A TDS system for realizing ultra-long frequency doubling scanning

Abstract

The present invention discloses a TDS system for realizing ultra-long frequency doubling scanning. By means of a first polarization-maintaining optical switch and a second polarization-maintaining optical switch, an optical fiber delay unit is connected between an optical fiber delay component of an existing TDS system and a terahertz detection antenna. The equivalent delay distance of the optical fiber delay component relative to the optical fiber is set as L0. The optical fiber delay unit includes N parallel optical fibers with different lengths. The length of the first optical fiber is L1, the length of the second optical fiber L2 = L1 + L0, the length of the third optical fiber L3 = L1 + 2L0,..., and the length of the Nth optical fiber LN = L1 + (N - 1)L0. Each optical fiber in the optical fiber delay unit is respectively connected to the corresponding transmission ports of the first polarization-maintaining optical switch and the second polarization-maintaining optical switch. The first polarization-maintaining optical switch and the second polarization-maintaining optical switch are respectively connected to a delay control module. It is first proposed to realize space-division multiplexing with different delay steps and extension of the total delay length by combining an optical switch with optical fiber paths of different fixed dispersions, which is a low-cost and large-delay TDS solution that is easy to implement.

Description

Technical Field:

[0001] The present invention belongs to the technical field of terahertz time-domain spectroscopy, and particularly relates to a TDS system for realizing ultra-long frequency doubling scanning. Background Art:

[0002] A terahertz time-domain spectroscopy system (hereinafter referred to as TDS) is a new type of spectroscopic measurement product, and its main components include a femtosecond laser, a fiber delay component, and a terahertz photoconductive emission antenna and a detector. TDS has many unique advantages, including a higher signal-to-noise ratio level compared with an infrared spectroscopy system, the unique penetrability and fingerprint spectrum characteristics in the terahertz band, etc. These advantages have made it widely concerned in various spectroscopic measurement fields and achieved excellent measurement results. Among them, when facing gases and other samples with fine absorption spectrum characteristics, TDS is often restricted by the delay range of the fiber delay component used (generally about 500 ps), unable to reach a wider delay range, that is, unable to achieve higher spectroscopic capabilities, and the fiber delay component with a longer delay range will greatly increase in size, while increasing the instability of the component. At present, it has been reported that a longer delay range is achieved by cascading multiple delay lines, but this will also greatly increase the hardware cost and debugging difficulty of the system.

[0003] In addition, the currently used fiber delay component can only achieve one-way scanning from 0 to the maximum delay, and no terahertz signal is scanned during the period from the maximum delay position back to zero delay, which is also one of the reasons for the generally low efficiency of current TDS spectral scanning. Summary of the Invention:

[0004] The purpose of the present invention is to overcome the shortcomings of the prior art and seek to design a TDS system for realizing ultra-long frequency doubling scanning.

[0005] To achieve the above purpose, the present invention adopts the following technical solutions:

[0006] The TDS system for realizing ultra-long frequency doubling scanning according to the present invention includes a laser, an optical fiber delay component, a first polarization-maintaining optical switch, a second polarization-maintaining optical switch, an optical fiber delay unit, a delay control module, and a terahertz transmitting and receiving unit. The output end of the laser is connected to the terahertz transmitting antenna. At the same time, the output end of the laser is sequentially connected to the terahertz detecting antenna through the optical fiber delay component, the first polarization-maintaining optical switch, the optical fiber delay unit, and the second polarization-maintaining optical switch. The equivalent delay distance of the optical fiber delay component relative to the optical fiber is set as L0. The optical fiber delay unit includes N parallel optical fibers with different lengths, where N is a positive integer greater than or equal to 2. The length of the first optical fiber is L1, the length of the second optical fiber L2 = L1 + L0, the length of the third optical fiber L3 = L1 + 2L0,..., the length of the Nth optical fiber LN = L1 + (N - 1)L0. The transmission ports of the first polarization-maintaining optical switch and the second polarization-maintaining optical switch correspond to the number of optical fibers in the optical fiber delay unit. Each optical fiber in the optical fiber delay unit is respectively connected to the corresponding transmission ports of the first polarization-maintaining optical switch and the second polarization-maintaining optical switch at both ends. The first polarization-maintaining optical switch and the second polarization-maintaining optical switch are respectively connected to the delay control module. The delay control module is used to control the first optical fiber, the second optical fiber,..., the Nth optical fiber in the optical fiber delay unit to be sequentially connected between the optical fiber delay component and the terahertz detecting antenna.

[0007] Further, the TDS system for realizing ultra-long frequency doubling scanning according to the present invention further includes a reverse processing module and a splicing module. The reverse processing module is connected to the terahertz acquisition module in the integrated control and signal acquisition and processing unit, and is used to reversely process the terahertz pulse signal acquired when the 2Mth optical fiber is connected. The splicing module is respectively connected to the terahertz acquisition module and the reverse processing module, and connects the starting ends of the forward terahertz pulse signals obtained when two adjacent optical fibers are connected.

[0008] Further explanation, except for the first optical fiber, other optical fibers all include polarization-maintaining optical fibers and polarization-maintaining dispersion compensation optical fibers, and the length ratio of the polarization-maintaining optical fiber to the polarization-maintaining dispersion compensation optical fiber is 1:5.

[0009] Further explanation, in order to reduce the loss of optical fiber connection, the first polarization-maintaining optical switch, the optical fiber delay unit, and the second polarization-maintaining optical switch are sequentially fused and encapsulated in a closed device.

[0010] Further, the TDS system for realizing ultra-long frequency doubling scanning according to the present invention further includes a power supply unit. The power supply unit is respectively connected to the laser, the optical fiber delay component, and the integrated control and signal acquisition and processing unit, and provides stable power for the active devices of each unit of the system.

[0011] The present invention has the following beneficial effects compared with the prior art: For the first time, it is proposed to combine an optical switch with optical fiber paths of different fixed dispersions to achieve space-division multiplexing with different delay steps and extend the total delay length, which is a low-cost and easy-to-implement large-delay TDS solution; while greatly expanding the delay range, for the first time, the return journey of the traditional delay line is utilized for scanning, doubling the actual scanning efficiency without increasing the scanning rate. Taking the scanning range of the fiber delay component 2 as an example, the solution achieves a two-fold comprehensive time delay without increasing the time, which is an efficient long-range delay solution. Description of the Drawings:

[0012] Figure 1 It is a schematic structural diagram of an existing TDS system.

[0013] Figure 2 It is a schematic structural diagram of a TDS system for realizing ultra-long frequency doubling scanning in Embodiment 1. Detailed Embodiment:

[0014] The present invention will be further described below through specific embodiments in conjunction with the drawings.

[0015] Embodiment 1

[0016] In order to collect high-frequency terahertz signals, existing TDS systems usually collect terahertz signals by synchronously locking and precisely adjusting the relative phase delay based on the periodicity of the signals to obtain a complete terahertz pulse signal. Specifically, as Figure 1 shown, the existing TDS system includes a laser 1, a fiber delay component 2, an integrated control and signal acquisition and processing unit 6, and a terahertz transmitting and receiving unit 8. The terahertz transmitting and receiving unit 8 includes a terahertz transmitting antenna and a terahertz detecting antenna. The laser 1 is used to output two femtosecond pulse lights with the same frequency and phase. One of the femtosecond pulse lights is output to the terahertz transmitting antenna, and the other femtosecond pulse light reaches the terahertz detecting antenna after being delayed by the fiber delay component 2. The femtosecond pulse light will excite photo-generated carriers inside the photoconductive material. Under the action of an external bias voltage, the carriers will migrate rapidly and radiate high-repetition-frequency pulsed terahertz waves into space. The integrated control and signal acquisition and processing unit 6, as the central control unit of the system, realizes functions such as the generation of synchronous trigger signals, the emission of modulation signals, and the acquisition and processing of terahertz signals.

[0017] As described above, the adjustment of the delay is achieved through the optical fiber delay component 2. The magnitude of the delay is usually expressed in time unit ps. Taking the delay range of a common optical fiber delay line of 500 ps as an example, a spectral resolution of 1 / 500 = 2 GHz can be obtained. The optical fiber delay component 2 is an existing optical fiber delay line (such as the compact high-speed oscillating optical fiber delay line in Patent CN111239908A), which is used to detect the time delay of the optical path. The delay time is converted into a delay distance as: L = 500e-12 * 3e8 = 15 cm. The equivalent delay distance of the optical fiber delay component 2 relative to the optical fiber is set as L0 = L / n, where n is the refractive index of the optical fiber. The refractive index of quartz optical fiber is about 1.5, and L0 = L / 1.5.

[0018] The TDS system for realizing ultra-long frequency doubling scanning involved in this embodiment expands the delay range through space-division multiplexing of the optical fiber delay unit 5 by adding an optical fiber delay unit 5 between the optical fiber delay component 2 and the terahertz detection antenna.

[0019] As Figure 2As shown in the figure, the TDS system for realizing ultra-long frequency doubling scanning involved in this embodiment includes a laser 1, an optical fiber delay component 2, a first polarization-maintaining optical switch 3, a second polarization-maintaining optical switch 4, an optical fiber delay unit 5, a delay control module, and a terahertz transmitting and receiving unit 8. The output end of the laser 1 is connected to the terahertz transmitting antenna. At the same time, the output end of the laser 1 is sequentially connected to the terahertz detecting antenna through the optical fiber delay component 2, the first polarization-maintaining optical switch 3, the optical fiber delay unit 5, and the second polarization-maintaining optical switch 4. The laser 1 is used to output two paths of femtosecond pulsed light with the same frequency and phase. One path of femtosecond pulsed light is output to the terahertz transmitting antenna, and the other path of femtosecond pulsed light reaches the terahertz detecting antenna after being delayed by the optical fiber delay component 2 and the optical fiber delay unit 5. The optical fiber delay component 2 is an existing optical fiber delay line (for example, the compact high-speed oscillating optical fiber delay line in patent CN111239908A), which is used for the time delay of the detection optical path. The equivalent delay distance of the optical fiber delay component 2 relative to the optical fiber is set as L0. The optical fiber delay unit 5 includes N parallel optical fibers with different lengths, where N is a positive integer greater than or equal to 2. The length of the first optical fiber is L1, the length of the second optical fiber L2 = L1 + L0, the length of the third optical fiber L3 = L1 + 2L0,..., the length of the Nth optical fiber LN = L1 + (N - 1)L0, where the number of N is set according to actual needs. The transmission ports of the first polarization-maintaining optical switch 3 and the second polarization-maintaining optical switch 4 correspond to the number of optical fibers in the optical fiber delay unit 5. Each optical fiber in the optical fiber delay unit 5 is respectively connected to the corresponding transmission ports of the first polarization-maintaining optical switch 3 and the second polarization-maintaining optical switch 4 at both ends. Each optical fiber in the optical fiber delay unit 5 corresponds to a channel. The first polarization-maintaining optical switch 3 and the second polarization-maintaining optical switch 4 are respectively connected to the delay control module. The delay control module is used to control the first optical fiber, the second optical fiber,..., the Nth optical fiber in the optical fiber delay unit 5 to be sequentially connected to the TDS system to realize different lengths of delay.

[0020] Furthermore, the TDS system for realizing ultra-long frequency doubling scanning involved in this embodiment further includes a reverse processing module and a splicing module. The reverse processing module is connected to the terahertz acquisition module in the integrated control and signal acquisition and processing unit, and is used for reverse processing of the terahertz pulse signal when the 2Mth optical fiber (where M is a positive integer greater than or equal to 1, such as the second optical fiber, the fourth optical fiber, the sixth optical fiber...) is connected. The splicing module is respectively connected to the terahertz acquisition module and the reverse processing module, and is used for connecting the starting ends of the forward terahertz pulse signals obtained when two adjacent optical fibers are connected (assuming that the terahertz pulse signal obtained when the (2M - 1)th optical fiber is connected is forward).

[0021] Specifically, the delay control module, the reverse processing module, and the splicing module belong to the functional modules added in the integrated control and signal acquisition and processing unit 6.

[0022] The specific working process of the TDS system for realizing ultra-long frequency doubling scanning involved in this embodiment is as follows:

[0023] (1) When the first polarization-maintaining optical switch 3 and the second polarization-maintaining optical fiber 4 are locked in channel 1 (i.e., connecting the first optical fiber to the TDS system), the optical fiber delay component 2 scans from 0 delay to the maximum delay, and the equivalent scanning length is L0. At this time, the optical fiber delay component 2 stays at the maximum delay position;

[0024] (2) When the first polarization-maintaining optical switch 3 and the second polarization-maintaining optical fiber 4 are synchronously switched to channel 2 (i.e., connecting the second optical fiber to the TDS system), the optical fiber delay component 2 starts to scan reversely from the maximum delay to 0 delay, and the total equivalent scanning length is 2L0. At this time, the optical fiber delay component 2 stays at the 0 delay position. It should be noted that the obtained terahertz pulse signal at this time is opposite to that in step (1);

[0025] (3) When the first polarization-maintaining optical switch 3 and the second polarization-maintaining optical fiber 4 are synchronously switched to channel 3 (i.e., connecting the third optical fiber to the TDS system), the optical fiber delay component 2 starts to scan from 0 delay to the maximum delay, and the total equivalent scanning length is 3L0. At this time, the optical fiber delay component 2 stays at the maximum delay position. It should be noted that the obtained terahertz pulse signal at this time is in the same direction as that in step (1);

[0026] (4) And so on. When the first polarization-maintaining optical switch 3 and the second polarization-maintaining optical fiber 4 are synchronously switched to channel N (i.e., connecting the Nth optical fiber to the TDS system), the delay line performs continuous scanning, so as to obtain a scanning range N*L0 that is N times that of the optical fiber delay component 2;

[0027] (5) After the scanning is completed according to the actual delay range requirement, the data in the reverse state are respectively reversed, and then the data collected in multiple channels are spliced to obtain the terahertz time-domain pulse waveform with an ultra-long delay range.

[0028] It should be further noted that due to the change of the optical fiber length, chromatic dispersion will be generated on the femtosecond pulse transmitted in the optical fiber, affecting the pulse width, and further affecting the terahertz pulse width and spectral width. In order to ensure that the change of the optical fiber length does not affect the total chromatic dispersion of the optical path, except for channel 1, other channels adopt a combination of polarization-maintaining optical fiber (PMF) and polarization-maintaining dispersion compensation optical fiber (PM-DCF) to determine the actual optical fiber length. Usually, the PM-DCF available on the market can achieve a 1:5 chromatic dispersion neutralization of the PMF. Taking channel 2 as an example, the lengths of the PM-DCF and PMF used are and That is, except for the first optical fiber, other optical fibers all include polarization-maintaining optical fiber (PMF) and polarization-maintaining dispersion compensation optical fiber (PM-DCF), and the length ratio of the polarization-maintaining optical fiber to the polarization-maintaining dispersion compensation optical fiber is 1:5.

[0029] Furthermore, it is necessary to adjust the external optical path of the system to adjust the main peak of the terahertz pulse into a segment to avoid the distortion of the main peak signal during the data splicing process.

[0030] Furthermore, to reduce the loss of fiber access, the fibers between the first polarization-maintaining optical switch 3, the fiber delay unit 5, and the second polarization-maintaining optical switch 4 are connected by precise length calculation and fusion splicing, and the three components or units are encapsulated in a closed device.

[0031] Furthermore, since there is a switching time of about 7 ms for the optical switch, a waiting window of about 10 ms should be reserved at the start of the scan after the control channel is switched.

[0032] Furthermore, the TDS system for realizing ultra-long frequency doubling scanning involved in this embodiment further includes a power supply unit 7. The power supply unit 7 is respectively connected to the laser 1, the fiber delay component 2, and the integrated control and signal acquisition and processing unit 6 to provide stable power for the active devices of each unit of the system.

Claims

1. A TDS system for realizing ultra-long frequency doubling scanning, characterized in that, It includes a laser, an optical fiber delay component, a first polarization-maintaining optical switch, a second polarization-maintaining optical switch, an optical fiber delay unit, a delay control module, and a terahertz transmitting and receiving unit. The output end of the laser is connected to the terahertz transmitting antenna. At the same time, the output end of the laser is sequentially connected to the terahertz detecting antenna through the optical fiber delay component, the first polarization-maintaining optical switch, the optical fiber delay unit, and the second polarization-maintaining optical switch. The equivalent delay distance of the optical fiber delay component relative to the optical fiber is set as L0. The optical fiber delay unit includes N parallel optical fibers with different lengths, where N is a positive integer greater than or equal to 2. The length of the first optical fiber is L1, the length of the second optical fiber L2 = L1 + L0, the length of the third optical fiber L3 = L1 + 2L0, ……, the length of the Nth optical fiber LN = L1 + (N - 1)L0. Each optical fiber in the optical fiber delay unit is connected to the corresponding transmission ports of the first polarization-maintaining optical switch and the second polarization-maintaining optical switch at both ends. The first polarization-maintaining optical switch and the second polarization-maintaining optical switch are respectively connected to the delay control module. The delay control module is used to control the first optical fiber, the second optical fiber, ……, the Nth optical fiber in the optical fiber delay unit to be sequentially connected between the optical fiber delay component and the terahertz detecting antenna. It also includes a reverse processing module and a splicing module. The reverse processing module is connected to the terahertz acquisition module and is used to reverse-process the terahertz pulse signal collected when the 2Mth optical fiber is connected. The splicing module is respectively connected to the terahertz acquisition module and the reverse processing module and connects the starting ends of the forward terahertz pulse signals obtained when two adjacent optical fibers are connected; The specific working process of the TDS system for realizing ultra-long frequency doubling scanning is as follows: (1) When the first polarization-maintaining optical switch and the second polarization-maintaining optical fiber are locked in the first channel, that is, the first optical fiber is connected to the TDS system, the optical fiber delay component scans from 0 delay to the maximum delay, and the equivalent scanning length is L0. At this time, the optical fiber delay component stays at the maximum delay position; (2) When the first polarization-maintaining optical switch and the second polarization-maintaining optical fiber are synchronously switched to the second channel, that is, the second optical fiber is connected to the TDS system, the optical fiber delay component starts to scan reversely from the maximum delay to 0 delay, and the total equivalent scanning length is 2L0. At this time, the optical fiber delay component stays at the 0 delay position. The terahertz pulse signal obtained at this time is opposite to that in step (1); (3) When the first polarization-maintaining optical switch and the second polarization-maintaining optical fiber are synchronously switched to the third channel, that is, the third optical fiber is connected to the TDS system, the optical fiber delay component starts to scan from 0 delay to the maximum delay, and the total equivalent scanning length is 3L0. At this time, the optical fiber delay component stays at the maximum delay position. The terahertz pulse signal obtained at this time is in the same direction as that in step (1); (4) And so on. When the first polarization-maintaining optical switch and the second polarization-maintaining optical fiber are synchronously switched to the Nth channel, that is, the Nth optical fiber is connected to the TDS system, the delay line performs continuous scanning, so as to obtain a scanning range N*L0 that is N times that of the optical fiber delay component; (5) After the scanning is completed according to the actual delay range requirement, the data in the reverse state are respectively reverse-processed, and then the data collected in multiple channels are spliced to obtain the terahertz time-domain pulse waveform with an ultra-long delay range.

2. The TDS system for realizing ultra-long frequency doubling scanning according to claim 1, wherein All optical fibers other than the first optical fiber include polarization-maintaining fibers and polarization-maintaining dispersion compensation fibers, and the length ratio of the polarization-maintaining fiber to the polarization-maintaining dispersion compensation fiber is 1:

5.

3. The TDS system for realizing ultra-long frequency doubling scanning according to claim 1, characterized in that, The first polarization-maintaining optical switch, the optical fiber delay unit, and the second polarization-maintaining optical switch are sequentially fused and encapsulated in a sealed device.

4. The TDS system for realizing ultra-long frequency doubling scanning according to claim 1, characterized in that, It also includes a power supply unit, which is respectively connected to the laser, the optical fiber delay component, and the integrated control and signal acquisition and processing unit to provide a stable power supply for the active devices of each unit of the system.

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