rus
Issue #7/2023
A. A. Sheinberger, M. V. Stepanenko, Yu. S. Zhidik, S. P. Ivanichko,A. V. Maykova
Study of the Systems for Laser Diode Radiation Output Into a Single-Mode Optical Fiber
Study of the Systems for Laser Diode Radiation Output Into a Single-Mode Optical Fiber
DOI: 10.22184/1993-7296.FRos.2023.17.7.526.538
An optical radiation output system of a laser diode based on a discrete ball lens and a fiber ball lens is proposed. The sensitivity of the following optical radiation output systems of a laser diode to the deviation of elements from the optimal position is determined: laser diode – cleaved optical fiber, laser diode – tapered optical fiber, laser diode – discrete molded lens – cleaved optical fiber, laser diode – discrete ball lens – fiber ball lens. The recommendations are given for the use of these systems in packaging the microwave-photonic modules including the photonic integrated circuits produced with an InP technology.
An optical radiation output system of a laser diode based on a discrete ball lens and a fiber ball lens is proposed. The sensitivity of the following optical radiation output systems of a laser diode to the deviation of elements from the optimal position is determined: laser diode – cleaved optical fiber, laser diode – tapered optical fiber, laser diode – discrete molded lens – cleaved optical fiber, laser diode – discrete ball lens – fiber ball lens. The recommendations are given for the use of these systems in packaging the microwave-photonic modules including the photonic integrated circuits produced with an InP technology.
Теги: microwave photonics optical lens optical radiation output optical systems photonic integrated circuits tapered optical fiber вывод оптического излучения коническое оптоволокно оптическая линза оптические системы радиофотоника фотонные интегральные схемы
Study of the Systems for Laser Diode Radiation Output
Into a Single-Mode Optical Fiber
A. A. Sheinberger, M. V. Stepanenko, Yu. S. Zhidik, S. P. Ivanichko, A. V. Maykova
Tomsk State University of Control Systems and Radioelectronics, Tomsk, Russia
An optical radiation output system of a laser diode based on a discrete ball lens and a fiber ball lens is proposed. The sensitivity of the following optical radiation output systems of a laser diode to the deviation of elements from the optimal position is determined: laser diode – cleaved optical fiber, laser diode – tapered optical fiber, laser diode – discrete molded lens – cleaved optical fiber, laser diode – discrete ball lens – fiber ball lens. The recommendations are given for the use of these systems in packaging the microwave-photonic modules including the photonic integrated circuits produced with an InP technology.
Keywords: optical radiation output, optical systems, optical lens, tapered optical fiber, photonic integrated circuits, microwave photonics
Article received: October 17, 2023
Article accepted: November 10, 2023
Introduction
When assembling any semiconductor photonic integrated circuits (PIC) in the form of microwave-photonic modules, some deviations of the optical path elements from the established position inevitably occur due to the technological capabilities of the positioning equipment, shrinkage of adhesive compounds, etc. All these phenomena lead to the unwanted optical power losses [1, 2]. In this regard, when selecting a radiation output system, it is necessary to know and consider the system sensitivity to the deviation of its elements from the optimal position. The subject of study is the sensitivity of various systems for laser diode radiation output into a single-mode optical fiber to the deviation of optical path elements from the optimal position.
There are well-known optical radiation output systems based on the optical fibers with a Fresnel lens at the end. The Fresnel lens ensures that the numerical apertures of the radiation source and the receiving optical fiber are matched [3, 4]. Moreover, a cylindrical lens on the emitting surface of the source can be applied to optically connect the radiation source to the receiving optical fiber [5, 6]. The disadvantage of such systems is complexity of the formation process for Fresnel lenses and cylindrical lenses, based on the electron beam lithography method or ion beam etching method. The simpler optical radiation output systems include various systems based on the tapered optical fibers made by chemical etching and electric arc melting of the optical fiber [6]. Some optical systems based on the lenses with a refractive index gradient (GRIN lenses) are also well-known [7]. However, due to the high cost of GRIN lenses, the discrete ball lenses often become the preferred option [8]. The simplest method for the optical radiation output from the source is a butt joint between a cleaved optical fiber with a radiation source/receiver [9].
Thus, for this study, the easiest systems for optical radiation output from a laser diode have been selected: laser diode – cleaved optical fiber, laser diode – tapered optical fiber, laser diode – lens – cleaved optical fiber, as well as laser diode – discrete ball lens – ball fiber lens.
Due to the need to package and assemble the PIC electrooptic modulator chips using InP produced by a team of authors [10] in the form of microwave-photonic modules, it has become necessary to determine the optimal design option for the laser diode optical radiation output system. Various designs of the systems for laser diode radiation output into a single-mode optical fiber were investigated (laser diode – cleaved optical fiber; laser diode – tapered optical fiber; laser diode – discrete molded lens – cleaved optical fiber; laser diode – discrete ball lens – fiber ball lens) following by the development of requirements to the optical path assembling process for the semiconductor photonic integrated circuits when they were packaged in the form of microwave-photonic modules.
To determine the sensitivity of laser diode optical radiation output systems to the deviation of their elements from the optimal position, the radiation source (laser diode) and radiation receiving elements (cleaved optical fiber, tapered optical fiber, optical fiber with a fiber ball lens) were placed on the micropositioners. The micropositioners were used to change the position of elements along three axes and rotation angles. At each position, the optical radiation power obtained by the receiving element was measured. The optimal position of the elements was a position providing the greatest optical power obtained by the receiving optical fiber. Further, sensitivity to the deviation of elements from the optimal position was determined as the range of the optical element deviation within which there was a drop in the obtained optical power by no more than 50% of the maximum value.
A laser diode optical output system based on a discrete ball lens and a fiber ball lens was proposed. The sensitivity to the deviation of elements from the optimal position was determined for the following systems for optical radiation output from a laser diode: laser diode – cleaved optical fiber, laser diode – tapered optical fiber, laser diode – discrete molded lens – cleaved optical fiber, laser diode – discrete ball lens – fiber ball lens.
The range of permissible deviation of the receiving element was selected as a criterion that determines optimal position of the emitter and the receiving device. The superiority of the optical radiation output system based on a discrete ball lens and a fiber ball lens over other optical radiation output devices being studied according to the selected criterion was established. The recommendations were given on the use of systems for optical radiation output from a laser diode to minimize the optical power losses in the photonic integrated circuits during their assembly in the form of microwave-photonic modules and operation.
The systems for optical radiation output from a laser diode under study
The study was performed in relation to the following optical output systems: laser diode – cleaved optical fiber (Fig. 1a), laser diode – tapered optical fiber (Fig. 1b), laser diode – discrete molded lens – cleaved optical fiber (Fig. 1c) and laser diode – discrete ball lens – fiber ball lens (Fig. 1d).
A semiconductor laser diode of the OL3502M‑2C1,2,3,4 series was selected as a source of optical radiation (NeoPhotonics Corporation). Its spot shape and the beam divergence angle met the operating requirements of most PICs produced using an InP technology. The laser diode applied had the following specifications: optical output power of at least 13 mW, radiation wavelength of 1310 nm, beam divergence angle of 20° along one axis and 40° along the other one. All systems under consideration used SMF‑28 optical fiber.
Laser diode – cleaved optical fiber system
In the laser diode – cleaved optical fiber system, optical radiation from the laser diode is transferred directly into the cleaved optical fiber. This system is the simplest one in terms of implementation [11]. The cleaved optical fiber means a single-mode, normally cleaved, smooth-end optical fiber without any antireflection coating. The disadvantage of this system includes the large loss of optical power when outputting laser diode radiation. This is due to the lack of matching between the numerical apertures of the cleaved optical fiber and the semiconductor laser diode [12].
Laser diode – tapered fiber system
A tapered optical fiber is an optical fiber with a tapered proximal (input) end. The tapered shape of the proximal end ensures that the numerical aperture of the radiation source and the receiving optical fiber are matched [13]. In this case, the tapered fiber (Raysung Photonic Inc) with a 90° cone vertex angle has been studied.
Laser diode – discrete molded lens – cleaved optical fiber system
The discrete lenses are used to increase the radiation output efficiency by its focusing or collimation [14]. In this case, a molded LightPath lens with a numerical aperture of 0.5 has been used. An antireflection coating has been applied to the lens to reduce optical loss within the wavelength range of 1100–1600 nm.
Laser diode – discrete ball lens – fiber ball lens system
In the laser diode – discrete ball lens – fiber ball lens system, the fiber ball lenses were used made by the arc melting method of the optical fiber end. A discrete ball lens was installed in the optical system in such a way that the optical fiber segment remaining after such melting being a solid structure with the lens, did not participate in the radiation propagation, but was a lens clamping device. The discrete ball lens diameter was 250 μm, the fiber ball lens diameter was 300 μm.
Study methodology for the laser diode radiation output systems
The optimal position of the system elements under study was determined using a test setup (Fig. 2), placed on a Thorlabs optical table with an active vibration isolation system.
The laser diode and radiation receiving elements (cleaved fiber, tapered fiber) were placed on the micropositioners MAX607/M and MAX609/M (Thorlabs), allowing their positions to be changed along the x, y and z axes, with simultaneous adjustment of their rotation angles. It was worth noting that the MAX609/M micropositioner included the built-in ultrasonic motors for precise movement of each of the axes used. The movement accuracy along any of the axes of the MAX609/M micropositioner was no less than 10 nm. Further, the optimal position of elements relative to each other was determined. For this purpose, the position of elements along all axes and rotation angles was subject to variation using the micropositioners. At each position the optical power of radiation obtained by the receiving element was measured by a PM20CH optical power meter (Thorlabs). The position providing the highest power was considered optimal. A similar method was applied to study the following systems: laser diode – discrete molded lens – cleaved optical fiber and laser diode – discrete ball lens – fiber ball lens. The discrete lens (molded or ball one) was placed on the MBT616D/M object table (Thorlabs), and the receiving element (cleaved fiber or fiber ball lens) and laser diode were mounted on the micropositioners. Thus, it was possible to separately adjust the laser diode’s optimal position relative to the discrete lens and the receiving element’s optimal position relative to the discrete lens.
After determining the optimal position of the system elements under study, sensitivity to the deviation of their individual elements was investigated. In this case, the system sensitivity to deviation of its elements meant dependence of the radiation output efficiency on any changes in the position of the system’s receiving or transmitting element.
During the sensitivity study, the radiation receiving element was moved at a certain pitch relative to a fixed laser diode or, conversely, a laser diode was moved relative to a fixed receiving element. When an element was shifted along one axis, its position along the other axes remained unchanged and was located at the optimal point determined earlier. With each movement at one pitch, the optical power obtained by the receiving element was recorded using an optical power meter. The radiation power of the laser diode was kept constant using a power control system.
Results
Laser diode – cleaved optical fiber system
In the laser diode – cleaved optical fiber system, the optimal position of the elements was achieved at a distance of 10 µm between the laser diode and the optical fiber along the x axis. The optical power obtained by the optical fiber at the optimal position of the elements was 1.87 mW. Consequently, the radiation output efficiency into the optical fiber () was equal to 0.144 relative to the optical power at the laser diode output. At this point, the study was performed in relation to the effect of the cleaved fiber deviation from the optimal position on the radiation output efficiency into the optical fiber. Further, two positions of the cleaved optical fiber along the x axis were selected at which the optical power was decreased by no more than 20% compared to the power in the optimal position. The first selected point was located 10 µm further than the obtained point of the optimal position along the x axis, the second point was 20 µm further. At these points, sensitivity of the system under consideration to the receiving element deviation (cleaved optical fiber) was also investigated.
The diagrams of radiation output efficiency versus the optical fiber displacement relative to the optimal position along the y and z axes for three various positions along the x axis are given in Fig. 3 and 4.
Based on the given dependencies, for each selected point along the x axis, the range of possible deviation of the cleaved optical fiber was determined relevant to the permissible halving of the obtained optical power relative to the maximum value for a given system (Table 1).
Having analyzed the system under consideration, it is possible to conclude that the influence of deviation along the y axis and along the z axis is equivalent. Moreover, displacement of the cleaved optical fiber from the optimal position along the x axis does not lead to a significant decrease in the system sensitivity, but it does lead to a power drop of the received optical radiation.
Laser diode – tapered fiber system
Optimal position of the tapered optical fiber relative to the laser diode was also determined in the laser diode – tapered fiber system, followed by the study of two other points along the x axis, for which the power reduction did not exceed 20% of the maximum value for this case. One of them is located 2 µm closer to the radiation source, and the second one is located 2.5 µm further away from the radiation source. The optimal position of the system elements was achieved with a distance between the laser diode and the tapered optical fiber along the x axis being equal to 20 μm. The optical power obtained by the tapered optical fiber at the optimal position was 12.95 mW, therefore, = 0.996. A similar value for the radiation output efficiency of a taoered fiber ( = 1) is given in the paper [15].
The system sensitivity under consideration was determined by the radiation output efficiency dropout level decreased by 2 times relative to the maximum value for this case (Table 2).
Based on the study results, the laser diode – tapered optical fiber system allows to gain the sufficiently high optical power being close to the maximum possible value. However, this system is highly sensitive to the displacement of the receiving element position relative to the optimal one. Displacement of the tapered optical fiber along the x axis relative to the optimal point does not lead to any changes in the system sensitivity under consideration. In addition, the laser diode – tapered fiber system is more sensitive to the displacement along the z axis than along the y axis that may be due to the elliptical shape of the light beam emerging from the radiation source.
Laser diode – discrete molded lens – cleaved optical fiber system
In the third system under study, it is necessary to coordinate three elements with each other, namely a laser diode, a lens and a cleaved optical fiber that receives radiation. Thus, this system can be considered as two subsystems: laser diode – lens and lens – cleaved optical fiber.
The optimal position of the system elements was achieved using the following parameters: the distance between the laser diode and the discrete molded lens along the x axis was 200 μm, the distance between the lens and the cleaved optical fiber was 3 mm. The radiation output efficiency with the optimal placement of system elements was 12 mW, that is, = 0.923. The well-known papers describing similar studies have shown the similar results, namely = 0.929 [16] and = 0.9 [17].
The technological capabilities of the lens and laser diode combination ensure the installation accuracy of not less than ± 15 microns relative to the required position. In this regard, an experiment was performed during which the dependence of changes in the optical signal power upon changing the lens position relative to the laser diode was determined. The position was changed in the range of ±15 μm from the optimal one along all axes under consideration. Change in the optical power along each of the axes was assessed when the laser diode was located in the optimal position relative to the lens along the other two axes. It was worth noting that when the lens and laser diode position was changed, the collecting cleaved optical fiber was adjusted each time to the point with the maximum obtained optical power for a given position.
It was found that when using the available technological capabilities of the laser diode and lens subsystem, the dropout radiation output efficiency was no more than 4% of the maximum possible value (from 0.791 to 0.75).
Further studies are aimed at determining the system sensitivity to the optical fiber displacement relative to the discrete molded lens and laser diode pre-fixed in the optimal positions. Similar to the radiation output studies performed for the laser diode – tapered optical fiber system, two additional points were selected along the x axis where the radiation output efficiency was decreased compared to the output efficiency in the optimal position by no more than 20%. These points were located at the distances of ±27 µm from the optimal position.
For each optical fiber position under consideration, the system sensitivity was determined by the dropout level of radiation output efficiency by 50% from the maximum value (Table 3).
Based on the study results, the laser diode – molded lens – cleaved optical fiber system allows to obtain a significant part of the optical power emitted by the laser diode, while the system sensitivity along any of the considered axes is not less than 7 microns.
Laser diode – discrete ball lens – fiber ball lens system
In this system under study, it is necessary to coordinate three elements with each other, namely a laser diode, a discrete ball lens and an optical fiber with a fiber ball lens that receives radiation. Thus, this system can be considered as two subsystems: laser diode – discrete ball lens and discrete ball lens – fiber ball lens.
The optimal position of the system elements was achieved using the following parameters: the distance between the laser diode and the discrete lens along the x axis was 56.9 μm. The radiation output efficiency with the optimal placement of system elements was 3.89 mW, that is, = 0.3.
The receiving element displacement along the x axis in the range from –50 to 50 μm did not lead to a drop in the radiation output efficiency. However, a larger displacement of the receiving fiber ball lens along the x axis was not provided for by the operating conditions of this optical system (the system must be compact).
The system sensitivity under consideration to the deviation of the fiber ball lens in terms of the dropout level of power obtained by 1 mW from the maximum value was determined (Table 4).
Based on the study results, the laser diode – discrete ball lens – fiber ball lens system has a permissible deviation range of the fiber ball lens of at least 7.9 μm. The significant difference between the allowable deviation ranges along the y and z axes indicates a significant elliptical beam shape. In the future, the laser diode optical radiation output system developed and studied in this paper is planned to be used with a radiation source and an integral beam expander. The integral beam expander converts the elliptical beam shape into a ball one that should increase the output efficiency. Moreover, the efficiency of radiation output into the optical fiber and the system sensitivity to deviation of its elements along the y axis can be increased by replacing the discrete ball lens and by increasing the diameter of both lenses in the system.
Comparative analysis of laser diode radiation output systems
Figures 5 and 6 show dependences of the radiation output efficiency for the laser diode radiation output systems under consideration when the receiving elements deviate from the optimal position. The optical fiber position along the x axis was selected to be optimal for each system under study.
In terms of minimizing optical losses when the radiation is transferred from the semiconductor waveguides of the InP-based photonic integrated circuits, the most efficient method is to use the system with a tapered optical fiber. In turn, the system with a lens has the least sensitivity to deviations among all considered systems.
The output efficiency of laser diode radiation into an optical fiber using a system based on a discrete ball lens and a fiber ball lens was equal to 0.3. It can be assumed that when using an integral beam converter that converts an elliptical beam shape to a ball one, an optical system based on a discrete ball lens and a fiber ball lens can provide a radiation output efficiency of more than 0.9.
Conclusion
The studies performed have shown that the tapered optical fiber provides high radiation output efficiency with the high sensitivity to deviation and can be used for testing the optoelectronic devices. The application of tapered optical fiber is impractical for packaging the photonic integrated circuits and implementing the hybrid integration technology. The use of a laser diode – discrete molded lens – optical fiber system is recommended when a possible displacement along any of the axes is up to 2.85 microns during assembly or operation of the optoelectronic devices. Our proposed laser diode – discrete ball lens – fiber ball lens system dominates over all studied optical systems in terms of the permissible deviation range. In turn, the laser diode – cleaved optical fiber system is not recommended for application, since this system is specified by significant losses of the optical power and high sensitivity to the deviation of its elements.
The obtained study results will be used in the further work for design and assembly of the photonic integrated circuits in the microwave-photonic module packages.
Acknowledgement
The work was performed by a team of the scientific laboratory of integrated optics and microwave-photonics with the financial support from the Ministry of Science and Higher Education of the Russian Federation under the agreement No. 075-03-2020-237/1 dated March 5, 2020 (internal project No. FEWM‑2020-0040).
AUTHORS
Anna A. Sheinberger, Junior Researcher, Laboratory of Integrated Optics and Radiophotonics (LIOR), Tomsk State University of Control Systems and Radioelectronics (TUSUR), Tomsk, Russia.
ORCID:0000-0001-9816-3294
Mikhail V. Stepanenko, Researcher, LIOR, TUSUR, Tomsk, Russia.
ORCID: 0000-0002-6608-5743
Yury S. Zhidik, Cand. of Sciences(Eng.), Leading Researcher, LIOR, TUSUR, Associate Professor of the department. Physical electronics TUSUR, Tomsk, Russia.
ORCID:0000-0001-7803-2086
Svetlana P. Ivanichko, Junior Researcher, LIOR, TUSUR, Tomsk, Russia.
ORCID: 0009-0000-9818-9646
Anastasiia V. Maykova, Junior Researcher, LIOR, TUSUR, Tomsk, Russia.
ORCID: 0009-0008-7074-4175
The contribution of each author
to the work
A. A. Sheinberger: conducting and discussing the results of the experiment; M. V. Stepanenko: setting the research goal and developing the experimental methodology; Yu. S. Zhidik: preparation of equipment for the experiment; S. P. Ivanichko: processing and discussion of the experimental results; A. V. Maykova: conducting the experiment.
Into a Single-Mode Optical Fiber
A. A. Sheinberger, M. V. Stepanenko, Yu. S. Zhidik, S. P. Ivanichko, A. V. Maykova
Tomsk State University of Control Systems and Radioelectronics, Tomsk, Russia
An optical radiation output system of a laser diode based on a discrete ball lens and a fiber ball lens is proposed. The sensitivity of the following optical radiation output systems of a laser diode to the deviation of elements from the optimal position is determined: laser diode – cleaved optical fiber, laser diode – tapered optical fiber, laser diode – discrete molded lens – cleaved optical fiber, laser diode – discrete ball lens – fiber ball lens. The recommendations are given for the use of these systems in packaging the microwave-photonic modules including the photonic integrated circuits produced with an InP technology.
Keywords: optical radiation output, optical systems, optical lens, tapered optical fiber, photonic integrated circuits, microwave photonics
Article received: October 17, 2023
Article accepted: November 10, 2023
Introduction
When assembling any semiconductor photonic integrated circuits (PIC) in the form of microwave-photonic modules, some deviations of the optical path elements from the established position inevitably occur due to the technological capabilities of the positioning equipment, shrinkage of adhesive compounds, etc. All these phenomena lead to the unwanted optical power losses [1, 2]. In this regard, when selecting a radiation output system, it is necessary to know and consider the system sensitivity to the deviation of its elements from the optimal position. The subject of study is the sensitivity of various systems for laser diode radiation output into a single-mode optical fiber to the deviation of optical path elements from the optimal position.
There are well-known optical radiation output systems based on the optical fibers with a Fresnel lens at the end. The Fresnel lens ensures that the numerical apertures of the radiation source and the receiving optical fiber are matched [3, 4]. Moreover, a cylindrical lens on the emitting surface of the source can be applied to optically connect the radiation source to the receiving optical fiber [5, 6]. The disadvantage of such systems is complexity of the formation process for Fresnel lenses and cylindrical lenses, based on the electron beam lithography method or ion beam etching method. The simpler optical radiation output systems include various systems based on the tapered optical fibers made by chemical etching and electric arc melting of the optical fiber [6]. Some optical systems based on the lenses with a refractive index gradient (GRIN lenses) are also well-known [7]. However, due to the high cost of GRIN lenses, the discrete ball lenses often become the preferred option [8]. The simplest method for the optical radiation output from the source is a butt joint between a cleaved optical fiber with a radiation source/receiver [9].
Thus, for this study, the easiest systems for optical radiation output from a laser diode have been selected: laser diode – cleaved optical fiber, laser diode – tapered optical fiber, laser diode – lens – cleaved optical fiber, as well as laser diode – discrete ball lens – ball fiber lens.
Due to the need to package and assemble the PIC electrooptic modulator chips using InP produced by a team of authors [10] in the form of microwave-photonic modules, it has become necessary to determine the optimal design option for the laser diode optical radiation output system. Various designs of the systems for laser diode radiation output into a single-mode optical fiber were investigated (laser diode – cleaved optical fiber; laser diode – tapered optical fiber; laser diode – discrete molded lens – cleaved optical fiber; laser diode – discrete ball lens – fiber ball lens) following by the development of requirements to the optical path assembling process for the semiconductor photonic integrated circuits when they were packaged in the form of microwave-photonic modules.
To determine the sensitivity of laser diode optical radiation output systems to the deviation of their elements from the optimal position, the radiation source (laser diode) and radiation receiving elements (cleaved optical fiber, tapered optical fiber, optical fiber with a fiber ball lens) were placed on the micropositioners. The micropositioners were used to change the position of elements along three axes and rotation angles. At each position, the optical radiation power obtained by the receiving element was measured. The optimal position of the elements was a position providing the greatest optical power obtained by the receiving optical fiber. Further, sensitivity to the deviation of elements from the optimal position was determined as the range of the optical element deviation within which there was a drop in the obtained optical power by no more than 50% of the maximum value.
A laser diode optical output system based on a discrete ball lens and a fiber ball lens was proposed. The sensitivity to the deviation of elements from the optimal position was determined for the following systems for optical radiation output from a laser diode: laser diode – cleaved optical fiber, laser diode – tapered optical fiber, laser diode – discrete molded lens – cleaved optical fiber, laser diode – discrete ball lens – fiber ball lens.
The range of permissible deviation of the receiving element was selected as a criterion that determines optimal position of the emitter and the receiving device. The superiority of the optical radiation output system based on a discrete ball lens and a fiber ball lens over other optical radiation output devices being studied according to the selected criterion was established. The recommendations were given on the use of systems for optical radiation output from a laser diode to minimize the optical power losses in the photonic integrated circuits during their assembly in the form of microwave-photonic modules and operation.
The systems for optical radiation output from a laser diode under study
The study was performed in relation to the following optical output systems: laser diode – cleaved optical fiber (Fig. 1a), laser diode – tapered optical fiber (Fig. 1b), laser diode – discrete molded lens – cleaved optical fiber (Fig. 1c) and laser diode – discrete ball lens – fiber ball lens (Fig. 1d).
A semiconductor laser diode of the OL3502M‑2C1,2,3,4 series was selected as a source of optical radiation (NeoPhotonics Corporation). Its spot shape and the beam divergence angle met the operating requirements of most PICs produced using an InP technology. The laser diode applied had the following specifications: optical output power of at least 13 mW, radiation wavelength of 1310 nm, beam divergence angle of 20° along one axis and 40° along the other one. All systems under consideration used SMF‑28 optical fiber.
Laser diode – cleaved optical fiber system
In the laser diode – cleaved optical fiber system, optical radiation from the laser diode is transferred directly into the cleaved optical fiber. This system is the simplest one in terms of implementation [11]. The cleaved optical fiber means a single-mode, normally cleaved, smooth-end optical fiber without any antireflection coating. The disadvantage of this system includes the large loss of optical power when outputting laser diode radiation. This is due to the lack of matching between the numerical apertures of the cleaved optical fiber and the semiconductor laser diode [12].
Laser diode – tapered fiber system
A tapered optical fiber is an optical fiber with a tapered proximal (input) end. The tapered shape of the proximal end ensures that the numerical aperture of the radiation source and the receiving optical fiber are matched [13]. In this case, the tapered fiber (Raysung Photonic Inc) with a 90° cone vertex angle has been studied.
Laser diode – discrete molded lens – cleaved optical fiber system
The discrete lenses are used to increase the radiation output efficiency by its focusing or collimation [14]. In this case, a molded LightPath lens with a numerical aperture of 0.5 has been used. An antireflection coating has been applied to the lens to reduce optical loss within the wavelength range of 1100–1600 nm.
Laser diode – discrete ball lens – fiber ball lens system
In the laser diode – discrete ball lens – fiber ball lens system, the fiber ball lenses were used made by the arc melting method of the optical fiber end. A discrete ball lens was installed in the optical system in such a way that the optical fiber segment remaining after such melting being a solid structure with the lens, did not participate in the radiation propagation, but was a lens clamping device. The discrete ball lens diameter was 250 μm, the fiber ball lens diameter was 300 μm.
Study methodology for the laser diode radiation output systems
The optimal position of the system elements under study was determined using a test setup (Fig. 2), placed on a Thorlabs optical table with an active vibration isolation system.
The laser diode and radiation receiving elements (cleaved fiber, tapered fiber) were placed on the micropositioners MAX607/M and MAX609/M (Thorlabs), allowing their positions to be changed along the x, y and z axes, with simultaneous adjustment of their rotation angles. It was worth noting that the MAX609/M micropositioner included the built-in ultrasonic motors for precise movement of each of the axes used. The movement accuracy along any of the axes of the MAX609/M micropositioner was no less than 10 nm. Further, the optimal position of elements relative to each other was determined. For this purpose, the position of elements along all axes and rotation angles was subject to variation using the micropositioners. At each position the optical power of radiation obtained by the receiving element was measured by a PM20CH optical power meter (Thorlabs). The position providing the highest power was considered optimal. A similar method was applied to study the following systems: laser diode – discrete molded lens – cleaved optical fiber and laser diode – discrete ball lens – fiber ball lens. The discrete lens (molded or ball one) was placed on the MBT616D/M object table (Thorlabs), and the receiving element (cleaved fiber or fiber ball lens) and laser diode were mounted on the micropositioners. Thus, it was possible to separately adjust the laser diode’s optimal position relative to the discrete lens and the receiving element’s optimal position relative to the discrete lens.
After determining the optimal position of the system elements under study, sensitivity to the deviation of their individual elements was investigated. In this case, the system sensitivity to deviation of its elements meant dependence of the radiation output efficiency on any changes in the position of the system’s receiving or transmitting element.
During the sensitivity study, the radiation receiving element was moved at a certain pitch relative to a fixed laser diode or, conversely, a laser diode was moved relative to a fixed receiving element. When an element was shifted along one axis, its position along the other axes remained unchanged and was located at the optimal point determined earlier. With each movement at one pitch, the optical power obtained by the receiving element was recorded using an optical power meter. The radiation power of the laser diode was kept constant using a power control system.
Results
Laser diode – cleaved optical fiber system
In the laser diode – cleaved optical fiber system, the optimal position of the elements was achieved at a distance of 10 µm between the laser diode and the optical fiber along the x axis. The optical power obtained by the optical fiber at the optimal position of the elements was 1.87 mW. Consequently, the radiation output efficiency into the optical fiber () was equal to 0.144 relative to the optical power at the laser diode output. At this point, the study was performed in relation to the effect of the cleaved fiber deviation from the optimal position on the radiation output efficiency into the optical fiber. Further, two positions of the cleaved optical fiber along the x axis were selected at which the optical power was decreased by no more than 20% compared to the power in the optimal position. The first selected point was located 10 µm further than the obtained point of the optimal position along the x axis, the second point was 20 µm further. At these points, sensitivity of the system under consideration to the receiving element deviation (cleaved optical fiber) was also investigated.
The diagrams of radiation output efficiency versus the optical fiber displacement relative to the optimal position along the y and z axes for three various positions along the x axis are given in Fig. 3 and 4.
Based on the given dependencies, for each selected point along the x axis, the range of possible deviation of the cleaved optical fiber was determined relevant to the permissible halving of the obtained optical power relative to the maximum value for a given system (Table 1).
Having analyzed the system under consideration, it is possible to conclude that the influence of deviation along the y axis and along the z axis is equivalent. Moreover, displacement of the cleaved optical fiber from the optimal position along the x axis does not lead to a significant decrease in the system sensitivity, but it does lead to a power drop of the received optical radiation.
Laser diode – tapered fiber system
Optimal position of the tapered optical fiber relative to the laser diode was also determined in the laser diode – tapered fiber system, followed by the study of two other points along the x axis, for which the power reduction did not exceed 20% of the maximum value for this case. One of them is located 2 µm closer to the radiation source, and the second one is located 2.5 µm further away from the radiation source. The optimal position of the system elements was achieved with a distance between the laser diode and the tapered optical fiber along the x axis being equal to 20 μm. The optical power obtained by the tapered optical fiber at the optimal position was 12.95 mW, therefore, = 0.996. A similar value for the radiation output efficiency of a taoered fiber ( = 1) is given in the paper [15].
The system sensitivity under consideration was determined by the radiation output efficiency dropout level decreased by 2 times relative to the maximum value for this case (Table 2).
Based on the study results, the laser diode – tapered optical fiber system allows to gain the sufficiently high optical power being close to the maximum possible value. However, this system is highly sensitive to the displacement of the receiving element position relative to the optimal one. Displacement of the tapered optical fiber along the x axis relative to the optimal point does not lead to any changes in the system sensitivity under consideration. In addition, the laser diode – tapered fiber system is more sensitive to the displacement along the z axis than along the y axis that may be due to the elliptical shape of the light beam emerging from the radiation source.
Laser diode – discrete molded lens – cleaved optical fiber system
In the third system under study, it is necessary to coordinate three elements with each other, namely a laser diode, a lens and a cleaved optical fiber that receives radiation. Thus, this system can be considered as two subsystems: laser diode – lens and lens – cleaved optical fiber.
The optimal position of the system elements was achieved using the following parameters: the distance between the laser diode and the discrete molded lens along the x axis was 200 μm, the distance between the lens and the cleaved optical fiber was 3 mm. The radiation output efficiency with the optimal placement of system elements was 12 mW, that is, = 0.923. The well-known papers describing similar studies have shown the similar results, namely = 0.929 [16] and = 0.9 [17].
The technological capabilities of the lens and laser diode combination ensure the installation accuracy of not less than ± 15 microns relative to the required position. In this regard, an experiment was performed during which the dependence of changes in the optical signal power upon changing the lens position relative to the laser diode was determined. The position was changed in the range of ±15 μm from the optimal one along all axes under consideration. Change in the optical power along each of the axes was assessed when the laser diode was located in the optimal position relative to the lens along the other two axes. It was worth noting that when the lens and laser diode position was changed, the collecting cleaved optical fiber was adjusted each time to the point with the maximum obtained optical power for a given position.
It was found that when using the available technological capabilities of the laser diode and lens subsystem, the dropout radiation output efficiency was no more than 4% of the maximum possible value (from 0.791 to 0.75).
Further studies are aimed at determining the system sensitivity to the optical fiber displacement relative to the discrete molded lens and laser diode pre-fixed in the optimal positions. Similar to the radiation output studies performed for the laser diode – tapered optical fiber system, two additional points were selected along the x axis where the radiation output efficiency was decreased compared to the output efficiency in the optimal position by no more than 20%. These points were located at the distances of ±27 µm from the optimal position.
For each optical fiber position under consideration, the system sensitivity was determined by the dropout level of radiation output efficiency by 50% from the maximum value (Table 3).
Based on the study results, the laser diode – molded lens – cleaved optical fiber system allows to obtain a significant part of the optical power emitted by the laser diode, while the system sensitivity along any of the considered axes is not less than 7 microns.
Laser diode – discrete ball lens – fiber ball lens system
In this system under study, it is necessary to coordinate three elements with each other, namely a laser diode, a discrete ball lens and an optical fiber with a fiber ball lens that receives radiation. Thus, this system can be considered as two subsystems: laser diode – discrete ball lens and discrete ball lens – fiber ball lens.
The optimal position of the system elements was achieved using the following parameters: the distance between the laser diode and the discrete lens along the x axis was 56.9 μm. The radiation output efficiency with the optimal placement of system elements was 3.89 mW, that is, = 0.3.
The receiving element displacement along the x axis in the range from –50 to 50 μm did not lead to a drop in the radiation output efficiency. However, a larger displacement of the receiving fiber ball lens along the x axis was not provided for by the operating conditions of this optical system (the system must be compact).
The system sensitivity under consideration to the deviation of the fiber ball lens in terms of the dropout level of power obtained by 1 mW from the maximum value was determined (Table 4).
Based on the study results, the laser diode – discrete ball lens – fiber ball lens system has a permissible deviation range of the fiber ball lens of at least 7.9 μm. The significant difference between the allowable deviation ranges along the y and z axes indicates a significant elliptical beam shape. In the future, the laser diode optical radiation output system developed and studied in this paper is planned to be used with a radiation source and an integral beam expander. The integral beam expander converts the elliptical beam shape into a ball one that should increase the output efficiency. Moreover, the efficiency of radiation output into the optical fiber and the system sensitivity to deviation of its elements along the y axis can be increased by replacing the discrete ball lens and by increasing the diameter of both lenses in the system.
Comparative analysis of laser diode radiation output systems
Figures 5 and 6 show dependences of the radiation output efficiency for the laser diode radiation output systems under consideration when the receiving elements deviate from the optimal position. The optical fiber position along the x axis was selected to be optimal for each system under study.
In terms of minimizing optical losses when the radiation is transferred from the semiconductor waveguides of the InP-based photonic integrated circuits, the most efficient method is to use the system with a tapered optical fiber. In turn, the system with a lens has the least sensitivity to deviations among all considered systems.
The output efficiency of laser diode radiation into an optical fiber using a system based on a discrete ball lens and a fiber ball lens was equal to 0.3. It can be assumed that when using an integral beam converter that converts an elliptical beam shape to a ball one, an optical system based on a discrete ball lens and a fiber ball lens can provide a radiation output efficiency of more than 0.9.
Conclusion
The studies performed have shown that the tapered optical fiber provides high radiation output efficiency with the high sensitivity to deviation and can be used for testing the optoelectronic devices. The application of tapered optical fiber is impractical for packaging the photonic integrated circuits and implementing the hybrid integration technology. The use of a laser diode – discrete molded lens – optical fiber system is recommended when a possible displacement along any of the axes is up to 2.85 microns during assembly or operation of the optoelectronic devices. Our proposed laser diode – discrete ball lens – fiber ball lens system dominates over all studied optical systems in terms of the permissible deviation range. In turn, the laser diode – cleaved optical fiber system is not recommended for application, since this system is specified by significant losses of the optical power and high sensitivity to the deviation of its elements.
The obtained study results will be used in the further work for design and assembly of the photonic integrated circuits in the microwave-photonic module packages.
Acknowledgement
The work was performed by a team of the scientific laboratory of integrated optics and microwave-photonics with the financial support from the Ministry of Science and Higher Education of the Russian Federation under the agreement No. 075-03-2020-237/1 dated March 5, 2020 (internal project No. FEWM‑2020-0040).
AUTHORS
Anna A. Sheinberger, Junior Researcher, Laboratory of Integrated Optics and Radiophotonics (LIOR), Tomsk State University of Control Systems and Radioelectronics (TUSUR), Tomsk, Russia.
ORCID:0000-0001-9816-3294
Mikhail V. Stepanenko, Researcher, LIOR, TUSUR, Tomsk, Russia.
ORCID: 0000-0002-6608-5743
Yury S. Zhidik, Cand. of Sciences(Eng.), Leading Researcher, LIOR, TUSUR, Associate Professor of the department. Physical electronics TUSUR, Tomsk, Russia.
ORCID:0000-0001-7803-2086
Svetlana P. Ivanichko, Junior Researcher, LIOR, TUSUR, Tomsk, Russia.
ORCID: 0009-0000-9818-9646
Anastasiia V. Maykova, Junior Researcher, LIOR, TUSUR, Tomsk, Russia.
ORCID: 0009-0008-7074-4175
The contribution of each author
to the work
A. A. Sheinberger: conducting and discussing the results of the experiment; M. V. Stepanenko: setting the research goal and developing the experimental methodology; Yu. S. Zhidik: preparation of equipment for the experiment; S. P. Ivanichko: processing and discussion of the experimental results; A. V. Maykova: conducting the experiment.
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