A terahertz long-distance detection method based on equivalent time sampling technology

Abstract

The invention discloses a terahertz long-distance detection method based on an equivalent time sampling technology. The laser emitted by a femtosecond laser is split into a pump laser and a detection laser by a beam splitter. The pump laser is incident on a terahertz transmitting antenna through an adjusted optical path delay line. The terahertz is incident on a receiving antenna after a long optical path. The detection laser is incident on a terahertz receiving antenna through a scanning optical path delay line. The optical path between the transmitting antenna and the receiving antenna is adjusted so that the optical path of the pump laser part in the pump optical path is approximately the same as the optical path of the detection laser part. At this time, the pump optical path is longer than the detection optical path by at least the distance traveled by the light in the air within a pump laser pulse cycle. In this way, the detection pulse starts detection after lagging the pump pulse by one or several pulses. A sample is taken at a single cycle of the detection pulse or at intervals of several cycles. Through this equivalent time sampling technology, long-distance detection of terahertz is realized.

Description

Technical Field

[0001] The present invention relates to the field of terahertz detection, and in particular to a terahertz long-distance detection method based on equivalent time sampling technology. Background Art

[0002] In the terahertz time-domain spectrometer system, although the photoconductive detection technology can obtain a transient current proportional to the terahertz radiation field, since the terahertz time-domain signal is about picoseconds or even shorter, and the rising edge of the current response time is basically in the sub-picosecond level, it is difficult for ordinary current detectors to measure the signal, so it is necessary to use equivalent time sampling technology to extract the terahertz time-domain electric field signal. However, in general terahertz time-domain spectrometer systems, equivalent time sampling is often the same detection light after beam splitting to sample the terahertz generated by the same pump light.

[0003] When terahertz needs to interact with rarefied gases, or when the object being detected is not suitable for close-range detection, it is necessary to increase the effective distance and perform long-distance detection of terahertz. However, in general terahertz time-domain spectrometer systems, the distance between the transmitting antenna and the receiving antenna is often short, which cannot meet the needs of long-distance detection.

[0004] Therefore, it is necessary to perform long-distance terahertz detection by improving the equivalent time sampling technology. Summary of the invention

[0005] In order to solve the above problems existing in the prior art, the present invention provides a terahertz long-distance detection method based on equivalent time sampling technology, by adjusting the optical path between the transmitting antenna and the receiving antenna, so that the optical path of the pump laser part in the pump optical path is approximately the same as the optical path of the detection laser part. At this time, the pump optical path is greater than the detection optical path by at least the distance traveled by the light in the air within one pump laser pulse cycle. In this way, the detection pulse will lag the pump pulse by one or several pulses to start detection, and a sample is taken at a single cycle of the detection pulse or at intervals of several cycles. Through this equivalent time sampling technology, long-distance terahertz detection is achieved.

[0006] The present invention is achieved through the following technical solutions:

[0007] A terahertz long-distance detection method based on equivalent time sampling technology, comprising:

[0008] The laser emitted by the femtosecond laser is split by a beam splitter, one beam is used as the pump laser and the other as the detection laser; the pump laser passes through an adjusted optical path delay line and is incident on the terahertz transmitting antenna, exciting a terahertz with a pulse width of the order of picoseconds and the same repetition frequency as the pump laser. The terahertz is incident on the receiving antenna after a long optical path, where the long optical path is the distance traveled by the light in the air during one pump laser pulse cycle; the detection laser passes through a scanning optical path delay line and is incident on the terahertz receiving antenna; from the beam splitter to the terahertz receiving antenna, the pump laser passes through a scanning optical path delay line and is incident on the terahertz receiving antenna. The pump optical path is the sum of the optical path of the pump laser part and the optical path of the terahertz optical path part, and the detection optical path is the optical path of the detection laser part; by adjusting the optical path delay line and the scanning optical path delay line, the optical path of the pump laser part in the pump optical path is made the same as the optical path of the detection laser part, and at this time, the pump optical path is at least longer than the detection optical path by the distance traveled by the light in the air within several pump laser pulse cycles; the detection pulse is made to lag the pump pulse by several pulses to start detection, and sampling is performed at each certain period of the detection pulse to achieve long-distance terahertz detection.

[0009] Furthermore, the femtosecond laser emits a laser with a pulse width of one hundred femtoseconds and a repetition frequency of one hundred megahertz.

[0010] Furthermore, the beam splitter is a free-space coated beam splitter or a fiber-coupled structure beam splitter.

[0011] Furthermore, the adjustable optical path delay line is a manual or electric adjustable optical path delay line, and the delayed optical path is greater than the distance traveled by light in the air within one laser pulse cycle, so as to achieve continuous detection of the long optical path of the terahertz part.

[0012] Furthermore, the scanning optical path delay line is an electric linear or rotational delay line, and the delayed optical path constitutes the scanning window width.

[0013] Furthermore, the terahertz transmitting antenna and receiving antenna are both photoconductive structures.

[0014] Furthermore, the optical path of the pump laser part is the optical path between the beam splitter and the transmitting antenna; the optical path of the detection laser part is the optical path between the beam splitter and the receiving antenna; the optical path of the terahertz part is the distance between the transmitting antenna and the receiving antenna, which is at least the distance traveled by the light in the air within one pump laser pulse cycle.

[0015] Furthermore, the detection is started by lagging the detection pulse by several pulses after the pump pulse, and sampling is performed at each certain period interval of the detection pulse. Specifically, the distance between the transmitting antenna and the receiving antenna is the distance traveled by the light in the air within i pump laser pulse periods, i≥1 and is an integer; therefore, when the first detection laser pulse is incident on the receiving antenna, the first terahertz pulse generated by the first pump laser pulse has not yet been incident on the receiving antenna; when the i-th detection laser pulse is incident on the receiving antenna, it meets the first terahertz pulse generated by the first pump laser pulse; then, with the scanning of the scanning optical path delay line, the detection pulse will start detection with a delay of i pulses after the pump pulse, and a sample is taken at each n period interval of the detection pulse, n≥1 and is an integer. Through this equivalent time sampling technology, long-distance terahertz detection is achieved.

[0016] The present invention has the following beneficial effects:

[0017] 1. The present invention uses the long optical path of the terahertz part, that is, the distance between the transmitting antenna and the receiving antenna is at least the distance that the light travels in the air within one pump laser pulse cycle, thus constituting the long distance required for detection.

[0018] 2. The present invention can realize long-distance terahertz detection by adopting an equivalent time sampling technology in which the detection pulse lags behind the pump pulse by one or several pulses.

[0019] 3. The present invention uses an adjustable optical path delay line, and the delayed optical path is slightly larger than the distance traveled by light in the air during one laser pulse cycle, so continuous detection of the long optical path of the terahertz part can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a schematic diagram of the equivalent time sampling principle.

[0021] Figure 2 It is a schematic diagram of long-distance detection of the terahertz time-domain spectrometer of the present invention.

[0022] Figure 3 This is a schematic diagram of the implementation of long-distance detection of the optical fiber-based terahertz time-domain spectrometer used in an embodiment of the present invention. DETAILED DESCRIPTION

[0023] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0024] like Figure 2As shown, a terahertz long-distance detection method based on equivalent time sampling technology includes: the laser emitted by a femtosecond laser is split by a beam splitter, one beam is used as a pump laser, and the other beam is used as a detection laser; the pump laser is incident on the terahertz transmitting antenna after adjusting the optical path delay line, and excites a terahertz with a pulse width of the order of picoseconds and the same repetition frequency as the pump laser, and the terahertz is transmitted through a long optical path of about three meters or a multiple of about three meters, that is, the long optical path is at least the distance traveled by the light in the air within a pump laser pulse cycle, and then incident on the receiving antenna; the detection laser is incident on the terahertz receiving antenna after scanning the optical path delay line; From the beam splitter to the terahertz receiving antenna, the pump optical path is the sum of the optical path of the pump laser part and the optical path of the terahertz optical path part, and the detection optical path is the optical path of the detection laser part; by adjusting the optical path delay line and the scanning optical path delay line, the optical path of the pump laser part in the pump optical path is made approximately the same as the optical path of the detection laser part. At this time, the pump optical path is longer than the detection optical path by at least the distance traveled by light in the air within one pump laser pulse cycle; in this way, the detection pulse will lag behind the pump pulse by one or several pulses to start detection, and a sample will be taken in a single cycle of the detection pulse or at intervals of several cycles. Through this equivalent time sampling technology, long-distance terahertz detection can be achieved.

[0025] Furthermore, the femtosecond laser emits a laser with a pulse width of about 100 femtoseconds and a repetition frequency of about 100 MHz.

[0026] Furthermore, the beam splitter is a free-space coated beam splitter or a fiber-coupled structure beam splitter.

[0027] Furthermore, the splitting ratio of the beam splitter is determined according to the attenuation of the two light paths after splitting. In the present invention, the splitting ratio of the beam splitter is 6:4, that is, the pump light path obtains 60% of the average laser power, and the detection light path obtains 40% of the average laser power.

[0028] Furthermore, the adjustable optical path delay line is a manual or electric adjustable optical path delay line, and the delayed optical path is slightly larger than the distance traveled by light in the air within one laser pulse cycle, so that continuous detection of the long optical path of the terahertz part can be achieved.

[0029] Furthermore, the scanning optical path delay line is an electric linear or rotational delay line, and the delayed optical path constitutes a scanning window width, which is determined according to the design of the delay line.

[0030] Furthermore, the terahertz transmitting antenna and receiving antenna are both photoconductive structures.

[0031] Furthermore, from the beam splitter to the terahertz receiving antenna, the pump optical path is the sum of the optical path of the pump laser part and the optical path of the terahertz optical path part, and the detection optical path is the optical path of the detection laser part.

[0032] Furthermore, the optical path of the pump laser part is the optical path from the beam splitter to the transmitting antenna.

[0033] Furthermore, the optical path of the detection laser part is the optical path from the beam splitter to the receiving antenna.

[0034] Furthermore, the optical path of the terahertz part is the distance between the transmitting antenna and the receiving antenna, which is at least the distance traveled by the light in the air within one pump laser pulse cycle.

[0035] Furthermore, the optical path of the pump laser part is approximately the same as the optical path of the detection laser part, that is, the pump optical path is longer than the detection optical path by the optical path of the terahertz part.

[0036] Furthermore, the detection pulse will lag the pump pulse by one or several pulses before starting detection, and a sample will be taken at a single cycle of the detection pulse or at intervals of several cycles. Through this equivalent time sampling technology, long-distance terahertz detection can be achieved.

[0037] Example

[0038] This embodiment is a terahertz long-distance detection method based on equivalent time sampling technology, which uses a terahertz time-domain spectrometer based on optical fiber to achieve terahertz long-distance detection, such as Figure 3 As shown, the optical fiber terahertz time-domain spectrometer includes a femtosecond laser, a beam splitter, a scanning optical path delay line, an adjustment optical path delay line, a transmitting antenna and a receiving antenna, and the connection between the optical fibers is in the form of a flange connection.

[0039] 1. Laser

[0040] The femtosecond laser in this embodiment is a fiber laser. The femtosecond laser emits a laser with a pulse width of about 100 femtoseconds and a repetition frequency of about 100 MHz. The laser is split into the following beams by a beam splitter: Figure 1 The pulse width of the pump laser and the detection laser incident on the transmitting antenna and the receiving antenna corresponds to Figure 1 Δt=100fs, repetition frequency is 100MHz, corresponding to Figure 1 The pulse period in is T = 10ns. The distance traveled by light in the air during one pulse period is the speed of light in a vacuum multiplied by the pulse period T, which is 3m. Figure 1 The n in it is a positive integer, indicating the number of cycles.

[0041] The central wavelength of the laser is 1560nm, which matches the wavelength required by the transmitting and receiving antennas.

[0042] 2. Beam splitter

[0043] In this embodiment, the beam splitter is a fiber-coupled beam splitter. The laser output, i.e., the pulse sequence before beam splitting, is set to Figure 2 1, 2, 3, 4, 5... in it.

[0044] The pulse sequence after beam splitting is Figure 1 The pulse sequence of the pump laser and the probe laser is 1, 2, 3, 4, 5, etc., where the frequency of the terahertz is the same as the frequency of the pump laser, that is, Figure 1 1, 2, 3, 4, 5... in it.

[0045] The pulse sequence after beam splitting is also Figure 2 The sequence 1 incident on the transmitting antenna and the receiving antenna,

[0046] 2, 3, 4, 5..., where the frequency of the terahertz is the same as the frequency of the pump laser, that is, Figure 2 1, 2, 3, 4, 5... in it.

[0047] In this embodiment, the distance between the transmitting antenna and the receiving antenna is the distance that light travels in the air during one pump laser pulse cycle, which constitutes the long distance required for detection. Figure 1 When the first detection laser pulse is incident on the receiving antenna, the first terahertz pulse generated by the first pump laser pulse has not yet been incident on the receiving antenna; when the second detection laser pulse is incident on the receiving antenna, it meets the first terahertz pulse generated by the first pump laser pulse. Figure 1 The detection pulse will start detection with a pulse lagging behind the pump pulse, and a sample will be taken at a single cycle T of the detection pulse or at intervals of several cycles nT. Through this equivalent time sampling technology, long-distance terahertz detection can be achieved.

[0048] Furthermore, if the distance between the transmitting antenna and the receiving antenna is the distance that light travels in the air during two, three, or even more pump laser pulse cycles, it constitutes the long distance required for detection. Figure 1 The a, b, c, d, e... in the detection laser pulse sequence will become 3, 4, 5, 6, 7..., 4, 5, 6, 7, 8..., and other delayed detection laser sequences.

[0049] 3. Beam splitter ratio

[0050] In this embodiment, the splitting ratio of the beam splitter is determined to be 6:4 according to the device losses of the subsequent pump optical path and detection optical path of the beam splitter, that is, the pump optical path obtains 60% of the average laser power and the detection optical path obtains 40% of the average laser power.

[0051] 4. Adjust the delay path of the optical path delay line

[0052] In this embodiment, the optical path delay line is manually adjusted, and the delayed optical path is slightly greater than the 3m distance that light travels in the air within a laser pulse cycle, and 4m is taken here, so that continuous detection of the long optical path of the terahertz part can be achieved. That is, in theory, any length of the optical path of the terahertz part greater than 3m can be used for terahertz detection. At the same time, short distances within 3m can still be detected, and at this time the terahertz time-domain spectrometer structure transitions to detecting laser pulses to collect the terahertz situation generated by the same pump pulse.

[0053] On the contrary, if the delay optical path of the optical path delay line is adjusted to be less than the 3m distance that light travels in the air during a laser pulse cycle, such as 2m. Then, when the optical path of the terahertz part is 3m, 1m will not be detected due to the limited distance of the optical path delay line. When the optical path of the terahertz part is 6m, 1m will not be detected in the first 3m and 1m will not be detected in the second 3m.

[0054] 5. Scanning optical path delay line scanning window

[0055] In this embodiment, the scanning optical path delay line is a rotating delay line, and the delayed optical path constitutes the scanning window width, which is designed to be 100 ps.

[0056] 6. Terahertz transmitting and receiving antenna structure

[0057] In this embodiment, the terahertz transmitting antenna and the receiving antenna are both photoconductive structures, and are both optical fiber coupled.

[0058] 7. Pump optical path and detection optical path

[0059] In this embodiment, from the beam splitter to the terahertz receiving antenna, the pump optical path is the sum of the optical path of the pump laser part and the optical path of the terahertz optical path part, and the detection optical path is the optical path of the detection laser part. The optical path part is a hybrid structure of free space and optical fiber.

[0060] 8. Optical path of the pump laser part

[0061] In this embodiment, the optical path of the pump laser part is the optical path from the beam splitter to the transmitting antenna, including air and the refractive index part, and the total optical path is 6m.

[0062] 9. Optical path of the detection laser part

[0063] In this embodiment, the optical path of the detection laser part is the optical path from the beam splitter to the receiving antenna, including air and the refractive index part, and the total optical path is 6m.

[0064] 10. Optical path of the terahertz part

[0065] In this embodiment, the optical path of the terahertz part is the distance between the transmitting antenna and the receiving antenna, which is 3 m, the distance traveled by the light in the air during one pump laser pulse cycle, constituting the long distance required for detection.

[0066] 11. Optical path of pump laser and detection part

[0067] In this embodiment, the optical path of the pump laser part is approximately the same as the optical path of the detection laser part.

[0068] 12. Relationship between optical paths of various parts

[0069] In this embodiment, the pump optical path is longer than the detection optical path by 3 m of the terahertz part.

[0070] 13. Equivalent Sampling Technology

[0071] like Figure 1 As shown, according to the above values, the detection pulse will lag the pump pulse by one pulse to start detection, and a sample will be taken at a single cycle of the detection pulse or at intervals of several cycles. Through this equivalent time sampling technology, long-distance terahertz detection can be achieved.

Claims

1. A terahertz long-distance detection method based on equivalent time sampling technology, characterized in that: include: The laser emitted by the femtosecond laser is split by a beam splitter, one beam is used as the pump laser and the other as the detection laser; the pump laser passes through an adjusted optical path delay line and is incident on the terahertz transmitting antenna, exciting a terahertz with a pulse width of the order of picoseconds and the same repetition frequency as the pump laser. The terahertz is incident on the receiving antenna after a long optical path, where the long optical path is the distance traveled by the light in the air during one pump laser pulse cycle; the detection laser passes through a scanning optical path delay line and is incident on the terahertz receiving antenna; from the beam splitter to the terahertz receiving antenna, the pump laser passes through a scanning optical path delay line and is incident on the terahertz receiving antenna. The pump optical path is the sum of the optical path of the pump laser part and the optical path of the terahertz optical path part, and the detection optical path is the optical path of the detection laser part; by adjusting the optical path delay line and the scanning optical path delay line, the optical path of the pump laser part in the pump optical path is made the same as the optical path of the detection laser part, and at this time, the pump optical path is at least longer than the detection optical path than the distance traveled by the light in the air within several pump laser pulse cycles; the detection pulse is made to lag the pump pulse by several pulses to start detection, and sampling is performed at each certain interval of the detection pulse to achieve long-distance terahertz detection; The optical path of the pump laser part is the optical path from the beam splitter to the transmitting antenna; the optical path of the detection laser part is the optical path from the beam splitter to the receiving antenna; the optical path of the terahertz part is the distance between the transmitting antenna and the receiving antenna, which is at least the distance traveled by the light in the air during one pump laser pulse cycle; The method of making the detection pulse lag the pump pulse by several pulses to start detection, and performing sampling at each certain interval of the detection pulse is specifically as follows: the distance between the transmitting antenna and the receiving antenna is the distance traveled by the light in the air within the i-th pump laser pulse period, i ≥ 1 and is an integer; therefore, when the first detection laser pulse is incident on the receiving antenna, the first terahertz pulse generated by the first pump laser pulse has not yet been incident on the receiving antenna; when the i-th detection laser pulse is incident on the receiving antenna, it meets the first terahertz pulse generated by the first pump laser pulse; Next, as the optical path delay line is scanned, the detection pulse will start detection with a delay of i pulses from the pump pulse, and a sample will be taken at every n cycles of the detection pulse, where n ≥ 1 and is an integer. Through this equivalent time sampling technology, long-distance terahertz detection can be achieved.

2. The terahertz long-distance detection method based on equivalent time sampling technology as claimed in claim 1, characterized in that: The femtosecond laser emits laser light with a pulse width of 100 femtoseconds and a repetition frequency of 100 MHz.

3. The terahertz long-distance detection method based on equivalent time sampling technology as claimed in claim 1, characterized in that: The beam splitter is a free-space coated beam splitter or a fiber-coupled structure beam splitter.

4. The terahertz long-distance detection method based on equivalent time sampling technology as claimed in claim 1, characterized in that: The adjustable optical path delay line is a manual or electric adjustable optical path delay line, and the delayed optical path is greater than the distance traveled by light in the air within a laser pulse cycle, so as to realize continuous detection of the long optical path of the terahertz part.

5. The terahertz long-distance detection method based on equivalent time sampling technology as claimed in claim 1, characterized in that: The scanning optical path delay line is an electric linear or rotary delay line, and the delayed optical path constitutes the scanning window width.

6. The terahertz long-distance detection method based on equivalent time sampling technology as claimed in claim 1, characterized in that: The terahertz transmitting antenna and receiving antenna are both photoconductive structures.

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