High-efficiency, narrow-linewidth vertical-external-cavity surface-emitting semiconductor laser

Abstract

The present application relates to the field of semiconductor laser, especially to a high-efficiency and narrow-linewidth vertical external cavity surface emitting semiconductor laser, comprising a hollow heat-conducting shell and a pump light anti-reflection film, a window layer and a gain chip which are sequentially stacked from top to bottom in the hollow heat-conducting shell; the gain chip comprises an active region, a distributed Bragg reflector and a substrate which are sequentially stacked from top to bottom, the substrate is bonded to the bottom of the hollow heat-conducting shell through a transition metal layer, the active region of the gain chip comprises staggered gradient pump light absorption layers and gradient quantum well layers, the thickness of each gradient pump light absorption layer gradually increases from the top to the bottom of the gain chip, and the thickness of each gradient quantum well layer gradually increases from the top to the bottom of the gain chip, while the composition of the well layer gradually decreases. The present application solves the temperature drift problem of the gain chip, realizes precise wavelength control, and further realizes narrow-linewidth and high-beam-quality laser output.

Description

Technical Field

[0001] The present invention belongs to the technical field of semiconductor lasers, and in particular relates to a high-efficiency, narrow-linewidth vertical external cavity surface emitting semiconductor laser. Background Art

[0002] Vertical external cavity surface emitting semiconductor lasers combine the advantages of semiconductor lasers and solid-state lasers, with the advantages of small size, good beam quality, and wide wavelength coverage. By integrating optical components into its unique external resonant cavity, it can achieve output performance control functions such as frequency conversion and linewidth compression. It is widely used in marine resource exploration, quantum information processing, laser medical treatment, laser precision industrial processing and other fields.

[0003] Based on the pumping method, vertical external cavity surface emitting semiconductor lasers can be divided into electrically pumped vertical cavity surface emitting lasers (VECSELs) or optically pumped vertical cavity surface emitting lasers (VECSELs). Currently, it is difficult to achieve high uniformity and large-area carrier injection with electrically pumped VESELs, and their output power is limited. Optically pumped VESELs can achieve large-area, uniform carrier injection, but the pump light absorption efficiency is low, resulting in low overall conversion efficiency. At high pump powers, poor heat dissipation in the gain chip leads to a temperature gradient distribution, causing a spatially uneven redshift in the output wavelength, which in turn stimulates multi-longitudinal mode oscillations. The differences in wavelength drift among the modes lead to spectral degeneration, ultimately broadening the laser linewidth and preventing narrow linewidth output.

[0004] At present, the fields of laser micro-size processing, precision spectral measurement, laser medical treatment, quantum information processing, etc. are in urgent need of small-sized, high-efficiency, narrow-linewidth vertical external cavity surface emitting semiconductor lasers, but the existing vertical external cavity surface emitting semiconductor lasers cannot meet the demand. Summary of the Invention

[0005] In view of this, the object of the present invention is to overcome the defects of the prior art and provide a high-efficiency, narrow-linewidth vertical external cavity surface emitting semiconductor laser.

[0006] To achieve the above object, the technical solution created by the present invention is implemented as follows:

[0007] A high-efficiency, narrow-linewidth vertical external cavity surface-emitting semiconductor laser comprises a hollow heat-conducting shell and a pump light anti-reflection film, a window layer, and a gain chip, which are respectively located in the hollow heat-conducting shell and stacked in sequence from top to bottom; the gain chip comprises an active region, a distributed Bragg reflector, and a substrate, which are stacked in sequence from top to bottom. The substrate is bonded to the bottom of the hollow heat-conducting shell through a transition metal layer. The active region comprises a first pump light absorption layer, a first barrier layer, a second pump light absorption layer, a first transition layer, a graded quantum well layer, a second transition layer, a third pump light absorption layer, which are periodically arranged from top to bottom. The first pump light absorption layer, the second pump light absorption layer, the third pump light absorption layer, and the fourth pump light absorption layer respectively constitute a graded pump light absorption layer; the thickness of each graded pump light absorption layer gradually increases from the top to the bottom of the gain chip, and the graded pump light absorption layer is located at a node position of a standing wave distribution of the gain chip; the thickness of each graded quantum well layer gradually increases from the top to the bottom of the gain chip, and the well layer component gradually decreases, and the graded quantum well layer is located at a peak position of a standing wave distribution of the gain chip.

[0008] Furthermore, the thickness of the graded quantum well layer gradually increases from the top to the bottom of the gain chip by 1% to 10%, and the composition of the graded quantum well layer gradually decreases from the top to the bottom of the gain chip by 1% to 5%.

[0009] Furthermore, the thickness of the well layer of the graded pump light absorption layer gradually increases from the top to the bottom of the gain chip by 1% to 10%.

[0010] Furthermore, a GaAs material system or an InP material system is adopted.

[0011] Furthermore, when the vertical external cavity surface emitting semiconductor laser adopts GaAs material system, the substrate adopts GaAs material with a thickness of 100nm-800nm; the distributed Bragg reflector adopts GaAs / AlAs material or Al X GaAs / Al y GaAs material, 0 <x<1,0<y<1,分布式布拉格反射器的单层材料厚度为1 / 4输出波长;渐变式泵浦光吸收层采用AlGaAs材料,厚度为20nm-400nm;渐变式量子阱层采用InGaAs材料,阱层厚度为1nm-20nm;第一势垒层与第二势垒层采用InGaP材料,厚度为20nm-200nm;窗口层采用Al z GaAs material, 0 <z<1,厚度为10nm-800nm。

[0012] Further, the In component of the gradual quantum well layer gradually decreases by 1-5% from the top to the bottom of the gain chip.

[0013] Further, the gain chip is in a cylindrical or prismatic structure, and the hollow heat-conducting shell is in a circular or prismatic structure.

[0014] Further, the gain chip is in a cylindrical structure with a diameter of 10-100 mm, and the hollow heat-conducting shell is in a circular structure with a wall thickness of 3-30 mm, an upper platform diameter of 10-100 mm, and a lower platform diameter of 30-300 mm.

[0015] Further, the hollow heat-conducting shell is made of a heat-conducting material.

[0016] Further, the heat-conducting material is any one of gold, silver, copper, and aluminum.

[0017] Further, the inner side wall of the hollow heat-conducting shell is plated with a high-reflection film.

[0018] Compared with the prior art, the application can achieve the following beneficial effects:

[0019] 1. The application adopts a gradual quantum well structure, and the quantum well gain peak wavelength is gradually reduced from the top to the bottom of the gain chip by gradient transformation of the quantum well component and thickness, so as to match the temperature gradient distribution of the gain chip, solve the temperature drift problem caused by excessively high pump power of the gain chip, realize precise wavelength control, and further realize narrow line width and high beam quality laser output.

[0020] 2. The application adopts a gradual pump light absorption structure, and the thickness of the pump light absorption layer is gradually increased from the top to the bottom of the gain chip, so as to match the gradually reduced pump light power density, increase the light absorption distance of the bottom quantum well with low pump light power density, realize efficient absorption of the bottom pump light, and further realize uniform injection of active region carriers.

[0021] 3. The high-reflection film plated on the inner side wall of the hollow heat-conducting shell reflects the pump light that is not absorbed by the gain chip, and the pump light is absorbed by the gain chip after multiple times of passing through the active region, thereby improving the overall absorption efficiency of the gain chip to the pump light. BRIEF DESCRIPTION OF DRAWINGS

[0022] The accompanying drawings, which form a part of the present application, are used to provide further understanding of the present application, and serve as an appropriate basis for the description of the illustrative embodiments of the present application and their explanation, and shall not constitute improper limitations to the present application. In the drawings:

[0023] Figure 1 is a front view of the high-efficiency and narrow-line-width vertical external cavity surface emitting semiconductor laser structure according to the embodiment of the present application.

[0024] Figure 2 3. A top view of a high-efficiency, narrow-linewidth vertical external cavity surface-emitting semiconductor laser structure according to an embodiment of the present invention;

[0025] Figure 3 This is a standing wave distribution diagram of a high-efficiency, narrow-linewidth vertical external cavity surface emitting semiconductor laser structure according to an embodiment of the present invention.

[0026] Explanation of the reference numerals: hollow thermally conductive shell 1, pump light anti-reflection film 2, window layer 3, gain chip 4, active area 41, distributed Bragg reflector 42, substrate 43, transition metal layer 44, first pump light absorption layer 411, first barrier layer 412, second pump light absorption layer 413, first transition layer 414, graded quantum well layer 415, second transition layer 416, third pump light absorption layer 417, second barrier layer 418, fourth pump light absorption layer 419, pump light 5, gain chip standing wave distribution 6. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and do not constitute a limitation of the present invention.

[0028] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other.

[0029] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention. In addition, the terms "first", "second" and the like are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, features defined as "first", "second" and the like may explicitly or implicitly include one or more of the features. In the description of the present invention, unless otherwise specified, "multiple" means two or more.

[0030] In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "installed," "connected," and "connected" should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to internal connections between two components. Those skilled in the art can understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0031] The present invention will be described in detail below with reference to the drawings and in combination with embodiments.

[0032] The present invention provides a high-efficiency, narrow-linewidth vertical external-cavity surface-emitting semiconductor laser. To better understand the objectives, technical solutions, and advantages of the present invention, its operating principle is first explained. Conventional vertical external-cavity surface-emitting semiconductor lasers (VECSELs) experience wavelength drift at high pump powers due to the temperature gradient distribution of the gain chip, resulting in multi-transverse-mode laser output and making it difficult to achieve high-beam-quality laser output.

[0033] The vertical external cavity surface emitting semiconductor laser provided by the present invention adopts a gradient quantum well structure in the active region of the gain chip. By gradiently changing the quantum well composition and thickness, the quantum well gain peak wavelength is gradually reduced from top to bottom, matching the temperature gradient distribution of the gain chip, solving the temperature drift problem caused by excessively high pump power, and achieving high-power, narrow linewidth, and high-beam quality laser output.

[0034] The gain chip has a bottom-emitting light pumping structure or a top-emitting light pumping structure. The bottom-emitting light pumping structure is used as an example for description.

[0035] like Figure 1 and Figure 2 As shown, the high-efficiency, narrow-linewidth vertical external cavity surface emitting semiconductor laser provided by the embodiment of the present invention includes: a hollow heat-conducting shell 1 and a pump light anti-reflection film 2, a window layer 3, and a gain chip 4, which are respectively located in the hollow heat-conducting shell 1 and stacked in sequence from top to bottom; the gain chip 4 includes an active area 41, a distributed Bragg reflector 42 and a substrate 43, which are stacked in sequence from top to bottom, and the substrate 43 is bonded to the bottom of the hollow heat-conducting shell 1 through a transition metal layer 44.

[0036] The hollow thermally conductive housing 1 is constructed from a highly thermally conductive material, including but not limited to gold, silver, copper, and aluminum. The inner wall of the hollow thermally conductive housing 1 is coated with a highly reflective film (not shown) to reflect pump light not absorbed by the gain chip 4. This allows the pump light to pass through the active region 41 multiple times before being absorbed by the gain chip 4, thereby improving the overall absorption efficiency of the gain chip 4 for the pump light 5. The hollow thermally conductive housing 1 is designed to match the shape of the gain chip 4, and the present invention is not limited to a specific shape. For example, if the hollow thermally conductive housing 1 has a truncated cone structure, the corresponding gain chip 4 has a cylindrical structure; or if the hollow thermally conductive housing 1 has a prism structure, the corresponding gain chip 4 has a prismatic structure.

[0037] The pump light anti-reflection film 2 is located on the top of the gain chip 4 . The pump light anti-reflection film 2 has a good anti-reflection effect on the pump light 5 , and is used to improve the efficiency of injecting the pump light 5 into the gain chip 4 .

[0038] The window layer 3, typically composed of a high-bandgap material, is located above the active region 41. The high-energy bandgap barrier of the window layer 3 prevents carriers from diffusing from the active region 41 to the surface of the gain chip 4, preventing non-radiative recombination of carriers on the surface of the gain chip 4 and reducing energy loss. Surface defects in the gain chip 4 significantly increase the probability of carrier recombination. The window layer acts as a physical barrier on the surface of the gain chip 4, effectively reducing the recombination rate on the surface of the gain chip 4 and improving quantum efficiency.

[0039] The active region 41 includes quantum well structures arranged periodically from top to bottom. Each group of quantum well structures includes a first pump light absorption layer 411, a first barrier layer 412, a second pump light absorption layer 413, a first transition layer 414, a graded quantum well layer 415, a second transition layer 416, a third pump light absorption layer 417, a second barrier layer 418 and a fourth pump light absorption layer 419.

[0040] For a group of quantum well structures, the purpose of distributing the first barrier layer 412 and the second barrier layer 418 on the upper and lower sides of the graded quantum well layer 415 is to confine the carriers within the graded quantum well layer 415 based on the high energy bands of the two barrier layers, and prevent the carriers from overflowing into other quantum well structures.

[0041] When the quantum well strain is large, the first transition layer 414 and the second transition layer 416 can serve as strain compensation layers to compensate for the quantum well strain effect and improve the growth quality of the quantum well structure material; when the quantum well strain effect is small, the first transition layer 414 and the second transition layer 416 can serve as gradient barrier layers to form a gradient energy band structure with the first barrier layer 412 and the second barrier layer 418, which is beneficial to the quantum well carrier injection.

[0042] The first pump light absorption layer 411 and the second pump light absorption layer 413 constitute a first graded-change pump light absorption layer, and the third pump light absorption layer 417 and the fourth pump light absorption layer 419 constitute a second graded-change pump light absorption layer. The thickness of the first graded-change pump light absorption layer of each quantum well structure is less than the thickness of the second graded-change pump light absorption layer, and the thickness of the second graded-change pump light absorption layer of the upper quantum well structure in two adjacent quantum well structures is less than the thickness of the first graded-change pump light absorption layer of the lower quantum well structure. Similarly, the thickness extends from the top to the bottom of the gain chip 4 to the entire gain chip 4.

[0043] For the gain chip 4 as a whole, each of the two pump light absorption layers separated by the barrier layer is used as a gradient pump light absorption layer, and the thickness of each gradient pump light absorption layer gradually increases from the top to the bottom of the gain chip, and the gradient pump light absorption layer is located at the node position of the standing wave distribution 6 of the gain chip (such as Figure 3 as shown), to avoid affecting laser oscillation.

[0044] After the pump light is absorbed, its power density decreases. Specifically, the pump light power density gradually decreases from the top to the bottom of the gain chip 4. The gradient pump light absorption structure designed in the present invention can match the gradually decreasing pump light power density, increasing the light absorption distance of the bottom quantum wells with lower pump light power density. This achieves efficient absorption of the lower-power-density pump light at the bottom, maintaining uniform carrier injection across each quantum well structure within the gain chip 4, and ultimately achieving uniform carrier injection into the active region 41 of the multi-quantum well structure.

[0045] There are two ways to gradually change the thickness of the pump light absorption layer, explained using a quantum well structure as an example. In the first thickness gradient, the thickness of the first pump light absorption layer 411 is the same as the thickness of the third pump light absorption layer 417, and the thickness of the second pump light absorption layer 413 is less than the thickness of the fourth pump light absorption layer 419. In the second thickness gradient, the thickness of the first pump light absorption layer 411 is less than the thickness of the second pump light absorption layer 413, and the thickness of the third pump light absorption layer 417 is less than the thickness of the fourth pump light absorption layer 419 and greater than the thickness of the second pump light absorption layer 413. To accommodate gradually decreasing pump light power density, the second thickness gradient is preferred.

[0046] The gradient quantum well layer 415 has a gradually increasing thickness from the top to the bottom of the gain chip 4, while the composition of the well layer gradually decreases. The gradient quantum well layer is located at the peak position of the gain chip standing wave distribution 6 (e.g. Figure 3 ), enhancing the periodic gain.

[0047] For quantum well structures with the same gain peak wavelength, the higher the quantum well thickness, the lower the composition. Therefore, by gradually increasing the thickness of the well layer while gradually reducing the composition of the well layer, the quantum well gain peak wavelength can be reduced, effectively reducing the strain accumulation during the growth process of the quantum well structure material. Under high pump power, the temperature of the gain chip 4 gradually increases from top to bottom, reducing the gain peak wavelength of the quantum well at the bottom of the gain chip 4. When the temperature is high, it can drift to the same wavelength range as the top of the gain chip 4, thereby matching the temperature gradient distribution of the gain chip 4 from top to bottom, solving the temperature drift problem of the gain chip 4 caused by excessive pump power, achieving precise control of the output laser wavelength, and facilitating narrowing the linewidth of the output laser, ultimately achieving laser output with narrow linewidth and high beam quality.

[0048] In one specific embodiment of the present invention, the gain chip 4 includes ten quantum well structures. The thickness of the graded quantum well layer increases gradually from the top to the bottom of the gain chip by 1% to 10%, and the composition of the graded quantum well layer decreases gradually from the top to the bottom of the gain chip by 1% to 5%. The thickness of the graded pump light absorption layer increases gradually from the top to the bottom of the gain chip by 1% to 10%.

[0049] Vertical external cavity surface emitting lasers (VECSELs) can choose the appropriate material system based on the application requirements of the emission band. For example, if the emission band is in the 600-1300nm range, the laser material system is GaAs; if the emission band is in the 1300-1700nm range, the laser material system is InP.

[0050] The present invention provides a specific vertical external cavity surface emitting laser based on the GaAs material system, wherein the hollow heat-conducting shell 1 is a truncated cone structure with a wall thickness of 3-30 mm, an upper table diameter of 10-100 mm, and a lower table diameter of 30-300 mm; the gain chip 4 is a cylindrical structure with a diameter of 10-100 mm; the substrate 43 is made of GaAs material with a thickness of 100 nm-800 nm; the distributed Bragg reflector 42 is made of GaAs / AlAs material or Al x GaAs / Al yGaAs material, 0 <x<1,0<y<1,分布式布拉格反射器42的单层材料厚度为1 / 4输出波长;渐变式泵浦光吸收层采用AlGaAs材料,厚度为20nm-400nm;渐变式量子阱层415采用InGaAs材料,阱层厚度为1nm-20nm,渐变式量子阱层的In组分沿增益芯片顶部至底部的方向以1%~5%的幅度逐渐降低;第一势垒层412与第二势垒层418采用InGaP材料,厚度为20nm-200nm;InGaP / AlGaAs / InGaAs / AlGaAs / InGaP材料中In组分为0-0.5,Al组分为0-0.2;窗口层采用Al z GaAs material, 0 <z<1,厚度为10nm-800nm;激光器的发光波段800nm-1300nm。

[0051] The above-mentioned vertical external cavity surface emitting laser can maintain high efficiency and narrow linewidth at high power output, meeting the needs of miniaturized high-performance laser light sources.

[0052] It should be understood that the various forms of the processes shown above can be used to reorder, add, or delete steps. For example, the steps described in the present disclosure can be performed in parallel, sequentially, or in a different order, as long as the desired results of the technical solutions disclosed in the present disclosure can be achieved. This is not limited herein.

[0053] The above specific embodiments do not limit the scope of protection of the present invention. Those skilled in the art will appreciate that various modifications, combinations, sub-combinations, and substitutions may be made based on design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention are intended to be included within the scope of protection of the present invention.

Claims

1. A high-efficiency, narrow-linewidth vertical external-cavity surface-emitting semiconductor laser, comprising a hollow heat-conducting housing and a pump light antireflection film, a window layer, and a gain chip, which are stacked sequentially from top to bottom within the hollow heat-conducting housing; the gain chip comprises an active region, a distributed Bragg reflector, and a substrate, which are stacked sequentially from top to bottom, and the substrate is bonded to the bottom of the hollow heat-conducting housing via a transition metal layer, characterized in that: The active region includes a first pump light absorption layer, a first barrier layer, a second pump light absorption layer, a first transition layer, a graded quantum well layer, a second transition layer, a third pump light absorption layer, a second barrier layer, and a fourth pump light absorption layer, which are periodically arranged from top to bottom. The first pump light absorption layer and the second pump light absorption layer, as well as the third pump light absorption layer and the fourth pump light absorption layer, respectively constitute a graded pump light absorption layer. The thickness of each graded pump light absorption layer gradually increases from the top to the bottom of the gain chip, and the graded pump light absorption layer is located at a node of a standing wave distribution in the gain chip. The thickness of each graded quantum well layer gradually increases from the top to the bottom of the gain chip, while the well layer composition gradually decreases, and the graded quantum well layer is located at a peak of a standing wave distribution in the gain chip.

2. The high-efficiency, narrow-linewidth vertical external cavity surface-emitting semiconductor laser according to claim 1, characterized in that: The thickness of the graded quantum well layer gradually increases from the top to the bottom of the gain chip by 1% to 10%, and the composition of the graded quantum well layer gradually decreases from the top to the bottom of the gain chip by 1% to 5%.

3. The high-efficiency, narrow-linewidth vertical external cavity surface-emitting semiconductor laser according to claim 1, characterized in that: The thickness of the well layer of the graded pump light absorption layer gradually increases from the top to the bottom of the gain chip by 1% to 10%.

4. The high-efficiency, narrow-linewidth vertical external cavity surface-emitting semiconductor laser according to claim 1, characterized in that: Vertical external cavity surface emitting semiconductor lasers use GaAs material system or InP material system.

5. The high-efficiency, narrow-linewidth vertical external cavity surface-emitting semiconductor laser according to claim 4, characterized in that: When a vertical external cavity surface emitting semiconductor laser adopts a GaAs material system, the substrate uses GaAs material with a thickness of 100 nm - 800 nm; the distributed Bragg reflector uses GaAs / AlAs material or Al X GaAs / Al y GaAs material, where 0 < x < 1 and 0 < y < 1, and the thickness of a single layer of the distributed Bragg reflector is 1 / 4 of the output wavelength; the tapered pump light absorption layer uses AlGaAs material with a thickness of 20 nm - 400 nm; the tapered quantum well layer uses InGaAs material with a well layer thickness of 1 nm - 20 nm; the first barrier layer and the second barrier layer use InGaP material with a thickness of 20 nm - 200 nm; the window layer uses Al z GaAs material, where 0 < z < 1, with a thickness of 10 nm - 800 nm.

6. The high-efficiency, narrow-linewidth vertical external cavity surface-emitting semiconductor laser according to claim 5, characterized in that: The In composition of the graded quantum well layer gradually decreases by 1% to 5% from the top to the bottom of the gain chip.

7. The high-efficiency, narrow-linewidth vertical external cavity surface-emitting semiconductor laser according to claim 1, characterized in that: The gain chip has a cylindrical structure or a prismatic structure, and the hollow heat-conducting shell has a truncated cone structure or a prism structure.

8. The high-efficiency, narrow-linewidth vertical external cavity surface-emitting semiconductor laser according to claim 7, characterized in that: The gain chip is a cylindrical structure with a diameter of 10-100mm; the hollow heat-conducting shell is a frustum structure with a wall thickness of 3-30mm, an upper table diameter of 10-100mm, and a lower table diameter of 30-300mm.

9. The high-efficiency, narrow-linewidth vertical external cavity surface-emitting semiconductor laser according to claim 8, characterized in that: The hollow heat-conducting shell is made of heat-conducting material.

10. The high-efficiency, narrow-linewidth vertical external cavity surface emitting semiconductor laser according to claim 9, characterized in that: The thermal conductive material is any one of gold, silver, copper and aluminum.

11. The high-efficiency, narrow-linewidth vertical external cavity surface emitting semiconductor laser according to claim 1, characterized in that: The inner wall of the hollow heat-conducting shell is plated with a high-reflective film.

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