RU2670719C9 - Fiber-optic photoelectronic ultrahigh frequency module - Google Patents

Abstract

FIELD: radio equipment; electronics.

SUBSTANCE: invention relates to the field of radio engineering, in particular to radiophotonics, and can be used in the design of antenna excitation systems and antenna arrays for communication, radar and electronic warfare. Fiber optic photoelectric ultra-high frequency module includes a symmetric fiber optic splitter, in the primary optical fiber of which powerful laser pulses of duration less than 2 ns are introduced in the spectral range of 820–860 nm, each of the secondary optical fibers is optically coupled to the AlGaAs-GaAs photodetector operating in the gate mode, photodetectors are electrically connected in series to the module, the number of photodetectors in the module being directly proportional to the magnitude of the effective wave resistance of the electrical load.

EFFECT: invention provides an increase in the power and speed of a fiber-optic photoelectric converter of high-power ultra-high frequency pulses of laser radiation.

4 cl, 3 ex, 4 dwg

Description

The invention relates to the field of radio engineering, in particular to radio photonics, and can be used in the design of excitation systems for antennas and antenna arrays for communication, radar and electronic warfare.

When designing modern airborne radar stations, electronic warfare systems and other devices, they place high demands on power, broadband, noise immunity, power consumption, size and weight. An integral part of such equipment are antenna devices and their excitation systems.

One of the most promising ways of developing this area is the creation and application of elements of radio photonics. To implement the above tasks, it is necessary to create powerful, ultra-wideband photoelectric converters operating in a pulsed mode, the power of which, when optimally matched with the load, will be sufficient to excite the transmitting antenna and emit the microwave signal into open space.

The principle of operation of the proposed device is based on the photoelectric conversion of high-power pulsed optical signals transmitted through optical fiber into electrical pulses using a high-speed powerful photoelectric microwave module.

Known fiber optic microwave photovoltaic module for calibrating and monitoring the phase of each radiating element in an active phased array (see https://www.researchgate.net/publication/235084654_RF_Photonic_In-Situ_Real-Time_Phased_Array_Antenna_Calibration_System MG D Parent. WM D. Ch.S. McDermitt, F. Bucholtz). This module uses a photon excitation device, which includes a low-power photodetector that converts the modulated optical signal into a high-frequency calibrated (reference) electric signal that can excite a small dipole antenna located in front of each radiating element of the phased array and operating in the near field.

The disadvantages of the known device is the small (1 mW) output power of the module, which excites a small dipole antenna that transmits a high-frequency calibrated signal to the radiating element of the antenna array, and the dipole antenna is located in the near zone of each element of the phased antenna array, i.e., the proposed device is not designed for working in the far zone for emitting a high-frequency signal into open space over long distances and serves as a source of high-frequency harmonic cal Low-power oscillations with a narrow passband.

Known fiber optic photoelectric microwave module (see http://masters.donntu.org/2012/fit/samoylenko/diss/index.htm, DA Samoilenko "Study of the characteristics of integrated microstrip active antennas in secure communication systems"), V This device implements the concept of a photon antenna, which differs from a traditional microwave antenna in that the coaxial cable is replaced by an optical fiber, the antenna excitation device is a photo detector, and the laser is the source of the optical signal. In the photodetector, the amplitude-modulated optical signal is converted into an electrical signal, which excites the microwave antenna. The disadvantage of the known device, according to the author, is the large conversion loss, which can exceed 10 dB and the low output power of the microwave signal, limited by the maximum photo current of the photodetector, which does not exceed several tens of milliamps.

Known optical fiber photoelectric microwave module (see http: //www.nict.go.ip/publication/shuppan/kihou-journal-vol51no.1.2/5-3.pdf, Li Keren and others. "Photonic Antennas and its Application to Radio-over-Fiber Wireless Communication Systems ”, Journal of the National Institute of Information and Communications Technology, Vol. 51, 2004), which presents an option for direct integration of a photodetector with a planar antenna through a coplanar waveguide. A disadvantage of the known device is the low output power of the device (10 mW).

A device is known (see US patent 20040145026 "Photonic transmitters Chi-Kuang Sun et al., Publication date July 29, 2004, https://www.google.ch/patents/US20040145026). in which the “printed” slot antenna is connected through a matching resistance section, through a coplanar waveguide and integrated with a photodetector. The disadvantages of the known device is the low power component of milliwatts.

The closest technical solution of the prototype is an optical fiber photoelectric microwave module (see patent US 2006140644, "High performance, high efficiency fiber optic link for analog and RF systems", A.Paolella, publication date June 29, 2006, http: // www.google.ch/patents/US20060140644). consisting of a photodetector, optical fiber and antenna, which describes the option of converting the modulated optical signal to a high-frequency electrical signal using one photodetector and transmitting the signal to the antenna due to direct conversion: modulated optical signal -optical fiber - detection-antenna.

A disadvantage of the known device is the small power (1 mW) of the excitation device, which is limited by the power of one photodetector, the impossibility of optimal matching of the photodetector with the antenna, which leads to loss of power of the generated antenna signal and distortion of the signal itself.

The objective of the present invention is to increase the power and speed of a fiber optic photoelectric converter of high-power microwave pulses of laser radiation.

This object is achieved in that the optical fiber photoelectric microwave module includes a symmetrical optical fiber splitter, into the primary optical fiber of which powerful pulses of optical radiation are introduced for a duration of less than 2 not in the spectral range of 810-860 nm, the length of the secondary optical fiber splitter is set to differ by no more than 3 mm, to ensure synchronized operation of photodetectors with a phase shift of no more than 10 ps. Each of the secondary optical fibers is optically coupled to an AlGaAs-GaAs photodetector, photodetectors They operate in the valve mode without applying external voltage, the photodetectors are soldered on an electrically insulating heat-conducting base so that the distance between adjacent photo detectors is set to no more than 150 μm, contacts to the i and p regions of the electron (p) and hole (p) types of conductivity each photodetector, are connected electrically in series to the assembly, and the contacts to the extreme photodetectors of the assembly are electrically connected using microwave matching devices, for example, in a microstrip design, to high frequencies oh load with impedance R _H, wherein the parameters N and the number of photodetectors in the module installed in accordance with the condition

R _n = kNR _PD,

where R _n is the effective load impedance of the module,

R _fd - the optimal load resistance of the photodetectors in the module,

меньше 0,5 не до

при

более 2 нс.k is an empirical coefficient that increases with increasing duration of optical pulses from a value of less than 0.8 when the pulse duration

less than 0.5 not to

at

more than 2 ns.

In the optical fiber photoelectric microwave module, contacts to the n- and p-regions of each photodetector can be made on its front surface, photodetectors can be interconnected using gold buses, the cross section of which is at least 0.01 mm ² , and the electrical connection to the load can be done using matching strip lines.

In the optical fiber photoelectric microwave module, the contacts to the n- and p-regions of each photodetector can be made on different, frontal and rear surfaces of the photodetector, photodetectors can be lapped together to form a step-like construction, and the electrical connection to the load can be carried out by strip matching lines (strip matching devices).

In the optical fiber photoelectric microwave module, the contacts to the edge photodetectors can be connected to the antenna feeder using matching coplanar waveguides.

This technical solution is illustrated by drawings, where:

in FIG. 1 shows the design of this fiber optic photoelectric microwave module (side view) based on 3 photodetectors arranged in a line with each other;

in FIG. 2 is a bottom view of the same structure as in FIG. 1, along section AA in FIG. one;

in FIG. 3 shows the design of a real fiber optic photoelectric microwave module with a serial connection of 8 “overlap” photodetectors in the form of a “ladder”;

in FIG. Figure 4 shows the pulse shapes of the generated voltage and power up to 4 W of a real fiber-optic photoelectric microwave module at an optical pulse repetition rate (up to 60 MHz) and a duration of 1.8 nsec. The duration of the electrical pulses at half maximum amplitude is 2 ns.

To increase the power and speed of the optical fiber photoelectric microwave module and the possibility of optimal matching of the output impedance of the photoelectric module and the wave resistance of the high-frequency load, an optoelectronic photoelectric microwave module is proposed, the structures of which are shown in FIG. 1 (side view), FIG. 2 (bottom view — along section AA in FIG. 1) and FIG. 3.

Fiber optic photoelectric microwave module includes (Fig. 1 and Fig. 2):

1 - primary fiber

2 - symmetric optical fiber splitter on N secondary optical fibers,

3 - secondary optical fibers,

4 - microwave photodetectors,

5 - electrically insulating, thermally conductive base,

6 - negative contact photodetector,

7 - positive contact photodetector,

8 - bus connection photodetectors,

9 - contacts for electrical connection of the module to the load, for example, to an antenna feeder,

10 - electrical connection of contacts 9, for example, an antenna feeder,

11 - antenna contacts for connecting the module, for example, through an antenna feeder,

12 - load, for example, an antenna emitter.

In FIG. Figure 3 shows a variant of a module based on an assembly of 8 overlap photodetectors that form a step structure of a ladder type. Here 1 - secondary optical fibers, 2 - optical fiber holder, 3 - electrical microwave connector SR-50-727, 4 - heat-conducting electrically insulating base, 5 - photodetectors, 6 - metallization, 7 - heat-saving copper case.

High-power optical pulses (of the order of 10 W or more) generated by a laser radiation source, and of a duration of the order of one nanosecond (or less) and a repetition rate of more than 100 MHz are transmitted via multimode optical fiber 1 to optical splitter 2, which evenly divides the power of the input optical signal between secondary optical fibers 3 with a diameter of d ₁ . Each of the secondary optical fibers in the amount of N is connected optically with photodetectors 4.

The length of the secondary optical fibers should not differ from each other by more than 3 mm. Under this condition, the phase shift between the optical pulses is not more than 15 ps during the propagation of the pulse through the secondary optical fibers 3, which are coupled to the photodetectors 4. This allows you to effectively convert without distortion as long optical pulses longer than 1 ns and optical pulses with a duration of less than 0.5 not.

Photodetectors are based on AlGaAs / GaAs heterostructures with maximum photosensitivity in the spectral range of 810-860 nm. The decrease in the photoresponse in the long-wavelength region is associated with the “through” passage of photons with a wavelength of more than 860 nm through the photoactive region of the photodetector, taking into account that the thickness of the region of absorption of photons and generation of current carriers in the developed photodetectors is less than 1.5 μm to reduce the separation time of the generated carriers current and improve the speed of photodetectors. The decrease in photosensitivity in the short-wavelength region of the spectrum at a wavelength of less than 810 nm is associated with an increase in the Stokes losses for the “cooling” of hot current carriers generated by photons with energies noticeably greater than the band gap of the semiconductor (gallium arsenide). Additional losses in the short-wavelength region occur due to absorption of radiation in the layer of a heavily doped wide-gap “window” made of Al _0.12 Ga _0.88 As solid solution having a short-wavelength absorption boundary at λ = 810 nm. This composition of this layer is optimal from the point of view of minimizing ohmic losses and maximizing photosensitivity in the spectral range of 810-860 nm radiation of developed high-power semiconductor lasers.

The photosensitive surface has the shape of a circle with a diameter of d ₂ (Fig. 1 and Fig. 2). To transmit the maximum power of the optical signal, the shape of the working surface of the photodetector is determined by the cross-sectional shape of the core of the optical fiber 3, i.e. has the shape of a circle, and the diameter of the photosensitive surface of the photodetector d _{2 is} larger than the diameter of the optical fiber d ₁ , which allows the photodetector to completely “capture” optical radiation from the optical fiber. In the developed devices, the diameter of the optical fibers is d ₁ = 200 μm, and the diameter of the photodetectors d ₂ = 250 μm.

Photodetectors 4 in the module (Fig. 1) are soldered on one heat-conducting electrically insulating base 5 and are located on it in a line next to each other at a distance of not more than 150 microns between them. In FIG. 1, FIG. 2 and FIG. 3, this distance is denoted by h. This range of distance h between the photodetectors is determined either by the range of minimum disk thicknesses used in machines for the mechanical precision separation of AlGaAs / GaAs heterostructures into photodetector chips when the photodetectors are in the assembly in the same plane (Fig. 1 and Fig. 2), or by the minimum thickness of the photodetector heterostructures when the location of the photo-detectors "overlap" in the form of a "ladder" (Fig. 3).

To ensure effective heat removal and minimal heating of photodetectors in a module operating at a pulsed laser radiation power of up to 20 W and a pulse repetition rate of more than 100 MHz, a heat-conducting electrically insulating base 5, made, for example, of aluminum nitride or beryllium oxide ceramic, is selected as the basis of photodetectors with a high coefficient of thermal conductivity: not less than 200 W / m⋅K.

Given the dynamic mode of operation of the device, for example, to transmit the maximum pulse power of the signal from the photovoltaic module to the antenna, it is necessary to fulfill the matching conditions between the feeder line and the excitation device, namely: the wave resistance of the antenna feeder should be equal to the output resistance of the photodetector. With full coordination, a constant current flows throughout the transmission line along its entire length and its value in the antenna depends only on the output power of the module.

Known fiber optic photoelectric microwave devices operate with bias voltage applied, i.e. photo detectors are powered by a constant voltage, which requires an additional electrical cable at the same time as the fiber optic cable. In the developed device, photodetectors work without the application of bias voltage (gate mode of photodetector operation). When implementing the gate operation mode, only the fiber optic cable is supplied to the microwave module itself. The use of optical fiber in the module without the use of electric cables allows to increase the noise immunity of the high-frequency path of the entire system, because the loss in signal transmission in the fiber optic line is much less than in electrical circuits. If you refuse the electric cable in the high-frequency path, the weight of the device decreases, the reliability of the device increases, because the optical fiber does not oxidize, does not get wet, does not close. In addition, the fiber has high security against interfiber influences, as interfiber radiation shielding level of more than 100 dB, i.e. a signal in one fiber does not affect a signal in another fiber. An important factor in the use of optical fiber and the refusal to apply reverse bias voltage to the photodetector (refusal from supplying an electric cable in addition to the optical fiber) is its fire safety and explosion safety even when the physical and chemical parameters of the optical fiber itself are changed.

At the optimal input optical pulse power, which is fed to the photosensitive surface of each photodetector, the series connection of photodetectors (in the amount of N) allows to increase the output resistance by a factor of N, which makes it possible to match the photoelectric module with the load. In this case, the condition

R _n = kNR _PD,

where R _n is the effective load impedance of the module,

R _fd - the optimal load resistance of the photodetectors in the module,

нс до k=0,9 при

нс и до величины, близкой к единице, при

более 3 нс.k is an empirical coefficient that increases with increasing duration of optical pulses from k = 0.8 for pulse duration

ns to k = 0.9 for

ns and to a value close to unity, at

more than 3 ns.

The physical value of the coefficient k is as follows. When the device is operating in pulsed mode, an important parameter is the broadband of the excitation device itself, which is determined by the time and frequency characteristics of the emitted pulse signal, namely the pulse duration and its repetition rate, which determine the frequency spectrum of the transmitted signal and, accordingly, the required bandwidth of the device itself .

It was experimentally established that when the photodetectors are operating in the present module at the optimum load R _fd , which provides the maximum generated power at a fixed optical power, the electrical pulse of the module is broadened by about 0.5 ns. This broadening is mainly determined by the diffusion capacitance of the photodetectors, which can be minimized by choosing the load resistance of the photodetectors lower than the value of R _fd , i.e. when the operation mode of the photodetectors is shifted from operation to the optimal load R _fd to the operation mode with a larger photocurrent and a lower voltage value. For optical pulses longer than 3 ns, this pulse broadening is insignificant. However, with a decrease in the duration of optical pulses, the role of this broadening increases.

до значений к<0,8 при длительности импульсов

. В результате такой оптимизации режима работы фотодетекторов в модуле, уширение импульсов фототока не превышает величины 0,2 нс в разработанных СВЧ модулях (фиг. 5) и частоте следования импульсов 100 МГц.Coefficient k is a “regulator” of the module operating mode, providing optimization of pulse broadening. An increase in the coefficient k means a decrease in the effective load resistance R _eff <R _fd ) and a transfer of the photodetector operation mode from the “optimal load” mode to a reduced load mode, in which the effect of the photodetector diffusion capacitance on the broadening of the photoresponse pulses is insignificant. The coefficient k decreases with decreasing pulse duration from a value close to unity for "long" pulses

to values k <0.8 with pulse duration

. As a result of this optimization of the photodetector operation mode in the module, the broadening of the photocurrent pulses does not exceed 0.2 ns in the developed microwave modules (Fig. 5) and pulse repetition rate of 100 MHz.

With a certain load impedance (R _n ) and the number (N) of photodetectors in the device, the maximum power of the device is determined by the expression

where P _m is the power of the module, expressed in watts,

U _m is the maximum operating voltage generated by each photodetector in the module, expressed in volts. When a photodetector based on the pAlGaAs-nGaAs heterostructure is operating in the gate mode, the maximum working voltage is in the range of U _M = 1 ... 1.1 V.

An increase in the number of (N) photodetectors in the module at a fixed load makes it possible to increase the power of the device in N ² times when the most accurate matching with the load is possible.

So at R _n = 50 Ohm and N = 2, the maximum power of the module is about 80 mW, and at R _n = 50 Ohm and N = 20 the maximum power of the module is about 8 W, i.e. 100 times more. In this case, the efficiency of the module is about 50%.

In accordance with claim 2 of the present invention, the electrical contacts 6 and 7 to the n- and p-regions of the photodetectors are made on the front, irradiated surface of each photodetector. The electrical contacts 6 and 7 (in Fig. 1 and Fig. 2) of the photo detectors are arranged as follows: the cathode of the photo detector ФД1 is located next to the output of the anode of the photodetector ФД2, the output of the cathode of the photodetector ФД2 is located next to the output of the anode of the photodetector ФД3, which allows them serial electrical connection with a minimum length (not more than 150 microns) of the conductive busbars 8. Minimization of the length of the conductive busbars 8 is necessary to reduce the overall stray inductance of the device. With this arrangement of photodetectors along one line, the electrical connection of the photodetectors and the optical docking of each secondary optical fiber 3 with the working surface of the corresponding photodetector 4 are carried out with minimal electrical and optical losses.

The distance h between the photodetectors is set in this case in the range of 30-150 microns. In the design of the device shown in FIG. 1, this range of h values is determined by the range of minimum disk thicknesses used in machines for mechanical separation of heterostructures into individual photodetector chips. To reduce ohmic and inductive losses with such a serial connection of photodetectors, gold conductors with a cross-sectional area of at least 0.01 mm ^{2 are used} , which provide increased electrical conductivity and reduced inductance of the photodetector assembly.

In accordance with paragraph 3 of the claims, the electrical contacts to the n- and p-regions of the photodetectors are made on different (back and irradiated) surfaces of the photodetectors, and their serial connection is "overlapped" in the form of a "ladder" (Fig. 3). In this case, the minimum distance between the photoactive regions of the photo detectors, located at a depth of 1-2 μm from the irradiated surface of the photodetectors, is set in the range of 100-150 μm, determined by the minimum achievable thickness of the photodetectors.

From the experiment it was found that such an overlapping photodetector switching provides a minimum inductance of the assembly and is more preferable when reducing the duration of optical pulses to values less than 1 ns.

In accordance with paragraph 4 of the claims, the contacts to the extreme photodetectors are connected to the antenna feeder using matching coplanar waveguides. Such a connection of the photoelectric microwave module with the antenna through the antenna feeder ensures minimization of power loss when transmitting pulsed signals from the photoelectric microwave module to the antenna.

нс, k=0,94. Количество фотодетекторов в модуле N=8. Максимальное рабочее напряжение, генерируемое единичными фотодетекторами в модуле, U_м=1 В. Максимальная пиковая импульсная мощность модуля

Вт. При длительности оптических импульсов 1,8 нс длительность электрических импульсов составляет 2 нс на полувысоте амплитуды мощности (фиг. 4).Example 1. The photoelectric microwave module is made in accordance with paragraph 1 and paragraph 2 of the claims. R _n = 15 Ohms, R _fd = 2 Ohms, the duration of the optical pulses

ns, k = 0.94. The number of photodetectors in the module N = 8. The maximum operating voltage generated by individual photodetectors in the module, U _m = 1 V. The maximum peak pulse power of the module

Tue With a duration of optical pulses of 1.8 ns, the duration of electrical pulses is 2 ns at half maximum of the power amplitude (Fig. 4).

нс, коэффициент k=0,9, количество фотодетекторов в устройстве N=22, U_м=1,04 В, максимальная импульсная мощность модуля, передаваемая в нагрузку через коаксиальный кабель, составляет Р_м=9,4 Вт. При длительности оптических импульсов 1 нс длительность электрических импульсов составляет 1,2 нс.Example 2. The photoelectric microwave module is made in accordance with paragraph 1 and paragraph 3 of the claims. The contacts to the extreme photodetectors are connected to a 50-ohm coaxial cable, R _n = 50 Ohms, R _fd = 2.5 Ohms, the duration of the optical pulses

ns, coefficient k = 0.9, the number of photodetectors in the device N = 22, U _m = 1.04 V, the maximum pulsed power of the module transmitted to the load through a coaxial cable is P _m = 9.4 W. With an optical pulse duration of 1 ns, the electrical pulse duration is 1.2 ns.

нс. Значение эмпирического коэффициента k=0,9. Количество фотодетекторов в модуле N=32. Максимальное рабочее напряжение, генерируемое единичными фотодетекторами в модуле, U_м=1,03 В. Максимальная мощность модуля, передаваемая в антенну, составляет

Вт. При длительности оптических импульсов 1 нс длительность электрических импульсов составляет 1,2 нс.Example 3. The photoelectric microwave module is made in accordance with paragraph 1, paragraph 2 and paragraph 4 of the claims. The module is connected to the antenna through an antenna feeder. The effective wave resistance of the feeder R _n = 100 Ohms, R _PD = 3.4 Ohms. Optical Pulse Duration

ns. The value of the empirical coefficient is k = 0.9. The number of photodetectors in the module N = 32. The maximum operating voltage generated by single photodetectors in the module, U _m = 1.03 V. The maximum power of the module transmitted to the antenna is

Tue With an optical pulse duration of 1 ns, the electrical pulse duration is 1.2 ns.

The result of the development of this optical fiber photoelectric microwave module is an increase in the pulse power of the microwave module to ~ 10 W with a duration of electric pulses at half maximum amplitude of 1-2 ns. In this case, the broadening of electric pulses amounted to 0.2 ns compared with the duration of optical pulses. This device provides the ability to control the output impedance to match it with the effective resistance of the high-frequency load.

Claims (7)

1. Optical fiber photoelectric microwave module, comprising a symmetrical optical fiber splitter, into the primary optical fiber of which powerful pulses of optical radiation are introduced for a duration of less than 2 ns in the spectral range of 810-860 nm, the length of the secondary optical fiber of the splitter is set to differ by no more than 3 mm, each of the secondary optical fibers optically docked to AlGaAs-GaAs photodetector, photodetectors operate in the gate mode without the application of external voltage, photodetectors are soldered to an electrically insulating heat For this reason, the distance between adjacent photodetectors is set to no more than 150 μm, the contacts to the n- and p-regions of the electronic (n) and hole (p) types of conductivity of each photodetector are electrically connected in series to the assembly, and the contacts to the extreme photodetectors of the assembly is electrically connected via matching microwave lines to an electrical load with an effective resistance R _n, wherein n number of photodetectors in the module installed in accordance with the condition _n = kNR R _PD, where R _n is the effective load impedance of the module, R _fd - the optimal load resistance of the photodetectors in the module, k - empirical coefficient increases with increasing optical pulse duration on the magnitude of less than 0.8 at a pulse duration of less than 0.5 ns and τ _imp k≅1 at more than 2 ns. 2. The optical fiber photoelectric microwave module according to claim 1, characterized in that the contacts to the n- and p-regions of each photodetector are made on its front surface, the photodetectors are interconnected using gold buses, the cross section of which is at least 0.01 mm ² , and the electrical connection to the load is made by matching strip lines. 3. The optical fiber photoelectric microwave module according to claim 1, characterized in that the contacts to the n- and p-regions of each photodetector are made on different, frontal and rear surfaces of the photodetector, the photodetectors are interconnected “overlapping”, forming a step design of the “ladder” type ”, And the electrical connection to the load is made by matching strip lines. 4. Fiber optic photoelectric microwave module according to paragraphs. 1-3, characterized in that the contacts to the extreme photodetectors are connected to the antenna feeder using matching coplanar waveguides.

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