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Currently, integrated silicon electronics has reached its limit (10 GHz), and for a long time it is time to move on photonics. The report focuses on photonic integrated circuits, components of the photonics element base, already existing and future developing devices and shows the advantage of photonics. Also some variants of optical transistors are discussed.
Теги: optical transistors photonic integrated circuits оптические транзисторы фотонные интегральные схемы
INTRODUCTION
In devices related to modern photonics, the signal carrier is an electromagnetic wave of the optical range, which can be modulated at a much higher rate than in the radio range. As in traditional radio electronics, the photonic element base has components such as a generator (most often it is a fiber or semiconductor laser); amplifier; connection lines, which can be waveguides; also photo detectors for converting an optical signal into an electrical signal. For the construction of optical calculating machines we need a logical element, with the oportunity to realize a tact (to change the state, when necessary, for example, write 1, read 1, delete 1, write 0, read 0, delete 0).
ADVANTAGES OF PHOTONICS
The technology of creating silicon integrated calculating elements and circuits with very small dimensions and high information processing speeds has approached a certain limit. This explains the interest in photonic integrated circuits, on the basis of which it is possible to create THz calculating machines [1, 2].
Let’s tell about the processing speed limits of the modern electronics circuits, where the logical element is a semiconductor silicon transistor. The maximum operation frequency determined the minimum time, which transistor needs to pass from one state to another, for this, the gate capacitance per channel – Cgс must be recharged, but it is limited by the time, that an electron needs to go throw the gate dielectric, it means, that the maximum frequency is ≈100 GHz (v – electron’s speed, L – length of gate dielectric), so it is the possible maximum of this transistor.
When using a metal waveguide thermal and shot noise, the magnitude of which is proportional to the frequency, the signal-to-noise ratio should be no less than 20 for reliable signal transmission, but then the maximum transmission frequency will be ~10 GHz. So, even we parallelize calculations to different logical chains (Fig. 1), the parallel channels must be multiplexed to the feedback, throughput of which must be more, than throughputs of the parallel channels, to make the feedback have time to work. Such way, for example, now the internet connections are being multiplexed to the optical fiber, which gives an opportunity for transmission signal on the wave near micron, which accords the frequency near THz. It increases the volume of the transmitted information per second more than 1 000 times.
If we exchange metal waveguides to optical fiber in computers and use an optical transistor, which can work with THz frequency, so the photonics will give:1) an opportunity to get as minimum, more than 3 order speed of data processing and transmission; 2) immunity against external radio interference; 3) an opportunity to transmit several modes through one fiber; 4) photonics can give an advantage in efficiency and the number of operations per joule.
As waveguides, many offer plasmonic waveguides, because surface plasmons can transmit a signal throw a channel, smaller wavelength. But because of much loss a channel need gain [8], which is not profitable. So, profitable could be plasmonic interconnects and splitters. Also good nanolasers could be done on plasmons, using Si-technology.
THE VARIANTS OF OPTICAL TRANSISTORS
There are several variants to realize the most important thing for THz computers – an optical transistor:
• semiconductor transistors on quantum dots, for example InP;
• single atom transistors;
• on the micro optical elements using interference or the Kerr effect;
• using magneto optical effects;
• photonic crystals with nanoadditives of Ag and Au.
For example, not a long time ago, German scientists from Max Plank institute [9] of quantum optics put the Rubidium atom between two thin mirrors located at a distance of half a millimeter of each other. Then they directed a laser beam on the construction, tuning it so that the atom began to reflect light. Then directed to the atom the second control laser beam another frequency at right angle to the first and tuned it so as to create transparent conditions for the passage through the construction the first laser beam (Fig. 2). So, the system started to have the two stages – transparent and opaque, like open and close stages of a classical transistor. The quantum mechanical transistor, like this (Tunneled transistor with a double electronic layer, Double Electron Layer Tunneling Transistor, DELTT) was developed by a team of Sandia Laboratory in Department of Energy (DOE). According Sandia, the device is able to perform a trillion operations per second, that is 10 times faster than the most advanced transistor circuits currently in use.
It is possible to build a switching on fiber or plasmonic amplifier, but because of inertness of the active medium the speeds of switching would be ~MHz.
Another variant of optical transistors is transistors on GaInPAs [3], which are produced by such companies as Oclaro and aXenic, for example a Mach-Zahnder modulator (Fig. 3–5), which is produced also on lithium niobate. Or on Si: SiO2 [4] with a ring resonator (Fig. 6–8).
The most perspective variant of an optical transistor is switching a magnetization in ferromagnetics by the interaction of different structures with femtosecond laser pulses (Fig.9). For example on MnFe, TmFeO3, FeBO3 or on GdFeCo (Fig. 10), however, these materials need high power, now is being developed low power switch on ferrite garnet [7].
There is such effect, as reversing in a reproducible manner magnetization by single circularly polarized laser pulse, without any applied magnetic field. This is fully optical ultra fast switching. There is such memory on ferromagnetic domens of spin-switching structures (Fig. 11), which reverse their magnetization by current (by electron’s spin). So the same effect we can use by photon’s spin, it also is described by Landau-Livshiz-Hilbert equation [6]:
,
where М – magnetization, current ,
magnetic field , γ, α and G – constants.
When we switch the magnetization using light (Fig. 12), the field of the laser pulse δHа turns the magnetization М [7]:
.
For example, one variant of realizing the optical switching, using the magnetization switching by the femtosecond laser pulses in ferromagnetics can be an electro optical Mach-Zehnder modulator, it works, changing the optical path of the field in the waveguide, changing the refractive index in one of the branches of the waveguide by voltage, so the output light interferes with different phases (or with the same phase without voltage). We can use magnetic films instead of electrodes, and use the magnetic field, instead of electric field, for the index of refraction modulation. So using the one optical fiber for index of refraction modulation by magnetic field, we can modulate the light field in the other optical fiber, which gives an optical-optic’s modulation – fully optical switching.
PERSPECTIVE DEVICES
The components of the photonics element base are necessary for the realization of optical calculating machines, which can be built on the basis of lithium niobate (waveguides can be created by a femtosecond laser), for photon analog-to-digital converters (ADC, Fig. 13), and for creating processors capable of multiplying a vector by a matrix.
In the late 90-es of the last century there were workings on the creation of an integral module of the optical computer with a logical matrix-tensor foundation called HPOC (High Performance Optoelectronic Communication) [9], (Fig. 14). The device would be used input matrix of VCSEL lasers (Vertical Cavity Surface-Emitting Laser – Surface Emitting Laser Vertical Cavity) connected planar waveguides and conventional optics with switching matrixes based on diffractive optical elements, and an output system consisting of an array of avalanche photodiodes, combined with a matrix of vertically emitting diodes. Prototypes have shown productivity 4.096 TB/s, and estimates indicate that the system is able to develop a speed of 1015 operations per second with an energy of less than 1 fJ per switch. However, due to the several reasons work was stopped. Currently the company "Opticomp Corporation" has developed a new integrated optical element, consisting of a matrix of VCSEL lasers and photo detectors connected waveguide, and plans to use these devices, how to process information, and to create a super fast switches super dense fiber communication lines.
The optical processor Enlight256 (Fig. 16,17) is a hybrid. It only changes the core, and everything else remains electric. It is based on the principle of operation of an analog optical computing device, and it is a developed hybrid digital-analog system, which contains both optical nodes and computer nodes necessary in engineering practice (for example, the implementation of in-system debugging popular in digital technology for embedded applications).
The core of the Enlight256 processor is optical, and the input and output information is presented in electronic form. The core consists of 256 VCSEL lasers, a spatial light modulator, a set of lenses and radiation receivers that form the Vector-Matrix Multiplication (VMM) matrix, which converts electrical information into light, then makes the necessary transformations of this information, directing light through a programmable internal Optics. The output radiation is registered by the receivers and converted again into an electrical signal. In Fig. 15 is a diagram of the operation of the core of this processor.
Inside its "calculational core," there is a parallel counting machine with a specialized architecture, optimal for performing the task of multiplying the matrix by a vector. For one clock, 8 ns duration, the Enlight256 processor is able to multiply a vector of 256 elements into a 256 Ч 256 matrix. Lenslet developers have limited the range of values of vector and matrix elements to 256, corresponding to traditional 8-bit integers. Thus, the performance of the Enlight256 processor is 8 · 1012 operations per second: for one clock cycle (8 ns) the processor multiplies the 256-byte vector by 256 Ч 256-byte matrix. Each element of the input vector is projected onto the column of the matrix. Each row of the matrix is projected onto one detector in the result (output) vector. The programming of the optical digital signal processor consists in changing the transmission values of the cells of the spatial modulator. Loading an application (or data inside a separate application) is done by replacing the matrix values in the spatial modulator (Fig. 18).
The optical matrix of VMM consists of three main elements:
1. Line of the 256 semiconductor VCSEL lasers, which are represented as a vector, which consists of 256 members and is one of the "registers" of the optical arithmetic logic unit, each element of which – is the number of 8 bits.
2. Control the light flux integrated-optical device based on GaAs / GaAlAs semiconductor quantum well structures (Multiple Quantum Well), which consists of a matrix of 256 Ч 256 spatial modulator working in reflection.
3. Lines of light photodetectors 256, which are integrated in an array of analog-to-light conversion (Analog to Digital Converters).
EnLight256 is already used for applications requiring high performance, such as a processor of the type capable of real-time process up to 15 video channels HDTV standard. It can be used for voice recognition, human faces, and image processing, etc. The device is ideal for use in military radar high resolution, as is able to process the data from the array antennas. In addition, EnLight256 dimensions (15 Ч 15 Ч 7 cm3) allow you to place it on the vehicle.
CONCLUSION
Not only transistor, which is being developed, also it is possible to use the existing systems and elements for realizing optical computers: generators – semiconductor and fiber lasers, amplifiers (fiber or metal-balls resonators), it’s also possible to construct a fiber capacitor, which will work as an electronics’ one for an alternating signal. These devices are necessary for the creation of new systems – photonic ADCs, processors, which can multiply a vector by a matrix and for perspective digital calculating machines, the next generation of which should be photonic integrated circuits, which are developed faster every year and get more and more investors.
In devices related to modern photonics, the signal carrier is an electromagnetic wave of the optical range, which can be modulated at a much higher rate than in the radio range. As in traditional radio electronics, the photonic element base has components such as a generator (most often it is a fiber or semiconductor laser); amplifier; connection lines, which can be waveguides; also photo detectors for converting an optical signal into an electrical signal. For the construction of optical calculating machines we need a logical element, with the oportunity to realize a tact (to change the state, when necessary, for example, write 1, read 1, delete 1, write 0, read 0, delete 0).
ADVANTAGES OF PHOTONICS
The technology of creating silicon integrated calculating elements and circuits with very small dimensions and high information processing speeds has approached a certain limit. This explains the interest in photonic integrated circuits, on the basis of which it is possible to create THz calculating machines [1, 2].
Let’s tell about the processing speed limits of the modern electronics circuits, where the logical element is a semiconductor silicon transistor. The maximum operation frequency determined the minimum time, which transistor needs to pass from one state to another, for this, the gate capacitance per channel – Cgс must be recharged, but it is limited by the time, that an electron needs to go throw the gate dielectric, it means, that the maximum frequency is ≈100 GHz (v – electron’s speed, L – length of gate dielectric), so it is the possible maximum of this transistor.
When using a metal waveguide thermal and shot noise, the magnitude of which is proportional to the frequency, the signal-to-noise ratio should be no less than 20 for reliable signal transmission, but then the maximum transmission frequency will be ~10 GHz. So, even we parallelize calculations to different logical chains (Fig. 1), the parallel channels must be multiplexed to the feedback, throughput of which must be more, than throughputs of the parallel channels, to make the feedback have time to work. Such way, for example, now the internet connections are being multiplexed to the optical fiber, which gives an opportunity for transmission signal on the wave near micron, which accords the frequency near THz. It increases the volume of the transmitted information per second more than 1 000 times.
If we exchange metal waveguides to optical fiber in computers and use an optical transistor, which can work with THz frequency, so the photonics will give:1) an opportunity to get as minimum, more than 3 order speed of data processing and transmission; 2) immunity against external radio interference; 3) an opportunity to transmit several modes through one fiber; 4) photonics can give an advantage in efficiency and the number of operations per joule.
As waveguides, many offer plasmonic waveguides, because surface plasmons can transmit a signal throw a channel, smaller wavelength. But because of much loss a channel need gain [8], which is not profitable. So, profitable could be plasmonic interconnects and splitters. Also good nanolasers could be done on plasmons, using Si-technology.
THE VARIANTS OF OPTICAL TRANSISTORS
There are several variants to realize the most important thing for THz computers – an optical transistor:
• semiconductor transistors on quantum dots, for example InP;
• single atom transistors;
• on the micro optical elements using interference or the Kerr effect;
• using magneto optical effects;
• photonic crystals with nanoadditives of Ag and Au.
For example, not a long time ago, German scientists from Max Plank institute [9] of quantum optics put the Rubidium atom between two thin mirrors located at a distance of half a millimeter of each other. Then they directed a laser beam on the construction, tuning it so that the atom began to reflect light. Then directed to the atom the second control laser beam another frequency at right angle to the first and tuned it so as to create transparent conditions for the passage through the construction the first laser beam (Fig. 2). So, the system started to have the two stages – transparent and opaque, like open and close stages of a classical transistor. The quantum mechanical transistor, like this (Tunneled transistor with a double electronic layer, Double Electron Layer Tunneling Transistor, DELTT) was developed by a team of Sandia Laboratory in Department of Energy (DOE). According Sandia, the device is able to perform a trillion operations per second, that is 10 times faster than the most advanced transistor circuits currently in use.
It is possible to build a switching on fiber or plasmonic amplifier, but because of inertness of the active medium the speeds of switching would be ~MHz.
Another variant of optical transistors is transistors on GaInPAs [3], which are produced by such companies as Oclaro and aXenic, for example a Mach-Zahnder modulator (Fig. 3–5), which is produced also on lithium niobate. Or on Si: SiO2 [4] with a ring resonator (Fig. 6–8).
The most perspective variant of an optical transistor is switching a magnetization in ferromagnetics by the interaction of different structures with femtosecond laser pulses (Fig.9). For example on MnFe, TmFeO3, FeBO3 or on GdFeCo (Fig. 10), however, these materials need high power, now is being developed low power switch on ferrite garnet [7].
There is such effect, as reversing in a reproducible manner magnetization by single circularly polarized laser pulse, without any applied magnetic field. This is fully optical ultra fast switching. There is such memory on ferromagnetic domens of spin-switching structures (Fig. 11), which reverse their magnetization by current (by electron’s spin). So the same effect we can use by photon’s spin, it also is described by Landau-Livshiz-Hilbert equation [6]:
,
where М – magnetization, current ,
magnetic field , γ, α and G – constants.
When we switch the magnetization using light (Fig. 12), the field of the laser pulse δHа turns the magnetization М [7]:
.
For example, one variant of realizing the optical switching, using the magnetization switching by the femtosecond laser pulses in ferromagnetics can be an electro optical Mach-Zehnder modulator, it works, changing the optical path of the field in the waveguide, changing the refractive index in one of the branches of the waveguide by voltage, so the output light interferes with different phases (or with the same phase without voltage). We can use magnetic films instead of electrodes, and use the magnetic field, instead of electric field, for the index of refraction modulation. So using the one optical fiber for index of refraction modulation by magnetic field, we can modulate the light field in the other optical fiber, which gives an optical-optic’s modulation – fully optical switching.
PERSPECTIVE DEVICES
The components of the photonics element base are necessary for the realization of optical calculating machines, which can be built on the basis of lithium niobate (waveguides can be created by a femtosecond laser), for photon analog-to-digital converters (ADC, Fig. 13), and for creating processors capable of multiplying a vector by a matrix.
In the late 90-es of the last century there were workings on the creation of an integral module of the optical computer with a logical matrix-tensor foundation called HPOC (High Performance Optoelectronic Communication) [9], (Fig. 14). The device would be used input matrix of VCSEL lasers (Vertical Cavity Surface-Emitting Laser – Surface Emitting Laser Vertical Cavity) connected planar waveguides and conventional optics with switching matrixes based on diffractive optical elements, and an output system consisting of an array of avalanche photodiodes, combined with a matrix of vertically emitting diodes. Prototypes have shown productivity 4.096 TB/s, and estimates indicate that the system is able to develop a speed of 1015 operations per second with an energy of less than 1 fJ per switch. However, due to the several reasons work was stopped. Currently the company "Opticomp Corporation" has developed a new integrated optical element, consisting of a matrix of VCSEL lasers and photo detectors connected waveguide, and plans to use these devices, how to process information, and to create a super fast switches super dense fiber communication lines.
The optical processor Enlight256 (Fig. 16,17) is a hybrid. It only changes the core, and everything else remains electric. It is based on the principle of operation of an analog optical computing device, and it is a developed hybrid digital-analog system, which contains both optical nodes and computer nodes necessary in engineering practice (for example, the implementation of in-system debugging popular in digital technology for embedded applications).
The core of the Enlight256 processor is optical, and the input and output information is presented in electronic form. The core consists of 256 VCSEL lasers, a spatial light modulator, a set of lenses and radiation receivers that form the Vector-Matrix Multiplication (VMM) matrix, which converts electrical information into light, then makes the necessary transformations of this information, directing light through a programmable internal Optics. The output radiation is registered by the receivers and converted again into an electrical signal. In Fig. 15 is a diagram of the operation of the core of this processor.
Inside its "calculational core," there is a parallel counting machine with a specialized architecture, optimal for performing the task of multiplying the matrix by a vector. For one clock, 8 ns duration, the Enlight256 processor is able to multiply a vector of 256 elements into a 256 Ч 256 matrix. Lenslet developers have limited the range of values of vector and matrix elements to 256, corresponding to traditional 8-bit integers. Thus, the performance of the Enlight256 processor is 8 · 1012 operations per second: for one clock cycle (8 ns) the processor multiplies the 256-byte vector by 256 Ч 256-byte matrix. Each element of the input vector is projected onto the column of the matrix. Each row of the matrix is projected onto one detector in the result (output) vector. The programming of the optical digital signal processor consists in changing the transmission values of the cells of the spatial modulator. Loading an application (or data inside a separate application) is done by replacing the matrix values in the spatial modulator (Fig. 18).
The optical matrix of VMM consists of three main elements:
1. Line of the 256 semiconductor VCSEL lasers, which are represented as a vector, which consists of 256 members and is one of the "registers" of the optical arithmetic logic unit, each element of which – is the number of 8 bits.
2. Control the light flux integrated-optical device based on GaAs / GaAlAs semiconductor quantum well structures (Multiple Quantum Well), which consists of a matrix of 256 Ч 256 spatial modulator working in reflection.
3. Lines of light photodetectors 256, which are integrated in an array of analog-to-light conversion (Analog to Digital Converters).
EnLight256 is already used for applications requiring high performance, such as a processor of the type capable of real-time process up to 15 video channels HDTV standard. It can be used for voice recognition, human faces, and image processing, etc. The device is ideal for use in military radar high resolution, as is able to process the data from the array antennas. In addition, EnLight256 dimensions (15 Ч 15 Ч 7 cm3) allow you to place it on the vehicle.
CONCLUSION
Not only transistor, which is being developed, also it is possible to use the existing systems and elements for realizing optical computers: generators – semiconductor and fiber lasers, amplifiers (fiber or metal-balls resonators), it’s also possible to construct a fiber capacitor, which will work as an electronics’ one for an alternating signal. These devices are necessary for the creation of new systems – photonic ADCs, processors, which can multiply a vector by a matrix and for perspective digital calculating machines, the next generation of which should be photonic integrated circuits, which are developed faster every year and get more and more investors.
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