RU2620261C1 - Transmitting device for fiber-optical soliton system of transferring synchronous digital channels - Google Patents

Abstract

FIELD: radio engineering, communication.

SUBSTANCE: framer (common frame shaper) is applied for hard synchronization of the system clock signal generator and generation of voltages for soliton pulse modulator keys, and also powerful multi-colored soliton pulses are used in combination with splitters of these pulses and an array of integrated optical delay lines is used to form group TDM-signal of each frame of the linear signal.

EFFECT: reducing the energy intensity, increasing the stability of the path elements involved in the formation of the frame, increasing the noise immunity.

3 dwg, 1 tbl

Description

The invention is a device and relates to the field of information transmission systems, can be used in the equipment of urban, regional, corporate and trunk telecommunication networks using the time multiplexing (TDM) method of a group of synchronous digital channels with the same clock frequency.

A fundamental condition for the implementation of such systems is the requirement to transmit all channel symbols of multiplexed messages during one clock interval, i.e. the use of ultrashort optical pulses providing the required multiplexing coefficient. Semiconductor lasers in mode-locked mode (mode-locked) are capable of generating pulse sequences up to subpicosecond and femtosecond durations [1]. Using such emitters, it is possible to form a linear TDM signal in the transmitting device, however, the spectrum of active transmissions (pulses) is so wide that transmission over distances characteristic of fiber systems is not feasible, at least because of the dispersion of the used optical fibers.

The optimal solution to this problem is the generation and use of the soliton mode of propagation of optical pulses along the fiber optic line. The basis of this mode is a nonlinear change in the refractive index of the medium in the electric field of the light wave, called the Kerr effect. The consequence is phase self-modulation (FSM), expressed in a change in the phase of the wave, depending on the intensity of the optical radiation. As a result of this, the frequencies of the leading edge of the pulse will be shifted to the low-frequency part of the spectrum, and the frequencies of the trailing edge to be shifted to the high-frequency part. If such a pulse propagates in a medium with a negative group velocity dispersion, and fibers with this property are easily realized, then the back of the pulse will catch up with the front, the pulse will be compressed. At a certain pulse power (fundamental soliton power), the combined effect of nonlinearity and negative dispersion exactly balances the pulse broadening due to the dispersion of the group velocity, and as a result, the ultra-narrow soliton packet propagates theoretically over very large distances (of the order of thousands of kilometers), if there is no light attenuation in the fiber or can be compensated by repeater amplifiers. Regarding the sources of the soliton pulses themselves, it should be noted that an extensive bibliography is available with data on research and industrial designs of emitters of powerful ultrashort pulses. But from the standpoint of building telecommunication equipment with TDM, the use of compact generators of dissipative solitons of hundreds of femtosecond durations formed in optical microcavities with high nonlinearity is especially promising. [2].

Fiber-optic systems are known that make it possible to increase the speeds and volumes of transmitted information through the use of ultrashort soliton pulses. In the transmitting devices of such systems, when generating a linear signal, either time division multiplexing (TDM) or mixed (WDM + TDM) is used.

Known experimental fiber optic line with time multiplexing (TDM), using soliton pulses [3]. In this line, temporary multiplexing of 10-gigabit channels transmitted by solitons at a wavelength of ~ 1550 nm was used.

The disadvantage of the proposed communication system is the use of a single wavelength for transmission, which significantly worsens the noise immunity of the system, due to soliton - soliton interaction with a large number of multiplexed channels, and therefore does not allow the system to work over long distances.

The known soliton system [4], in which the 4-channel transmission of OS-192 signals was carried out by the method of mixed multiplexing, was used (Pirelli and MCI) the existing fiber-optic communication line (FOCL) (Chicago-St. Louis) using SONET equipment total length of 450 km. Soliton pulse generators were installed after the existing OS-192 commercial transmitters, RZ / NRZ converters installed in front of the OS-192 commercial receivers, and dispersion compensating fiber (DCF) inserts at terminal and 5 transit junction stations

The disadvantages of this, although successfully tested, system are:

- the need for a separate generator of soliton pulses for each channel 10 Gbit / s, which increases the complexity and energy intensity of the system;

- with an increase in the number of multiplexed channels, the system characteristics approach the properties of the DWDM path, mutual interference and the degradation of linear signals increase.

A known soliton system [5] also applying the principle of mixed multiplexing, which uses a set of tunable fiber optic delay lines to form one aggregate serial stream from parallel information streams, the data and the synchronization signal being transmitted at different wavelengths of optical radiation. The disadvantages of this transmission method are:

- limited possibilities of increasing the system capacity and range of non-regenerative transmission, due to the limits of shortening pulses of optoelectronic modules, the exposure of a linear TDM signal to nonlinear distortion, polarization mode dispersion along the path;

- transmission of all information signals at one wavelength;

- non-optimal design and technological approach to the implementation of the proposed system, focused not on promising modular-integrated block layouts, but on bulk fiber, including mechanical nodes for adjusting optical delay lines.

As a prototype of the present invention, the closest analogue is selected - the transmitter unit stated in the system [6], which includes an input unit, which includes N photodetectors that form channel electrical signals for controlling integral optical modulators-keys of solitons, and a clock isolator the frequency of the input digital channels, the input of which is connected to the output of one of the N photodetectors, and the output is connected to the input of the block of synchronization signals of the system, the signals being controlled Integrated-optical modulators-keys are distributed in groups of M channel symbol streams, where M is the number of carrier wavelengths of the generated optical solitons, the block of optical emitters includes N / M blocks of signal grouping generators, each of which includes soliton pulse generators, inputs of which are connected to the system timing signal unit, the soliton pulse generator generates M groups of multicolored solitons (λ ₁ ... λ _M), simultaneously emitted clocked subscriber FIR systems wherein each generator soliton pulses output connected to the input of their integrated optical key modulator, the output of each of which is connected to an input of a corresponding integrated-optic delay line so that in the aggregate generated soliton pulses relative time shift Δτ = T ₀ / N, where T ₀ - clock period of the input user signals, then outputs integrated optical delay lines are connected to M inputs of the integrated-optical wavelength multiplexer channel group, wherein the yield ka of an integrated optical-spectral spectral multiplexer of a group of channels is connected to the input of the corresponding integrated-optical delay line of a block of optical delay lines that create a relative time shift between groups of channels, the outputs of integrated optical delay lines are connected to the N / M inputs of a group integrated optical-optical adder with a matrix transmitting N / M × 1, forming a group N-channel TDM clock signal by combining the input signals, the output of the group integrated optical sum a matrix with a transmission matrix N / M × 1 is connected to the input of an integrated optical shift delay line Δτ / 2, the output of which is connected to a second input of a linear integrated optical-adder with a transmission matrix 2 × 1, the first input of which is connected to the output of a generator of synchronizing soliton pulses emitting a synchronization soliton of a TDM clock signal with a wavelength of λ _s , and the input of the generator of synchronizing soliton pulses is connected to the output of the system synchronization signal block, while the output of the linear integrated optical about the adder with a 2 × 1 transmission matrix is connected to the input of the linear signal input block into the fiber optic line, containing the optical isolator in series and the input of the optical booster amplifier, the output of which is connected to the input of the fiber optic path.

The disadvantages of the prototype include:

- the need to use in the transmitter a large number (equal to the number of multiplexed channels) of soliton pulse generators, which leads to increased energy intensity and cost of the transmitting device;

- the application of a group TDM signal generation scheme in the form of several separate sequentially joined integrated optical elements, which complicates the technological requirements for the elements and the conditions for design implementation, as well as increases the requirements for operating conditions under which the necessary structure stability of the multiplexed signal is ensured;

- the use of only one of the input channels to synchronize all system devices with a TDM clock selector. Strict frame-by-frame synchronization of many subscriber channels should be ensured by common timing for all channels in the input unit. In addition, there is no device that provides optimal levels of electrical signals that control the operation of keys - modulators of optical solitons.

The task to be solved by the claimed invention is directed is the creation of a transmitting device for equipment of a soliton system with mixed multiplexing, which differs from the prototype analogue by eliminating the indicated disadvantages of the latter, in particular:

- a decrease (several times) in the required number of generators of a periodic sequence of ultrashort soliton optical pulses, which entails a decrease in energy intensity and cost;

- the combination of all devices involved in the channel-by-channel formation of information symbol paths and their temporal arrangement on the interval of the clock time interval, into a single integrated optical block for generating a clock (N-channel) linear optical signal, and this block can be produced in a single technological cycle, without intermediate docking;

- the use of a framer (common frame shaper) in the input unit for tight synchronization of the system synchronization signal generator, generation of electrical voltages for controlling the modulator keys of soliton pulses in accordance with the signals received from the output ports of photodetectors of subscriber signals in the input unit.

The technical result consists in reducing the energy intensity, increasing the stability of the path elements involved in the formation of the clock frame, increasing noise immunity.

The technical result provided by the combination of features given in the formula and aimed at eliminating the above disadvantages of the prototype analogue is achieved by the fact that the proposed transmitting device of a fiber-optic soliton transmission system of synchronous digital channels consists of an input unit (WB), a block of optical emitters (BOI) , a block for generating a linear clock signal (BFTS) and a block for inputting a linear signal into a fiber optic line (BWLS),

the input unit consists of N photodetectors, the inputs of which are connected to N ports of subscriber signals, and the outputs are connected to N channel inputs of the frame, the (N + 1) -th input of the frame is connected to an external clock source F _{T of} input synchronous digital channels, The N channel outputs of the framer are connected to the control electrodes of the key modulators included in the linear clock generation unit, the (N + 1) -th output of the framer is connected to the input of the system clock signal generator, and the MU includes M generator powerful soliton pulse generators, the inputs of which are connected via a delay line Δτ / 2 to the system synchronization signal generator, and the soliton pulse generators generate M multi-colored solitons, each at one of the wavelengths λ ₁ ... λ _M corresponding to ITU-T G.694.1, simultaneously emitted from the generator of the system synchronization signals with a clock frequency, the output of each generator of powerful soliton pulses being connected to the input of its integrated optical splitter with a 1 × N / M matrix from the BFTS unit, and the output requirements the power of the soliton pulse generators and the power branching coefficient N / M must ensure that the conditions for maintaining in each channel a power level sufficient to maintain the soliton mode after all dissipative losses in the BFTS path, up to the input of the output booster amplifier of the transmitting device, are met in this unit BFTS output of each splitter is connected to one input from the corresponding group of integrated optical modulator keys, the output of each of which is connected to the integrated optical input ESK line delay a certain value, in accordance with the ratios:

where i = serial number of the emitter of solitons of the group signal,

i = 1, ... M,

j = serial number of the channel in the group from each splitter,

j = 1, ... N / M,

M - the number of generators of synchronous soliton pulses;

N is the number of input multiplexed channels;

k = the number of sequences already included in the composition of the generated frame λ ₁ , ... λ _M , k = 0, ... (N / M) -1,

so that the first channel as part of a clock frame (with a wavelength of λ ₁ ) is transmitted to the input of the integrated-optical adder with zero delay, and all other channels are transmitted to the inputs of their adders with a delay of Δτ relative to the previous one, the outputs of the adders are connected to the input port for the corresponding the wavelength of the integrated optical spectral multiplexer, in which a group N-channel TDM clock signal is generated by combining M channel groups, the input port of the multiplexer for the wavelength is connected to the output generator of synchronizing soliton pulses on a wave of λ _s , the input of which is connected to the output of the generator of the system synchronization signal, while the pulse that starts generating powerful solitons with λ ₁ ... λ _M has a delay Δτ / 2 relative to the pulse that starts the generator of solitons with λ _s , the output is integral the optical multiplexer is connected to the input of the optical gate of the BWLS unit, the output of which is connected to the output booster amplifier, the output of which is connected to the input of the fiber optic path.

It should be noted that the number N in this scheme of the optical transmitting device cannot be taken arbitrarily; it turns out to be equal to the product of the number of optical soliton emitters and the branching coefficient of the splitters, which is determined from the above conservation conditions in all channels of the soliton mode. Therefore, with the known power of the output soliton pulses, it is necessary to select the corresponding number M of emitters in the transmitter, with a minimum redundancy of the number of channels multiplexed in this case.

The invention is illustrated by drawings.

In FIG. 1 is a structural diagram of a transmitting device.

In FIG. 2 is a diagram of a generated linear TDM signal.

In FIG. 3 is a detailed diagram of a transmitting device, specific for an example of generating a 20-channel linear signal.

A transmitting device of a fiber optic soliton transmission system of synchronous digital channels includes:

input unit 1 for connecting and converting signals of N synchronous digital subscriber channels, consisting of N photodetectors 2, the outputs of which are connected to N channel inputs of the frame 3, the (N + 1) -th input of the frame is connected to an external clock source F _T synchronous input digital channels, N channel outputs of the frame 3 are connected to the control electrodes of the key modulators 10 included in the linear signal generating unit 8, the (N + 1) -th frame output is connected to the input of the system 4 clock signal generator ;

the block of optical emitters 5, consisting of generators of powerful soliton pulses 6 and a generator of synchronizing soliton pulses 7, the input of which is connected to the first output of the generator of synchronization signals of system 4, and the inputs of M emitters are connected to the second output of the generator of synchronization signals of system 4 through the delay line Δτ / 2 powerful soliton pulses 6, simultaneously emitting one of M multi-colored solitons with wavelengths λ ₁ ... λ _M , and all wavelengths are different and there are no repetitive ones,

a clock linear signal shaper unit 8, consisting of integrated optical N / M 9 splitters, the input of each of which is connected to the output of the corresponding powerful soliton pulse generator 6, and the outputs of each of the N / M 9 splitters are connected to the inputs of the corresponding integral optical modulator keys 10, the output of each of which is connected to the input of the integrated optical delay line 11, and the delay values are determined by the relations:

where i is the serial number of the emitter of solitons of the group signal,

i = 1, ... M;

j is the serial number of the channel in the group from each splitter,

j = 1 ... N / H;

M - the number of generators of synchronous soliton pulses;

N is the number of input multiplexed channels;

k is the number of sequences already included in the composition of the generated frame λ ₁ , ... X _M , k = 0, ... (N / M) -1;

Δτ = T ₀ / N = channel spacing in a linear TDM signal, and the outputs of the integrated optical delay lines 11 are connected to the inputs of the integrated optical adders 12 of their groups of output signals of the delay lines, and the outputs of the adders 12 are connected to the input ports for the corresponding lengths waves of the integrated optical spectral multiplexer (λ ₁ , ... λm, λ _s ) × 1 13, in which a group N-channel clock TDM signal is formed by combining M channel groups, so that channels from all the multi-colored emitters of solitons are inserted into the group signal in turn, with a delay by Δτ relative to the previous one, and the input port of the multiplexer 13 for wavelength λ _{s is} connected to the output of the generator 7 synchronizing soliton pulses on the wave λ _s , the output of the integrated optical spectral multiplexer 13 is connected to the input of the linear signal input unit 14 in fiber optic line consisting of an optical gate 15 and an output booster amplifier 16.

A transmission device of a fiber optic soliton transmission system of synchronous digital channels, the structural diagram of which is shown in FIG. 1, works as follows.

The equipment of the input unit 1 receives N subscriber synchronous digital channels simultaneously supplied to the N input ports of the photodetector devices 2 included in the WB. The electrical output ports of the photodetectors are connected to the N channel inputs of the frame 3, the purpose of which is to form at each clock frame interval, in the form of electrical signals with edges and levels corresponding to the received symbols and optimal for controlling modulator keys 10, as well as a reset command for the generated combination at the end of the beat. In addition, the frame 3 generates a trigger signal generator of the synchronization signal system 4, and the operation of the frame is controlled by an external clock frequency signal F _T. At the time of receiving the start pulse from the generator 4, the powerful soliton generators 6 and the synchronizing soliton pulse generator 7 generate single ultra-narrow optical pulses with the corresponding wavelengths. Generator 7 with a wavelength of λ _s generates its soliton earlier than the others by Δτ / 2, this pulse opens the beginning of the clock frame during operation of the entire information transmission system. It should be noted that here and in the future it is assumed that the conditions for the equidistance of the optical paths connecting the individual elements of all system blocks, i.e. connecting lines (in the diagrams) by themselves do not introduce any time shifts.

Emitters 6 of powerful solitons with λ ₁ , ... λ _M can be made, for example, in the form of compact soliton generators based on microresonators [2], while the energy of the output femtosecond pulses must be sufficient to ensure the soliton regime of active symbols at the input of the booster amplifier 16 unit BVLS 14, i.e. taking into account losses in the elements of the integrated optical path of block 8 BFTS. The emitters 6 synchronously provide pulses to the input ports of the splitters 9, which divide the pulse into N / M parts of equal power, each of which is fed to the input of one of the integrated optical modulator keys 10, made according to the well-known Mach-Zander scheme and operating in either transmitting a pulse from splitter 9 (active sending, "1") or locking (pause, "0"). The output channels of the modulator keys 10 are connected to the inputs of the integrated optical delay lines 11, the delay time of the symbols in which are determined in accordance with formula (1), and these lines form the relative time shifts of channel symbols (pulses or pauses) by Δτ, which provide their arrangement with cyclic alternation of wavelengths of solitons in the interval of the clock period T ₀ (see the structure of the clock frame in Fig. 2). From the output channels of the delay lines 11, channel symbols are fed to the inputs of the N / M waveguide adders 12, made according to the passive association of channels, and from the output of each adder, the signal is fed to the input port (for its wave) of the integrated optical multiplexer 13 made according to the AWG scheme ( Arrayed Waveguide Grating), the output of which is a linear signal combining the synchronization soliton λ _c with a group signal of N channel symbols forming a TDM clock signal, which is fed to the input of the WLAN unit 14 and, after the necessary amplification soliton symbols, is transmitted to the linear path of the system.

Let us show the operation of the proposed transmission device using the specific example shown in FIG. 3. Let it be necessary to design a transmitting device for a soliton transmission system in the C-band of 20 synchronous subscriber channels with a speed of ₀ = 11.2 Gbit / s (STM-64 + FEC) each. For a speed of 11.2 Gbit / s, the duration of the clock interval is T ₀ = 1 / V ₀ = 89.28 ps. At this interval, 20 channels of the TDM frame and a synchronization signal, which starts the start of the next clock cycle, should be placed as part of the linear clock signal. In accordance with the principle of operation of the proposed device, the number of transmitted subscriber channels should be determined by the product of the number of "multi-colored" soliton pulse generators M by the division ratio of the splitters 12 for each wavelength, λ ₁ , ... λ _M. In our case, at N = 20, it is advisable to choose either M = 5, and the division ratio 1: 4, or M = 4, and the division ratio 1: 5 It is technologically convenient to carry out integrated-optical splitters with coefficients 1: 2 ⁿ , or In our case, 1: 2 ² = 1: 4. Thus, the number of soliton pulse generators for the formation of a group signal is taken equal to M = 5.

We set the central wavelengths λ ₁ , ... λ ₅ corresponding to the ITU-T G.694.1 standard grid, moreover, in the range of the working range of well-developed and widely used erbium-based optical amplifiers (EDFA), i.e. approximately λ≈1530 ... 1560 nm.

In our example, we select the central wavelengths for soliton pulse generators:

λ ₁ = 1545, 32 nm (194.0 THz);

λ ₂ = 1546.92 nm (193.8 THz);

λ ₃ = 1548.52 nm (193.6 THz);

λ ₄ = 1550.12 nm (193.4 THz);

λ ₅ = 1551.72 nm (193.2 THz);

λ _C = 1553.33 nm (193.0 THz).

The central wavelengths are spaced 200 GHz, which, together with the alternation of the working wavelengths of the symbols in the linear signal, practically eliminates the possibility of soliton-soliton interaction of the pulses of the linear signal;

In addition, in the range of application of EDFA, it is possible to work with the same system in the opposite direction, with a shift of all waves of the oncoming pulse stream by 100 GHz relative to the pulses of this system.

6 generators solitons are launched pulses are generated by the system timing signal generator 4 and emit powerful synchronous continuous sequence of ultrashort pulses with the frequency F _T = 1 / T _0, and the pulse duration τ _hydrochloric must satisfy the condition: τ _hydrochloric << Δτ = T _0/20 = 4.46 ps, Δτ is the intersymbol interval on the clock interval. For example, τ _sol may be ≈ 400 femtoseconds or shorter.

From the outputs of the splitters 9, channel symbols are fed to the inputs of the channel modulator keys 10, to the control electrodes of which binary signals of each of the 20 channels are applied during the clock interval. These signals (feed, reset and value) are generated by the frame 3 in accordance with the electrical signals from the output ports of the photodetectors 2 of the input unit 1 WB. The modulator keys 10 are part of the integrated circuit of the BFTS block 8, they are performed according to the Mach-Zander scheme, and the control electrodes of each key are electrically connected to one of the channel output ports of the frame 3. The modulator keys are open for the passage of the soliton pulse during the clock interval the control signal on the electrodes is “1” and closed when the signal is “0”.

The output channels of the keys 10 are connected to an array of waveguide delay lines 11, which, in fact, create the arrangement of channel symbols (pulses or pauses) in time in the proposed transmitting device, which ensures the formation of a group clock signal. The values of these delays, determined by the rule f-crystals (1), will for this particular example have the numerical values indicated in the following table:

The frame structure of a 20-channel linear signal generated in accordance with the rules of the invention is shown in FIG. 2.

Thus, the present invention fully realizes the task, technologically feasible and industrially applicable.

LINKS

1. P.G. Kryukov “Lasers of ultrashort pulses” // Quantum Electronics, 31, No. 2, 2001, p. 95-119.

2. T. Herr, V. Brasch, J.D. Jost, C.Y. Wang, N.M. Kondratiev, M.L. Gorodet-sky & T.J. Kippenberg.

Nature Photonics 8, 145-152 (2014) doi: 10.1038 / nphoton.2013.343.

3 .. Straight Line 10 Gb / s Soliton Transmission over 1000 km of Standard Fiber with In-Line Chirped Fiber Grating for Partial Dispersion Compensation Straight Line 10 Gb / s Soliton Transmission over 1000 km of Standard Fiber with In-Line Chirped Fiber Grating for Partial Dispersion Compensation (Optoelectronics Research Center University of Southampton.

4. Harbor, Stephen "Soliton Research May Improve Optical Fiber Transmission" // EvroPhotonics, Oct / Nov 1998, p. 38-39.

5. Patent No. 2384955, cl. IPC H04J 14/08, H04B 10/12 B.H. Tsukanov, M.Ya. Yakovlev "Optical information transmission system."

6. Patent No. 2574338, cl. IPC H04J 14/08, Н04В 10/12 I.A. Lukin, V.N. Udovichenko "Fiber-optic soliton transmission system of synchronous digital channels."

Claims (11)

Transmitter of a fiber-optic soliton system for transmitting synchronous digital channels, including an input unit for connecting and converting signals of N synchronous subscriber digital channels, N photodetectors, a block of optical emitters, integrated optical key modulators, soliton pulse generators, optical delay lines, block input a linear signal into a fiber optic line, consisting of an optical valve and an output booster amplifier, characterized in that the outputs of N photodetectors included in the input unit are connected to N channel inputs of the frame, the (N + 1) -th input of the frame is connected to an external clock source F _{T of} input synchronous digital channels, N channel outputs of the frame are connected to the control electrodes of the key modulators included in the block for generating a linear clock signal, the (N + 1) th output of the framer is connected to the input of the system clock signal generator, the block of optical emitters includes powerful soliton generators pulses and a generator of synchronizing soliton pulses, the input of which is connected to the first output of the system clock signal generator, and the inputs of M emitters of powerful soliton pulses simultaneously emitting one of the M multi-colored solitons with wavelengths λ ₁ ... λ _M , and all wavelengths are different and there are no repetitive ones, the output of each radiator is connected to the input of its integrated optical soliton energy splitter on N / M channels of equal power, and the outputs of each splitter are connected to the inputs of the corresponding group of integrated optical key modulators, the output of each of which is connected to the input of the integrated optical delay line, and the delay values are determined by the relations:

where i is the serial number of the emitter of solitons of the group signal, i = 1, ... M; j is the serial number of the channel in the group from each splitter, j = 1, ... M; M - the number of generators of synchronous soliton pulses; N is the number of input multiplexed channels; k is the number of sequences already included in the structure of the generated frame λ1, ... λm, k = 0, ... (N / M) -1, the outputs of the lines of the integrated optical delay lines are connected to the inputs of the integrated optical adders of their groups of output signals of the delay lines, and the outputs of the adders are connected to the input ports for the corresponding wavelengths of the integrated optical spectral multiplexer, in which a group N-channel TDM clock signal is generated by combining M groups of channels, so that channels from all multi-colored emitters of solitons are inserted into the group signal in turn, the input port of the multiplexer for wavelength λ _s connected to the output of the generator of synchronizing soliton pulses on the wave λ _s , the output of the integrated optical spectral multiplexer is connected to the input of the input block of the linear signal into the fiber optic line.

Images