rus
Issue #3/2017
A.Andryieuski, V.F.Andryieuski
Laser Diode Modules: Optical Coupling And Parts Bonding
Laser Diode Modules: Optical Coupling And Parts Bonding
In the article are presented the main technologies of optical coupling from semiconductor laser diode to optical fiber and methods of module parts bonding.
Теги: laser diode modules laser parts bonding optical coupling ввод излучения в волокно лазерные диодные модули фиксация элементов лазерных диодных модулей
INTRODUCTION
The semiconductor laser and optical fiber are the ones of the most important discoveries of the last century that had a great technical and societal impact. That fact was recognized by Nobel prizes of 2000 and 2009, respectively.
The first laser diode samples in 1962 by the group of R. N. Hall were GaAs homostructures and operated at cryogenic temperatures in pulse mode. However, already in 1970 the group headed by Zh. I. Alferov demonstrated a heterostructure-based semiconductor laser working at room temperature in CW mode. Further on in 1970-ies rapid development and improvement occurred and new types of lasers were suggested, including single-frequency lasers with distributed feedback (DBR and DFB), quantum wells lasers and vertical cavity surface emitting lasers (VCSEL). However, only in the years 1990–2000 the manufacturing technology allowed to produce cheap and reliable laser diodes for a broad range of applications in telecommunication, sensors, security systems and biomedical equipment. Nowadays, the most popular types of laser diodes are the lasers with a Fabry-Perot resonator with a broad spectrum containing several modes, the lasers with distributed feedback DFB working in a single-mode regime and the lasers with vertical resonator VCSEL (Fig. 1)
The experiments on optical radiation transmission through a glass fiber started in the beginning of XX century, but only by 1970 Corning Inc. decreased optical losses to 17 dB/km that made an optical fiber acceptable for communication. The following technology development allowed to decrease optical losses by a factor of 100. Nowadays, multiple types of optical fibers on the basis of quartz glass are used, namely, single and multi-mode fibers, polarization maintaining fibers, photonic crystal fibers, spun-fibers, fibers with metal filler etc.
Coupling light from semiconductor laser diode to optical fiber is the task of extreme importance for practical applications of laser diodes. Even though it is possible to use a system of lenses and micromanipulators in an optical lab for light collimation and focusing of the fiber end, usage of bulky optical systems is unacceptable in the equipment subject to mechanical vibrations and temperature variations and under strict constraints on weight and size. This is the main reason why a fiber, a focusing lens and a laser diode have to be carefully aligned and fastened, thus making a laser diode module. Such module is convenient to be used in an equipment, it can be quickly installed and removed multiple times. Laser diode modules are subject to various requirements, including small weight, compactness, large optical coupling efficiency, stability of work under vibrations, high and low temperatures. All these can be ensured by accuracy of optical alignment as well as by construction of package and methods of bonding the modules parts.
In this article we consider two important aspects of laser diode modules assembly: coupling to an optical fiber and bonding the parts of a laser diode module.
COUPLING TO AN OPTICAL FIBER
Not all the radiation coming out of a laser diode couples to a guided mode (Fig. 2a). A part of it goes outside of the fiber, a part enters the cladding and is coupled out further on, a part is reflected from the end. The main characteristic of coupling is the coupling efficiency η that is the ratio of power coupled to the waveguided mode to an incident power
.
In 1990-ies a complicated technology of optical alignment was used. It required covering the fiber with a layer of gold and then alignment in a melted solder and then fixation until the solder is cooled down (Fig. 2b). The lens was most commonly formed on the fiber end by an electric arc discharge. In order to simplify the alignment the solder was pre-deposited into a tube where the fiber was then placed [1]. A typical coupling efficiency by this method was 30–40%. This method was, however, expensive and difficult to implement. Nevertheless, it has a limited employment up to nowadays.
In the end of 1990-ies the laser diodes with a small ball lens in a protective cap started to be produced [2] (Fig. 2c). This way allowed to couple only 10–20% of radiation in a fiber, but the main advantage of this technology was cheapness.
The experiment on light coupling with the help of a graded-index (GRIN) lens were conducted already in 1980-ies [3], but only in the beginning of the XXI century the laser diodes with the GRIN lenses became commercially available (Fig. 2d). Even though the laser diodes with such lenses were more expensive, the coupling efficiency was larger and reached 30%. As for a disadvantage, such systems could work in a narrower temperature range.
In 2010 the laser diodes in aspherical lenses of more complicated shape and with antireflection coatings appeared on the market (Fig. 2e). Such lenses are used nowadays in quality laser diodes and provide coupling efficiency up to 40% in the modules.
LasersCom LLC chose another method of optical coupling that uses a lens formed on the end of an optical fiber by selective etching. This allows to couple up to 80% of optical power in the mass-produced modules. This technology does not use a lens as a separate optical element that makes optical alignment simpler and more reliable. The achieved coupling efficiency is 2 time larger than for typical laser diode modules for optical time-domain reflectometry and 5 times larger than for the typical modules for optical communication. Large coupling efficiency results in a large slope of the PI-curve and allows for using smaller currents that is of extreme importance for reducing power consumption in communication systems.
MODULE PARTS BONDING
As important as optical coupling is the technology of the laser diode module parts bonding. This factor ensures reliability, durability and thermal stability of the modules parameters.
Up to 2000 the main method of module parts bonding was employment of epoxy glues [4] (Fig. 3a). Even though working with epoxy glues is convenient, after module assembly one needs to perform thermal cycling and accelerated aging tests. A considerable part of the devices is rejected due to parameters worsening.
Starting from 2000 the technology of laser beam welding was introduced into laser diode modules assembly [5] (Fig. 3b). The manufacturers of the laser diode modules even tried to emphasize in advertisements that the modules did not contain epoxy glue. Even though laser beam welding increased modules yield, it also has some constraints. The most important is that the welded parts have to be firmly fixed during welding in order to avoid mechanical deformations [6] and the influence of laser welding on optical parameters has to be thoroughly controlled.
The technology by LasersCom LLC has a significant advantage the essence of which is to use instead of glue a special compound with a low thermal expansion coefficient (Fig. 3c). This ensures high strength as well as excellent thermal stability.
CONCLUSIONS
Combining together the technology of lens formation on the end of the fiber with the technology of parts bonding allows LasersCom LLC manufacturing laser diode modules with excellent parameters. For example, stability of the optical power P under temperature T variation is characterized by the value of maximal deviation of the power from the reference value, the so-called tracking error
,
where T0 = 25 °C is the reference temperature, Tmin and Tmax are the minimal and maximal temperatures, respectively. All the modules produced by LasersCom LLC pass the thermal control in the temperature range from –20 °C to +50 °C, and some modules are also controlled in the range from –60 °C to +80 °C. The maximal tracking error Er does not exceed 0.04 dB. The LasersCom modules successfully pass the vibration tests as well as confirm ionizing radiation resistance tests. Small size and low power consumption make the laser diode modules suitable for portable devices and equipment, for which strict size and weight requirements are imposed. An additional feature of the LasersCom modules is the high stability of optical power in a broad temperature range and small level of low-frequency noise that allows to use the modules for optical radiation sources with high stability.
Except for the laser diode modules at the wavelengths in a broad spectral range from 400 to 1650 nm, LasersCom LLC produces superluminescent LED modules, PIN and avalanche photodiode modules, photodiode modules with transimpedance amplifiers and automatic gain control, as well as hybrid modules including optical splitters, isolators, multiplexors and other passive components [7].
Compact optoelectronic modules find multiple applications in fiber optical measurement and communication systems, metrology, range finding, security systems, microwave photonics, medicine, life science and scientific research. The following development of technology of the laser diode modules is targeted to broadening the spectral range, for example, to mid-infrared, as well as to increasing the power efficiency, reliability and resistance to extreme conditions.
The semiconductor laser and optical fiber are the ones of the most important discoveries of the last century that had a great technical and societal impact. That fact was recognized by Nobel prizes of 2000 and 2009, respectively.
The first laser diode samples in 1962 by the group of R. N. Hall were GaAs homostructures and operated at cryogenic temperatures in pulse mode. However, already in 1970 the group headed by Zh. I. Alferov demonstrated a heterostructure-based semiconductor laser working at room temperature in CW mode. Further on in 1970-ies rapid development and improvement occurred and new types of lasers were suggested, including single-frequency lasers with distributed feedback (DBR and DFB), quantum wells lasers and vertical cavity surface emitting lasers (VCSEL). However, only in the years 1990–2000 the manufacturing technology allowed to produce cheap and reliable laser diodes for a broad range of applications in telecommunication, sensors, security systems and biomedical equipment. Nowadays, the most popular types of laser diodes are the lasers with a Fabry-Perot resonator with a broad spectrum containing several modes, the lasers with distributed feedback DFB working in a single-mode regime and the lasers with vertical resonator VCSEL (Fig. 1)
The experiments on optical radiation transmission through a glass fiber started in the beginning of XX century, but only by 1970 Corning Inc. decreased optical losses to 17 dB/km that made an optical fiber acceptable for communication. The following technology development allowed to decrease optical losses by a factor of 100. Nowadays, multiple types of optical fibers on the basis of quartz glass are used, namely, single and multi-mode fibers, polarization maintaining fibers, photonic crystal fibers, spun-fibers, fibers with metal filler etc.
Coupling light from semiconductor laser diode to optical fiber is the task of extreme importance for practical applications of laser diodes. Even though it is possible to use a system of lenses and micromanipulators in an optical lab for light collimation and focusing of the fiber end, usage of bulky optical systems is unacceptable in the equipment subject to mechanical vibrations and temperature variations and under strict constraints on weight and size. This is the main reason why a fiber, a focusing lens and a laser diode have to be carefully aligned and fastened, thus making a laser diode module. Such module is convenient to be used in an equipment, it can be quickly installed and removed multiple times. Laser diode modules are subject to various requirements, including small weight, compactness, large optical coupling efficiency, stability of work under vibrations, high and low temperatures. All these can be ensured by accuracy of optical alignment as well as by construction of package and methods of bonding the modules parts.
In this article we consider two important aspects of laser diode modules assembly: coupling to an optical fiber and bonding the parts of a laser diode module.
COUPLING TO AN OPTICAL FIBER
Not all the radiation coming out of a laser diode couples to a guided mode (Fig. 2a). A part of it goes outside of the fiber, a part enters the cladding and is coupled out further on, a part is reflected from the end. The main characteristic of coupling is the coupling efficiency η that is the ratio of power coupled to the waveguided mode to an incident power
.
In 1990-ies a complicated technology of optical alignment was used. It required covering the fiber with a layer of gold and then alignment in a melted solder and then fixation until the solder is cooled down (Fig. 2b). The lens was most commonly formed on the fiber end by an electric arc discharge. In order to simplify the alignment the solder was pre-deposited into a tube where the fiber was then placed [1]. A typical coupling efficiency by this method was 30–40%. This method was, however, expensive and difficult to implement. Nevertheless, it has a limited employment up to nowadays.
In the end of 1990-ies the laser diodes with a small ball lens in a protective cap started to be produced [2] (Fig. 2c). This way allowed to couple only 10–20% of radiation in a fiber, but the main advantage of this technology was cheapness.
The experiment on light coupling with the help of a graded-index (GRIN) lens were conducted already in 1980-ies [3], but only in the beginning of the XXI century the laser diodes with the GRIN lenses became commercially available (Fig. 2d). Even though the laser diodes with such lenses were more expensive, the coupling efficiency was larger and reached 30%. As for a disadvantage, such systems could work in a narrower temperature range.
In 2010 the laser diodes in aspherical lenses of more complicated shape and with antireflection coatings appeared on the market (Fig. 2e). Such lenses are used nowadays in quality laser diodes and provide coupling efficiency up to 40% in the modules.
LasersCom LLC chose another method of optical coupling that uses a lens formed on the end of an optical fiber by selective etching. This allows to couple up to 80% of optical power in the mass-produced modules. This technology does not use a lens as a separate optical element that makes optical alignment simpler and more reliable. The achieved coupling efficiency is 2 time larger than for typical laser diode modules for optical time-domain reflectometry and 5 times larger than for the typical modules for optical communication. Large coupling efficiency results in a large slope of the PI-curve and allows for using smaller currents that is of extreme importance for reducing power consumption in communication systems.
MODULE PARTS BONDING
As important as optical coupling is the technology of the laser diode module parts bonding. This factor ensures reliability, durability and thermal stability of the modules parameters.
Up to 2000 the main method of module parts bonding was employment of epoxy glues [4] (Fig. 3a). Even though working with epoxy glues is convenient, after module assembly one needs to perform thermal cycling and accelerated aging tests. A considerable part of the devices is rejected due to parameters worsening.
Starting from 2000 the technology of laser beam welding was introduced into laser diode modules assembly [5] (Fig. 3b). The manufacturers of the laser diode modules even tried to emphasize in advertisements that the modules did not contain epoxy glue. Even though laser beam welding increased modules yield, it also has some constraints. The most important is that the welded parts have to be firmly fixed during welding in order to avoid mechanical deformations [6] and the influence of laser welding on optical parameters has to be thoroughly controlled.
The technology by LasersCom LLC has a significant advantage the essence of which is to use instead of glue a special compound with a low thermal expansion coefficient (Fig. 3c). This ensures high strength as well as excellent thermal stability.
CONCLUSIONS
Combining together the technology of lens formation on the end of the fiber with the technology of parts bonding allows LasersCom LLC manufacturing laser diode modules with excellent parameters. For example, stability of the optical power P under temperature T variation is characterized by the value of maximal deviation of the power from the reference value, the so-called tracking error
,
where T0 = 25 °C is the reference temperature, Tmin and Tmax are the minimal and maximal temperatures, respectively. All the modules produced by LasersCom LLC pass the thermal control in the temperature range from –20 °C to +50 °C, and some modules are also controlled in the range from –60 °C to +80 °C. The maximal tracking error Er does not exceed 0.04 dB. The LasersCom modules successfully pass the vibration tests as well as confirm ionizing radiation resistance tests. Small size and low power consumption make the laser diode modules suitable for portable devices and equipment, for which strict size and weight requirements are imposed. An additional feature of the LasersCom modules is the high stability of optical power in a broad temperature range and small level of low-frequency noise that allows to use the modules for optical radiation sources with high stability.
Except for the laser diode modules at the wavelengths in a broad spectral range from 400 to 1650 nm, LasersCom LLC produces superluminescent LED modules, PIN and avalanche photodiode modules, photodiode modules with transimpedance amplifiers and automatic gain control, as well as hybrid modules including optical splitters, isolators, multiplexors and other passive components [7].
Compact optoelectronic modules find multiple applications in fiber optical measurement and communication systems, metrology, range finding, security systems, microwave photonics, medicine, life science and scientific research. The following development of technology of the laser diode modules is targeted to broadening the spectral range, for example, to mid-infrared, as well as to increasing the power efficiency, reliability and resistance to extreme conditions.
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