The water-to-air heat exchanger, supported by two alternating current (AC) fans, transfers the heat dissipated from the laser into the atmosphere. The digital thermometer monitors the temperature of the cooling water. The thermometer is located at the water tube coming out right after the laser head, where the temperature of the cooling liquid is the maximum. The temperature information reaches the sub processing unit (SPU) Board and is a primary signal for the cooling fan speed regulation circuit. The speed will reach maximum while the cooling water temperature achieves a program value set (nominal value is 50°C).

The flow switch is constantly monitoring the cooling water flow rate. The flow rate of switch status information is fed to the CPU Board. If the cooling water flow rate drops below the minimum allowed value, the laser system becomes inactive, and a “No flow” error message appears on the screen.

The water pump contains a two-level mechanical switch for water flow rate regulations from low to high. The switch position choice and the corresponding flow level depend on the external temperature conditions to provide a good and reliable cooling regime for operating laser (normally set to the highest speed).

Laser articulated arm design

The articulated arm delivers the Nd:YAG laser treatment beam and the diode laser guide beam to the patient skin surface. The output of the built-in diode laser is a visible low-power beam at a wavelength of 650 nm. Since the treatment beam is invisible, the guide beam allows the operator to see the treatment area to which the Nd:YAG beam will be delivered. It consists of high reflection coated seven mirrors of 1,064 nm & 532 nm mounted at the special tubes freely rotated in different directions and diode laser as shown in Fig. 7. The handpiece with the focusing lens is mounted at the end of the arm. It provides the adjustability of the beam spot size on the patient’s skin. The spot size of the handpiece is automatically detected by changing the handpiece dial. The articulated arm-handpiece unit must always be perfectly aligned.

Figure 7. Arm design of Nd:YAG laser. It consists of high reflection coated seven mirrors of 1,064 nm & 532 nm mounted at the articulated points. The handpiece (H/P) with the focusing lens is mounted at the end of the arm.
RESULTS

We have developed a picosecond 1,064 nm/532 nm Nd:YAG laser system called the “PICOCARE MAJESTY,” as shown in Fig. 8A. The MFDS (Republic of Korea) and FDA (U.S.) approved the MAJESTY laser device in July 2021 and January 2022, respectively. The MAJESTY laser has 250 ps pulse duration and 2 GW peak power at 1,064 nm wavelength. We developed six types of handpieces: Zoom, MLA handpieces with spot sizes ranging from 2 to 10 mm and Collimation, Dye 595 nm/660 nm, and DOE handpieces. Fig. 8B shows the MAJESTY laser handpiece that can use for both wavelengths of 1,064 nm & 532 nm. Table 1 shows the features of developed 1,064 nm picosecond laser specifications in detail. The maximum energy of 500 mJ and 250 mJ was achieved at the wavelength of 1,064 nm and 532 nm, respectively. Fig. 9 presents the measured stability during 30 minutes at the output energy of 500 mJ with a repetition rate of 10 Hz of 8 mm Zoom handpiece. The recorded maximum and minimum energies were 530 mJ and 489 mJ at an average energy of 508 mJ, respectively. The energy instability at the maximum energy of 500 mJ was below 5%, with a standard deviation of 7.7 mJ for operation time. The laser output pulse was obtained with stable linear polarization of output radiation with single longitude and transverse modes. The developed laser typically can be operated at a pulse width of 250 ps with maximum energy of 500 mJ. Inset figure (upper left) in Fig. 9 shows laser output pulse of 1,064 nm measured using a digital oscilloscope (DSA70804; Tektronix, USA) and InGaAs photo-detectors (ET-3500; EOT, USA).

Table 1 . Developed laser system specifications

SpecificationsValue
Wavelength (nm)1,064/532
Laser typePicosecond Nd:YAG laser
Pulse energy (mJ)500/250
Pulse duration (ps)250/190
Peak power2 GW (250 ps), 1.3 GW (190 ps)
Pulse rate (Hz)1-10
Spot size (mm)2-10 (1 mm step)
Start up time (min)<2
Dimensions (width × depth × height)500 × 968 × 907
Weight (kg)100


Figure 8. Picosecond Nd:YAG laser system and handpiece. (A) Laser device has 250 ps pulse duration and 2 GW peak power at 1,064 nm wavelength. (B) Six types of the handpiece: Zoom handpieces with spot sizes ranging from 2 to 10 mm, Collimation and Micro Lens Array, Diffractive Optical Element, and Dye 660 nm/595 nm handpieces from the right.

Figure 9. The measured output energy of 500 mJ during 30 minutes at 10 Hz. Inset figure (upper left) represents the measured laser pulse duration of 250 ps at 1,064 nm. Inset figure (upper right) represents the measured laser beam profile with tophat mode at 1,064 nm.

The thermal lens effect must be compensated to increase of repetition rate. Therefore, we performed a task to stabilize the temperature of the laser head and raised the repetition rate to 10 Hz. Inset figure (upper right) in Fig. 9 shows the beam profile measured using laser cam-HR (Coherent, USA). It offers a tophat-shaped beam pattern and means a uniform spatial distribution.

DISCUSSION

WONTECH developed the first commercially available Nd:YAG 250 picosecond laser in Republic of Korea. Making a high-efficiency Nd:YAG picosecond laser can achieve it through a comprehensive combination of complex optical techniques, thermodynamic design, high power design, high precision driver, and precise control technique. The uniform energy of the beam profile allows safe and effective treatment results to be predicted. When the more powerful peak power is given to the tissue, the temperature of the tissue rises rapidly and it expands by that thermal effect, which causes breaking down the tissue with the stronger shock wave of photomechanical effect. The smaller the broken pigmented particles, the easier it is to be removed by macrophages. Majesty laser delivers a stronger photoacoustic effect to the pigmented particles with the shoreened pulse duration. By the result of breaking down pigments very finely, it gets a faster and better effect in removal of tattoo pigments. Moreover, it is expected to be applied to clinical application as follows: For Stress Relaxation Time (SRT) of 1 μm size melanosome, 250 ps is suitable to be treat the lesion safely by destroying melanosome olny without damaging surrounding tissue, the pulse duration should be shorter than 300 ps [9]. The Thermal Relaxation Time (TRT) for the 10-100 nm Tattoo particles is 100-10 nanoseconds and more safe and effective treatment without thermal damage, the pulse duration needs to be shorter due to the small size of the Tattoo particles. The excellent scar treatment and skin rejuvenation effects can be induced by forming Laser Induced Optical Breakdown (LIOB) more effectively with the shorter the pulse duration and the higher the peak power. Regarding MAJESTY laser, we are currently conducting several clinical trials and the results of these studies will be presented in the near future.

ACKNOWLEDGMENTS

None.

AUTHOR CONTRIBUTIONS

Conceptualization: HR. Data curation: HR, JL. Investigation: HR, JL. Methodology: HR. Project administration: HR. Software: YS. Validation: HR, YS. Visualization: HR, JL. Writing–original draft: HR. Writing–review & editing: all authors.

CONFLICT OF INTEREST

No potential conflict of interest relevant to this article was reported.

FUNDING

None.

DATA AVAILABILITY

Contact the corresponding author for data availability.

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