Variable spot size optical system for a dual-wavelength laser therapy device

1. Introduction

The development of laser technology has expanded the potential applications of lasers in the field of medicine.^1,2,3 For example, laser treatment of vascular diseases, which is based on the selective photolysis of light, employs a specific laser wavelength to focus on the hemoglobin in blood in an attempt to minimize damage to surrounding tissues.⁴ A 532-nm laser is effectively absorbed by hemoglobin; thus, it is the most suitable treatment for superficial vascular and capillary dilation.⁵ Conversely, 940-nm semiconductor lasers penetrate to greater depths and represent the highest absorption wavelength of hemoglobin in the infrared band and the best absorption wavelength for water. Compared with the other laser wavelengths, the 940-nm semiconductor laser exhibits less absorption of melanin so is more suitable for dark skin and deeper blood vessel lesions with a diameter of more than 0.3mm.⁶ At present, the majority of high-power laser therapeutic instruments used in clinical applications are single wavelength; therefore, they cannot fully meet the requirements for clinical treatment.⁷ For example, Dornier Medilas D series for varicose veins, 1470-nm semiconductor laser therapeutic instrument of Biolitec company in Germany for prostate treatment, etc. If two devices are used, the operation is tedious and expensive. At present, most of the existing dual-wavelength laser therapy devices are weak lasers. For example, the 635-nm and 808-nm double wavelength laser medical devices independently developed by Institute of Biomedical Engineering, Chinese Academy of Medical Sciences, the maximum output of multi-channel laser acupuncture instrument is only 0.1W; the 650-nm/830-nm dual-wavelength semiconductor laser therapy device has a maximum output of only 1W for clinical application.

Therefore, to resolve this problem, this study designs and experimentally analyzes an optical system for a dual-wavelength high-power laser therapeutic system with a variable spot size. The therapeutic device has both visible wave and near-infrared wave characteristics. As mentioned before, during treatment, different wavelengths, and spot sizes can be used to accurately target both superficial blood vessels and deeper vessels with larger diameters according to different lesion positions and vessel diameters of the patient. The proposed dual-wavelength laser therapy device has significant clinical applications.

2. Overall Design of the Optical System

The design parameters of the optical system are as follows: laser $\mathrm{\text{wavelength}}=532$nm or 940nm; output $\mathrm{\text{spot}}=1.4$mm or 2.8mm; output power stability of 532nm $\mathrm{\text{beam}}\le 5\%$; the loss of both $\mathrm{\text{wavelengths}}\le 20\%$. A schematic of the optical system of the dual-wavelength high-power laser therapy device is shown in Fig. 1. It comprises an 808-nm laser pump source, a 532-nm resonator, a 940-nm semiconductor laser, a beam combining device, and a spot conversion device. The 532-nm laser adopts semiconductor end pump technology, an 808-nm laser is used for pumping, and the resonant cavity has an internal frequency-doubled folded cavity structure. The 532-nm and 940-nm lasers pass through the beam combining device and enter the optical fiber; the two laser wavelengths are then transmitted to the treatment handpiece through the optical fiber. Then, through the spot transformation device in the treatment handpiece, a laser spot output of different sizes can be produced using different arm lengths.

2.1. The 940-nm laser

The 40-W, 940-nm laser diode medical module of Dilas, Germany, was used in this study with a threshold current equal to 10A. When the working current is 55A, the output power is 40W and the voltage is 1.8V. The module adopts an optical fiber output, the matched optical fiber core diameter is 200$\mu$m, and the numerical aperture is 0.22. The module has red light indication, optical power monitoring photodiode (PD), and optical fiber access monitoring functions.

2.2. High-power intracavity frequency-doubled 532-nm laser

The high-power intracavity frequency-doubled 532-nm laser employs an 808-nm semiconductor laser from the pump source, which is absorbed by an Nd: YVO4 laser crystal and produces 1064-nm oscillation in the optical resonance cavity. It is then transformed into a 532-nm laser output through the nonlinear crystal LBO (LiB₃O${}_5)$. Also, 532-nm and 650-nm indicator lights are coupled into a 400-$\mu$m multimode fiber through the optical coupling system. Finally, a treatment light spot with a diameter of 1.4mm or 2.8mm is formed by the aggregation system in the treatment handpiece. The resonant cavity adopts the internal frequency-doubled folded cavity structure shown in Fig. 2. Prior to the experiments, the lengths of the two arms of the folded cavity are calculated, ignoring the thermal lens effect, and set as fixed values. The parameters of the waist (narrowest part of the beam) of the short arm and G1G2 are calculated using MATLAB software, as shown in Fig. 3.

2.3. Beam combination device

The structure of the beam combining device^8,9,10,11 is shown in Fig. 4. Lens L1 is used for collimation, lens L2 is used for focusing, and a $45^{\circ }$ angle beam combiner is placed at the waist position behind the first lens. The beam combiner is installed near the green laser, and the green light after collimation is directly input to the beam combiner. The 940-nm laser is output by the optical fiber with a SMA905 connector. The coating is anti-reflective at 940nm and highly reactive at 532nm.

The 940-nm semiconductor laser is output by a multimode fiber. Its core diameter is 200$\mu$m and its numerical aperture is 0.22. Propagation of a Gaussian-like beam in a mixed mode is considered for this laser beam, which satisfies the ABCD matrix calculation method. By knowing the fiber parameters, the parameters of this type of Gaussian beam can be obtained; i.e., waist radius ${\omega }_M^{\prime }(0)=100\mu$m, divergence angle ${\theta }_M^{\prime }(0)=\text{arcsin }0.22$. According to the numerical aperture of the optical fiber, the beam parameter product (BPP) is calculated as 22$\mathrm{\text{mm}}\cdot \mathrm{\text{m}}$rad of the semiconductor laser with $\lambda =0.94\mu$m. Then, considering that $M^2=\pi$ BPP/$\lambda$, $M^2=74$ is obtained. Therefore, BPP is the product of spot radius multiplied by far-field divergence angle, $M^2$ is the beam quality factor and $\lambda$ is the wavelength.

Because the beam quality of the 532-nm laser is much higher than that of the 940-nm semiconductor laser, we only need to consider the coupling spot size, which can be optimized according to the design demands of the 532-nm laser. Therefore, the key consideration is coupling the 940-nm laser with the output fiber. The distance $l_1$ between the input fiber and the lens L1 should be at the same focal length as lens L1; i.e., $l_1=f_1$. The complex parameters can be obtained from the following equation :

The size and divergence angle of the light spot on the lens are calculated by the ABCD matrix. The distance $l{2}^{\prime }$ between the output fiber and the lens L2 should refer to the waist position of the output fiber. As the minimum core diameter of the fiber is 400$\mu$m, the beam on the output fiber should meet the following requirements: waist spot radius ${\omega }_M^{\prime }=200\mu$m; divergence angle ${\theta }_M^{\prime }=\text{arcsin }0.22$. If a 20-mm focal length lens is used for collimation, the 940-nm beam waist radius at the position of the beam combining mirror is 4.4mm. When the aberration is not considered, the focal length of the focusing lens should be 40mm; however, the spot diameter will be greater than 400$\mu$m due to the existence of aberration. A lens with a focal length of 30mm is used for focusing because the coupling condition of the multimode fiber is considered. For a Gaussian beam, in order to ensure transmission of the majority of power, the lens size should be 1.5 times larger than the spot size; therefore, the lens diameter is selected as 15mm and the beam combination diameter is 20mm.

2.4. Spot transformation device

A spot transformation device is designed to adjust the size of the output spot so that the spot size after output of the optical fiber can meet the system requirements.¹² A schematic diagram of the treatment handpiece is shown in Fig. 5. Considering that the beam quality of the two wavelengths is significantly different, it is necessary to make the waist size of the output spot independent of the beam quality.¹³ According to the ABCD matrix, the output spot radius is ${\omega }_i^2=A^2{\omega }_0^2+B^2{\theta }^2$, where ${\omega }_0$ is the beam waist radius and $\theta$ is the beam divergence angle. If the size of the incident spot is constant and the waist of the output laser beam should also be constant, then $B$ must be 0 to ensure that the transformation matrix meets the following requirements :

If a double lens focusing telescope system is used, the spot size is only related to the input spot radius and magnification. The defocusing amount is satisfied by $\mathrm{\Delta }=f_1+f_2-l=0$, so the transformation matrix is

where $M_T=-\frac{f_2}{f_1}$, $l=f_1+f_2$. According to the spot size, the focal length of the lens can be calculated, as shown in Table 1.

2.5. Security design

The mature and stable XVD medical switching power supply of Excelsys company switching power supply is selected in this paper, and its safety performance meets the requirements of the national general standards GB 9706.1-2007 for Medical electrical equipment-Part1: General requirements for safety. The requirements of electromagnetic compatibility (EMC) standards YY 0505-2012 also need to be considered in the design, which can test the ability of electromagnetic radiation and immunity of equipment, mainly including surge impact immunity, conducted emission, radiation emission, and other test items. The surge current and peak voltage of the system power supply in the working process, the charging and discharging of the large capacitance used in the power frequency rectifier filter, the voltage switching during the high-frequency operation of the switch tube and the reverse recovery current of the output rectifier diode will produce a lot of electromagnetic interference. The system power supply selected and the whole device adopt the anti-interference processing of shielding, grounding and adding EMI filter in this paper which meet the requirements of EMC standard.

3. Experimental Results and Discussion

Stable output of laser power for good therapeutic effect provides effective experimental data for clinical research. The dual-wavelength laser treatment system is built and its output power stability is tested, as shown in Figs. 6–8.

3.1. Development and testing of the 532-nm laser device

The whole cavity was sealed and surrounded by thermal insulation materials. The laser crystal was wrapped in indium foil and connected with copper bracket. The copper bracket and copper plate were connected by indium foil. The copper plate and thermo electric cooler (TEC) were connected by thermal conductive silicone grease to achieve heat dissipation. The mature and stable medical system power supply in the market was used to power the laser power module, the circuit part of the main controller and the touch screen. The main controller controlled the laser module to output the laser that could meet the treatment requirements, so that it could work stably.

The 532-nm laser was manufactured and debugged with the above design, and the experimental device was shown in Fig. 7. The maximum output power was 7.29W. The 1064-nm wavelength was filtered by a reflection filter. The output power of the 532-nm laser reached 6.98W and higher. At this time, the working current of the pumped semiconductor laser was 40A. Preliminary experimental set-up using the 532-nm laser is shown in Fig. 6 and the power stability results of the 532-nm laser are shown in Fig. 8. The power fluctuation over 150min was less than 2%, which could ensure the stability of laser output within the allowable power deviation range.

3.2. Experimental results of the beam combination device

The experimental results of coupling the 532-nm laser with the optical fiber using the beam combining mirror and lens are shown in Fig. 8. The power loss was less than 20%. When the 940-nm laser was coupled with the optical fiber using beam combining mirror and lens, the loss was also less than 20%, which meets the output requirements of the instrument. The power of the therapeutic device is higher than that of the existing dual-wavelength therapeutic device, it has dual characteristics of visible light and near-infrared. It can accurately target both superficial vessels and vessels with a larger diameter and deeper position, and can meet the comprehensive clinical treatment demands of vascular diseases.

3.3. Safety testing of the whole device

The leakage current was tested by CS550 series medical leakage current tester of Nanjing Changsheng: the enclosure leakage current: normal state: 0.001mA, single fault: 0.001mA; earth leakage current: normal state: 0.193mA, single fault: 0.377mA; patient leakage current: normal state: 0.001mA, single fault: 0.001mA; withstand voltage ability of the whole device was tested by CS50 series withstand voltage tester: the withstand voltage between the live part and the unprotected grounded shell parts reached 4000V; the protective earth resistance of the whole device was tested by CS5800/Y series earth resistance tester: the protective earth resistance was 0.02$\mathrm{\Omega }$. The test results conform to the relevant requirements of GB 9706.1-2007 medical electrical equipment Part 1: General safety standards for the protection of electric shock hazards, and ensure the safety of the therapeutic device.

4. Conclusion

This study designs an optical system and test method for a dual-wavelength high-power laser therapeutic device with a variable spot size, according to the requirement of stable output of medical high-power dual-wavelength laser. The testing results of the whole device show that the output power stability of the 532-nm beam varies by less than 2% over 150 min, and the loss of both wavelengths is less than 20%, which can satisfy the stable output of dual-wavelength high-power laser. The safety testing results show that the safety of the whole device meets the requirements of national general standards for medical electrical safety. It ensures laser stable output and high safety. The therapeutic device has dual characteristics of visible light and near-infrared. It can accurately target both superficial vessels and vessels with a larger diameter and deeper position, and can meet the comprehensive clinical treatment demands of vascular diseases.

Acknowledgments

This research was supported by the National Key R&D Program of China (No. 2017YFB0403802), the Technology Cooperation High-tech Industrialization Program of Jilin Province of China and the Chinese Academy of Sciences (No. 2018SYHZ0023), the Key Technology R&D Program of Jilin Province of China (No. 20180201047YY), the Scientific Research Program of Shanghai Science and Technology Commission (No. 18441904300), and the Technology Cooperation High-tech Industrialization Program of Jilin Province of China and the Chinese Academy of Sciences (No. 2019SYHZ0032).

Conflict of Interest

The authors have no conflicts of interest relevant to this article.