Laser diode including Anti-reflection (AR) coating layer for improving beam quality, and ar coating layer

Abstract

The present invention relates to a laser diode comprising: an active layer on which light amplification of the laser diode is performed; and an anti-reflection (AR) coating layer formed on an emission facet of the laser diode, wherein the AR coating layer is configured to have a relatively high reflectance at a target wavelength of the laser diode and a relatively low reflectance at wavelengths in the vicinity of the target wavelength.

Description

Laser diode including an AR (ANTI-REFLECTION) coating layer for improving beam quality and AR coating layer

[0001] The present invention relates to a laser diode including an AR (Anti-Reflection) coating layer for improving beam quality and an AR coating layer.

[0002] In conventional semiconductor lasers, AR coatings are applied to control the reflectivity of the emission facet. Existing AR coating technology typically features a U-shaped (or V-shaped) reflectivity curve and utilizes a method that enhances oscillation efficiency by minimizing reflectivity at specific wavelengths.

[0003] However, conventional AR coatings exhibit low reflectivity only at a specific center wavelength while maintaining constant reflectivity in the surrounding wavelength range, making them highly susceptible to multimode oscillation. In particular, in Large Optical Cavity (LOC) structures, as the resonator lengthens, multiple resonant modes are more likely to form, which can lead to the problem of multimode oscillation occurring over a wide spectral range.

[0004] The present invention aims to solve the problem that when multimode oscillation occurs, beam quality deteriorates, laser focusing performance is reduced, and it becomes difficult to secure the desired precision in the application field.

[0005] This invention aims to solve the problem in which oscillation current increases at specific wavelengths if mirror loss is not properly adjusted, because conventional U-shaped or V-shaped AR coatings reduce reflectance only at specific wavelengths.

[0006] The present invention aims to solve the limitations of existing reflectance control methods, which have the limitation that reflectance is maintained constant over a wide band or is controlled locally only at specific wavelengths.

[0007] The present invention relates to a laser diode comprising: an active layer on which optical amplification of the laser diode is performed; and an anti-reflection (AR) coating layer formed on an emission facet of the laser diode; wherein the AR coating layer is configured such that the reflectance at a target wavelength of the laser diode is relatively high and the reflectance at a surrounding wavelength relative to the target wavelength is relatively low.

[0008] In addition, the AR coating layer is configured such that one or more layers of a high-refractive-index material layer having a refractive index greater than or equal to a critical threshold and a low-refractive-index material layer having a refractive index less than or equal to a critical threshold are stacked.

[0009] In addition, the AR coating layer is configured such that one or more high-refractive-index material layers having a refractive index greater than or equal to a critical threshold and low-refractive-index material layers having a refractive index less than or equal to a critical threshold are stacked, and the high-refractive-index material layers and the low-refractive-index material layers are stacked alternately.

[0010] In addition, the AR coating layer is configured such that the high refractive index material layer has a thickness of 50 nm or more, and the low refractive index material layer has a thickness of 300 nm or less.

[0011] In addition, the AR coating layer is configured to have a reflectance of 5% or more and 15% or less at the target wavelength, and to have a reflectance of 1% or less at the surrounding wavelength.

[0012] In addition, the AR coating layer is configured to have a W-shaped reflectance shape.

[0013] In addition, the AR coating layer is configured to reduce mirror loss at the target wavelength.

[0014] The present invention relates to an anti-reflection (AR) coating layer formed on the emission surface of a laser diode for improving beam quality, comprising: a high-refractive-index material layer having a refractive index greater than or equal to a critical value; and a low-refractive-index material layer having a refractive index less than or equal to a critical value; and having a multilayer structure formed such that one or more of the high-refractive-index material layer and the low-refractive-index material layer are stacked.

[0015] In addition, the high refractive index material layer and the low refractive index material layer are formed to be alternately stacked.

[0016] In addition, the reflectance of the AR coating layer is adjusted to be relatively high at the target wavelength and relatively low at the surrounding wavelengths relative to the target wavelength.

[0017] In addition, the high refractive index material layer has a thickness of 50 nm or less, and the low refractive index material layer is configured to have a thickness of 300 nm or less.

[0018] The present invention provides the effect of inducing single-mode oscillation and preventing multi-mode oscillation by applying a W-shape reflectance.

[0019] The present invention provides the effect of reducing mirror loss and inducing oscillation at the corresponding wavelength.

[0020] The present invention can provide the effect of reducing the power consumption of a laser diode by lowering the oscillation current and extending the lifespan of the device by minimizing heat generation.

[0021] The present invention can provide the effect of reducing the optical spectrum width (FWHM, Full Width at Half Maximum) by applying W-shape reflectance, which facilitates oscillation at a specific central wavelength and suppresses oscillation at surrounding wavelengths.

[0022] The present invention can provide the effect of improving beam quality and increasing the consistency of laser output by maintaining single-mode oscillation through the application of W-shape reflectance.

[0023] The present invention can provide the effect of stably maintaining single-mode oscillation even in an LOC structure by applying W-shape reflectance.

[0024] The present invention can provide the effect of exhibiting optimal performance at a specific wavelength by effectively controlling the reflectance in a multi-wavelength range through the application of W-shaped reflectance.

[0025] Figure 1 is a diagram illustrating the effect of the reflectance characteristics of an anti-reflection (AR) coating on the oscillation mode and spectrum width of a laser in a large optical cavity (LOC) structure.

[0026] FIG. 2 is a diagram illustrating the structure of an InGaAs / InGaAsP-based semiconductor laser according to the present invention.

[0027] Figure 3 is a diagram for visually explaining the change in laser oscillation characteristics when the W-shape AR reflectance of the present invention is applied.

[0028] Figure 4 is a graph showing the reflectance (%) of an AR coating layer according to the present invention as a function of wavelength.

[0029] Specific details of the embodiments are included in the detailed description and drawings.

[0030] The advantages and features of the present invention and the methods for achieving them will become clear by referring to the embodiments described below in detail together with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below but may be implemented in various different forms. These embodiments are provided merely to ensure that the disclosure of the present invention is complete and to fully inform those skilled in the art of the scope of the invention, and the present invention is defined only by the scope of the claims. Throughout the specification, the same reference numerals refer to the same components.

[0031] According to the present invention, the Large Optical Cavity (LOC) structure is a method for realizing a high-power laser by expanding the length and volume of the resonator compared to a conventional optical resonator. The LOC structure is used in the design of diode lasers, high-power solid-state lasers, fiber lasers, etc., and has the advantage of increasing the intensity of the output light by increasing the gain as the size of the resonator increases. However, as the length of the resonator increases, a problem may arise in which the possibility of multi-mode oscillation increases.

[0032] A characteristic of the LOC structure is that as the length of the resonator increases, the path for light to travel back and forth lengthens, thereby increasing the beam size and output power. However, as the resonator length increases, the likelihood of oscillation in multiple modes (lasing modes) increases, and it may become difficult to maintain single-mode oscillation that occurs only at specific wavelengths. Additionally, while the LOC structure enables greater optical amplification by increasing the length of the active region, a wider gain envelope increases the likelihood of multi-mode phenomena where multiple resonant modes oscillate.

[0033] In particular, maintaining single-mode oscillation is a critical technical challenge in LOC structures. As the resonator length increases, the likelihood of multiple resonant modes existing also increases; consequently, this can lead to a broader optical spectrum and degraded beam quality. When such multi-mode oscillation occurs, the beam contains multiple wavelengths, resulting in spatial spreading or increased divergence. This degrades the laser beam's focusing performance, which can cause performance degradation in applications such as cutting, welding, and medical lasers.

[0034] According to the present invention, an Anti-Reflection (AR) coating is a thin film coating designed to minimize reflection on the surface of an optical element to reduce light loss and to allow light to transmit through the element to the maximum extent. Generally, in lasers or optical devices, light loss occurs when a portion of the incident light is reflected from the surface, which can reduce the efficiency of the system. Therefore, applying an AR coating can reduce unnecessary reflection and enable the optical element to transmit more light effectively.

[0035] An ideal AR coating must exhibit very low reflectivity at specific wavelengths while maintaining consistent characteristics at other wavelengths. In other words, rather than having low reflectivity only in a specific frequency band, it must maintain uniform reflectivity across various wavelengths to ensure stable performance of the optical system. Particularly in laser systems, reflectivity flatness is a critical design factor, as low reflectivity at a single wavelength or within a narrow band can make it difficult to select the desired laser mode.

[0036] However, if the reflection curve has a U- or V-shape, problems may arise where the reflectance decreases or changes abruptly only at specific wavelengths. This can result in non-uniform performance of the optical system, as light may be excessively transmitted at certain wavelengths or, conversely, reflected in specific regions. In particular, if the reflection curve in an LOC structure takes a U- or V-shape, the decrease in reflectance at specific wavelengths increases the likelihood of multiple lasing modes being amplified; ultimately, this can lead to a wider spectrum width of the laser and induce multi-mode oscillation.

[0037] Figure 1 is a diagram illustrating the effect of the reflectance characteristics of an anti-reflection (AR) coating on the oscillation mode and spectrum width of a laser in a large optical cavity (LOC) structure.

[0038] As shown in FIG. 1(a), a gain curve of a laser diode (100) indicated by a dotted line can be shown on a graph with wavelength on the x-axis and intensity on the y-axis. The gain envelope indicated by the dotted line represents the degree of laser amplification occurring in a specific wavelength range. The black vertical bar representing each lasing mode indicates the lasing mode of the laser diode (100) at a specific wavelength and shows how the specific resonance mode amplified in the resonator is distributed.

[0039] As shown in Fig. 1(b), in a graph with wavelength on the x-axis and AR (Anti-Reflection) on the y-axis, the reflection ratio (%) represents the proportion of reflected light among incident light; generally, 0% indicates complete transmission (no reflection), and 100% indicates complete reflection. The shape of the graph in Fig. 1(b) forms a U- or V-shaped curve, indicating a structure where the reflection is minimized at a specific center wavelength and increases towards both ends. The goal of AR coating is to maintain maximum transmittance at a specific wavelength in various optical devices such as lasers, lenses, displays, and optical fibers. Therefore, it is designed to minimize reflection at a specific center wavelength; however, since it is important to maximize efficiency in a specific wavelength band even if reflection increases as it moves away from the center wavelength, it mainly forms a U- or V-shaped curve.

[0040] As shown in Fig. 1(c), in a graph with wavelength on the x-axis and intensity on the y-axis, as the reflectance curve decreases at a specific wavelength, multiple oscillation modes are formed around that wavelength. As a result, the spectrum width of the laser diode (100) widens, and multi-mode oscillation increases. When the reflectance of the AR coating decreases at a specific wavelength, the likelihood of multiple resonance modes being amplified around that wavelength increases. That is, the spectrum width of the laser diode (100) widens, and the likelihood of changing from single-mode to multi-mode increases.

[0041] Therefore, the oscillation mode is not concentrated on a specific single wavelength but is widely distributed across multiple wavelengths. If single-mode oscillation is maintained, the laser beam quality is high and can be sharply focused. However, when multi-mode oscillation occurs, light is emitted at multiple wavelengths, increasing laser beam divergence and degrading focusing performance. This can be a problem, particularly in high-power lasers or lasers used for precision processing. In other words, beam quality deteriorates, and it may become difficult to achieve the desired laser performance.

[0042] FIG. 2 is a diagram illustrating the structure of an InGaAs / InGaAsP-based semiconductor laser according to the present invention.

[0043] As described, the laser diode (100) includes an active layer (110) and an AR coating layer (120). Specifically, the AR coating layer (120) can achieve W-shaped AR (anti-reflection) reflectance characteristics at a specific wavelength, such as 1430 nm, by stacking a high-refractive-index material layer and a low-refractive-index material layer. While conventional AR coatings have a U-shaped or V-shaped curve that minimizes reflectance at a specific center wavelength, the AR coating layer (120) of the present invention can optimize reflectance at multiple wavelengths and suppress reflection in a specific wavelength range by repeatedly stacking a high-refractive-index (n ≥ 1.80 @ 1430 nm) material and a low-refractive-index (n < 1.80 @ 1430 nm) material. As a result, the present invention can achieve the effect of suppressing multi-mode oscillation that may occur in a Large Optical Cavity (LOC) structure and improving beam quality.

[0044] As illustrated in FIG. 2, the laser diode (100) includes an active layer (110). Specifically, the active layer (110) is a region where optical amplification occurs in the laser diode and can perform optimal oscillation at a specific wavelength, such as 1430 nm. Additionally, the active layer (110) can be configured to form InGaAs / InGaAsP on an InP (Indium Phosphide) substrate. Specifically, the InGaAsP of the active layer (110) is easy to adjust in wavelength and has excellent optical properties, so it is used in optical communication and laser devices in the 1300–1600 nm band.

[0045] According to one embodiment of the present invention, a laser diode (100) may apply an AR coating layer (120) through multilayer thin film stacking to control the reflectivity of the laser emission facet. That is, the AR coating layer (120) is formed by combining a high refractive index material layer and a low refractive index material layer and stacking them to a specific thickness, and an amorphous silicon (α-Si) layer may be added to prevent oxidation as needed. The amorphous silicon (α-Si, ① in FIG. 2) layer serves to prevent oxidation of the laser emission facet and can be deposited with a thin thickness of 50 nm or less, and can be optionally omitted.

[0046] The AR coating layer (120) of the laser diode (100) of the present invention can achieve a W-shape reflectance with reduced reflectance at a specific wavelength by forming a multilayer structure by combining a high refractive index material layer and a low refractive index material layer. As a specific example, the high refractive index material layer (②, ④, ⑥ in FIG. 2) generally uses a material such as TiO2 (Titanium Dioxide) or Ta2O5 (Tantalum Pentoxide) and can be formed with a thickness of 50 nm or more. That is, the structure of the laser diode (100) of the present invention can provide a minimized reflectance and an optimized effect at multiple wavelengths by utilizing a high refractive index material (n ≥ 1.80 @ 1430 nm) to match phase conditions at a specific wavelength.

[0047] The AR coating layer (120) of the laser diode (100) of the present invention can achieve a W-shape reflectance with reduced reflectance at a specific wavelength by forming a multilayer structure by combining a high refractive index material layer and a low refractive index material layer. As a specific example, the low refractive index material (n < 1.80 @ 1430 nm) uses a material such as SiO2 (Silicon Dioxide) or MgF2 (Magnesium Fluoride), and is formed with a thickness of 300 nm or less, so that the reflectance can be controlled by combining it with the high refractive index material.

[0048] The AR coating layer (120) of the laser diode (100) of the present invention can adjust the number of stacked layers of a high refractive index material layer and a low refractive index material layer. That is, the AR coating layer (120) may include at least one high refractive index and one low refractive index layer, respectively, and the number of stacked layers may be changed to match a specific reflectance. Through this, the structure of the laser diode (100) of the present invention can be designed to lower reflectance not only at a specific wavelength but also across a wide wavelength range centered around 1430 nm. This is advantageous for suppressing multimode oscillation in the LOC structure and improving beam quality.

[0049] According to one embodiment of the present invention, the AR coating layer (120) of the laser diode (100) can optimize reflectance at multiple wavelengths by forming a W-shaped reflectance, unlike conventional simple U-shaped or V-shaped reflectances, and can induce stable optical output in an LOC structure. Since the LOC structure is prone to multi-mode oscillation due to the large size of the resonator, applying the W-shaped reflectance structure of the present invention effectively lowers reflectance at specific multiple wavelengths to maintain single-mode oscillation and optimize the spectral width. In addition, the effect of increasing the output stability of the laser and improving beam quality can be obtained.

[0050] Figure 3 is a diagram for visually explaining the change in laser oscillation characteristics when the W-shape AR reflectance of the present invention is applied.

[0051] As described above, according to the structure of the laser diode (100) of the present invention, unlike conventional general AR reflectance, a W-shape AR reflectance is applied so that laser oscillation is advantageously formed only at a specific center wavelength, and oscillation is suppressed at surrounding wavelengths. Through this, the oscillation current of the laser can be reduced, and the beam quality can be improved by reducing the full width of the spectrum (FWHM).

[0052] As shown in FIG. 3(b), in a graph according to the present invention where the x-axis represents wavelength and the y-axis represents reflectance (AR %), the reflectance characteristics of the AR coating having a W-shape reflectance show a shape in which the reflectance is high at a specific wavelength and relatively low in the surrounding wavelength range. Unlike general AR coatings which have simple U- or V-shaped reflectances, the W-shape reflectance maintains high reflectance only in a specific wavelength region. In the region of high reflectance, the mirror loss is low, so laser oscillation is favorably formed at that wavelength; conversely, in the surrounding wavelengths with low reflectance, the mirror loss increases, resulting in the effect of suppressing oscillation.

[0053] According to the present invention, for a laser to oscillate, the gain amplified in the active layer must be greater than the loss. There are two main types of loss: mirror loss and internal loss. Mirror loss is related to the reflectivity of the laser emission surface; the higher the reflectivity, the smaller the mirror loss, and the lower the reflectivity, the larger the mirror loss. Internal loss refers to losses such as absorption or scattering occurring within the active layer, which are determined by the characteristics of the material. Therefore, for the laser to oscillate stably, the gain in the active layer must be greater than the sum of the mirror loss and the internal loss.

[0054] According to the present invention, as Mirror Loss increases, a higher gain is required for the laser to oscillate, and a higher current must be supplied to secure this gain. In other words, the greater the Mirror Loss, the higher the critical current required for laser oscillation. Conversely, if Mirror Loss is small, the laser can oscillate even at a lower current. Therefore, reducing Mirror Loss at a specific wavelength makes laser oscillation at that wavelength advantageous, while conversely, increasing Mirror Loss at undesirable wavelengths can suppress oscillation.

[0055] The laser diode (100) of the present invention is characterized by controlling mirror loss by applying a W-shape reflectance. When a W-shape reflectance is applied, the reflectance becomes relatively higher at a specific target wavelength, resulting in a smaller mirror loss, and consequently, the current required for oscillation is reduced. On the other hand, at surrounding wavelengths, the reflectance is low, causing a larger mirror loss, which makes oscillation difficult. In other words, the laser diode (100) of the present invention can provide the effect of strongly oscillating the laser only at the target wavelength and suppressing oscillation at unwanted wavelengths.

[0056] According to the present invention, in a general laser, the reflectance of the emission surface has a constant shape, so it tends to oscillate in multiple modes over a wide wavelength range. However, the laser diode (100) of the present invention applies a W-shape reflectance so that oscillation is advantageous only at a specific wavelength and oscillation becomes difficult at surrounding wavelengths, so as a result, the spectral width is reduced and single-mode oscillation becomes possible. The laser diode (100) of the present invention has a reduced spectral width, which improves beam quality and increases the focusing performance and stability of the laser.

[0057] In addition, the laser diode (100) of the present invention can suppress multimode oscillation even in a Large Optical Cavity (LOC) structure. In a typical LOC structure, multimode oscillation is more likely to occur as the length of the resonator increases; however, the laser diode (100) of the present invention applies a W-shape reflectance to induce oscillation only in a specific mode and suppress the remaining modes, thereby maintaining a high-quality laser beam. Therefore, the laser diode (100) of the present invention can provide the effect of not only simply lowering the oscillation current but also improving the beam quality of the laser and securing a stable output at a specific wavelength.

[0058] FIGS. 3(c) and FIGS. 3(d) show how laser oscillation characteristics change after applying W-shape reflectance. In the conventional state exhibiting a broad oscillation mode distribution, multi-mode oscillation occurs at various wavelengths, which causes the spectral width of the laser to widen and degrades beam quality. However, when W-shape reflectance is applied, as shown in FIGS. 3(d), the present invention can provide the effect of enhancing the oscillation mode only at a specific center wavelength and suppressing oscillation in the surrounding wavelength range.

[0059] In other words, Figure 3(c) shows the W-shaped reflectance combined with the conventional Gain Envelope graph. The conventional Gain Envelope represents the region where laser oscillation can occur within a specific wavelength band, and generally, multiple lasing modes exist across a wide wavelength range. However, when the W-shaped reflectance is applied to the Gain Envelope graph, the reflectance becomes relatively higher at a specific central wavelength and lower at surrounding wavelengths. The dotted line in Figure 3(c) represents the W-shaped reflectance curve; as this reflectance is applied, mirror loss decreases at a specific wavelength while conversely increasing at surrounding wavelengths. That is, laser oscillation becomes more favorable in the central specific wavelength band, while oscillation is suppressed at surrounding wavelengths.

[0060] Figure 3(d) shows the final laser oscillation characteristics after applying W-shape reflectance. Here, the x-axis represents wavelength and the y-axis represents intensity. It indicates that when W-shape reflectance is applied, the optical spectrum width (FWHM, Full Width at Half Maximum) is reduced and the single-mode oscillation characteristics are enhanced. Previously, there was a tendency for multi-mode oscillation to occur with a broad spectral distribution, but as a result of applying W-shape reflectance, strong oscillation occurs only at a specific center wavelength, and oscillation at surrounding wavelengths is suppressed, resulting in a narrower spectral width.

[0061] Figure 4 is a graph showing the reflectance (%) of an AR coating layer according to the present invention as a function of wavelength.

[0062] As described, unlike the reflectance curve of a typical AR coating layer which has a U- or V-shape, the AR coating layer (120) forms a W-shape reflectance in which the reflectance is relatively high at a target wavelength and lower at shorter and longer wavelengths. This provides the effect of making laser oscillation advantageous only at a specific target wavelength and suppressing oscillation at surrounding wavelengths.

[0063] The horizontal axis (X-axis) of the graph in Fig. 4 represents wavelength (nm) and shows the change in reflectance of the AR coating layer in the range from 1200 nm to 1600 nm. The target wavelength of 1430 nm is highlighted, and the AR reflectance is characterized by being relatively higher at this wavelength. The vertical axis (Y-axis) represents reflectance (%) and varies in the range from 0% to 0.40%. A lower reflectance reduces optical loss, providing an effect that is advantageous for laser oscillation at specific wavelengths.

[0064] As described, the reflectance of the laser diode (100) of the present invention increases to approximately 0.17% at the target wavelength of 1430 nm. When the reflectance increases, the mirror loss at that wavelength decreases, resulting in an effect that is favorable for laser oscillation. This serves to induce oscillation only at a specific target wavelength and suppress oscillation at surrounding wavelengths. On the other hand, the reflectance approaches 0% around 1350 nm and 1500 nm, and through this control of reflectance, the mirror loss increases at wavelengths surrounding the target wavelength, making oscillation difficult. Consequently, multi-mode oscillation is suppressed, and oscillation becomes possible only at a specific single wavelength.

[0065] The W-shape reflectance shape of the laser diode (100) of the present invention is advantageous for maintaining single-mode oscillation and improving beam quality compared to conventional general U-shaped or V-shaped reflectances. In conventional AR coating layers, the reflectance is lowered only at a specific wavelength and remains constant at surrounding wavelengths; however, by applying the W-shape reflectance of the present invention, a structure is formed in which the reflectance is higher only at a specific target wavelength and lower at surrounding wavelengths. Through this, multi-mode oscillation is suppressed, and laser oscillation can be optimized only at the target wavelength.

[0066] In other words, as the reflectivity at the target wavelength becomes relatively higher, mirror loss is reduced, and consequently, the threshold current is reduced. As a result, the efficiency of the laser diode (100) is increased, and the effect of reduced power consumption can be obtained. In addition, since multi-mode oscillation is suppressed and oscillation is induced only at a specific target wavelength, there is an advantage of being able to maintain single-mode oscillation. When single-mode oscillation is maintained, the full width at half maximum (FWHM) of the laser is reduced, which increases the output stability of the laser and allows for the provision of a high-quality beam.

[0067] The scope of the present invention is not limited to the embodiments described above but may be implemented in various forms of embodiments within the scope of the appended claims. It is deemed that the scope of the claims of the present invention includes various modifications that are possible by anyone with ordinary knowledge in the technical field to which the invention pertains, without departing from the essence of the invention claimed in the claims.

[0068] [Explanation of the symbol]

[0069] 100: Laser diode

[0070] 110: Active layer

[0071] 120: AR coating layer

Claims

1. In a laser diode, An active layer on which optical amplification of the above laser diode is performed; and It includes an AR (Anti-Reflection) coating layer formed on the emission facet of the laser diode; and The above AR coating layer is, The laser diode is configured such that the reflectance at the target wavelength is relatively high, and the reflectance at surrounding wavelengths relative to the target wavelength is relatively low. Laser diode.

2. In Paragraph 1, The above AR coating layer is, A structure configured such that one or more layers of a high-refractive-index material having a refractive index greater than a critical threshold and a low-refractive-index material having a refractive index less than a critical threshold are stacked. Laser diode.

3. In Paragraph 1, The above AR coating layer is, One or more layers of a high-refractive-index material having a refractive index greater than a critical threshold and a low-refractive-index material having a refractive index less than a critical threshold are stacked, and The above high-refractive-index material layer and the above low-refractive-index material layer are configured to be alternately stacked. Laser diode.

4. In Paragraph 2, The above AR coating layer is, The above high-refractive-index material layer is configured to have a thickness of 50 nm or more, and The above low-refractive-index material layer is configured to have a thickness of 300 nm or less, Laser diode.

5. In Paragraph 1, The above AR coating layer is, It is configured to have a reflectance of 5% or more and 15% or less at the above target wavelength, and Configured to have a reflectance of 1% or less at the above surrounding wavelengths, Laser diode.

6. In Paragraph 1, The above AR coating layer is, Configured to have a W-shaped reflectivity shape, Laser diode.

7. In Paragraph 1, The above AR coating layer is, Configured to reduce mirror loss at the above target wavelength, Laser diode.

8. An AR (Anti-Reflection) coating layer formed on the emission surface of a laser diode to improve beam quality, A high-refractive-index material layer having a refractive index greater than a critical threshold; and A low-refractive-index material layer having a refractive index below a critical threshold; comprising A multilayer structure having one or more layers formed such that the high-refractive-index material layer and the low-refractive-index material layer are stacked AR coating layer formed on the emission surface of a laser diode to improve beam quality.

9. In Paragraph 8, The above high-refractive-index material layer and the above low-refractive-index material layer are, formed to be alternately stacked AR coating layer formed on the emission surface of a laser diode to improve beam quality.

10. In Paragraph 8, The reflectance of the above AR coating layer, Adjusted to be relatively high at the target wavelength and relatively low at surrounding wavelengths relative to the target wavelength, AR coating layer formed on the emission surface of a laser diode to improve beam quality.

11. In Paragraph 8, The above high-refractive-index material layer is, It has a thickness of 50 nm or less, and The above low-refractive-index material layer is, Configured to have a thickness of 300 nm or less, AR coating layer formed on the emission surface of a laser diode to improve beam quality.

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