CN205719255U - Terahertz time-domain spectroscopy radiation and detection device - Google Patents

Abstract

The utility model relates to a terahertz time-domain spectral radiation and detection device. The femtosecond laser radiated by the femtosecond laser in the terahertz time-domain spectral radiation and detection device is divided into pump light and probe light by the beam splitter, and the pump light and probe light are transmitted in the hollow photonic crystal fiber. In the above-mentioned terahertz time-domain spectral radiation and detection device, by reasonably selecting the pulse center wavelength output by the femtosecond laser, the second-order fiber dispersion generated when the laser pulse is transmitted in the hollow-core photonic crystal fiber is negative. , beam splitter and other positive dispersion neutralize each other. It avoids the inconvenience and expense of grating or a pair of prisms to compensate dispersion in traditional solid fiber. Secondly, because the pulse is basically transmitted in the hollow, for a pulse with a pulse width of tens of femtoseconds, its energy can be as high as hundreds of nanojoules, and there will be no pulse broadening due to self-phase modulation, which provides a great opportunity for terahertz radiation devices and detection devices High-energy laser pulses.

Description

Terahertz time-domain spectral radiation and detection device

technical field

The utility model relates to the technical field of terahertz, in particular to a terahertz time-domain spectral radiation and detection device.

Background technique

The terahertz (THz, 1THz=10 ¹² Hz) frequency band refers to the electromagnetic radiation range with a frequency from 0.1 THz to 10 THz and a wavelength between microwave and infrared light. Due to its short time scale, terahertz radiation can provide the ability of ultrafast time-resolved spectroscopy. It can be used to penetrate some materials such as organisms, dielectric materials, and gas phase substances. It can be obtained by analyzing the transflective terahertz signal of the sample material. Information about the composition of the material, as well as its physical, chemical, biological state, etc. Also because of the wide frequency band of terahertz wave and the small photon energy, it will not cause damage to the detection material, so terahertz technology can be applied to many fields such as imaging, spectral analysis, nondestructive testing and high-speed wireless communication.

In order to miniaturize and enhance the stability of the terahertz spectrometer system, the method of optical fiber transmission is generally adopted. However, the traditional solid fiber technology must use a grating or a pair of prisms for dispersion compensation due to dispersion problems; secondly, the pulse broadening due to self-phase modulation limits the highest pulse energy used in terahertz radiation devices and detection devices.

Utility model content

Based on this, it is necessary to provide a terahertz time-domain spectral radiation and detection device with dispersion compensation and high laser pulse transmission efficiency to solve the above problems.

A terahertz time-domain spectral radiation and detection device, including a femtosecond laser, a beam splitter, a sample stage for placing a sample to be measured, an information processing device, a first optical path component and a second optical path component, the femtosecond laser The radiated femtosecond laser is divided into pump light and probe light by the beam splitter; wherein,

The first optical path assembly includes a first optical fiber coupler, a first optical fiber, and a terahertz emitting device;

The second optical path assembly includes a second optical fiber coupler, a second optical fiber and a terahertz detection device;

The pump light generates a terahertz pulse through the first optical path assembly, and the terahertz pulse is received by the terahertz detection device after being irradiated on the sample to be tested; the detection light is coupled through the second optical fiber The detector and the second optical fiber irradiate the terahertz detection device to generate a detection signal, and the detection signal is transmitted to the information processing device for further signal processing;

Both the first optical fiber and the second optical fiber are hollow-core photonic crystal fibers.

In one of the embodiments, the hollow photonic crystal fiber comprises a hollow core and a cladding surrounding the hollow core with an array of cylindrical holes; the diameter of the hollow core is greater than or equal to the diameter of the cylindrical holes.

In one of the embodiments, the ratio of the diameter of the hollow core to the diameter of the cylindrical hole is in the range of 1-4.

In one of the embodiments, the cross section of the cylindrical hole is one of circular, hexagonal or hexagonal.

In one of the embodiments, the terahertz time-domain spectral radiation and detection device further includes an optical isolator, and the optical isolator is placed between the femtosecond laser and the beam splitter.

In one of the embodiments, the first optical path assembly further includes a silicon chip, which is used to filter out stray light and only allow the terahertz pulse to pass through; the silicon chip is arranged between the terahertz emitting device and the between the sample stages.

In one of the embodiments, the first optical path assembly further includes a first half-wave plate for adjusting the polarization direction of the femtosecond laser; the first half-wave plate is arranged on the between the beamsplitter and the first fiber coupler.

In one embodiment, the terahertz emitting device is one of an active photoconductive antenna or a passive frequency doubling crystal.

In one of the embodiments, the second optical path assembly further includes a delay line device along the propagation direction of the probe light, a chopper, and a second half-wave plate; the chopper is used for the The information processing device provides modulation frequency; the second half-wave plate is used to adjust the polarization direction of the detection light.

In one embodiment, the femtosecond laser is a titanium-sapphire laser that radiates laser pulses of 65 femtoseconds, and the pulse center wavelength ranges from 768 to 788 nanometers.

In the above terahertz time-domain spectral radiation and detection device, the laser output from the femtosecond laser is coupled to the hollow-core photonic crystal fiber through the fiber coupler, so that the second-order fiber dispersion generated when the laser pulse is transmitted in the hollow-core photonic crystal fiber is negative The value is neutralized with the positive dispersion generated by optical isolators, beam splitters, etc. In this way, the inconvenience and expense of gratings or a pair of prisms to compensate for dispersion in traditional solid fibers can be avoided. Secondly, because the pulse is basically transmitted in the hollow, for a pulse with a pulse width of tens of femtoseconds, its energy can be as high as hundreds of nanojoules, and there will be no pulse broadening due to self-phase modulation, which provides a great opportunity for terahertz radiation devices and detection devices High-energy laser pulses.

Description of drawings

Figure 1 is an optical path diagram of a terahertz time-domain spectroscopy detection device;

Fig. 2 is a schematic cross-sectional view of a hollow-core photonic crystal fiber under a scanning electron microscope.

Symbols in the figure: femtosecond laser 1, reflective silver mirror 2, 3, 14, optical isolator 4, beam splitter 5, first half-wave plate 6, first fiber coupler 7, first optical fiber 8, Terahertz emission device 9, silicon wafer 10, sample stage 11, delay line device 12, chopper 13, second half-wave plate 15, second fiber coupler 16, second optical fiber 17, terahertz detection device 18 and information processing device 19.

detailed description

In order to facilitate the understanding of the utility model, the utility model will be described more fully below with reference to the relevant drawings. Preferred embodiments of the utility model are provided in the accompanying drawings. However, the invention can be embodied in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present utility model more thorough and comprehensive.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the technical field of this invention. The terminology used in the description of the utility model herein is only for the purpose of describing specific embodiments, and is not intended to limit the utility model. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

As shown in Figure 1, it is an optical path diagram of a terahertz time-domain spectral radiation and detection device, which includes a femtosecond laser 1, reflective silver mirrors 2, 3, an optical isolator 4, a beam splitter 5, and A sample stage 11 for placing the sample to be tested, an information processing device 19, and a first optical path assembly and a second optical path assembly. The femtosecond laser radiated by the femtosecond laser 1 is divided into pump light and probe light by the beam splitter 5 . Wherein, the first optical path assembly includes a first half-wave plate 6 , a first optical fiber coupler 7 , a first optical fiber 8 , a terahertz emission device 9 , and a silicon chip 10 along the transmission direction of the pumping light. The second optical path assembly includes a delay line device 12 , a chopper 13 , a second half-wave plate 15 , a second fiber coupler 16 , a second optical fiber 17 , and a terahertz detection device 18 along the transmission direction of the detection light. The pump light passes through the first optical path component to generate a terahertz pulse, and the terahertz pulse is irradiated on the sample and is received by the terahertz detection device 18; the detection light is irradiated on the terahertz detection device 18 to generate a detection signal, and the detection signal is transmitted to the information processing device 19 for further signal processing, wherein the first optical fiber 8 and the second optical fiber 17 are both hollow-core photonic crystal fibers.

The specific working process is as follows: the femtosecond laser 1 is a titanium-sapphire laser, which emits 65 femtosecond laser pulses, and the pulse center wavelength ranges from 768 to 788 nanometers. In the present embodiment, the pulse center wavelength is 778nm, and when the pulse center wavelength is 778nm, the second-order fiber dispersion generated when the laser pulse is transmitted in the hollow photonic crystal fiber is negative, so that the optical isolator 4 and the beam splitter The positive dispersion produced by the 5th class neutralizes each other, and the effect of neutralizing the negative dispersion and positive dispersion is the best. In this way, the inconvenience and expense of grating or a pair of prisms to compensate for dispersion can be avoided in traditional solid optical fibers, the optical path is simplified, and the cost is saved. In other embodiments, the pulse center wavelength can also fluctuate 10 nm up and down from 778 nm, and the pulse center wavelength can be adjusted according to a specific optical path.

In this embodiment, due to space limitations, several reflective silver mirrors 2, 3, 14 are arranged in the entire optical path to turn the corresponding optical path. In other embodiments, according to the size of the space, the number and position of the reflecting silver mirrors can be set according to actual needs.

The laser second pulse is reflected by the reflective silver mirrors 2 and 3 into the optical isolator 4. The optical isolator 4 is mainly to prevent the influence of the reflected laser pulse in the subsequent optical path on the mode locking of the femtosecond laser, and can also calibrate the entire optical path and security protection. In this embodiment, the optical isolator 4 is arranged between the reflective silver mirror 3 and the beam splitter mirror 5 . Since the pulse is transmitted by a hollow-core photonic crystal fiber, the reflection of the pulse is very weak, so in other embodiments, the optical isolator 4 can be omitted.

The laser second pulse after passing through the optical isolator 4 is divided by the beam splitter 5 into pump light and probe light, the pump light is used to generate terahertz pulses, and the probe light is used to detect terahertz pulses. The pump light passes through the first half-wave plate 6, and the first half-wave plate 6 is used to change the polarization direction of the laser pulse. Next, the pump light is coupled into the first optical fiber (hollow photonic crystal fiber) 8 by the first fiber coupler 7, and the laser pulse transmitted in the first optical fiber (hollow photonic crystal fiber) 8 is irradiated in the terahertz emission device 9, generating Terahertz pulses.

The terahertz emitting device 9 is one of a terahertz emitting device with a photoconductive antenna or a passive nonlinear optical rectifying crystal. In this embodiment, the terahertz emitting device 9 is an active photoconductive antenna, and laser pulses are irradiated on the photoconductive antenna to generate photogenerated carriers, which are driven by the electric field applied by the DC bias voltage to radiate terahertz pulses. Wherein, the information processing device 19 provides a DC bias voltage for the terahertz transmitting device 9 . At the same time, the pump light passes through the first half-wave plate 6 so that the polarization direction of the pump light is perpendicular to the metal groove of the photoconductive antenna.

In other embodiments, the terahertz emission device can also be a combination of a focusing lens and a frequency-doubling crystal. After the pump light is focused by the focusing lens, it radiates on the frequency-doubling crystal to generate frequency-doubling light, which is superimposed with the fundamental frequency light. A two-color field is formed. After the femtosecond pulse light is focused, the ionized air forms a plasma filament. Due to the asymmetry of the electric field of the two-color field, the photogenerated carriers are driven to radiate terahertz pulses, and the frequency-doubling crystal can be a barium metaborate crystal ( BBO).

The terahertz pulse passes through the silicon chip 10 to filter out the laser pulses of other frequencies and only passes through the terahertz pulse, wherein the silicon chip 10 is a hyper-hemispherical silicon substrate lens with high resistivity. After the filtered terahertz pulse is collimated and expanded, it is irradiated on the sample, and is received by the terahertz detection device 18 through the sample.

Probe light passes through delay line device 12, and delay line device 12 comprises optical delay line control device and optical delay line, and information processing device 19 controls optical delay line (grating ruler) to move by optical delay line control device (voice coil motor), and delay line The device 12 detects the terahertz pulse point by point in space by using a step-scan method to obtain a terahertz time-domain waveform. The detection light processed by the delay line device 12 provides a modulation frequency for the following terahertz detection device 18 through the chopper 13 . After passing through the second half-wave plate 15, it is coupled into the second optical fiber (hollow-core photonic crystal fiber) 17 by the second fiber coupler 16 for transmission.

Hollow-core photonic crystal fibers 8 and 17 have high damage threshold and high coupling efficiency. Laser pulses are basically transmitted in the hollow center of hollow-core photonic crystal fibers. Self-phase modulation results in pulse broadening. This is several orders of magnitude higher than the 0.1 nanojoule pulse energy of traditional solid fibers. The hollow-core photonic crystal fiber can conduct high-energy laser pulses for terahertz generation and detection, thereby improving the dynamic range of the whole machine.

The probe light transmitted from the second optical fiber 17 is irradiated in the terahertz detection device 18 . In the terahertz detection device 18, the detection light generates a weak current signal under the excitation of the terahertz pulse. The change in the strength of this weak current signal can reflect the change in the strength of the terahertz field, and the terahertz time-domain spectrum can be deduced through the amplification circuit and calculation program in the post-information processing device 19 . By comparing the difference between the terahertz spectrum with and without the sample, some information such as the characteristic parameters and internal structure of the sample can be obtained, so as to realize some practical applications such as non-destructive detection of special materials, pharmaceutical testing and medical diagnosis. This set of devices is also the basic structural diagram of the terahertz time-domain microscope, with a resolution of tens of microns.

FIG. 2 is a schematic cross-sectional view of a hollow-core photonic crystal fiber under a scanning electron microscope. A hollow-core photonic crystal fiber (Hollow-core PCF, HC-PCF) is a common bandgap photonic crystal fiber. The hollow-core photonic crystal fiber includes a hollow core 11 , a glass microcapillary cladding and an outermost cladding 15 surrounding a cylindrical hole 13 array around the hollow core. Hollow-core photonic crystal fiber (HC-PCF) is a hollow core surrounded by a cladding of glass microcapillaries and creates a photonic band gap (PBG) that traps electromagnetic waves in a specific optical wavelength range in the core defect (core defect) middle. Wherein, the apertures of the hollow core 11 and the cylindrical hole 13 in the hollow photonic crystal fiber can be uniform, and the diameter D1 of the hollow core 11 can also be greater than the diameter D2 of the cylindrical hole 13, wherein the diameter D1 of the hollow core 11 and the diameter D1 of the cylindrical hole 13 The ratio of the diameter D2 ranges from 1 to 4. In this embodiment, the ratio of the diameter D1 of the hollow core 11 to the diameter D2 of the cylindrical hole 13 is about 3:1, wherein the diameter of the hollow photonic crystal fiber is about 40 microns, and the diameter D1 of the hollow core 11 is about 8 microns , the wall thickness of the hollow core 11 is about 76 nanometers; the diameter D2 of the cylindrical hole 13 is about 2.33 microns, and the wall thickness of the cylindrical hole 12 is about 70 nanometers. In this embodiment, the cross-section of the cylindrical holes 13 is a regular hexagon, that is, the hollow core 11 is surrounded by a cladding of cylindrical holes 13 in a regular hexagonal array. In other embodiments, the cross-section of the cylindrical hole 13 is circular, hexagram-shaped or the like.

In this embodiment, the ratio of hollow core to glass in the surrounding cladding is about 92:8, so that 99% of the laser pulse energy can be transmitted in the hollow core, and only about 1% is transmitted in the glass fiber. The contribution of nonlinear effects due to glass fibers is basically ignored. At the same time, the large-scale hollow design of the hollow core further improves the transmission rate of the laser pulse energy in the air, thereby weakening the phase shift and group velocity dispersion caused by the nonlinear Kerr effect. The numerical aperture of the hollow-core photonic crystal fiber is 0.15, and the throughput ranges between 40% and 50%. The transmission efficiency of the fiber is limited by the imperfect coupling at the interface and its intrinsic attenuation, which is about 1.5dB/m. The drawing of the hollow-core photonic crystal fiber is carried out in a dust filter and dry air environment, which can prevent dust and water vapor from penetrating into the fiber, resulting in deterioration of the performance of the hollow-core photonic crystal fiber and attenuation of transmission efficiency.

In the above-mentioned terahertz time-domain spectral radiation and detection device, by reasonably selecting the central wavelength of the pulse output by the femtosecond laser, the second-order fiber dispersion generated when the laser pulse is transmitted in the hollow-core photonic crystal fiber is negative. The positive dispersion produced by beam splitters, lenses, etc. neutralizes each other. In this way, the inconvenience and expense of gratings or a pair of prisms to compensate for dispersion in traditional solid fibers can be avoided. Secondly, since the laser pulse is basically transmitted in the hollow core, for a pulse with a pulse width of 100 femtoseconds, its energy can be as high as several hundred nanojoules without causing pulse broadening due to self-phase modulation. This is several orders of magnitude higher than the 0.1 nanojoule pulse energy of traditional solid fibers.

The technical features of the above-mentioned embodiments can be combined arbitrarily. To make the description concise, all possible combinations of the technical features in the above-mentioned embodiments are not described. However, as long as there is no contradiction in the combination of these technical features, should be considered as within the scope of this specification.

The above-mentioned embodiments only express several implementation modes of the utility model, and the description thereof is relatively specific and detailed, but it should not be understood as limiting the scope of the utility model patent. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the scope of protection of the utility model patent should be based on the appended claims.

Claims (10)

1. A terahertz time-domain spectral radiation and detection device, comprising a femtosecond laser, a beam splitter, a sample stage for placing a sample to be measured, an information processing device, a first optical path assembly and a second optical path assembly, the femtosecond The femtosecond laser radiated by the second laser divides the femtosecond laser into pump light and probe light through the beam splitter; it is characterized in that, The first optical path assembly includes a first optical fiber coupler, a first optical fiber, and a terahertz emitting device; The second optical path assembly includes a second optical fiber coupler, a second optical fiber and a terahertz detection device; The pump light generates a terahertz pulse through the first optical path assembly, and the terahertz pulse is received by the terahertz detection device after being irradiated on the sample to be tested; the detection light is coupled through the second optical fiber The detector and the second optical fiber irradiate the terahertz detection device to generate a detection signal, and the detection signal is transmitted to the information processing device for further signal processing; Both the first optical fiber and the second optical fiber are hollow-core photonic crystal fibers. 2. The terahertz time-domain spectral radiation and detection device according to claim 1, wherein the hollow-core photonic crystal fiber comprises a hollow core and a cladding of a cylindrical hole array surrounding the hollow core; the hollow core The diameter is greater than or equal to the diameter of the cylindrical hole. 3 . The terahertz time-domain spectral radiation and detection device according to claim 2 , wherein the ratio of the diameter of the hollow core to the diameter of the cylindrical hole is in the range of 1-4. 4 . 4. The terahertz time-domain spectral radiation and detection device according to claim 2, wherein the cross section of the cylindrical hole is one of circular, hexagonal or hexagonal. 5. The terahertz time-domain spectral radiation and detection device according to claim 1, characterized in that, the terahertz time-domain spectral radiation and detection device further comprises an optical isolator, and the optical isolator is placed on the Between the femtosecond laser and the beamsplitter. 6. The terahertz time-domain spectral radiation and detection device according to claim 1, characterized in that, the first optical path assembly further includes a silicon chip for filtering out stray light, and only allowing the terahertz pulse to transmit through; the silicon wafer is disposed between the terahertz emission device and the sample stage. 7. The terahertz time-domain spectral radiation and detection device according to claim 1, wherein the first optical path assembly also includes a first half-wave plate for adjusting the femtosecond laser Polarization direction: the first half-wave plate is arranged between the beam splitter and the first fiber coupler. 8. The terahertz time-domain spectral radiation and detection device according to claim 1, wherein the terahertz emitting device is one of an active photoconductive antenna or a passive frequency doubling crystal. 9. The terahertz time-domain spectral radiation and detection device according to claim 1, characterized in that, the second optical path assembly further includes a delay line device along the propagation direction of the probe light, a chopper, a second a half-wave plate; the chopper is used to provide modulation frequency for the information processing device; the second half-wave plate is used to adjust the polarization direction of the detection light. 10. The terahertz time-domain spectral radiation and detection device according to claim 1, wherein the femtosecond laser is a titanium-sapphire laser that radiates 65 femtosecond laser pulses, and the pulse center wavelength ranges from 768 to 788 nm.