JP4243506B2 - Semiconductor laser and optical module using the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical transmission device, and more particularly to a distributed feedback semiconductor laser that oscillates at a single wavelength and is suitable for application to an optical communication module, an optical communication system, and an optical network.
[0002]
[Prior art]
At present, the optical communication not only of the trunk line but also of the subscriber network is progressing rapidly. Distributed feedback (hereinafter referred to as “DFB”) type semiconductor lasers capable of realizing a single longitudinal mode operation are stable in operation even at high-speed modulation, and are actively put into practical use as basic transmission devices.
[0003]
As elements for realizing a stable single longitudinal mode operation with good reproducibility, there are a phase shift type DFB laser using a phase shift type diffraction grating and a gain coupling type DFB laser using a gain diffraction grating. In the phase shift DFB laser, a λ / 4 phase shift DFB semiconductor laser (for example, see Non-Patent Document 1) in which a phase shift of a quarter wavelength (λ / 4) is introduced into the laser resonator has a mainstream structure. However, a basic technique for mass-producing λ / 4 phase shift type diffraction gratings with a high yield has not been established yet. Therefore, at present, the λ / 4 phase shift DFB semiconductor laser has not been widely put into practical use.
[0004]
Further, as a gain-coupled DFB laser, a part of a gain diffraction grating structure in which a diffraction grating is directly cut into a multiple quantum well structure made of an InGaAsP material has been partially put into practical use (for example, see Non-Patent Document 2). This structure has a feature that a DFB laser can be manufactured with a high yield by a relatively easy method.
[0005]
On the other hand, recently, it has become known to use InGaAlAs as a method for improving various characteristics of a semiconductor laser for communication in place of InGaAsP which has been conventionally used. Since InGaAlAs has a deep conductor band structure unique to the material, electron confinement is strong, and the quantum size effect is increased compared to InGaAsP.
[0006]
[Non-Patent Document 1]
Optical Fiber Communication Conference and Exhibition 2002 (Optical Fiber Communication Conference and Exhibit, 2002) No. 17-22, 417-418 (Takiguchi, T et al. “1.3 μm uncooled InGaInAs-MQW DFB laser with λ / 4-shifted grating ") (March 2002)
[Non-Patent Document 2]
14th IEEE International Semiconductor Laser Conference (1994, 14th IEEE International) 19-23 No. 51-52 (Lu, H. et al. “High-power and high-speed performance of gain-coupled” 1.3μm strained-layer MQW DFB lasers ”) (September 1994)
[0007]
[Problems to be solved by the invention]
From the above-described characteristics of the InGaAlAs material, if this is used for the light emitting layer, a semiconductor laser that maintains high performance characteristics up to a high temperature is realized. However, it is difficult to apply the InGaAlAs material to the gain-coupled DFB laser from the viewpoint of the manufacturing process. This is because it is difficult to re-grow in a state where the InGaAlAs layer containing Al that is chemically unstable and easily oxidized is exposed. Such a situation is also the same for InAlAs materials containing Al.
[0008]
An object of the present invention is to provide a semiconductor laser having a light emitting layer made of a compound semiconductor containing Al, which can be realized by a simple manufacturing method, an optical module using the semiconductor laser, and a function integrated laser.
[0009]
[Means for Solving the Problems]
In order to achieve the above object, the inventor of the present invention has a two-layer structure in which an active layer serving as a light emitting layer of a laser is composed of an InGaAlAs material active layer and an InGaAsP material active layer laminated thereon, and a gain diffraction grating is formed on the InGaAsP active layer. The structure which forms is devised. As a result, a gain-coupled distributed feedback laser can be realized while maintaining the high temperature characteristics of the InGaAlAs active layer. As a result, a semiconductor laser having high temperature characteristics can be manufactured with a high yield.
[0010]
Note that the combination of the high gain characteristic of the InGaAlAs active layer and the gain diffraction grating greatly improves the laser return light resistance. This eliminates the need for an optical isolator that has been conventionally required when a distributed feedback laser is modularized, so that the optical module can be made smaller and more economical.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The optical transmitter according to the present invention will be described below in more detail with reference to some embodiments of the invention shown in the drawings.
<Embodiment 1 of the Invention>
FIG. 1 shows an example in which a distributed feedback semiconductor laser having a wavelength band of 1.3 μm is manufactured using the present invention. An InGaAlAs active layer 105 is formed on the n-type semiconductor substrate 101, and an InGaAsP active layer 109 is further stacked thereon. A diffraction grating 111 is formed on the InGaAsP active layer 109 and the surface is protected by the surface protective layer 131. A p-type electrode 132 is provided on the ridge structure, and an n-type electrode 133 is provided on the back surface of the substrate 101. A low reflection film 151 is formed on the front end surface perpendicular to the substrate 101, and a high reflection film 152 is formed on the rear end surface. Main laser light is emitted from the front end surface, and monitoring laser light is emitted from the rear end surface.
[0012]
A method for manufacturing the semiconductor laser of this embodiment having such a structure will now be described. As shown in FIG. 2, an n-type InP buffer layer 102 having a thickness of 0.5 μm is formed on an InP semiconductor substrate 101 having a conductivity type of n-type and having a (100) crystal plane by a metal organic chemical vapor deposition method. N-type InAlAs buffer layer 103, 0.05 μm-thick n-type InGaAlAs lower graded guide layer 104, undoped seven-period InGaAlAs-based multiquantum well active layer [5 nm thickness 1.2% compressive strained InGaAlAs (composition Wavelength 1.37 μm) Well layer, 8 nm thick InGaAlAs (composition wavelength 1.00 μm) barrier layer] 105, 0.01 μm thick undoped InAlAs electron leakage prevention layer 106, 0.02 μm thick undoped InGaAlAsP composition gradient layer 107, 0 0.03 μm-thick undoped InGaAsP (composition wavelength 1.05 μm) intermediate guide layer 108, undoped 3-period InGaA P-type multiple quantum well active layer [5 nm thick 1.0% compressive strain InGaAsP (composition wavelength 1.37 μm) well layer, 8 nm thick InGaAsP (composition wavelength 1.00 μm) barrier layer] 109, 0.01 μm thick p The first InP cladding layer 110 is grown sequentially. The emission wavelengths of the multiple quantum well active layers 105 and 109 are both about 1.31 μm. In the composition gradient layer 107, the composition gradually changes between the electron leakage prevention layer 106 and the intermediate guide layer 108.
[0013]
The band structure near the active layer is shown in FIG. As shown in the figure, by introducing an electron leakage prevention layer 106 and a composition gradient layer 107, an InGaAsP quantum well active layer (MQW) 109 is formed while maintaining supply and confinement of electrons to the InGaAlAs multiquantum well layer (MQW) 105. It becomes possible to introduce. In addition, the barrier against holes enables a small and uniform carrier injection.
[0014]
Next, a part of the InGaAsP quantum well active layer 109 and the p-type first InP cladding layer 110 are periodically removed by etching as shown in FIG. 3 by photolithography and dry etching using a normal interference exposure method. 111 is formed on the entire surface of the substrate. The period of the diffraction grating 111 is 201 nm. The depth of the diffraction grating is about 50 nm so that the diffraction grating penetrates the InGaAsP quantum well active layer 109 and reaches the InGaAsP intermediate guide layer 108.
[0015]
The formed diffraction grating 111 is a periodic diffraction along the light traveling direction for the distributed reflection of the light emitted to the multi-quantum well active layer 109 that is a layer near the multi-quantum well active layer 105 that is a light-emitting layer. The grating is a gain diffraction grating that includes an InGaAsP material having an optical gain at the oscillation wavelength of the laser, and that the refractive index and optical gain are periodically perturbed.
[0016]
Subsequently, after removing several nm of the damaged layer on the etching surface automatically formed by dry etching by known wet etching, as shown in FIG. 4, the p-type second InP cladding layer 112 having a thickness of 0.05 μm, 4 nm thick p-type InGaAsP (composition wavelength 1.3 μm) etching stop layer 113, 1.7 μm thick p-type third InP cladding layer 114, 0.03 μm thick p-type InGaAsP (composition wavelength 1.3 μm) barrier relaxation layer 115 and 0.2 μm thick high-concentration p-type InGaAs electrode contact layer 116 are successively regrown. At this time, since the InGaAlAs layer is not exposed on the wafer surface, there is no problem of oxidation of the Al layer, and a good regrowth interface can be obtained.
[0017]
Through the above steps, a basic structure of a gain-coupled distributed feedback laser in which an InGaAsP-based gain diffraction grating 111 is formed on the InGaAlAs-based multiple quantum well layer 105 is fabricated. The normalized optical coupling coefficient of this laser is about 4.0.
[0018]
Subsequently, this wafer is processed into a well-known ridge waveguide type laser, and the laser diode chip shown in FIG. 1 is completed. The resonator length is 200 μm, and a low-reflection film 151 having a reflectance of 1% and a high-reflection film 152 having a reflectance of 70% are formed on the front end face and the rear end face by a normal method.
[0019]
The manufactured 1.3 μm band distributed feedback semiconductor laser element had a threshold current of 6 mA and an oscillation efficiency of 0.30 W / A at room temperature and in continuous conditions. Reflecting simple fabrication, good oscillation characteristics such as a threshold current of 15 mA and an oscillation efficiency of 0.22 W / A were obtained even at a high temperature of 85 ° C.
[0020]
When a forward bias is applied below the oscillation threshold, a typical spectral shape in which an oscillation dominant mode appears on the long wavelength side of the stop band reflecting a gain-coupled diffraction grating is obtained. As a result, a stable single mode operation with a submode suppression ratio of 40 dB or more was realized with a high production yield of 95% or more in a wide temperature range of −40 ° C. to 85 ° C.
[0021]
Thus, according to the present invention, it is possible to achieve both the high-temperature high-performance characteristics of the conventional InGaAlAs quantum well structure and the high yield of the distributed feedback laser with the gain diffraction grating structure. This structure is applicable not only to the 1.3 μm band but also to the distributed feedback semiconductor laser in the 1.55 μm band and other wavelength bands. Further, as will be described later, this structure can be applied not only to a single laser but also to a function-integrated integrated optical device.
[0022]
In this embodiment, the active layer 105 has a quantum well structure made of an InGaAlAs-based material, but this may be a semiconductor layer containing at least one of InAlAs and InGaAlAs, and a gain diffraction grating is formed. Although the active layer 109 has a quantum well structure composed of an InGaAsP-based material, it can be a semiconductor layer including at least one of InP, InGaAsP, and InGaAs materials having optical gain at the laser oscillation wavelength. Alternatively, a bulk structure including at least one of InGaAsP and InGaAs can be used.
<Embodiment 2 of the Invention>
FIG. 6 shows an example in which a distributed feedback semiconductor laser having a wavelength of 1.3 μm and having a different structure is manufactured using the present invention. The main difference from the first embodiment is that a p-type substrate is used and a buried heterostructure is employed. By using the p-type substrate, the efficiency of hole injection into the active layer is improved as compared with the device of the first embodiment.
[0023]
In the semiconductor laser having this structure, an InGaAlAs active layer 205 is formed on a p-type semiconductor substrate 201, and an InGaAsP active layer 209 is further stacked thereon. A diffraction grating 211 is formed on the InGaAsP active layer 209, and the surface is protected by the surface protective layer 231. The active layers 205 and 209 are surrounded by a p-type buried layer 241, a high-resistance buried layer 242, an n-type buried layer 243, and a trench 244, thereby forming a buried heterostructure. An n-type electrode 132 is provided on the top of the structure, and a p-type electrode 133 is provided on the back surface of the substrate 101. A low reflection film 251 is formed on the front end face perpendicular to the substrate 101, and a high reflection film 252 is formed on the rear end face. Main laser light is emitted from the front end face, and monitoring laser light is emitted from the rear end face.
[0024]
A method for manufacturing the semiconductor laser of this embodiment having such a structure will now be described. As shown in FIG. 7, a p-type InP buffer layer 202 having a thickness of 1.5 μm is formed on an InP semiconductor substrate 201 having a conductivity type of p-type and having a (100) crystal plane by a metal organic chemical vapor deposition method. p-type InAlAs buffer layer 203, 0.1 μm-thick undoped InGaAlAs lower guide layer 204, undoped seven-period InGaAlAs-based multi-quantum well active layer [5 nm-thick 1.2% compression strained InGaAlAs (composition wavelength 1.37 μm) Well layer, 8 nm thick InGaAlAs (composition wavelength 1.00 μm) barrier layer] 205, 0.1 μm thick undoped InGaAlAs intermediate guide layer 206, undoped InGaAlAsP composition gradient layer 207, 0.05 μm thick undoped InGaAsP (composition wavelength 1) .05 μm) intermediate guide layer 208, undoped three-period InGaAsP-based multiple quantum well active layer [5n m-thick 1.0% compressive strain InGaAsP (composition wavelength 1.37 μm) well layer, 8 nm-thick InGaAsP (composition wavelength 1.00 μm) barrier layer] 209, 0.01 μm-thick n-type first InP cladding layer 210 Grows sequentially. The emission wavelengths of the multiple quantum well active layers 205 and 209 are both about 1.31 μm.
[0025]
FIG. 10 is a band structure diagram in the vicinity of the active layer. In this embodiment, since a p-type substrate is used, a band structure that can prevent electron supply and leakage of electrons to the p-type layer is a feature. Furthermore, the introduction of the composition gradient layer 207 reduces the barrier against electrons from the InGaAsP quantum well active layer (MQW) 209 to the InGaAlAs quantum well active layer (MQW) 205.
[0026]
Next, a part of the InGaAsP quantum well active layer 209 and the n-type first InP cladding layer 210 are periodically removed by etching as shown in FIG. 8 by photolithography and dry etching using a normal interference exposure method. Is formed on the entire surface of the substrate. The period of the diffraction grating 211 is 201 nm. The depth of the diffraction grating 211 is about 50 nm so that the diffraction grating penetrates the InGaAsP quantum well active layer 209 and reaches the InGaAsP intermediate guide layer 208.
[0027]
Subsequently, after removing the damaged layer number nm on the etching surface automatically formed by dry etching by known wet etching, the undoped second InP cladding layer 212 having a thickness of 0.4 μm, n-type InGaAsP having a thickness of 0.1 μm ( The cap layer 213 is sequentially regrown (composition wavelength 1.3 μm) (FIG. 9).
[0028]
Through the above steps, a basic structure of a gain-coupled distributed feedback laser in which an InGaAsP gain diffraction grating 211 is formed on the InGaAlAs multiple quantum well layer 205 is fabricated. The normalized optical coupling coefficient of this laser is about 4.0.
[0029]
Subsequently, this wafer is processed into a known buried hetero laser, and the laser diode chip shown in FIG. 6 is completed. The resonator length is 200 μm, and a low reflection film 251 having a reflectance of 1% and a high reflection film 252 having a reflectance of 70% are formed on the front end face and the rear end face by a known method.
[0030]
The manufactured 1.3 μm band distributed feedback semiconductor laser element had a threshold current of 3 mA and an oscillation efficiency of 0.35 W / A at room temperature and under continuous conditions. Reflecting the simple fabrication, good oscillation characteristics were obtained with a threshold current of 9 mA and an oscillation efficiency of 0.25 W / A even at a high temperature of 85 ° C.
[0031]
The spectrum shape when a forward bias is applied below the oscillation threshold reflects a gain-coupled diffraction grating, and a typical shape in which an oscillation main mode appears on the long wavelength side of the stop band was obtained. As a result, a stable single mode operation with a submode suppression ratio of 40 dB or more was realized with a high production yield of 95% or more in a wide temperature range of −40 ° C. to 85 ° C.
[0032]
As described above, according to the present invention, it is possible to achieve both high-temperature high-performance characteristics of a conventional InGaAlAs quantum well structure and high yield of a distributed feedback laser having a gain diffraction grating structure. This structure is applicable not only to the 1.3 μm band but also to the distributed feedback semiconductor laser in the 1.55 μm band and other wavelength bands. Further, as will be described later, this structure can be applied not only to a single laser but also to a function-integrated integrated optical device.
<Third Embodiment of the Invention>
FIG. 11 shows an optical lens 503, a rear end surface light output monitor photodiode 504, an optical fiber 505, and a laser driving driver after the distributed feedback semiconductor laser 501 of the first or second embodiment is mounted on a heat sink 502. It is a perspective view of the module which integrated IC506. The module is housed in a housing 507, and a fiber sleeve 508, which is a connector for connecting the optical fiber 505 to the housing 507, and a high-frequency connector 509 for inputting a high-frequency signal supplied to the driver IC 506 are attached.
[0033]
The manufactured module realized a stable single mode operation with a submode suppression ratio of 40 dB or more in a wide temperature range of −40 ° C. to 85 ° C. with a high manufacturing yield of 95% or more. A clear eye opening with an extinction ratio of 8 dB or more was obtained at an operating speed of 10 Gbit / s.
[0034]
In this semiconductor laser 501, since the combination of the high gain characteristics of the InGaAlAs active layer and the gain diffraction grating and the normalized optical coupling coefficient are set as high as 4.0, the return light from the fiber end, which is a problem in module mounting, is set. The characteristic is that there is little deterioration of oscillation characteristics due to. For this reason, this module can achieve a predetermined return light resistance without using an expensive optical isolator normally used for preventing return light, which is very effective for the economics of the module.
<Embodiment 4 of the Invention>
FIG. 12 shows an embodiment of a function integrated laser using the gain coupled semiconductor laser of the present invention. In this example, an electroabsorption optical modulator and a gain-coupled laser operating in a wavelength band of 1.55 μm are monolithically integrated by the butt joint method. As a method for manufacturing the element, first, the diffraction grating structure of the laser part is manufactured by the same method as that described in the first embodiment.
[0035]
In FIG. 12, 702 and 703 are respectively an InGaAlAs multiquantum well layer and an InGaAsP multiquantum well layer formed on an InP semiconductor substrate 701 having an n-type (100) crystal plane, and the emission wavelengths are both about 1560 nm. A gain diffraction grating having a period of 241 nm is formed in the InGaAsP-based multiple quantum well layer 703.
[0036]
After forming the diffraction grating, the light absorption layer 704 of the electroabsorption optical modulator is formed by a known butt joint connection method, and then the InP clad layer 705 is regrown and buried to produce an integrated structure.
[0037]
Thereafter, the integrated structure is processed into a ridge waveguide structure as in the first embodiment, and then upper electrodes 711 and 712 and a lower electrode 713 are formed. Through the cleavage process, a non-reflective film 714 is formed on the front end face, and a highly reflective film 715 is formed on the rear end face, thereby completing the chip. This element serves as a heart component of a compact transmission light source at 2.5 to 40 Gbit / s.
<Embodiment 5 of the Invention>
A functionally integrated laser using the gain-coupled semiconductor laser of the present invention can also be manufactured by an optical modulator / laser integration process using band gap energy control by a known selective growth method shown in FIG. In this embodiment, the use of InGaAlAs-based materials for both the laser part and the modulator part makes it possible to simultaneously improve the characteristics of both elements.
[0038]
In FIG. 13, reference numerals 802 and 803 denote an InGaAlAs multiquantum well layer and an InGaAsP multiquantum well layer formed on an InP semiconductor substrate 801 having an n-type (100) crystal plane, respectively. Reference numeral 804 denotes a light absorption layer of an electroabsorption optical modulator that also employs an InGaAlAs-based material. A gain diffraction grating is formed in the InGaAsP-based multiple quantum well layer 803. After forming the diffraction grating, the InP clad layer 805 is regrown and buried to produce an integrated structure.
[0039]
Thereafter, the integrated structure is processed into a ridge waveguide structure as in the first embodiment, and then upper electrodes 811 and 812 and a lower electrode 813 are formed. Through the cleavage process, a non-reflective film 814 is formed on the front end face, and a highly reflective film 815 is formed on the rear end face, thereby completing the chip.
[0040]
【The invention's effect】
According to the present invention, it is possible to realize a distributed feedback semiconductor laser having a gain diffraction grating structure that has high-performance characteristics at a high temperature and can be easily manufactured at a high yield. By using this semiconductor laser, it is possible to realize an optical module and a function integrated laser that are stable in a wide temperature range. Furthermore, if the present invention is used, not only the device performance and the yield can be dramatically improved, but also it is possible to contribute to the reduction in cost, capacity, and distance of an optical communication system to which this device is applied.
[Brief description of the drawings]
FIG. 1 is a perspective view for explaining an embodiment of a first invention of an optical transmission apparatus according to the present invention.
FIG. 2 is a first diagram for explaining a manufacturing method of the optical transmission device in FIG. 1;
FIG. 3 is a second diagram for explaining a method for manufacturing the optical transmission device in FIG. 1;
4 is a third diagram for explaining a method for manufacturing the optical transmission device in FIG. 1; FIG.
5 is a diagram for explaining a band structure of the optical transmission device in FIG. 1; FIG.
FIG. 6 is a perspective view for explaining an embodiment of the second invention of the optical transmission apparatus of the present invention.
7 is a first diagram for explaining a method for manufacturing the optical transmission device in FIG. 6; FIG.
8 is a second diagram for explaining a method for manufacturing the optical transmission device in FIG. 6. FIG.
9 is a third diagram for explaining a method for manufacturing the optical transmission device in FIG. 6. FIG.
10 is a diagram for explaining a band structure of the optical transmission device in FIG. 6;
FIG. 11 is a perspective view for explaining an embodiment of a third invention by an optical module using the optical transmitter of the present invention.
FIG. 12 is a cross-sectional view for explaining an embodiment of a fourth invention by a function integrated laser using the optical transmitter of the present invention.
FIG. 13 is a cross-sectional view for explaining an embodiment of a fifth invention by a function integrated laser using the optical transmitter of the present invention.
[Explanation of symbols]
101 ... n-type InP semiconductor substrate, 102 ... n-type InP buffer layer, 103 ... n-type InAlAs buffer layer, 104 ... n-type InGaAlAs lower graded guide layer, 105 ... InGaAlAs-based multiple quantum well active layer, 106 ... InAlAs Electron leakage prevention layer, 107 ... InGaAlAsP composition gradient layer, 108 ... InGaAsP intermediate guide layer, 109 ... InGaAsP-based multiple quantum well active layer, 110 ... p-type first InP cladding layer, 111 ... diffraction grating, 112 ... p-type second InP clad layer, 113 ... p-type InGaAsP etching stop layer, 114 ... p-type third InP clad layer, 115 ... p-type InGaAsP barrier relaxation layer, 116 ... high-concentration p-type InGaAs electrode contact layer, 131 ... surface protective layer, 132 ... p-type electrode, 133 ... n-type electrode, 151 ... low reflection film, 152 ... high reflection film, 201 ... p-type InP semiconductor substrate, 202 ... p-type InP buffer layer, 203 ... p-type I AlAs buffer layer, 204... InGaAlAs lower graded guide layer, 205. InGaAlAs-based multiple quantum well active layer, 206. InGaAlAs intermediate guide layer, 207. InGaAlAsP composition gradient layer, 208. InGaAsP intermediate guide layer, 209. Multiple quantum well active layer, 210 ... n-type first InP clad layer, 211 ... diffraction grating, 212 ... second InP clad layer, 213 ... n-type InGaAsP cap layer, 231 ... surface protective layer, 232 ... n-type electrode, 233 ... p-type electrode, 241 ... p-type buried layer, 242 ... high-resistance buried layer, 243 ... n-type buried layer, 251 ... low reflection film, 252 ... high reflection film.

Claims (3)

Progress of light for distributed reflection of light emitted to a first conductivity type InP semiconductor substrate, a first conductivity type buffer layer, a light emitting layer, and a semiconductor layer disposed in the vicinity of the light emitting layer At least a periodic diffraction grating along the direction, and a cladding layer of a second conductivity type,
The buffer layer, the light emitting layer, the periodic diffraction grating, and the cladding layer are formed in this order on the semiconductor substrate,
The EML, InAlAs, is composed of an undoped active layer comprised of any material of InGaAlAs in including a multiple quantum well structure,
The periodic diffraction grating includes an undoped active layer made of any one of InP, InGaAsP, and InGaAs having an optical gain at a laser oscillation wavelength, and the refractive index and the optical gain are periodically perturbed. A gain diffraction grating,
Different undoped film compositions wavelength from the active layer of the gain grating, provided under the active layer of the gain grating,
A semiconductor laser, wherein a composition gradient layer in which the composition gradually changes between two adjacent layers having different compositions is provided between the undoped film and the final layer of the light emitting layer . 2. The semiconductor laser according to claim 1 , wherein the first conductivity type is p-type and the second conductivity type is n-type. The semiconductor laser according to claim 1 or 2 and an optical lens for condensing the laser beam emitted from the semiconductor laser, wherein the laser beam emitted from the semiconductor laser is passed through an optical isolator. An optical module that is incident on an optical fiber that guides light to the outside through the optical lens.

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