CN101813619A - Method utilizing polarization-controllable T-Hz wave to measure optical axis direction of birefringent crystal - Google Patents
Method utilizing polarization-controllable T-Hz wave to measure optical axis direction of birefringent crystal Download PDFInfo
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Abstract
The method of the invention comprises the following steps: (1) moving a BBO crystal to change the distance from the BBO crystal to a plasma to obtain the relationship between the angle theta of the polarization direction of T-Hz wave relative to horizontal direction and the distance d from the BBO crystal to the plasma; (2) substituting the refractive indexes of T-Hz wave under different frequencies to a formula for calculating phase delay, thus obtaining the relationship of phase delay between the two axes of the birefringent crystal along with frequency change; (3) respectively selecting a corresponding T-Hz wave frequency f1 when the phase delay is odd times of pi and a corresponding T-Hz wave frequency f2 when the phase delay is even times of pi, moving the BBO crystal, and measuring the motion distance delta d of the BBO crystal corresponding to the T-Hz waves of frequency f1 and f2 at the nearest neighbor part with the minimum amplitude; and (4) calculating the included angle alpha between the birefringent crystal optical axis and the horizontal direction. Therefore, the invention can measure the optical axis direction of the birefringent crystal without rotating the birefringent crystal, thereby ensuring the measurement along the optical axis direction of the birefringent crystal to be more convenient and accurate.
Description
Technical field
The present invention relates to a kind of optical axis measuring method of birefringece crystal, specifically the present invention relates to a kind of method of utilizing the controlled THz wave spectrum of polarization that the optical axis direction of birefringece crystal is measured.
Background technology
Terahertz emission has the character of uniquenesses such as transient state, low energy and coherence owing to it, and has great scientific value and wide application prospect at aspects such as satellite communication, Non-Destructive Testing, military radars.The making of terahertz polarization measurement, Terahertz communication, biomedical imaging, military target identification, chemical composition analysis and Terahertz filter plate etc. all needs the polarization direction of THz wave is accurately controlled.
Excite the polarization direction of the terahertz emission that air plasma produces by double-frequency laser by research, can control the polarization direction of the THz wave of radiation by the relative phase between change fundamental wave and the second harmonic, utilize the controlled tera-hertz spectra of this polarization to carry out high-acruracy survey the optical axis of birefringece crystal.
Existing when utilizing terahertz time-domain spectroscopy that the optical axis of birefringece crystal is measured need to birefringece crystal at least the Rotate 180 degree measure, not only time-consuming in actual applications but also complicated operation utilizes the controlled terahertz light spectrometry of polarization can overcome these shortcomings.
Summary of the invention
The invention provides a kind of method of utilizing the controlled T-Hz wave to measure optical axis direction of birefringent crystal of polarization, when measuring to solve the optical axis that exists in the above-mentioned background technology to birefringece crystal need to birefringece crystal at least the Rotate 180 degree measure the technical matters of not only time-consuming in actual applications but also complicated operation.
A kind of method of utilizing the controlled T-Hz wave to measure optical axis direction of birefringent crystal of polarization, comprise the steps: (1) mobile bbo crystal to change the distance of bbo crystal to plasma, the polarization direction that obtains THz wave with respect to the angle θ of horizontal direction and bbo crystal to plasma apart from the relation between the d;
(2) refractive index of THz wave under the different frequency is brought into the computing formula of phase delay, draw phase delay between two axles of birefringece crystal with the relation of frequency change;
A THz wave frequency f 2 of correspondence during the even-multiple of THz wave frequency f 1 of correspondence and π when (3) to choose phase delay respectively be the odd-multiple of π, mobile bbo crystal, the THz wave of survey frequency f1 and f2 is at the corresponding bbo crystal displacement Δ d of the amplitude minimum value place of arest neighbors;
(4) angle of birefringece crystal optical axis and horizontal direction
Wherein, in the step (1) between the maximal value and minimum value of THz wave amplitude, bbo crystal between the plasma apart from linear monotonic relationshi between the angle θ of polarization direction with respect to horizontal direction of d and THz wave.
Wherein, the computing formula of phase delay is described in the step (2)
Wherein f is the Terahertz wave frequency, Δ n=n
e-n
o, n
eAnd n
oThe refractive index of extraordinary ray and ordinary light during for corresponding different frequency f, δ is a phase delay, and a is the thickness of birefringece crystal, and c is the light velocity.
Wherein, bbo crystal moves 13mm, and the polarization direction of corresponding THz wave turns over 90 °.
Wherein, adopt accurate translation stage control to move described bbo crystal.
The present invention passes through technique scheme, make and to rotate birefringece crystal, can measure the optical axis direction of birefringece crystal, its calculating and drawing all can adopt computer program to carry out automatically, make that the measurement of optical axis of crystal direction is more easy and accurate, reached beneficial technical effects.
Description of drawings
Fig. 1 is the synoptic diagram of experimental provision of the present invention;
Fig. 2 is that the angle of the polarization direction of THz wave and horizontal direction and bbo crystal are to the graph of a relation between the distance of plasma;
Fig. 3 is under the situation of horizontal polarization for the THz wave of incident, the THz wave of ordinary THz wave, extraordinary THz wave and intermediate angle 0.2 Terahertz in the scope of 2.2 Terahertzs refractive index and the graph of a relation of frequency.
Fig. 4 is the graph of a relation of phase delay with frequency change;
Fig. 5 is that the amplitude of Terahertz electric field of different frequency is with the variation diagram of bbo crystal position.
Description of reference numerals
The 1-lens; The 2-BBO crystal; The 3-polyfluortetraethylene plate; 4-throws the face mirror; The tested birefringent material of 5-; The 6-electro-conductive glass; The 7-ZnTe crystal; The 8-quarter-wave plate; The 9-Wollaston prism; The 10-differential detector; The 11-catoptron; The L1-pump light; L2-surveys light.
Embodiment
For shape of the present invention, structure and characteristics can be understood better, below will enumerate preferred embodiment and also be elaborated in conjunction with the accompanying drawings.
Fig. 1 is the synoptic diagram of experimental provision of the present invention, terahertz polarization analyzer of the present invention comprises LASER Light Source, in the present embodiment, the femtosecond laser amplifier that this LASER Light Source can select for use Spectra-Physics to produce, its laser pulse average output power is 3.5W, repetition frequency 1KHz, centre wavelength 800nm, pulsewidth 50fs.LASER Light Source can also be selected the laser instrument that produces other wavelength lasers for use.
This LASER Light Source light path rear is coaxially arranged with beam splitting chip, and the laser pulse of generation is divided into two-beam through behind the beam splitting chip, pump light L1 and detection light L2.Pump light L1 has most energy of incident laser, produces THz wave through the Terahertz generation device.Surveying light L2 is low power laser, as detecting light beam.
The THz wave that pump light L1 produces through the Terahertz generation device behind throwing face mirror 4 collimations that are provided with on the light path of Terahertz generation device rear, focuses on tested birefringent material 5 places on the light path of rear.Via the birefringence effect of this tested birefringent material 5, THz wave is divided into two orthogonal o light of direction of vibration and e light on time domain.
Survey light L2 as detecting light beam with the THz wave separately, after being located at optical path adjustment device on the light path separately and adjusting its light path, conllinear arrives crystal detection 7 places in the same way respectively.
In the present embodiment, optical path adjustment device is embodied as: o light and the e light THz wave after separately through throwing face mirror 4 collimation and focus on after, reflex to crystal detection 7 places by the electro-conductive glass 6 at the face of throwing mirror 4 rears.This crystal detection 7 is selected the electro-optic crystal as ZnTe for use, and this electro-conductive glass 6 can be selected ITO for use.Detecting light beam after catoptron 11 and lens 1 are adjusted light path and are focused on, see through electro-conductive glass 6 and THz wave by quartz crystal in the same way conllinear by crystal detection 7.
Because pump light L1 and detection light L2 are homology light, so when conllinear was by crystal detection 7 in the same way, THz wave and detecting light beam overlapped on crystal detection.In crystal detection 7, the Terahertz electric field changes the index ellipsoid of crystal detection 7, thereby the polarization state of the detecting light beam of outgoing is changed, and making originally, the polarization direction of the 800nm detecting light beam of horizontal polarization changes.Sniffer is located at this crystal detection 7 light path rears, the detectable Terahertz electric field waveform that draws.
Concrete, this sniffer mainly is made up of the quarter-wave plate 8 that sets gradually on light path, wollaston prism 9 and difference detector 10.From the detecting light beam of these crystal detection 7 outgoing, 8 zeroings of process quarter-wave plate, the light intensity of the horizontal polarization light of detecting light beam and vertical polarized light equates when making beginning.Through wollaston prism 9 horizontal polarization light and vertical polarized light are separated then, the two-beam that separates is surveyed by difference detector 10.
Horizontal polarization light separately and the vertical pairing difference between current of polarized light are proportional to the Terahertz electric field, use difference detector 10 light intensity difference of this two bundles polarized light can be converted to difference between current, machine computed in software as calculated, thus detect the time dependent time-domain spectroscopy of Terahertz electric field.
Terahertz generation device of the present invention is to adopt double-frequency laser to excite air to produce plasma, thereby the method generation of generation THz wave.The Terahertz generation device is included in lens 1, barium metaborate crystal 2 and the polyfluortetraethylene plate 3 that sets gradually on the pump light L1 light path.
Pump light L1 carries out frequency multiplication through BBO (barium metaborate) crystal 2 after passing through the set lens 1 in beam splitting chip rear, and part light is the frequency doubled light of 400nm by frequency multiplication, still is the fundamental frequency light of 800nm after part light sees through from bbo crystal.This frequency doubled light focuses on mutually mixing in the distance bbo crystal apart from the d place with fundamental frequency light, produces plasma, thereby excites THz wave.Bbo crystal is installed on the accurate translation stage, by accurate translation stage can accurately control bbo crystal to plasma apart from d.
Bbo crystal rear light path is provided with polyfluortetraethylene plate 3, and the THz wave of generation can see through from polyfluortetraethylene plate 3, thereby other light waves of elimination obtain testing required THz wave.
Excite air to produce in the mechanism of plasma generation THz wave based on double-frequency laser, when fundamental frequency only during linearly polarized light, the THz wave that excites also is a linear polarization.Along with the polarization direction of the variation THz wave of the relative phase of fundamental frequency light and frequency doubled light can change continuously.The relative phase of fundamental frequency light and frequency doubled light along with bbo crystal to the changing of plasma apart from the different of d, and the variation of the relative phase of fundamental frequency light and frequency doubled light is proportional to the variation of the position of bbo crystal.When the position of bbo crystal recurred variation, the polarization direction of THz wave also can recur variation.
The method of utilizing the controlled T-Hz wave to measure optical axis direction of birefringent crystal of polarization of the present invention adopts above-mentioned experimental provision to finish, and comprises the steps:
1, mobile bbo crystal to be to change the distance of bbo crystal to plasma, the polarization direction that obtains THz wave with respect to the angle θ of horizontal direction and bbo crystal to plasma apart from the relation between the d.
Fig. 2 is that the polarization direction of THz wave and the angle between horizontal direction and bbo crystal are to the graph of a relation between the distance of plasma, shown among the figure that the polarization direction of THz wave is with respect to the consequential variation of the angle of horizontal direction when changing the position of bbo crystal 2.In experimental provision shown in Figure 1, continuously change bbo crystal to plasma apart from d, measure the polarization angle of THz wave under the different distance d respectively, can obtain curve shown in Fig. 2.The method of wherein measuring the polarization angle of THz wave is a prior art, does not repeat them here.
From Fig. 2, can draw, the polarization direction of THz wave with respect to the angle of horizontal direction at 0 ° between 90 ° the time, bbo crystal to plasma apart from the polarization direction of d and THz wave with respect between the angle θ of horizontal direction being the linear monotonic relation.When bbo crystal arrives the variable in distance Δ d=13mm of plasma, the polarization direction of THz wave turns over Δ θ=90 °, also can intuitively find out from experiment, when mobile bbo crystal, when very big and minimal value appearred in the electric field of THz wave, bbo crystal moved past about 13mm.
2, bring the refractive index of THz wave under the different frequency into formula
Draw phase delay between two axles of birefringece crystal with the relation of frequency change.
Fig. 3 is under the situation of horizontal polarization for the THz wave of incident, the THz wave of ordinary THz wave, extraordinary THz wave and intermediate angle 0.2 Terahertz in the frequency range of 2.2 Terahertzs refractive index and the graph of a relation of frequency, the refractive index n of e light
eRefractive index n with o light
oCan table look-up and obtain or calculate out.The measuring and calculating of refractive index is a state of the art, does not give unnecessary details in the present invention.
Fig. 4 be phase delay between two axles of birefringece crystal with the graph of a relation of frequency change, this curve is by formula
Calculate.F Terahertz wave frequency wherein, Δ n=n
e-n
o, n
eAnd n
oThe refractive index of extraordinary ray and ordinary light can be read according to Fig. 3 during for corresponding different frequency f, also can table look-up according to actual needs to obtain or calculate out, and δ is a phase delay, and a is the thickness of birefringece crystal, and c is the light velocity.
As shown in Figure 4, in the present embodiment, when the Terahertz wave frequency 0.2 when between 2.2, changing, phase delay changes between 8 π at 2 π.When phase delay is the odd-multiple of π, this moment, the effect of birefringece crystal was equivalent to 1/2nd wave plates, polarization direction by the THz wave behind the birefringece crystal can change, and the angle of its polarization direction and the optical axis of crystal becomes the polarization direction of incident THz wave and two times of optical axis of crystal angle; And when phase delay was the even-multiple of π, the polarization direction of the THz wave by birefringece crystal did not change, and is identical with the polarization direction of inciding the THz wave on the birefringece crystal.
A THz wave frequency f 2 of correspondence during the even-multiple of THz wave frequency f 1 of correspondence and π when 3, to choose phase delay respectively be the odd-multiple of π, mobile bbo crystal, the THz wave of survey frequency f1 and f2 is at the corresponding bbo crystal displacement Δ d of the amplitude minimum value place of arest neighbors.
Fig. 5 is that the Terahertz electric field amplitude is with the variation diagram of bbo crystal position under the THz wave frequency of correspondence when choosing phase delay and being respectively the odd-multiple of π and even-multiple.Measure THz wave behind the curve map on the time domain by the device of Fig. 1, can obtain the amplitude curve figure of THz wave on frequency domain among Fig. 5 through Fourier transform.The THz wave frequency of curve in the present embodiment is respectively 0.47THz, 0.91THz, 1.26THz, 1.67THz and 1.93THz.
The amplitude curve of the THz wave frequency of correspondence when choosing phase delay arbitrarily and be odd-multiple of π in Fig. 5 and during an even-multiple can read the alternate position spike of the THz wave transmitance minimum value BBO of arest neighbors under these two frequencies from figure.In the present embodiment, choose the amplitude curve of the THz wave under 1.26THz and the 0.91THz frequency respectively, the amplitude curve of the THz wave of respective frequencies when promptly phase delay is respectively 5 π and 4 π.As can be seen from the figure, the position of its corresponding bbo crystal is moved and is about Δ d=4mm.
4, the angle of birefringece crystal optical axis and horizontal direction
In conjunction with Fig. 2, because between the maximal value and minimum value of THz wave amplitude, bbo crystal is to linear monotonic relationshi between the angle θ with respect to horizontal direction apart from d and terahertz polarization direction between the plasma, so when bbo crystal moved Δ d, the angle that the polarization direction of THz wave turns over was
Therefore especially, by all can obtaining among Fig. 2 and in the practice, when bbo crystal moved 13mm, the polarization direction of corresponding THz wave turned over 90 °, and when bbo crystal moved Δ dmm, the polarization angle of THz wave turned over
In the present embodiment
For the incident THz wave of horizontal polarization, if the angle of its polarization direction and the optical axis of crystal is α, then pass through after the birefringece crystal, be the THz wave of the even-multiple of π for phase delay, its polarization direction does not change, and still is horizontal polarization; For phase delay is the THz wave of the odd-multiple of π, and the angle of its polarization direction and the optical axis of crystal becomes 2 α.Therefore relatively phase delay is the odd-multiple of π and the THz wave of even-multiple, and the polarization direction that can obtain between the two turns over angle delta θ=2 α.
So angle of the optical axis of crystal and incident terahertz polarization direction
Because the terahertz polarization direction is identical with horizontal direction, i.e. the angle of the optical axis of crystal and horizontal direction
Bring data in the present embodiment into and draw α ≈ 13.85 degree.
The present invention passes through technique scheme, make and to rotate birefringece crystal, can measure the optical axis direction of birefringece crystal, its calculating and drawing all can adopt computer program to carry out automatically, make that the measurement of optical axis of crystal direction is more easy and accurate, reached beneficial technical effects.
The above description of this invention is illustrative, and nonrestrictive, and those skilled in the art is understood, and can carry out many modifications, variation or equivalence to it within spirit that claim limits and scope, but they will fall within the scope of protection of the present invention all.
Claims (5)
1. a method of utilizing the controlled T-Hz wave to measure optical axis direction of birefringent crystal of polarization is characterized in that, comprises the steps:
(1) mobile bbo crystal to be to change the distance of bbo crystal to plasma, the polarization direction that obtains THz wave with respect to the angle θ of horizontal direction and bbo crystal to plasma apart from the relation between the d;
(2) refractive index of THz wave under the different frequency is brought into the computing formula of phase delay, draw phase delay between two axles of birefringece crystal with the relation of frequency change;
A THz wave frequency f 2 of correspondence during the even-multiple of THz wave frequency f 1 of correspondence and π when (3) to choose phase delay respectively be the odd-multiple of π, mobile bbo crystal, the THz wave of survey frequency f1 and f2 is at the corresponding bbo crystal displacement Δ d of the amplitude minimum value place of arest neighbors;
(4) angle of birefringece crystal optical axis and horizontal direction
2. the method for utilizing the controlled T-Hz wave to measure optical axis direction of birefringent crystal of polarization as claimed in claim 1, it is characterized in that, in the step (1) between the maximal value and minimum value of THz wave amplitude, bbo crystal between the plasma apart from linear monotonic relationshi between the angle θ of polarization direction with respect to horizontal direction of d and THz wave.
3. the method for utilizing the controlled T-Hz wave to measure optical axis direction of birefringent crystal of polarization as claimed in claim 2 is characterized in that, the computing formula of phase delay is described in the step (2)
Wherein f is the Terahertz wave frequency, Δ n=n
e-n
o, n
eAnd n
oThe refractive index of extraordinary ray and ordinary light during for corresponding different frequency f, δ is a phase delay, and a is the thickness of birefringece crystal, and c is the light velocity.
4. the method for utilizing the controlled T-Hz wave to measure optical axis direction of birefringent crystal of polarization as claimed in claim 2 is characterized in that bbo crystal moves 13mm, and the polarization direction of corresponding THz wave turns over 90 °.
5. the method for utilizing the controlled T-Hz wave to measure optical axis direction of birefringent crystal of polarization as claimed in claim 2 is characterized in that, adopts accurate translation stage control to move described bbo crystal.
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| CN102096269B (en) * | 2011-01-18 | 2012-11-28 | 南京邮电大学 | Terahertz surface plasma wave optical modulator and modulation method thereof |
| CN102882107A (en) * | 2012-10-22 | 2013-01-16 | 上海理工大学 | Method capable of adjusting polarization and intensity of terahertz wave rapidly and continuously |
| CN103194788A (en) * | 2013-04-17 | 2013-07-10 | 清华大学 | Preparation, characterization and application methods of anisotropic medium crystal in THz (Terahertz) frequency band |
| CN103194788B (en) * | 2013-04-17 | 2016-03-23 | 清华大学 | The preparation of Terahertz frequency range anisotropic medium crystal, sign and application method |
| CN103411903A (en) * | 2013-07-29 | 2013-11-27 | 南开大学 | THz radiation reinforcing method through light line array |
| CN103712781A (en) * | 2013-12-25 | 2014-04-09 | 天津大学 | Device and method for measuring multi-incidence-angle polarization interference in birefringence optical wedge optical axis direction |
| CN103712781B (en) * | 2013-12-25 | 2016-03-30 | 天津大学 | The multiple angles of incidence polarization interference measurement mechanism of birefringent wedge optical axis direction and method |
| CN105841816A (en) * | 2016-04-18 | 2016-08-10 | 深圳市太赫兹系统设备有限公司 | Terahertz time-domain spectroscopy system |
| US10408679B2 (en) | 2016-04-18 | 2019-09-10 | Shenzhen Institute Of Terahertz Technology And Innovation | Terahertz time-domain spectroscopy system |
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