RU2642814C1 - Automated equalizer of output signal levels of optical amplifiers of multichannel systems - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: essence of the proposed invention is to use the branch devices based on micro resonators in the signal fibre, after the chain amplifier, coupled with the micro actuators controlled by the electronic circuits that use the signals of the photodetectors-sensors of the power level distribution in the signal fibre to produce commands controlling the position of the micro actuators, with the purpose of automatic adjustment of the power levels across the operating range of the equalizer by changing the connection of the micro resonators with a signal fibre.

EFFECT: possibility of constructing a universal equalizer, operable without the individual configuration with any type of the optical amplifier, the automatic alignment operations of the power levels passing through the signal fibre, in the framework of the project working range of the transmitted signals and providing the technological capabilities of the integrated implementation of the device design, with a small amount of the power consumption and the possibility of the remote control of the linear signal uniformity at the amplifier output when embedded in the telemechanic system algorithm.

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Description

The invention is a device and relates to the field of information transmission systems, can be used in the linear equipment of broadband backbone telecommunication networks with optical amplifiers, requiring support for the high flatness of the power levels of all transmitted channels that make up the linear signal, under dynamic operating conditions of multichannel systems, including channel input / output, routing, switching effects ROADM (Reconfigurable Optical Add Drop Multiplexer - reconfigurable optical ultipleksor IO), etc.

In the literature, periodic and patent, various methods and schemes for flattening the output spectral characteristics of optical amplifiers are known, mainly with respect to widely used erbium doped fiber amplifiers EDFA (Erbium Doped Fiber Amplifier). As examples illustrating well-known devices of a similar purpose, but different in principle of operation, circuits, and components used, we can consider:

a) equalization of the gain of the amplifier in the entire operating range by means of dopants in the active fiber

[S. Yoshida, S. Kuwano, and K. Iwashita, "Gain-flattened EDFA with high A1 concentration for multistage repeatered WDM transmission systems," Electron. Lett. 31, 1765-1767 (1995)];

b) the use of Mach-Zehnder filters (MZ) in a chain of EDFA amplifiers [J.Y. Pan, M.A. Ali, A.F. Elrefaie, and R.E. Wagner, "Multiwavelength fiber-amplifier cascades with equalization employing Mach-Zehnder optical filter," IEEE Photon. Technol. Lett. 7, 1501-1503 (1995)];

c) the use of variable controlled attenuators with adjustable average attenuation per channel through an electronic controller: [M. Lelic, G.J. Cowley, and N. Menon, "Dynamic controller for a multichannel optical amplifier," US Patent No. 6,535,330 (2003);

Gee-Kung Chang, Lars E. Esklldsen et al. "Self-Regulating Multi-wavelength Optical Amplifier Module for Scalable Lightwave Communications Systems", US Patent No. 5,392,154 (1995);

d) the use of an array of branches based on Bragg gratings [. R. Kashyap, R. Wyatt, and P.F. McKee, "Wavelength flattened saturated erbium amplifier using multiple side-tap Bragg gratings," Electron. Lett. 29, 1025-1026 (1993)];

e) spectrum equalization using multilayer filter combinations mλ _c / 2 - mλ _s / 4 separated by adjustable gaps [Keith Wayne Goossen, "Enchanced Tilt Optical Power Equalizer", US Patent No. 6,704, 513 B1 (2004)];

f) using EDFA gain slope equalization schemes by pump control [Yamaguchi, Shohei, “Optical Gain Equalizer and Optical Gain Equaliizing Method”, European patent EP 1137129 A2 (2001)];

g) the use of tunable acousto-optic notch filters with electronic control of the EDFA spectrum profile:

[H.S. Kim, S.H. Yun, H.K. Kim, N. Park, and B.Y. Kim, "Actively Gain-flattened erbium-doped fiber amplifier over 35 nm by using all-fiber acousto-optic tunable filters," IEEE Photon. Technol. Lett. 10, 790-792 (1998);

B.Y. Kim, S.H. Yun, and B.W. Lee, "Acousto-optic filter", US Patent No. 6,532,323 B2 (2003);

Hyo Sang Kim, Seok Hyun Yun et al. "Dynamic gain equalization of erbium-doped fiber amplifier with all-fiber acousto-optic tunable filters", Optical Fiber Communication Conference and Exhibit, 1998. OFC '98., Technical Digest];

h) the use of flattening frequency response of the filter amplifier on long-period fiber gratings (LPFG):

[J.R. Qian and H.F. Chen, "Gain flattening fiber filters using phase shifted long period fiber gratings," Electron. Lett. 34, 1132-1133 (1998);

P.D. Greene and H.N. Rourke, "Tailoring long period optical fiber gratings for flattening EDFA gain spectra," Electron. Lett. 35, 1373-1374 (1999);

M.K. Pandit, K.S. Chiang, Z.H. Chen, and S.P. Li, "Tunable Long-Period Fiber Gratings for EDFA gain and ASE equalization," Microwave and Opt. Technol. Lett. 25, 181-184 (2000);

i) the use for broadband alignment of the EDFA spectrum of fiber sections with side polishing of the sheath associated with tunable optical filters: [R.K. Varshney, A. Singh, K. Pande, and B.P. Pal, "Side-polished fiber-based gain-flattening filter for erbium doped fiber amplifiers," Opt. Commun. 271, 441-444 (2007);

K.T. Kim, S. Hwangbo, J.P. Mah, and K.R. Sohn, "Widely tunable filter based on coupling between a side-polished fiber and a tapered planar waveguide," IEEE Photon. Technol. Lett. 17, 142-144 (2005)].

The disadvantages, to one degree or another, are characteristic of the listed well-known adjustable equalizers of broadband DWDM (Dense Wavelength Division Multiplexing) signal at the EDFA output, are:

- individual adjustment of the equalizer for a specific type of amplifier;

- the complexity of the control system that provides the necessary settings, especially when dynamically rebuilding information traffic (input / output channels, reconfiguring traffic, switching to redundancy);

- a significant amount of insertion loss.

According to the results of analysis and comparison of the above sources, the most promising options for the use in practical equipment of broadband long-distance communication systems are options based on the use of a linear fiber with areas with side polished optical fiber (SPF), which are characterized by:

- low insertion loss,

- good mechanical stability,

- easy integration;

- tunability.

As a prototype of the invention, as the closest device of the same purpose to the claimed device for the totality of features, an analogue [R.K. Varshney and B. Nagaraju, A. Singh, B.P. Pall, and A.K. Kar "Design and implementation of an all-fiber broadband tunable gain equalization filter for DWDM signals", Opt Express. 2007. Oct. 17; 15 (21): 13519-30].

The prototype device is built on the basis of a coupler block with side polishing of the signal fiber (SPF) associated with a multimode beveled MMW (multimode waveguide), which serves as a tunable notch filter for the SPF. An effective coupling occurs under the condition of resonance between the mode of the SPF coupler and one of the higher-order modes of the MMW waveguide, which are close to the cutoff conditions, and either a narrow-band channel output or band-pass filters for wavelengths at which resonances occur are achieved. It was shown that by appropriate selection of the opto-geometric parameters of the SPF and the tapering MMW, as well as the communication region between them, one can select the loss spectrum in the linear fiber so that the transmit power level remains almost constant over the entire EDFA operating range (~ 1530 ... 1560 nm) ( non-uniformity ~ 0.35 dB in the 35 nm band).

The disadvantages of the prototype include:

1. The individuality of the operating parameters - with other EDFA data, the replacement or reconfiguration of the waveguide resonator is required.

2. The device requires ensuring translational movements of the SPF block (in the y and z directions) and adjusting the angle of inclination relative to the MMW waveguide, and the requirements for the functional relationship between these shifts and the equalizer parameters are not indicated.

3. The operational adjustment of the equalizer with real dynamic traffic of the DWDM linear signal seems to be a very difficult algorithm.

The problem to which the invention is directed is to create a device for leveling the average power levels of a linear signal at the output of an optical amplifier, which differs from the prototype analogue by eliminating the indicated disadvantages of the latter.

The technical result provided by the combination of features given in the formula is:

- the ability to build a universal equalizer, workable without individual settings with any type of optical amplifier (EDFA, FRA, etc.);

- automatic performance of operations of leveling the power passing through the signal fiber within the design working range of the transmitted signals;

- providing technological feasibility of integral design of the device, with a small amount of power consumption and with the ability to remotely control the uniformity of the levels of the linear signal at the amplifier output when embedded in the algorithm of the telemechanics system.

The specified technical result, provided by the combination of features given in the formula, is achieved by the fact that the automated equalizer of the output signal levels of optical amplifiers contains a signal fiber with a coupler of transmitted power and a device for variable coupling of fibers with an external optical resonator, which together form a broadband notch filter that allows leveling levels to be achieved power in the working range of the transmitted signal, while the composition of the variable communication device fibers with an external resonator include N identical branch devices, each of which rejects power in the 1 / N band of the equalizer operating range, so that together they cover the entire operating range, each branch device includes a side polished SPF signal fiber and an optical a microresonator tuned to the middle frequency of the strip (zone) of its branch device, with each optical microresonator mechanically connected to the movable element of its microactuator, o providing the ability to change communication with the signal fiber on its branch device due to the movement of the microcavity relative to its SPF portion of the signal fiber, the output of the Nth branch device through the signal fiber is connected to a broadband coupler with low power take-off, the output of the broadband coupler is connected to the input of the spectrum splitting demultiplexer the transmitted signal into N bands (zones) uniformly distributed over the entire operating range, the output N ports of the demultiplexer are connected to responsibly with N photodetectors-detectors of power levels, the output signals of which are connected respectively to the N inputs of the comparator of power levels in the zones of the operating range, N output ports of the comparator are connected respectively to the N inputs of microcircuits generating electrical control signals of N microactuators according to the criterion for the distribution of the degree of notch across all N branch devices corresponding to the equalization of power levels in all N spectral bands in the signal fiber, N control signals are fed to N in one port of the interface circuit, forming electrical joints with input buses of actuators of microactuators, and the synchronous operation of all electronic equalizer circuits is provided by the local clock generator, the value of which is selected taking into account the inertia of all elements of the branch devices, and the output of the clock generator is connected to the input ports synchronization of the comparator, microcircuits forming the control signals of microactuators, photodetectors-detectors and interface circuits. In this case, the comparator can be equipped with a buffer memory and an output port, which makes it possible to monitor the distribution of power in the working spectrum of the signal fiber when connected to external control means, for example, to a telemechanics system. At the same time, N branch devices with optical microresonators can be structurally combined on a single chip with thermal stabilization (Peltier elements). In this case, all the communication areas of the fiber with microresonators can be installed in a heat chamber filled with immersion liquid.

The claimed invention "Automated equalizer of the output signal levels of optical amplifiers of multichannel systems" is illustrated by the drawings.

In FIG. 1 is a structural diagram of an automated equalizer of output signal levels of optical amplifiers of multichannel systems;

in FIG. 2 shows the structure of a variable coupling device of a fiber with external optical resonators.

The automated equalizer of the output signal levels of optical amplifiers, contains (Fig. 1) a signal fiber with a coupler of transmitted power 2 and a variable-link device of the fiber with an external optical resonator 1, which together form a broadband notch filter that allows alignment of power levels in the operating range of the transmitted signal, at the same time, the structure of the variable-coupling device of the fiber with the external resonator 1 includes (Fig. 2) N identical branch devices 9, each of which It provides power notch in the 1 / N band of the equalizer working range, so that in total they cover the entire working range, each branch device includes a section with side polishing of the signal fiber SPF and an optical microcavity 10 tuned to the middle frequency of the band (zone) of its device branches, each optical microcavity 10 is mechanically connected to the movable element of its microactuator 11, providing the possibility of changing communication with the signal fiber on its device branch By moving the microcavity 10 relative to its SPF portion of the signal fiber, the output of the Nth branch device 9 through the signal fiber is connected (Fig. 1) with a broadband coupler 2 with low power take-off, the output of the wideband coupler 2 is connected to the input of the demultiplexer 3, dividing the spectrum of the transmitted signal into N bands (zones) uniformly distributed over the entire operating range, the output N ports of the demultiplexer 3 are connected respectively to N photodetectors - power level detectors 4, the output signals of which are connected respectively to the N inputs of the comparator of power levels 5 in the zones of the operating range, N output ports of the comparator 5 are connected respectively to the N input microchips 6, forming electrical control signals for N microactuators 11 according to the criterion for the distribution of the degree of rejection on all N branch devices 9, corresponding to the alignment of power levels in all N spectral bands in the signal fiber, N control signals with microchip 6 are fed respectively to the N input ports of the circuit- interface 7, forming electrical joints with input buses of actuators microactuators 11, and the synchronous operation of all electronic equalizer circuits is provided by local a clock frequency 8, the value of which is selected taking into account the inertia of the operation of all elements of the branch devices, and the output of the clock 8 is connected to the input synchronization ports of the comparator 5, microcircuits 6, photodetectors-detectors 4 and the interface circuit 7. In this case, the comparator 5 can be equipped with a buffer memory and an output port that provides the ability to monitor the distribution of power in the working spectrum of the signal fiber when connected to external control means, for example, a telemechanical system and. Moreover, N branch devices 9 with optical microresonators can be structurally combined on a single chip with thermal stabilization (Peltier elements). Moreover, all the communication areas of the fiber with microresonators 10 can be installed in a heat chamber filled with immersion liquid.

The automated equalizer of the output signal levels of optical amplifiers works as follows.

The optical range to be aligned is divided into N zones, where N≥1, and N SPF sections are formed on the signal fiber. Each SPF section is associated with an optical microresonator 10, the parameters of which satisfy the conditions: the resonant frequency ω _0i is equal to the frequency of the average wavelength of the ith zone (i = 1, ... N); the loaded Q factor Q _ci = ω _0i / 2δ _s , where the damping decrement δ _c determines the width of the resonance curve of the resonator (the full width at the level at the boundaries of the half-maximum zone is FHWM (Full width at half maximum), i.e., the band-pass filter band) . Each microcavity 10 is mechanically connected to the microactuator 11, which provides a variable connection of the microcavity 10 with the SPF _i section, forming in aggregate a branch device 9.

The initial position of the microactuators 11 of all branch devices 9 corresponds to the lowest coupling coefficient of the microresonators 10 with the SPF section. The photodetector detectors of levels 4 are calibrated so that, with a uniform distribution of power over all areas of the working range of the automated equalizer, they give, taking into account the characteristics of the zonal strip separator, the same readings at the outputs.

With the start of the F _T 8 clock, the analog-to-digital converter of each photodetector-detector 4, digital devices of the microcircuit 6, and comparator 5 are started. At the initial clock of the automated equalizer, the distribution of power levels over the spectrum of the operating range in the variable communication device 1 is the same as from the output of EDFA, operating in the normal mode in the absence of alignment. The received signal from the device 1 is fed to the input of the wideband coupler 2, which allocates approximately 1-2% of the input signal power for analysis and transmits the selected signal to the demultiplexer 3, dividing the spectrum of the received signal into N zones corresponding to the zones of partitioning the optical range. Calibration of the demultiplexer 3 corresponds to the equality of powers at all output ports of the demultiplexer with a uniform distribution of powers over the entire working optical range of the automated equalizer.

Photodetectors-detectors 4 convert the optical signals into electrical ones, which are fed to the comparator 5 to compare the values of the signals in all zones with each other. From the comparator 5, the signals are fed to the inputs of the microcircuit 6, which processes the signal level ratios in all N zones, selects the zone with the lowest signal level, takes it as the reference value, and generates control signals for all other zones. The control signals from the microcircuit 6 are fed to the interface circuit 7, which forms electrical joints with the input buses of the actuators of the microactuators 11. The mode of correction of power levels in the signal fiber to complete relative alignment is set by tick-wise adjustment of the notch due to a change in the coupling of the SPF microcavity 10 in all areas of the working range and is supported automatically during the operation of the automated equalizer. Thus, the power levels of the N-channels are equalized to the selected reference level.

An embodiment is possible in which the comparator 5 is equipped with a buffer memory and an output port, which makes it possible to monitor the distribution of power in the working spectrum of the signal fiber when connected to external control means, for example, to a telemechanics system. This makes it possible to connect the equalizer to the signal quality control system at the output of the variable communication device 1 by means of a telemechanics system. An embodiment is also possible in which N branch devices 9 with optical microresonators 10 are structurally combined on a single chip with thermal stabilization (Peltier elements). It is also possible in branch devices 9 to connect the SPF connection region of the linear fiber with zonal microresonators 10 in a heat chamber to fill the removed sheath SPF with immersion (in this case, the losses introduced into the fiber itself and the thermal stabilization of microresonators are reduced).

We show the work of the proposed automated equalizer using a specific example.

Let it be necessary to design a leveling device for a repeater of a DWDM transmission system of 32 channels spaced 100 GHz apart in the C-band (1530-1560 nm).

To implement the claimed equalizer, the purpose of which is to automatically correct the characteristics of the output signal of the repeater, we use the following solutions:

1) As an output directional coupler 2, we use a broadband splitter with a tap to the zone separator of an insignificant part of the signal (~ 0.5-1% _Pout ).

2) Taking advantage of the fact that the output amplitude-frequency characteristic of industrial EDFAs has the character of a fairly smooth function, we choose the number N of spectral regions of the spectrum as small, with central frequencies through 500 GHz, i.e. in our example, N = 7. The center frequencies of the zones and thus the resonant frequencies of the microresonators 10 will be:

ω ₀₁ = 195.7 THz (λ = 1531.9 nm), channel 57 according to a standard DWDM grid;

ω ₀₂ = 195.2 THz (λ = 1535.82 nm), channel 52,

ω ₀₃ = 194.7 THz (λ = 1539.77 nm) channel 47,

ω ₀₄ = 194.2 THz (λ = 1543.73 nm) channel 42,

ω ₀₅ = 193.7 THz (λ = 1547.72 nm) channel 37,

ω ₀₆ = 193.2 THz (λ = 1551.72 nm) channel 32,

ω ₀₇ = 192.7 THz (λ = 1555.75 nm) channel 27,

where ω is the frequency (THz), 01 ... 07 is the zone number, λ is the microcavity tuning wavelength (nm).

3) Microresonators can be used in various types - spherical, disk, spheroid, toroidal, Fabry-Perot type, their features, excitation methods and manufacturing techniques are described in detail in [M.L. Gorodetsky, special course “Optical microcavities”, Moscow State University, 2009; “Optical microresonators with giant Q-factor”, Fiz-MatLit, 2011]. For use in the equalizer, i.e. as a notch band filter, an extremely high Q factor is not required, the Q factor should be limited to the total attenuation coefficient δ = δ ₀ + δ _s - intrinsic and load losses (absorption, scattering), which determines the width of the resonance curve or filter band (in our case - band of the zone) by the ratio: 1 / Q _i = 2 (δ ₀ + δ _s ) / ω _0i . In our example, it is advisable to choose spherical microcavities 10 with whispering gallery modes (WGMs) made of a polymer (for example, of the PDMS type [AL Martin et al. "Replica-molded high-Q polimer microresonators", Optics Letters, 29 (6) 533- 535 (2004)]. For thermal stabilization, it is advisable to place the optical elements of the zone corrector block in a heat chamber filled with immersion liquid.

4) Change in each section of the branch 9 of the degree of connection of the SPF _i fiber section with “one's own” microcavity 10 is carried out by moving this microcavity rigidly connected to the movable element of the microactuator 11, according to the control signal from the interface circuit 7. The highest degree of communication corresponding to the maximum selection excess power microresonator 10 in the strip of its zone, is achieved by contact of the microresonator 11 with a portion of fiber with side polishing SPF, i.e. with the light guide core of the signal fiber. When the microresonator is moved away (by a gap of the order of a micrometer), this connection decreases sharply, so that smooth and stable adjustment can be carried out only by means of a precision microactuator, preferably based on MEMS technologies (microelectromechanical systems) that are well developed at present.

Claims (5)

1. An automated equalizer of the output signal levels of optical amplifiers, containing a signal fiber with a coupler of transmitted power and a variable fiber coupling device with an external optical resonator, which together form a broadband notch filter that allows alignment of power levels in the operating range of the transmitted signal, characterized in that the structure of the variable-fiber communication device with an external resonator includes N identical branch devices, each of which performs power notch in the 1 / N band of the equalizer working range, so that together they cover the entire working range, each branch device includes a section with side polishing of the signal fiber SPF and an optical microcavity tuned to the middle frequency of the strip (zone) of its branch device, each optical microresonator being mechanically the ki is connected to the moving element of its microactuator, which provides the possibility of changing communication with the signal fiber on its branch device by moving the microcavity relative to its SPF section of the signal fiber, the output of the Nth branch device through the signal fiber is connected to a broadband coupler with low power take-off, broadband output the coupler is connected to the input of the demultiplexer, dividing the spectrum of the transmitted signal into N bands (zones), uniformly distributed throughout the working in the range, the output N ports of the demultiplexer are respectively connected to N photodetectors-detectors of power levels, the output signals of which are connected respectively to the N inputs of the comparator of power levels in the zones of the operating range, the N output ports of the comparator are connected respectively to the N inputs of microcircuits forming the electrical control signals of N microactuators by the criterion for the distribution of the degree of rejection on all N branch devices corresponding to the equalization of power levels in all N spectral bands in the signal fiber, N control signals are fed to the N input ports of the interface circuit, forming electrical joints with the input buses of the actuators of the microactuators, and the synchronous operation of all electronic equalizer circuits is provided by the local clock generator, the value of which is selected taking into account the inertia of the operation of all elements of the branch devices , and the output of the clock generator is connected to the input ports of the synchronization of the comparator, microcircuits, photodetectors, detectors and inter face 2. The device according to claim 1, in which the comparator is equipped with a buffer memory and an output port that provides the ability to monitor the distribution of power in the working spectrum of the signal fiber when connected to external control means, for example, to a telemechanics system. 3. The device according to claim 1, in which N branch devices with optical microresonators are structurally combined on a single chip with thermal stabilization (Peltier elements). 4. The device according to claim 1, in which all the communication areas of the fiber with microresonators are installed in a pressure chamber filled with immersion liquid.

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