WO2018214158A1

WO2018214158A1 - Self-referencing terahertz electro-optic sampling spectral interferometer and measurement system

Info

Publication number: WO2018214158A1
Application number: PCT/CN2017/086137
Authority: WO
Inventors: 徐世祥; 郑水钦; 刘俊敏; 蔡懿; 曾选科; 上官煌城
Original assignee: 深圳大学
Priority date: 2017-05-26
Filing date: 2017-05-26
Publication date: 2018-11-29

Abstract

A self-referencing terahertz electro-optic sampling spectral interferometer and measurement system. The self-referencing terahertz electro-optic sampling spectral interferometer comprises a pulse stretcher (102), a broadband half-wave plate (103), lenses (106, 110), a terahertz electro-optic crystal (109), a birefringent crystal (111), a linear polarizer (112), and a spectrometer (113). The optical elements have a smart structural design related to self-referencing interference, and chirped pulses are employed, on the basis of electro-optic sampling and the spectral interference principle, as a probe to complete a single measurement of high-intensity terahertz time-domain spectroscopy.

Description

Self-reference terahertz electro-optical sampling spectral interferometer and measuring system

Technical field

The invention belongs to the technical field of terahertz measurement, and in particular relates to a self-referencing terahertz electro-optical sampling spectral interferometer and a measuring system.

Background technique

Terahertz waves have unique properties such as high transparency, high security, and high spectral resolution, so they have important academic value and application potential. With the continuous advancement of terahertz optical technology, the intensity of terahertz signal sources has increased, resulting in terahertz strong fields and nonlinear spectroscopy. However, since the terahertz pulse intensity is already strong enough, the conventional two techniques of conventional terahertz photoconductive sampling and electro-optic sampling are no longer applicable. The conventional technology cannot effectively measure strong terahertz pulses with high signal-to-noise ratio.

Summary of the invention

The invention provides a self-referencing terahertz electro-optical sampling spectral interferometer aiming at solving the problem that a strong terahertz pulse with high signal to noise ratio cannot be effectively measured.

In order to solve the above technical problem, the present invention is achieved by the present invention. The present invention provides a self-referencing terahertz electro-optic sampling spectral interferometer, the interferometer comprising:

a pulse stretcher for widening the incident detection pulse into a chirp pulse, the probe pulse having the widened pupil pulse incident on the broadband half wave plate;

The broadband half-wave plate is configured to adjust a polarization direction of the probe pulse that has been widened into a chirped pulse, and the detection pulse of the adjusted polarization direction is incident on the lens;

The lens for focusing the detection pulse of the adjusted polarization direction to a terahertz electro-optical crystal;

The terahertz electro-optic crystal is configured to cause the detecting pulse to coincide with an incident terahertz pulse to be loaded, so as to load a terahertz field strength signal of the terahertz pulse to be detected into the detecting pulse, a detection pulse carrying a terahertz field strength signal is incident on the birefringent crystal;

The birefringent crystal is configured to generate a pulse pair that is loaded with a terahertz field strength signal to generate time delays, which are an o light pulse and an e light pulse, respectively, and the o light pulse and the e light pulse The polarization directions are perpendicular to each other, and the pulse pair is incident on the linear polarizer;

The linear polarizer is configured to transmit half of the o-optical pulse and the e-optical pulse of the pulse pair to the spectrometer, and the pulse passing through the linear polarizer is consistent with the polarization direction;

The spectrometer is configured to record spectral interference data of a pulse pair after the linear polarizer to implement terahertz electro-optic phase modulation measurement based on the spectral interference data.

Further, the interferometer further includes a spectral phase coherent electric field reconstruction method (SPIDER) technology measuring device, configured to measure a time spectrum characteristic of the detecting pulse before the detecting pulse is incident on the stretcher, to obtain the The time spectral characteristics of the probe pulse.

Further, the interferometer further includes a silicon wafer; the silicon wafer is interposed between the lens and the terahertz electro-optic crystal for transmitting the incident terahertz pulse to be measured to a low loss condition A terahertz electro-optical crystal is described, and a detection pulse emitted from the lens is reflected to the terahertz electro-optical crystal.

Further, the terahertz pulse to be measured incident to the terahertz electro-optical crystal coincides with the detection pulse in time and space.

Further, the mutual time delay of the pair of pulses generated by the birefringent crystal is proportional to the refractive index difference of the o-optical pulse and the e-optical pulse of the birefringent crystal and its thickness.

Further, the detection pulse is a linearly polarized ultrashort femtosecond pulse.

Further, the pulse stretcher is specifically configured to broaden the incident detection pulse into a picosecond pulse.

Further, the transmission axis of the linear polarizer is at 45° to the polarization direction of the o-light pulse and the e-light pulse of the pulse pair.

The present invention also provides a self-referencing terahertz electro-optical sampling spectral interferometry system comprising all of the components in the self-referencing terahertz electro-optic sampling spectrometer described above, and the self-referencing terahertz electro-optical sampling spectral interferometer Has the function.

Compared with the prior art, the invention has the following advantages:

The present invention provides a self-referencing terahertz electro-optic sampling spectral interferometer comprising a pulse stretcher, a broadband half-wave plate, a lens, a terahertz electro-optic crystal, a birefringent crystal, a linear polarizer, and a spectrometer. The above optical components are designed by a clever self-reference interference structure. Based on the principle of electro-optical sampling and spectral interference, a single measurement of the high-intensity terahertz pulse time-domain spectrum is realized by using a chirped pulse as a probe, which improves the strongness. The terahertz pulse measures the signal-to-noise ratio in a single pass.

DRAWINGS

FIG. 1 is a schematic structural diagram of a self-referencing terahertz electro-optical sampling spectral interferometer according to an embodiment of the present invention.

detailed description

The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It is understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

As a first embodiment of the present invention, as shown in FIG. 1, the present invention provides a self-referencing terahertz electro-optic sampling spectral interferometer, the interferometer comprising a pulse stretcher 102, a broadband half-wave plate 103, and a first A lens 106, a terahertz electro-optic crystal 109, a birefringent crystal 111, a linear polarizer 112, and a spectrometer 113. In addition, as shown in FIG. 1 , THz pulse represents a terahertz pulse, and Probe pulse represents a detection pulse. The interferometer provided in this embodiment further includes a measurement device SPIDER 101 based on a spectral phase coherent electric field reconstruction technique, a second lens 110 , A silicon wafer 108, a mirror 104/105 for reflection, and an off-axis parabolic mirror (OAPM) 107.

The pulse stretcher 102 is configured to broaden the incident probe pulse Probe pulse into a chirp pulse, and the probe pulse which has been broadened into a chirp pulse is incident on the broadband half-wave plate 103. In the present embodiment, the probe pulse Probe pulse is a polarized ultrashort femtosecond pulse, and the pulse stretcher 102 broadens the incident probe pulse to a picosecond pulse.

The broadband half-wave plate 103 is for adjusting the polarization direction of the probe pulse which has been broadened into a chirped pulse, and the detection pulse of the adjusted polarization direction is incident on the lens 106.

a lens 106 for focusing the above-mentioned adjusted polarization direction of the detection pulse to the terahertz electro-optic crystal 109;

The terahertz electro-optic crystal 109 is configured to temporally and spatially coincide with the incident THz pulse to be measured in the terahertz electro-optic crystal 109 to achieve the terahertz field of the THz pulse to be measured. The strong signal is loaded into the detection pulse, that is, electro-optic modulation is realized, and the detection pulse loaded with the terahertz field strength signal is incident on the birefringent crystal 111. The axial surface of the terahertz electro-optic crystal 109 is parallel to the incident surface. The THz pulse to be measured is focused by the off-axis parabolic mirror OAPM107, and then incident on the terahertz electro-optical crystal 109. The off-axis parabolic mirror 107 is mainly used for focusing the terahertz pulse.

The birefringent crystal 111 is configured to generate the above-described probe pulses loaded with the terahertz field strength signal to generate o-light pulses and e-light pulses that are time-delayed with each other, and the o-light pulses and the e-light pulses are a pair of pulse pairs. o The polarization directions of the light pulses and the e-light pulses are perpendicular to each other, and the pair of pulses are incident on the linear polarizer 112. The mutual time delay of the pulse pairs generated by the birefringent crystal 111 is proportional to the refractive index difference between the o optical pulse and the e optical pulse of the birefringent crystal, and is also proportional to the thickness of the o optical pulse and the e optical pulse of the birefringent crystal, and The intensity of the pair of pulses is approximately equal.

The linear polarizer 112 is configured to transmit half of the o-optical pulse and the e-light pulse of the pulse pair to the spectrometer 113, and the pulse transmitted through the linear polarizer 112 is aligned with respect to the polarization direction. The transmission axis of the linear polarizer 112 is 45 degrees from the polarization direction of the o-light pulse and the e-light pulse of the pulse pair.

The spectrometer 113 is configured to record spectral interference data of the pulse pairs (i.e., the o-optical pulse and the e-optical pulse) after the above-described warp polarizer 112 to implement terahertz electro-optical phase modulation measurement based on the spectral interference data. The spectral interference data includes terahertz electro-optical phase modulation information.

In the embodiment, the interferometer further comprises a measuring device SPIDER101 based on the spectral phase coherent electric field reconstruction technique, which is used for measuring the time spectrum characteristic of the probe pulse Probe pulse before the detecting pulse Probe pulse is incident on the stretcher. Time spectrum of pulse probe pulse Sexuality, used to assist in the implementation of terahertz time domain spectral measurements.

In this embodiment, the interferometer further includes a silicon wafer 108 interposed between the lens 106 and the terahertz electro-optic crystal 109 for transmitting the incident terahertz pulse to be transmitted to the terahertz electro-optic crystal with low loss. 109, and the detection pulse emitted from the lens 106 is reflected to the terahertz electro-optic crystal 109.

It should be noted that to complete the terahertz time-domain spectroscopy measurement, two experimental measurements of spectral interference data are required. The specific process is as follows:

(1) In the absence of terahertz pulse interference, after the series of components having only the probe pulse passing through the interferometer is measured, the probe pulse interference fringe data collected by the spectrometer is used, and the probe pulse interference fringe data is used as reference data.

(2) In the case of interference with terahertz pulses, the interference fringe data obtained with the terahertz signal is measured, and the interference fringe data with the terahertz signal is used as the probe data.

(3) Combining the time-spectrum characteristics of the pre-measured probe pulse, and the reference data and the probe data, the terahertz time domain waveform can be obtained by Fourier transform processing.

In summary, the interferometer provided by the first embodiment of the present invention has an optical component designed by an ingenious self-reference interference structure, and based on the principle of electro-optical sampling and spectral interference, using a chirped pulse as a probe. A single measurement of the high-intensity terahertz pulse time-domain spectrum improves the signal-to-noise ratio of the single-measurement of the strong terahertz pulse; and in this embodiment, the interference spectrum is recorded by the spectrometer to realize the electro-optical phase modulation measurement, thereby obtaining the terahertz time domain. Waveforms have the advantages of real-time, high linearity and distortion-free compared to traditional time-domain spectrometers. In addition, for terahertz sources, the stronger the terahertz pulse, the lower the pulse repetition rate, which leads to the use of conventional The measurement by the pump-probe technique brings a large measurement error, and the interferometer provided in this embodiment can be measured in a single time, effectively avoiding measurement errors.

As a second embodiment of the present invention, as shown in FIG. 1, an ultrashort pulse laser system is used as a system for emitting a detection pulse. In this embodiment, a 800 nm titanium gemstone femtosecond laser system is selected, and a part of the ultrashort pulse laser is outputted. Time spectral characteristics were measured using a SPIDER 101 device. The ultrashort pulsed laser is spread as a probe pulse through the stretcher 102 to a chirped pulse of about 10 ps. The widened detection pulse is rotated by an achromatic half-wave plate 103 to rotate the polarization direction of the detection pulse from the original level (or Rotate 45° in the vertical direction. Then, it is focused on a terahertz electro-optical crystal via a lens 106. In this embodiment, the terahertz electro-optical crystal is a <110> degree ZnTe crystal. Inside the terahertz electro-optic crystal, the detection pulse and the terahertz pulse signal to be tested are The time and space coincide, and the terahertz field strength signal is loaded into the detection pulse by the electro-optical effect. After passing through the terahertz electro-optic crystal, the detection pulse loaded with the terahertz field strength signal passes through a 200 mm thick α-BBO crystal, and generates a pulse pair having almost equal intensity but perpendicular polarization directions and mutual time delay of 600 fs. That is, o light pulse and e light pulse. The pulse is received by a spectrometer to obtain an interference spectrum signal after a linear polarizer 112 having a transmission axis that is at 45° to the polarization direction of the o-optical pulse and the e-light pulse. The corresponding spectral interference ring can be expressed as:

Where I _ref (ω) represents the reference light spectrum, I _pro (ω) represents the probe light, β represents the interference fringe contrast, τ represents the time difference between the probe light and the reference light, and φ _NL (ω) represents the nonlinearity introduced by the ZnTe crystal. Phase, φ _THz (ω) represents the phase introduced by the terahertz signal. Before the system measures the terahertz signal, the spectral interference fringes (ie, reference data) in the case of a set of terahertz-free signals are measured and stored. The phase of the interference fringes includes the time difference between the reference light and the probe light. The introduced phase ωτ and the nonlinear phase φ _NL (ω) introduced by the ZnTe crystal. Then, when there is a terahertz signal, the phase of the interference fringe includes the phase φ _THz (ω) introduced by the terahertz signal in addition to ωτ and φ _NL (ω). Combined with the pre-measured pulse time/spectral characteristics, the terahertz time domain waveform can be obtained by Fourier transform, and the terahertz pulse spectrum information can be obtained accordingly.

In summary, the self-referencing terahertz electro-optical sampling spectral interferometer provided by the second embodiment of the present invention can directly measure the phase, and has the advantages of high linearity and no distortion compared with the conventional time domain spectrometer. The structure is simple and the measurement method is simple.

The present invention also provides a self-referencing terahertz electro-optical sampling spectral interferometry system comprising all of the components in the self-referencing terahertz electro-optic sampling spectrometer described above, and the self-referencing terahertz electro-optical sampling spectral interferometer The functions that are available are not described here.

The above is only the preferred embodiment of the present invention, and is not intended to limit the invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention are included in the present invention. Within the scope of protection.

Claims

A self-referencing terahertz electro-optical sampling spectral interferometer, characterized in that the interferometer comprises:

a pulse stretcher for widening the incident detection pulse into a chirp pulse, the probe pulse having the widened pupil pulse incident on the broadband half wave plate;

The broadband half-wave plate is configured to adjust a polarization direction of the probe pulse that has been widened into a chirped pulse, and the detection pulse of the adjusted polarization direction is incident on the lens;

The lens for focusing the detection pulse of the adjusted polarization direction to a terahertz electro-optical crystal;

The terahertz electro-optical crystal is configured to cause the detection pulse to coincide with an incident terahertz pulse to be loaded, so as to load a terahertz field strength signal of the terahertz pulse to be detected into the detection pulse, which is loaded A detection pulse having a terahertz field strength signal is incident on the birefringent crystal;

The birefringent crystal is configured to generate a pulse pair that is loaded with a terahertz field strength signal to generate a time-delayed pulse pair, which are an o-light pulse and an e-light pulse, respectively, the pulse pair being incident on the linear polarizer;

The linear polarizer is configured to transmit half of the o-optical pulse and the e-optical pulse of the pulse pair to the spectrometer, and the pulse passing through the linear polarizer is consistent with the polarization direction;

The spectrometer is configured to record spectral interference data of a pulse pair after the linear polarizer to implement terahertz electro-optic phase modulation measurement based on the spectral interference data.
The interferometer according to claim 1, wherein said interferometer further comprises spectral phase coherent electric field reconstruction technique measuring means for time of said detecting pulse before said detecting pulse is incident on said stretcher The spectral characteristics are measured to obtain the time spectral characteristics of the probe pulses.
The interferometer of claim 1 wherein said interferometer further comprises a silicon wafer;

The silicon wafer is interposed between the lens and the terahertz electro-optic crystal for transmitting incident terahertz pulses to be transmitted to the terahertz electro-optic crystal with low loss, and The detection pulse of the lens is reflected to the terahertz electro-optical crystal.
The interferometer according to claim 1, wherein said terahertz pulse to be measured incident on said terahertz electro-optic crystal coincides with said detection pulse in time and space.
The interferometer according to claim 1, wherein said mutual time delay of said pair of pulses generated by said birefringent crystal is proportional to a refractive index difference between an o-optical pulse and an e-optical pulse of said birefringent crystal and thickness.
The interferometer of claim 1 wherein said detection pulse is a linearly polarized ultrashort femtosecond pulse.
The interferometer of claim 1 wherein said pulse stretcher is specifically for broadening the incident detection pulse to a picosecond pulse.
The interferometer of claim 1 wherein the pass axis of said linear polarizer is at 45 with the direction of polarization of the o and p pulses of said pulse pair.
A self-referencing terahertz electro-optical sampling spectral interferometry system, comprising the self-referencing terahertz electro-optical sampling spectral interferometer of any of claims 1-8.