CN109921872B

CN109921872B - Optical fiber time transmission system and transmission method

Info

Publication number: CN109921872B
Application number: CN201811440812.2A
Authority: CN
Inventors: 吴龟灵; 左发兴; 陈建平
Original assignee: Shanghai Jiaotong University
Current assignee: Shanghai Jiaotong University
Priority date: 2018-11-29
Filing date: 2018-11-29
Publication date: 2020-11-10
Anticipated expiration: 2038-11-29
Also published as: CN109921872A

Abstract

A high-precision large-range optical fiber time transmission system and method is composed of a master clock, a master time transmission unit, several optical splitters/combiners, several bidirectional photoelectric optical relay units, several intermediate time transmission units, several bidirectional optical amplification units and several slave time transmission units. The invention realizes high-precision large-range optical fiber time transmission by passive optical shunt, time division multiple access, bidirectional photoelectric optical relay, bidirectional optical amplification and a bidirectional time transmission mode.

Description

Optical fiber time transmission system and transmission method

Technical Field

The invention relates to time frequency transmission, in particular to a high-precision large-range optical fiber time system and a transmission method.

Background

The high-precision time synchronization technology has important application value in the fields of satellite navigation, aerospace, deep space exploration, geological mapping, scientific research and measurement and the like. At present, the traditional high-precision time synchronization technology mainly comprises a GPS common view and a satellite bidirectional ratio which are equal, and can reach the time transfer precision of ns magnitude.

Optical fiber transmission has the advantages of low loss, large capacity, high speed, high stability, safety and reliability, and has been widely used in the field of communication. Optical fiber-based time transfer is an effective way to achieve high-precision long-distance and wide-range time transfer. The high-precision optical fiber time transmission faces the problem that the transmission delay of an optical fiber link changes along with the changes of factors such as temperature, stress, transmission wavelength and the like. In order to realize high-precision time transfer, the same-fiber bidirectional transmission scheme is generally adopted at present. In long-haul bidirectional fiber time transfer, bidirectional optical amplification is necessary to compensate for optical signal attenuation. We have previously proposed a two-way light amplification scheme [ see wu guiling; zhang Hao; chenjianpei, "high precision optical fiber time transfer bidirectional optical amplification method and apparatus," application number: CN201610073321.3,2016.6], the symmetry of the link can be ensured to the greatest extent, and the influence of multiple amplification of noise such as Rayleigh scattering and the like on the time transfer performance of the optical fiber can be effectively avoided.

For distributed fiber-optic time transfer, the university of polish AGH physic [ see p.krehrik, l.sliwczynski, l.buczek, and m.lipinski, "Multipoint cancellation of RF frequency in fibrous link with stabilized amplification delay," IEEE transactions on ultrasound, ferroelectrics, and frequency control, vol.60, pp.1804-1810,2013 ] proposes to insert a 2 × 2 optical coupler in the main link, coupling out part of the forward and backward transferred optical signals for distributed time transfer. But this reduces the power of the optical signal delivered by the main link and also degrades the stability of the time delivery of the main link. We have previously proposed a distributed time transfer scheme [ see wu-guiling; zhang Hao; chenjianpai, "high-precision long-distance distributed optical fiber time transmission method and system," application number: CN201610781482.8,2016.8], it can realize high precision long distance distributed optical fiber time transmission, but this kind of line distributed system can only realize the time synchronization of users on single optical fiber link, and can not realize large range optical fiber time transmission.

Disclosure of Invention

The invention aims to provide a high-precision large-range optical fiber time transmission system and a transmission method for an optical fiber time transmission scheme based on same-fiber same-wave bidirectional time division multiplexing aiming at the defects of the prior art.

The technical solution of the invention is as follows:

a high precision large range optical fiber time transfer system comprising: the system comprises a main clock, a main time transfer unit, an optical distribution network and a plurality of slave time transfer units; the optical distribution network is composed of a plurality of optical splitters/combiners, a plurality of bidirectional photoelectric optical relay units, a plurality of intermediate time transmission units, a plurality of bidirectional optical amplification units and a plurality of optical fiber links. The bidirectional photoelectric optical relay unit, the intermediate time transfer unit and the bidirectional optical amplification unit are connected through an optical fiber link. Each bidirectional photoelectric optical relay unit performs photoelectric optical relay on forward and backward optical signals, each intermediate time transfer unit performs photoelectric optical relay on the forward and backward optical signals and decodes the forward and backward optical signals to obtain timing signals in an optical fiber link, and each bidirectional optical amplification unit performs optical amplification on the forward and backward optical signals; the main time transfer unit is connected with the main clock and is also connected with the combining end of the optical distribution network; each slave time transfer unit is connected with a branch end of the optical distribution network;

the timing signal of the main clock is sent through the main time transmission unit and is broadcasted to each intermediate time transmission unit and each slave time transmission unit through the optical distribution network;

each slave time transmission unit delays the timing signal from the master time transmission unit, then sends the timing signal to the master time transmission unit in the reverse direction of the original optical fiber link through the optical distribution network, and measures the time interval between the timing signal from the master time transmission unit and the timing signal after the delay;

the master time transfer unit respectively measures the time interval between the timing signal from each slave time transfer unit and the timing signal of the master clock, and broadcasts the time interval to each intermediate time transfer unit and each slave time transfer unit through an optical distribution network;

each intermediate time transmission unit measures the time interval between the timing signal from the master time transmission unit and the timing signal from the slave time transmission unit;

and the slave time transmission units and the intermediate time transmission units obtain clock difference between each unit and the master clock according to the time interval received from the master time transmission unit and the locally measured time interval, so that high-precision and large-range optical fiber time transmission is realized.

The bidirectional photoelectric optical relay unit comprises a 2 multiplied by 2 optical switch, a photoelectric conversion module, a signal processing and decoding control module and an electro-optical conversion module.

The port 1 and the port 2 of the 2 x 2 optical switch are respectively connected with the forward input and the backward input of the optical fiber link, the port 3 is connected with the photoelectric conversion module, and the port 4 is connected with the electro-optical conversion module; the photoelectric conversion module inputs the electric signal to the signal processing and decoding control module; under the control of the control signal of the signal processing and decoding control module, the 2 × 2 optical switch switches the forward optical signal of the optical fiber link input port 1 to the port 3, outputs the forward optical signal to the photoelectric conversion module, and outputs the forward optical signal to the port 2 through the port 4 after the photoelectric conversion of the photoelectric conversion module; under the control of the control signal of the signal processing and decoding control module, the 2 × 2 optical switch switches the backward optical signal of the optical fiber link input port 2 to the port 3, outputs the backward optical signal to the photoelectric conversion module, and outputs the backward optical signal to the port 1 through the port 4 after the photoelectric conversion of the photoelectric conversion module.

The photoelectric conversion module converts the optical signal from the 2 x 2 optical switch port 3 into an electric signal and then inputs the electric signal into the photoelectric conversion module and the signal processing and decoding control module.

The signal processing and decoding control module decodes the time code in the electric signal, extracts the timing signal and outputs a state control signal to the 2 x 2 optical switch according to the timing signal.

The electro-optical conversion module converts the electrical signal from the photoelectric conversion module into an optical signal and outputs the optical signal to the port 4 of the 2 × 2 optical switch.

The initial state of the 2 x 2 optical switch is a forward conducting state.

The forward conduction state of the 2 × 2 optical switch means that the port 1 is connected to the port 3, the port 2 is connected to the port 4, and the backward conduction state means that the port 1 is connected to the port 4, and the port 2 is connected to the port 3.

The intermediate time transfer unit comprises a 2 multiplied by 2 optical switch, a photoelectric conversion module, a signal processing and decoding control module, a time interval measuring module and an electro-optical conversion module.

The port 1 and the port 2 of the 2 x 2 optical switch are respectively connected with the forward input and the backward input of the optical fiber link, the port 3 is connected with the photoelectric conversion module, and the port 4 is connected with the electro-optical conversion module; the photoelectric conversion module inputs the electric signal to the signal processing and decoding control module; under the control of a control signal of the signal processing and decoding control module, the 2 x 2 optical switch switches a forward optical signal of the input port 1 of the optical fiber link to the port 3, outputs the forward optical signal to the photoelectric conversion module, and outputs the forward optical signal to the port 2 through the port 4 after the photoelectric conversion of the photoelectric conversion module; under the control of the control signal of the signal processing and decoding control module, the 2 × 2 optical switch switches the backward optical signal of the optical fiber link input port 2 to the port 3, outputs the backward optical signal to the photoelectric conversion module, and outputs the backward optical signal to the port 1 through the port 4 after the photoelectric conversion of the photoelectric conversion module.

The signal processing and decoding control module decodes the time code in the electric signal, extracts the timing signal, outputs a state control signal to the 2 x 2 optical switch according to the timing signal, outputs the timing signal to the time interval measuring module and outputs time information.

The time interval measuring module receives the forward and backward timing signals input by the signal processing and decoding control module and measures the time intervals of the forward and backward timing signals.

The initial state of the 2 x 2 optical switch is a forward conducting state.

The control method of the 2 x 2 optical switch in the bidirectional photoelectric optical relay unit and the intermediate time transfer unit is as follows:

after starting, the initial state of the 2 multiplied by 2 photoswitch is a forward conducting state;

the forward transmitted optical signal is divided into two paths after passing through the photoelectric conversion module. One path is subjected to electro-optic conversion by an electro-optic conversion module and then output to a 2 x 2 optical switch, and the output of the 2 x 2 optical switch is transmitted to a next unit through an optical fiber; the other path of the forward timing signal is identified by the signal processing and decoding control module, and the time t when the forward timing signal is identified is recorded_fAnd based on the time t at which the forward timing signal is identified_fDetermining the time t when the 2 x 2 optical switch is set to the backward conducting state next time_b2＝t_f+t_cAnd the forward conduction state at time t_f2＝t_f+T-t_s2Wherein T isPeriod of the transmitted timing signal, t_cLength of time code for transfer, t_s2Is the switching time of a 2 x 2 optical switch; updating t after each new time code is received_fAnd storing the value of (t)_f2、t_b2A value of (d);

within one period T of the transferred timing signal, at T_f2At the moment, the 2 multiplied by 2 optical switch is set to be in a forward conducting state; at t_b2At the moment, the 2 x 2 optical switch is set to be in a backward conducting state;

the time transmission method of the optical fiber time transmission system has two working modes:

mode one, a small capacity static time transfer mode,

mode two, large capacity dynamic time transfer mode.

The method works in a mode one, and comprises the following steps:

(1) before the system is started, each slave time transmission unit is numbered and assigned with address information, and the delay time T of the timing signal of each slave time transmission unit is calculated_di＝(2T_M+ Δ T) xi, i being the number of slave time transfer units, T_MThe transmission delay (obtained by measurement) is the maximum main time transfer unit and the maximum slave time transfer unit in the optical distribution network, and the delta T is a redundancy value which is set for ensuring the stable and reliable work of the system;

(2) when the main time transfer unit detects a timing signal input by a main clock, starting optical signal transmission, broadcasting an optical signal containing the timing signal of the main clock and a time code of control and state information to an optical distribution network through an optical fiber link, and closing the optical signal transmission after the transmission is finished;

(3) in each optical fiber link, the bidirectional photoelectric optical relay unit performs photoelectric optical relay on the arriving forward optical signal; the bidirectional optical amplification unit optically amplifies the arriving forward optical signal;

(4) the intermediate time transfer unit extracts a forward timing signal from the received time code and performs photoelectric optical relay on the arriving forward optical signal;

(5) the ith slave time transfer unit transfers the time code from the master time transfer unitDecoding to extract timing signal, delaying the decoded timing signal by time T_diThen, the delayed timing signal and the ith slave time transfer unit address information are coded into a time code, modulated to the same optical wavelength as the forward transmission, sent to the same optical fiber link in the reverse direction, and the optical signal sending is closed after the sending is finished;

(6) in each link, the bidirectional photoelectric optical relay unit performs photoelectric optical relay on the arriving backward optical signal; the bidirectional optical amplification unit optically amplifies the arriving backward optical signal;

(7) the intermediate time transfer unit extracts the backward timing signal and the address information of the slave time transfer unit from the received time code, and measures the time interval T between the received forward timing signal and the backward timing signal from each slave time transfer unit_MTransmitting the address information of each slave time transfer unit and the measured corresponding time interval T_MStoring, and simultaneously carrying out photoelectric optical relay on the arriving backward optical signals;

(8) the master time transmission unit receives the time codes from the slave time transmission units and decodes the time codes to obtain the backward timing signals and the address information of the slave time transmission units; measuring the time interval T between the timing signal of the master clock and each decoded timing signal_AB(ii) a Transmitting each slave time transfer unit address information and corresponding measured time interval T_ABStoring;

(9) when the main time transfer unit detects a timing signal input by a main clock, the optical time transfer unit broadcasts and sends a time code carrying the timing signal of the main clock, the address information of each saved slave time transfer unit and the corresponding measured time interval and control and state information to an optical distribution network through an optical fiber link, and the optical signal transmission is closed after the transmission is finished;

(10) in each link, the bidirectional photoelectric optical relay unit performs photoelectric optical relay on an arriving forward optical signal; the bidirectional optical amplification unit optically amplifies the arriving forward optical signal;

(11) the intermediate time transfer unit extracts the forward timing signal, the address information of each slave time transfer unit andeach time interval T measured by the master time transfer unit_ABStoring the address information and the corresponding time interval, and simultaneously carrying out photoelectric optical relay on the arriving forward optical signal; the intermediate time transfer unit transfers the time interval T corresponding to the stored address information_ABAnd T_MCalculating the clock difference between the received forward timing signal and the main clock:

wherein the content of the first and second substances,

and

the transmission delay and the reception delay of the master time transfer unit,

and

obtaining the sending time delay and the receiving time delay of the intermediate time transmission unit through equipment calibration;

(12) the ith slave time transfer unit decodes the time code from the master time transfer unit, extracts the timing signal and the time interval T measured by the master time transfer unit corresponding to the ith slave time transfer unit_ABDelaying the decoded timing signal by a time T_diThen, the delayed timing signal and the ith slave time transfer unit address information are coded into a time code, modulated to the same optical wavelength as the forward transmission, sent to the same optical fiber link in the reverse direction, and the optical signal sending is closed after the sending is finished; meanwhile, the ith slave time transfer unit calculates the clock difference between the received timing signal and the master clock timing signal:

wherein

And

the transmission and reception delays of the master time transfer unit,

and

the time delay of sending and receiving from the time transfer unit is obtained by calibrating the equipment.

(13) Returning to the step (6), after the main time transmission unit stops working is detected, the step (14) is carried out

(14) End up

The method works in a mode one, and the system can perform addition and deletion operation on the slave time transfer unit according to the time transfer application requirement.

When a slave time transfer unit (denoted as a K +1 th slave time transfer unit) is added to the system, the K +1 th slave time transfer unit extracts a timing signal from a received time code and delays the received timing signal by a time T_d(K+1)＝(2T_MAnd after + delta T) x (K +1), starting optical signal transmission, encoding the delayed timing signal and the K +1 th slave time transfer unit address information into a time code, modulating the time code to the same optical wavelength as forward transmission, transmitting the time code to the same optical fiber link in a reverse direction, and closing optical signal transmission after the transmission is finished. Then repeating the steps: (6) - (12)

When the slave time transfer unit is deleted in the system, in a plurality of continuous measuring periods, the master time transfer unit does not obtain the backward timing signal and the address information for deleting the slave time transfer unit from the received time code, then the master time transfer unit judges that the slave time transfer unit is deleted from the system, and deletes the information of the slave time transfer unit which is locally stored.

When the method works in the mode two, the method comprises the following steps:

(1) before system start, master timeInter-delivery unit setting recognition window t from time delivery unit₀Comparing the window initial value t and the window width delta t by each slave time transfer unit to identify the window t₀Comparing the window initial value t and the comparison window width delta t, and storing;

(2) when the main time transfer unit detects a timing signal input by the main clock, the optical signal transmission is started, a time code containing the timing signal of the main clock and control and state information is broadcasted to the optical distribution network through the optical fiber link, and the optical signal transmission is closed after the transmission is finished.

(3) In each link, the bidirectional photoelectric optical relay unit performs photoelectric optical relay on an arriving forward optical signal; the bidirectional optical amplification unit optically amplifies the arriving forward optical signal;

(4) the intermediate time transfer unit extracts a forward timing signal from the received time code and performs photoelectric optical relay on the arriving forward optical signal.

(5) Each slave time transfer unit decodes the time code from the master time transfer unit to extract a timing signal, and randomly delays the decoded timing signal by a time T_d＝τ_cAfter xn, the delayed timing signal and the address information of the unit are coded into a time code, modulated to the same optical wavelength as the forward transmission, sent to the same optical fiber link in the reverse direction, and the optical signal sending is closed after the sending is finished; wherein, tau_eN is not greater than (t) for the length of the time code to be transmitted₀-2T_abm)/τ_cRandom integer of (1), T_abmThe maximum unidirectional link transmission delay (measured) in the optical distribution network.

(7) the intermediate time transfer unit extracts the backward timing signal and the address information of the slave time transfer unit from the received time code, and measures the time interval T between the received forward timing signal and the backward timing signal from each slave time transfer unit_MTransmitting each slave time to the unitAddress information and measured corresponding time interval T_MAnd storing and carrying out photoelectric optical relay on the backward optical signals.

(8) The master time transfer unit decodes the successfully received slave time transfer unit time code to obtain a backward timing signal and address information from the slave time transfer unit; measuring the time interval T between the timing signal of the master clock and each decoded timing signal_AB(ii) a Transmitting each slave time transfer unit address information and corresponding measured time interval T_ABStoring; the master time transfer unit calculates the time delay adjustment quantity delta tau of the timing signal corresponding to the slave time transfer unit which stores the address information according to the comparison window T of each distributed slave time transfer unit_ABAnd storing the time delay adjustment amount.

(9) When the main time transfer unit detects a timing signal input by a main clock, a time code carrying the timing signal of the main clock, locally stored address information of a slave time transfer unit, a corresponding measured time interval and time delay adjustment quantity and control and state information is broadcast and transmitted to an optical distribution network through an optical fiber link, and optical signal transmission is closed after the transmission is finished;

(11) the intermediate time transfer unit extracts the forward timing signal, the carried address information of each slave time transfer unit and each time interval T measured by the master time transfer unit from the received time code_ABStoring the address information and the corresponding time interval, and simultaneously carrying out photoelectric optical relay on the arriving forward optical signal; if the address information of a certain slave time transfer unit is stored in both forward and backward transmission, the intermediate time transfer unit transfers the time interval T corresponding to the address information_ABAnd T_MCalculating the clock difference between the received forward timing signal and the main clock:

wherein

And

the transmission and reception delays of the master time transfer unit,

and

the time delay for transmitting and receiving the intermediate time transmission unit is obtained by calibrating equipment.

(12) Each slave time transfer unit decodes the time code from the master time transfer unit. If the received time code does not contain the address information of the slave time transmission unit, the decoded timing signal is randomly delayed for a time T_d＝τ_cXn, coding the delayed timing signal and the address information of the unit into a time code, modulating the time code to the same optical wavelength as the forward transmission, reversely sending the time code to the same optical fiber link, and closing the optical signal transmission after the completion of the sending; if the received time code contains the address information of the slave time transfer unit, extracting the time delay adjustment quantity delta tau of the timing information corresponding to the slave time transfer unit and the time interval T measured by the master time transfer unit_ABControl and status information, delaying the received timing signal by a time T_dAnd the + delta tau codes the delayed timing signal and the address information of the unit into a time code, modulates the time code to the same optical wavelength as the forward transmission, reversely sends the time code to the same optical fiber link, and closes the optical signal sending after the sending is finished. The slave time transfer unit which simultaneously contains the address information calculates the clock difference between the received timing signal and the timing signal of the master clock:

wherein

And

the transmission and reception delays of the master time transfer unit,

and

(13) In each link, the photoelectric optical relay unit performs photoelectric optical relay on the arriving backward optical signal; the bidirectional optical amplification unit optically amplifies the arriving backward optical signal;

(14) the intermediate time transfer unit extracts the backward timing signal and the address information of the slave time transfer unit from the received time code, and measures the time interval T between the received forward timing signal and the backward timing signal from each slave time transfer unit_MTransmitting each slave time transfer unit address information and the corresponding measured time interval T_MAnd storing and carrying out photoelectric optical relay on the backward optical signals.

(15) The master time transfer unit decodes the successfully received slave time transfer unit time code to obtain a backward timing signal and address information from the slave time transfer unit; measuring the time interval T between the timing signal of the master clock and each decoded timing signal_AB(ii) a If the address information of the slave time transfer unit is stored locally, updating the time interval T corresponding to the slave time transfer unit_ABWhile simultaneously comparing T_ABThe size of the time slot T corresponding to the slave time transfer unit is equal to or less than T if T-T is satisfied_ABIf the time delay adjustment quantity delta t is less than or equal to t + delta t + t, the value of the time delay adjustment quantity delta t of the slave time transfer unit is kept unchanged; if T-T is not satisfied, T is less than or equal to_ABT + delta T + T, then the value delta tau of the time delay adjustment of the slave time transfer unit is recalculated to T-T_ABAnd updating the value of the delay adjustment, wherein t is the time-dependent transmission allowed by the systemThe delivery unit timing signal is offset from the maximum of the alignment window. If the slave time transfer unit address information is not stored locally, the new slave time transfer unit is considered to be identified, and the slave time transfer unit address information and the corresponding time interval T are transmitted_ABStoring, and calculating the time delay adjustment amount delta tau of the address information corresponding to the timing signal of the slave time transfer unit according to the set comparison window T_ABAnd storing the time delay adjustment amount. If the address information of the slave time transfer unit which has been stored is not included in the received address information, the slave time transfer unit corresponding to the address information is considered to be deleted from the system, and the address information corresponding to the slave time transfer unit and the time interval T are set_ABAnd deleting the timing signal time delay adjustment quantity delta tau.

(16) Returning to the step (9), after the main time transmission unit stops working is detected, the step (17) is carried out

(17) End up

Compared with the prior art, the invention has the beneficial effects that:

the invention is based on mature technology, and ensures that bidirectional time delay is symmetrical to the maximum extent by leading bidirectional time signals to pass through the same optical fiber link; the time transmission distance and range are effectively expanded through a special bidirectional optical amplifier, and the bidirectional optical amplification symmetry is ensured to the maximum extent; the influence of multiple times of optical amplification of Rayleigh scattering and other noises on the time transfer stability of the optical fiber is effectively avoided through the optical-electrical-optical process; distributed time transfer is achieved by passive optical splitting and time division multiplexing from time transfer units.

Drawings

FIG. 1 is a schematic structural diagram of an embodiment of a high-precision large-range optical fiber time transfer system according to the present invention;

FIG. 2 is a schematic structural diagram of a bi-directional optical-electrical relay unit according to an embodiment of the present invention;

FIG. 3 is a schematic diagram of an intermediate time transfer unit in accordance with an embodiment of the present invention;

FIG. 4 is a timing diagram illustrating operation of one embodiment of the present invention;

FIG. 5 is a schematic diagram of an intermediate time transfer unit extracting a backward timing signal from a received time code and address information from the time transfer unit in an embodiment of the present invention;

FIG. 6 is a timing diagram illustrating operation of mode two according to the present invention.

Detailed Description

An embodiment of the present invention is given below with reference to the accompanying drawings. The present embodiment gives a detailed implementation and a specific workflow of the present invention, but the scope of the present invention is not limited to the following embodiments.

In this embodiment, the high-precision large-area optical fiber time transfer system (as shown in fig. 1) includes: the system comprises a main clock 1-1, a main time transfer unit 1-2, a plurality of optical splitting/combining devices 1-5, a plurality of bidirectional photoelectric optical relay units 1-6, a plurality of intermediate time transfer units 1-7, a plurality of bidirectional optical amplification units 1-8 and a plurality of slave time transfer units 1-4; the light distribution network 1-3 is composed of a plurality of light splitting/combining devices 1-5, a plurality of bidirectional photoelectric light relay units 1-6, a plurality of intermediate time transmission units 1-7 and a plurality of bidirectional light amplification units 1-8. Each bidirectional photoelectric optical relay unit 1-6 performs photoelectric optical relay on forward and backward optical signals, and each bidirectional optical amplification unit 1-8 performs optical amplification on the forward and backward optical signals; the main time transmission unit 1-2 is connected with the main clock 1-1 and is connected with the combining end of the optical distribution network 1-3; each slave time transfer unit 1-4 is connected with a branch end of the optical distribution network 1-3; the bidirectional optical amplification units 1-8 employ bidirectional amplifiers comprising 2 × 2 optical switches and unidirectional amplifiers [ see wu guiling; zhang Hao; chenjianpei, "high precision optical fiber time transfer bidirectional optical amplification method and apparatus," application number: CN201610073321.3,2016.6 ]. The master time transfer unit 1-2 and the slave time transfer unit 1-4 respectively adopt a first optical fiber time transfer unit and a second optical fiber time transfer unit in our applied patent [ see wu-guiling; zhang Hao; chenjianpai, "high-precision long-distance distributed optical fiber time transmission method and system," application number: CN201610781482.8,2016.8 ]. The transmission direction from the master time transfer unit 1-2 to the slave time transfer unit 1-4 is forward; the direction of transmission from the slave time transfer unit 1-4 to the master time transfer unit 1-2 is backwards. The timing signal delivered is 1PPS and the time code length is approximately 6 us. In the embodiment, 2 × 2 mechanical optical switches with switching time of 1ms are adopted as the 2 × 2 optical switches in the bidirectional photoelectric optical relay units 1-6 and the intermediate time transfer units 1-7.

The bidirectional photoelectric optical relay unit (as shown in fig. 2) comprises a 2 × 2 optical switch 2-1, a photoelectric conversion module 2-2, a signal processing and decoding control module 2-3, and an electro-optical conversion module 2-4. The port 1 and the port 2 of the 2 x 2 optical switch 2-1 are respectively connected with the forward input and the backward input of the optical fiber link, the port 3 is connected with the photoelectric conversion module 2-2, and the port 4 is connected with the photoelectric conversion module 2-4; the photoelectric conversion module 2-2 inputs the electric signal to the signal processing and decoding control module 2-3; under the control of the control signal of the signal processing and decoding control module, the 2-3, 2 × 2 optical switch 2-1 switches the forward optical signal of the input port 1 of the optical fiber link to the port 3, outputs the forward optical signal to the photoelectric conversion module 2-2, and outputs the forward optical signal to the port 2 through the port 4 after the photoelectric conversion of the photoelectric conversion module 2-4; under the control of the control signal of the signal processing and decoding control module 2-3, the 2 × 2 optical switch 2-1 switches the backward optical signal of the optical fiber link input port 2 to the port 3, outputs the backward optical signal to the photoelectric conversion module 2-2, and outputs the backward optical signal to the port 1 through the port 4 after the photoelectric conversion of the photoelectric conversion module 2-4. The photoelectric conversion module 2-2 converts the optical signal from the port 3 of the 2 × 2 optical switch 2-1 into an electrical signal and inputs the electrical signal to the photoelectric conversion module 2-2 and the signal processing and decoding control module 2-3. The signal processing and decoding control module 2-3 decodes the time code in the electrical signal, extracts the timing signal, and outputs a state control signal to the 2 × 2 optical switch 2-1 according to the timing signal. The electro-optical conversion module 2-4 converts the electrical signal from the electro-optical conversion module 2-2 into an optical signal and outputs the optical signal to the port 4 of the 2 × 2 optical switch 2-1. The 2 x 2 optical switch 2-1 is initially in a forward conducting state. The forward conducting state of the 2 × 2 optical switch 2-1 means that the port 1 is connected to the port 3, the port 2 is connected to the port 4, and the backward conducting state means that the port 1 is connected to the port 4, and the port 2 is connected to the port 3.

The intermediate time transfer unit (as shown in fig. 3) includes a 2 × 2 optical switch 3-1, an optical-to-electrical conversion module 3-2, a signal processing and decoding control module 3-3, a time interval measurement module 3-4, and an electrical-to-optical conversion module 3-5. The port 1 and the port 2 of the 2 x 2 optical switch 3-1 are respectively connected with the forward input and the backward input of the optical fiber link, the port 3 is connected with the photoelectric conversion module 3-2, and the port 4 is connected with the photoelectric conversion module 3-5; the photoelectric conversion module 3-2 inputs the electric signal to the signal processing and decoding control module 3-3; under the control of a control signal of the signal processing and decoding control module 3-3, the 2 × 2 optical switch 3-1 switches a forward optical signal of the optical fiber link input port 1 to the port 3, outputs the forward optical signal to the photoelectric conversion module 3-2, and outputs the forward optical signal to the port 2 through the port 4 after the photoelectric conversion of the photoelectric conversion module 3-5; under the control of a control signal of the signal processing and decoding control module 3-3, the 2 × 2 optical switch 3-1 switches a backward optical signal of the optical fiber link input port 2 to the port 3, outputs the backward optical signal to the photoelectric conversion module 3-2, and outputs the backward optical signal to the port 1 through the port 4 after the photoelectric conversion of the photoelectric conversion module 3-5. The photoelectric conversion module 3-2 converts the optical signal from the port 3 of the 2 x 2 optical switch 3-1 into an electrical signal and inputs the electrical signal to the photoelectric conversion module 3-5 and the signal processing and decoding control module 3-3. The signal processing and decoding control module 3-3 decodes the time code in the electrical signal, extracts the timing signal, and outputs a state control signal to the 2 × 2 optical switch 3-1 according to the timing signal, and simultaneously outputs the timing signal to the time interval measuring module 3-4 and outputs the time information. The time interval measuring module 3-4 receives the forward and backward timing signals inputted from the signal processing and decoding control module 3-3 and measures the time intervals of the forward and backward timing signals. The electro-optical conversion module 3-5 converts the electrical signal from the electro-optical conversion module 3-2 into an optical signal, and the 2 x 2 optical switch 3-1 has a port 4. The 2 x 2 optical switch 3-1 is initially in a forward conducting state. The forward conducting state of the 2 × 2 optical switch 3-1 means that the port 1 is connected to the port 3, the port 2 is connected to the port 4, and the backward conducting state means that the port 1 is connected to the port 4, and the port 2 is connected to the port 3.

The control method of the 2 × 2 optical switch in the bidirectional photoelectric optical relay unit and the intermediate time transfer unit in the present embodiment is as follows:

the forward transmitted optical signal is divided into two paths after passing through the photoelectric conversion module. One path is subjected to electro-optic conversion by an electro-optic conversion module, then is output to a 2 x 2 optical switch, and is output by the 2 x 2 optical switch through an optical fiberTransmitting to the next unit; the other path of the signal processing and decoding control module identifies the forward timing signal and records the time t of identifying the forward 1PPS timing signal_fAnd based on the time t at which the forward 1PPS timing signal is identified_fDetermining the time t of setting the 2 x 2 optical switch to the backward conducting state next time_b2＝t_f+0.000006 and time of forward conduction state t_f2＝t_f+ 0.999; updating t after each new time code is received_fAnd storing the value of (t)_f2、t_b2A value of (d);

the time transfer method of the optical fiber time transfer system of the embodiment operates in mode one (as shown in fig. 4), and includes the following steps:

(1) before the system is started, each slave time transmission unit is numbered and assigned with address information, and the delay time of the timing signal of each slave time transmission unit is calculated, T_diI is the number of the slave time transfer unit, wherein the transmission delay of the largest master time transfer unit and the largest slave time transfer unit in the optical distribution network is measured to be about 5ms, and the redundancy value for ensuring the stable and reliable operation of the system is set to be 1 ms.

(2) When the main time transfer unit detects the timing signal input by the main clock, the optical signal transmission is started, the optical signal containing the timing signal of the main clock and the time code of the control and state information is broadcasted to the optical distribution network, and the optical signal transmission is closed after the transmission is finished.

(5) As shown in fig. 4, the ith slave time transfer unit pair is fromDecoding the time code of the main time transmission unit to extract the timing signal, delaying the decoded timing signal by a time T_diThen, the delayed timing signal and the ith slave time transfer unit address information are coded into a time code, modulated to the same optical wavelength as the forward transmission, sent to the same optical fiber link in the reverse direction, and the optical signal sending is closed after the sending is finished;

(7) as shown in fig. 5, the intermediate time transfer unit extracts the backward timing signal and the slave time transfer unit address information from the received time code, and measures the time interval T between the received forward timing signal and the backward timing signal from each slave time transfer unit_MTransmitting the address information of each slave time transfer unit and the measured corresponding time interval T_MAnd storing and carrying out photoelectric optical relay on the backward optical signals.

(8) The master time transmission unit receives the time codes from the slave time transmission units and decodes the time codes to obtain the backward timing signals and the address information of the slave time transmission units; measuring the time interval T between the timing signal of the master clock and each decoded timing signal_AB(ii) a The address information and the corresponding measured time interval T_ABStoring;

(9) when the main time transfer unit detects a timing signal input by a main clock, the optical distribution network broadcasts and transmits a time code carrying the timing signal of the main clock, locally stored address information of the slave time transfer unit and corresponding measured time interval, control and state information, and the optical signal transmission is closed after the transmission is finished;

(11) the intermediate time transfer unit extracts the forward timing signal from the received time code and carries the forward timing signal to each slave time transfer unitAddress information and time intervals T measured by the master time transfer unit_ABStoring the address information and the corresponding time interval, and simultaneously carrying out photoelectric optical relay on the arriving forward optical signal; the intermediate time transmission unit transmits a time interval T corresponding to the address information_ABAnd T_MCalculating the clock difference between the received forward timing signal and the main clock:

wherein

And

the transmission and reception delays of the master time transfer unit,

and

(12) The ith slave time transfer unit decodes the time code from the master time transfer unit, extracts the timing signal and the time interval T measured by the master time transfer unit corresponding to the slave time transfer unit_ABDelaying the decoded timing signal by a time T_diCoding the delayed timing signal and the address information of the unit into a time code, modulating the time code to the same optical wavelength as the forward transmission, reversely transmitting the time code to the same optical fiber link, and closing optical signal transmission after the transmission is finished; meanwhile, the ith slave time transfer unit calculates the clock difference between the received timing signal and the master clock timing signal:

wherein

And

the transmission and reception delays of the master time transfer unit,

and

(13) Repeating the step (6) -12 until the main time transmission unit stops working and then entering the step (14)

(14) End up

In the wide-range optical fiber time transmission system in this embodiment, when the slave time transmission unit (denoted as the K +1 th slave time transmission unit) is added to the system, the K +1 th slave time transmission unit extracts the timing signal from the received time code, and at the same time, delays the received timing signal by the time T_d(K+1)And after 0.011 (K +1), starting optical signal transmission, encoding the delayed timing signal and the K + 1-th slave time transfer unit address information into a time code, modulating the time code to the same optical wavelength as the forward transmission, transmitting the optical signal to the same optical fiber link in a reverse direction, and closing optical signal transmission after the transmission is finished. Then repeating the steps: (6) - (13)

In the wide-range optical fiber time transfer system in this embodiment, when the slave time transfer unit is deleted in the system, in several consecutive measurement periods, the master time transfer unit determines that the slave time transfer unit has been deleted from the system if the master time transfer unit does not obtain the backward timing signal and the address information of the slave time transfer unit from the received time code, and deletes the information of the slave time transfer unit stored locally.

The time transfer method of the optical fiber time transfer system of the embodiment operates in the second mode (as shown in fig. 6), and includes the following steps:

(1) before the system is started, the master time transfer unit sets a slave time transfer unit identification window 0.05, a comparison window initial value t of each slave time transfer unit to be 0.05+ i multiplied by 0.0001 and a comparison window width to be 0.0001, and stores values of the identification window 0.05, the comparison window initial value t and the comparison window width to be 0.0001.

(5) Each slave time transfer unit decodes the time code from the master time transfer unit to extract a timing signal, and randomly delays the decoded timing signal by a time T_dAfter 0.000006 × n, coding the delayed timing signal and the address information of the unit into a time code, modulating the time code to the same optical wavelength as the forward transmission, reversely transmitting the time code to the same optical fiber link, and closing optical signal transmission after the transmission is finished; wherein n is a random integer no greater than 100.

(7) the intermediate time transfer unit extracts the backward timing signal and the address information of the slave time transfer unit from the received time code, and measures the time interval T between the received forward timing signal and the backward timing signal from each slave time transfer unit_MTransmitting the address information of each slave time transfer unit and the measured corresponding time interval T_MAnd storing and carrying out photoelectric optical relay on the backward optical signals.

(8) Master time transfer unit to successfully received slave timeDecoding the time code of the time transfer unit to obtain a backward timing signal and address information from the time transfer unit; measuring the time interval T between the timing signal of the master clock and each decoded timing signal_AB(ii) a The address information and the corresponding measured time interval T_ABStoring; the master time transfer unit calculates the time delay adjustment quantity delta tau of the timing signal corresponding to the slave time transfer unit which stores the address information according to the comparison window T of each distributed slave node time transfer unit_ABAnd storing the time delay adjustment amount.

(9) When the main time transfer unit detects a timing signal input by a main clock, a time code carrying the timing signal of the main clock, locally stored address information of the slave time transfer unit, a corresponding measured time interval and time delay adjustment quantity and control and state information is broadcast and transmitted through an optical distribution network, and optical signal transmission is closed after the transmission is finished;

(11) as shown in fig. 5, the intermediate time transfer unit extracts the forward timing signal, the address information of each slave time transfer unit and the time interval T measured by the master time transfer unit from the received time code_ABStoring the address information and the corresponding time interval, and simultaneously carrying out photoelectric optical relay on the arriving forward optical signal; if the address information of a certain slave time transfer unit is stored in both forward and backward transmission, the intermediate time transfer unit stores the corresponding time interval T of the address information according to the stored address information_ABAnd T_MCalculating the clock difference between the received forward timing signal and the main clock:

wherein

And

the transmission and reception delays of the master time transfer unit,

and

(12) Each slave time transfer unit decodes the time code from the master time transfer unit. If the received time code does not contain the address information of the slave time transmission unit, the decoded timing signal is randomly delayed for a time T_dWhen the time code is 0.000006 × n, the delayed timing signal and the address information of the unit are encoded into the time code, modulated to the same optical wavelength as the forward transmission, and sent to the same optical fiber link in the reverse direction, and the optical signal transmission is closed after the transmission is finished; if the received time code contains the address information of the slave time transfer unit, extracting the time delay adjustment quantity delta tau of the timing information corresponding to the slave time transfer unit and the time interval T measured by the master time transfer unit_ABControl and status information, delaying the received timing signal by a time T_dAnd the + delta tau codes the delayed timing signal and the address information of the unit into a time code, modulates the time code to the same optical wavelength as the forward transmission, reversely sends the time code to the same optical fiber link, and closes the optical signal sending after the sending is finished. The slave time transfer unit which simultaneously contains the address information calculates the clock difference between the received timing signal and the timing signal of the master clock:

wherein

And

the transmission and reception delays of the master time transfer unit,

and

(14) the intermediate time transfer unit extracts the backward timing signal and the address information of the slave time transfer unit from the received time code, and measures the time interval T between the received forward timing signal and the backward timing signal from each slave time transfer unit_MTransmitting the address information of each slave time transfer unit and measuring the corresponding time interval T_MAnd storing and carrying out photoelectric optical relay on the backward optical signals.

(15) The master time transfer unit decodes the successfully received slave time transfer unit time code to obtain a backward timing signal and address information from the slave time transfer unit; measuring the time interval T between the timing signal of the master clock and each decoded timing signal_AB(ii) a If the address information of the slave time transfer unit is stored locally, updating the time interval T corresponding to the slave time transfer unit_ABWhile simultaneously comparing T_ABIf the time slot T corresponding to the slave time transfer unit is 0.05+ i × 0.0001, T-0.00003 ≦ T_ABIf t +0.00013 is less than or equal to t, the value of the time delay adjustment quantity delta t of the slave time transfer unit is kept unchanged; if T-0.00003 is not more than T_ABT +0.00013, the value delta tau of the time delay adjustment of the slave time transfer unit is recalculated to T-T_ABAnd updates the value of the delay adjustment amount. If the slave time transfer unit address information is not stored locally, the new slave time transfer unit is considered to be identified, and the slave time transfer unit address information and the corresponding time interval T are transmitted_ABStoring, and counting according to the set comparison window tCalculating the time delay adjustment quantity delta tau of the timing signal of the slave time transfer unit corresponding to the address information_ABAnd storing the time delay adjustment amount. If the address information of the slave time transfer unit which is stored locally is not contained in the received address information, the slave time transfer unit corresponding to the address information is considered to be deleted from the system, and the address information corresponding to the slave time transfer unit and the time interval T are used_ABAnd deleting the timing signal time delay adjustment quantity delta tau

(16) Repeating the steps (9) -15 until the main time transmission unit stops working and then entering the step (17)

(17) And (6) ending.

Claims

1. A time transfer method for a high-precision large-range optical fiber time transfer system, characterized by being implemented with a high-precision large-range optical fiber time transfer system (1), the system comprising: the system comprises a main clock (1-1), a main time transfer unit (1-2), an optical distribution network (1-3) and a plurality of slave time transfer units (1-4); the optical distribution network (1-3) is composed of a plurality of optical splitters/combiners (1-5), a plurality of bidirectional photoelectric optical relay units (1-6), a plurality of intermediate time transmission units (1-7), a plurality of bidirectional optical amplification units (1-8) and a plurality of optical fiber links, the bidirectional photoelectric optical relay units (1-6), the intermediate time transfer units (1-7) and the bidirectional optical amplification units (1-8) are connected through optical fiber links, each bidirectional photoelectric optical relay unit (1-6) performs photoelectric optical relay on forward and backward optical signals, each intermediate time transfer unit (1-7) performs photoelectric optical relay on the forward and backward optical signals and decodes the forward and backward optical signals to obtain timing signals in the optical fiber links, and each bidirectional optical amplification unit (1-8) performs optical amplification on the forward and backward optical signals; the main time transfer unit (1-2) is connected with the main clock (1-1) and is also connected with the combining end of the optical distribution network (1-3); each slave time transfer unit (1-4) is connected with the branch end of the optical distribution network (1-3); in the optical distribution network (1-3), the positions, the sequences and the number of the bidirectional photoelectric optical relay units (1-6) and the bidirectional optical amplification units (1-8) are arbitrary; the timing signal of the master clock (1-1) is sent through the master time transfer unit (1-2) and sent to each intermediate time transfer unit (1-7) and each slave time transfer unit (1-4) through the optical distribution network (1-3); each slave time transfer unit (1-4) delays the timing signal from the master time transfer unit (1-2), then sends the timing signal to the master time transfer unit (1-2) through the optical distribution network (1-3) along the original optical fiber link in the reverse direction, and measures the time interval between the timing signal from the master time transfer unit (1-2) and the timing signal after delay; the master time transfer unit (1-2) respectively measures the time interval between the timing signal from each slave time transfer unit (1-4) and the timing signal of the master clock (1-1), and sends the time interval to each intermediate time transfer unit (1-7) and each slave time transfer unit (1-4) through the optical distribution network (1-3); each of said intermediate time transfer units (1-7) measuring the time interval of the timing signal from the master time transfer unit (1-2) and the timing signal from the slave time transfer unit (1-4); each slave time transmission unit (1-4) and each intermediate time transmission unit (1-7) obtain the clock difference between each unit and the master clock (1-1) according to the time interval received from the master time transmission unit (1-2) and the locally measured time interval, and realize high-precision and wide-range optical fiber time transmission in point-to-multipoint picosecond magnitude; the method comprises a small-capacity static time transfer mode and a large-capacity dynamic time transfer mode;

the small-capacity static time transfer mode comprises the following steps:

1) before the system is started, each slave time transmission unit is numbered and assigned with address information, and the delay time T of the timing signal of each slave time transmission unit is calculated_di＝(2T_M+ Δ T) xi, i being the number of slave time transfer units, T_MThe transmission time delay of the largest main time transfer unit and the largest slave time transfer unit in the optical distribution network is measured, and delta T is a redundancy value which is set for ensuring the stable and reliable work of the system;

2) when the main time transfer unit (1-2) detects a timing signal input by a main clock, starting optical signal transmission, transmitting an optical signal containing the timing signal of the main clock and a time code of control and state information to an optical distribution network through an optical fiber link, and closing optical signal transmission after the transmission is finished;

3) in each optical fiber link, the bidirectional photoelectric optical relay unit performs photoelectric optical relay on the arriving forward optical signal; the bidirectional optical amplification unit optically amplifies the arriving forward optical signal;

4) the intermediate time transfer unit extracts a forward timing signal from the received time code and performs photoelectric optical relay on the arriving forward optical signal;

5) the ith slave time transfer unit decodes the time code from the master time transfer unit to extract a timing signal, and delays the decoded timing signal by a time T_diThen, the delayed timing signal and the ith slave time transfer unit address information are coded into a time code, modulated to the same optical wavelength as the forward transmission, sent to the same optical fiber link in the reverse direction, and the optical signal sending is closed after the sending is finished;

6) in each link, the bidirectional photoelectric optical relay unit performs photoelectric optical relay on the arriving backward optical signal; the bidirectional optical amplification unit optically amplifies the arriving backward optical signal;

7) the intermediate time transfer unit extracts the backward timing signal and the address information of the slave time transfer unit from the received time code, and measures the time interval T between the reception of the forward timing signal and the reception of the backward timing signal from each slave time transfer unit_MTransmitting the address information of each slave time transfer unit and the measured corresponding time interval T_MStoring, and simultaneously carrying out photoelectric optical relay on the arriving backward optical signals;

8) the master time transfer unit (1-2) receives the time codes from the slave time transfer units, and decodes the time codes to obtain the backward timing signals and the address information of the slave time transfer units; measuring the time interval T between the timing signal of the master clock and the decoded backward timing signal from each slave time transfer unit_AB(ii) a Transmitting each slave time transfer unit address information and corresponding measured time interval T_ABStoring;

9) when the main time transfer unit detects a timing signal input by a main clock, the optical time transfer unit broadcasts and sends a time code carrying the timing signal of the main clock, the address information of each saved slave time transfer unit and the corresponding measured time interval and control and state information to an optical distribution network through an optical fiber link, and the optical signal transmission is closed after the transmission is finished;

10) in each link, the bidirectional photoelectric optical relay unit performs photoelectric optical relay on an arriving forward optical signal; the bidirectional optical amplification unit optically amplifies the arriving forward optical signal;

11) the intermediate time transfer unit extracts the forward timing signal, the carried address information of each slave time transfer unit and each time interval T measured by the master time transfer unit from the received time code_ABStoring the address information and the corresponding time interval, and simultaneously carrying out photoelectric optical relay on the arriving forward optical signal; the intermediate time transfer unit transfers the time interval T corresponding to the stored address information_ABAnd T_MCalculating the clock difference between the received forward timing signal and the main clock according to the formula (1):

wherein the content of the first and second substances,

and

and

12) the ith slave time transfer unit decodes the time code from the master time transfer unit, extracts the timing signal and the time interval T measured by the master time transfer unit corresponding to the ith slave time transfer unit_ABiDelaying the decoded timing signal by a time T_diTiming after delayThe signal and the ith slave time transfer unit address information are coded into a time code, modulated to the same optical wavelength as the forward transmission, sent to the same optical fiber link in the reverse direction, and the optical signal transmission is closed after the transmission is finished; meanwhile, the ith slave time transfer unit calculates the clock difference between the received timing signal and the timing signal of the master clock according to the formula (2):

wherein the content of the first and second substances,

and

and

the sending time delay and the receiving time delay of the slave time transfer unit are obtained by calibrating equipment;

13) returning to the step (6), and entering a step 14 after detecting that the main time transmission unit stops working;

14) finishing;

the large-capacity dynamic time transfer mode comprises the following steps:

1) before the system is started, the master time transfer unit (1-2) sets a slave time transfer unit identification window t₀Comparing the window initial value t and the window width delta t by each slave time transfer unit to identify the window t₀Comparing the window initial value t and the comparison window width delta t, and storing;

2) when the main time transfer unit (1-2) detects a timing signal input by the main clock (1-1), starting optical signal transmission, transmitting a time code containing the timing signal of the main clock and control and state information to an optical distribution network through an optical fiber link, and closing optical signal transmission after the transmission is finished;

3) in the optical fiber link, a bidirectional photoelectric optical relay unit performs photoelectric optical relay on an arriving forward optical signal; the bidirectional optical amplification unit optically amplifies the arriving forward optical signal;

5) each slave time transfer unit decodes the time code from the master time transfer unit to extract a timing signal, and randomly delays the decoded timing signal by a time T_d＝τ_cAfter xn, the delayed timing signal and the address information of the unit are coded into a time code, modulated to the same optical wavelength as the forward transmission, sent to the same optical fiber link in the reverse direction, and the optical signal sending is closed after the sending is finished; wherein, tau_cN is not greater than (t) for the length of the time code to be transmitted₀-2T_abm)/τ_cRandom integer of (1), T_abmThe maximum unidirectional link transmission time delay obtained by measurement in the optical distribution network is obtained;

8) the master time transfer unit decodes the successfully received slave time transfer unit time code to obtain a backward timing signal and address information from the slave time transfer unit; measuring the time interval T between the timing signal of the master clock and the decoded backward timing signal from each slave time transfer unit_AB(ii) a Transmitting each slave time transfer unit address information and corresponding measured time interval T_ABStoring; the master time transfer unit calculates the time delay adjustment quantity delta tau of the timing signal corresponding to the slave time transfer unit which stores the address information according to the comparison window T of each distributed slave time transfer unit_ABAnd storing the time delay adjustment quantity;

9) when the main time transfer unit detects a timing signal input by a main clock, a time code carrying the timing signal of the main clock, locally stored address information of a slave time transfer unit, a corresponding measured time interval and time delay adjustment quantity and control and state information is broadcast and transmitted to an optical distribution network through an optical fiber link, and optical signal transmission is closed after the transmission is finished;

11) the intermediate time transfer unit extracts the forward timing signal, the carried address information of each slave time transfer unit and each time interval T measured by the master time transfer unit from the received time code_ABStoring the address information and the corresponding time interval, and simultaneously carrying out photoelectric optical relay on the arriving forward optical signal; if the address information of a certain slave time transfer unit is stored in both forward and backward transmission, the intermediate time transfer unit transfers the time interval T corresponding to the address information_ABAnd T_MCalculating the clock difference between the received forward timing signal and the main clock:

wherein the content of the first and second substances,

and

the transmission and reception delays of the master time transfer unit,

and

obtaining the sending and receiving time delay of the intermediate time transmission unit through equipment calibration;

12) each slave time transfer unit decodes the time code from the master time transfer unit:

if the received time code does not contain the address information of the slave time transmission unit, the decoded timing signal is randomly delayed for a time T_d＝τ_cXn, coding the delayed timing signal and the address information of the unit into a time code, modulating the time code to the same optical wavelength as the forward transmission, reversely sending the time code to the same optical fiber link, and closing the optical signal transmission after the completion of the sending;

if the received time code contains the address information of the slave time transfer unit, extracting the time delay adjustment quantity delta tau of the timing information corresponding to the slave time transfer unit and the time interval T measured by the master time transfer unit_ABControl and status information, delaying the received timing signal by a time T_d+ delta tau, coding the delayed timing signal and the address information of the unit into a time code, modulating the time code to the same optical wavelength as the forward transmission, reversely sending the time code to the same optical fiber link, and closing optical signal sending after the sending is finished; the slave time transfer unit containing address information calculates the clock difference between the received timing signal and the timing signal of the master clock according to the following formula:

wherein the content of the first and second substances,

and

and

13) in each link, the photoelectric optical relay unit performs photoelectric optical relay on the arriving backward optical signal; the bidirectional optical amplification unit optically amplifies the arriving backward optical signal;

14) the intermediate time transfer unit extracts the backward timing signal and the address information of the slave time transfer unit from the received time code, and measures the time interval T between the reception of the forward timing signal and the reception of the backward timing signal from each slave time transfer unit_M', passing each slave time transfer unit address information and the corresponding measured time interval T_M' storing and simultaneously performing photoelectric optical relay on the arriving backward optical signals;

15) the master time transfer unit decodes the successfully received slave time transfer unit time code to obtain a backward timing signal and address information from the slave time transfer unit; measuring the time interval T between the timing signal of the master clock and each decoded timing signal_AB’；

If the address information of the slave time transfer unit is stored locally, updating the time interval T corresponding to the slave time transfer unit_AB', simultaneously comparing T_AB' size of time slot T corresponding to the slave time transfer unit, if T-T ≦ T_AB' < t + delta t + t, then the value of the time delay adjustment quantity delta t of the slave time transfer unit is kept unchanged; if T-T is not satisfied, T is less than or equal to_AB' T + delta T + T, then the value delta tau of the time delay adjustment of the slave time transfer unit is recalculated to T-T_AB' and updating the value of the delay adjustment, wherein t is the maximum value of the deviation ratio window from the timing signal of the time transfer unit allowed by the system;

if the slave time transfer unit address information is not stored locally, the new slave time transfer unit is considered to be identified, and the slave time transfer unit address information and the corresponding time interval T are transmitted_ABStoring, and calculating the time delay adjustment amount delta tau of the address information corresponding to the timing signal of the slave time transfer unit according to the set comparison window T_AB', and storing the time delay adjustment amount;

if the address information of the slave time transfer unit which has been stored is not included in the received address information, the slave time transfer unit corresponding to the address information is considered to be deleted from the system, and the address information corresponding to the slave time transfer unit and the time interval T are set_AB' and deleting the timing signal time delay adjustment quantity delta tau;

16) returning to step 9), entering step 17) after detecting that the main time transmission unit stops working;

17) and (6) ending.

2. The method of claim 1, wherein when the system is operating in a low capacity static time transfer mode, the system automatically identifies and adds or deletes slave time transfer units when the slave time transfer units are manually added or deleted in the distribution network according to the time transfer application requirements:

when a slave time transfer unit is added in the system, the slave time transfer unit is marked as K +1 th, and the K +1 th slave time transfer unit extracts a timing signal from a received time code and delays the received timing signal by a time T_d(K+1)＝(2T_MAfter + delta T) x (K +1), starting optical signal transmission, encoding the delayed timing signal and the K +1 th slave time transfer unit address information into a time code, modulating the time code to the same optical wavelength as forward transmission, reversely transmitting the optical signal to the same optical fiber link, and closing optical signal transmission after the transmission is finished; then repeating the steps in the small-capacity static time transfer mode: 6) -12)

When the slave time transfer unit is deleted in the system, in a plurality of continuous measurement periods, the master time transfer unit cannot obtain the backward timing signal and the address information for deleting the slave time transfer unit from the received time code, and then judges that the slave time transfer unit is deleted from the system, and deletes the information of the slave time transfer unit which is locally stored.