CN103135263A - Electric light wave guider capable of being used for ultra-high-speed electric light sampling - Google Patents

Abstract

The invention is suitable for the field of optical elements and provides an electric light wave guider capable of being used for ultra-high-speed electric light sampling. The electric light wave guider comprises an AlGaAs core layer and AlGaAs wrapping layers located on two sides of the AlGaAs core layer, wherein the outer sides of the AlGaAs wrapping layers located on two sides are respectively provided with electrodes. The AlGaAs wrapping layers located on two sides have the same refractive index to input femtosecond laser, and the refractive index of the AlGaAs wrapping layers is lower than that of the AlGaAs core layer to the input femtosecond laser. The AlGaAs light wave guider serves as an electric light crystal to transmit the femtosecond laser, accordingly half-wave voltage can be greatly reduced, and the detecting sensitivity is improved.

Description

A kind of electro-optical transducer that can be used for Ultra-high-speed Electro-optic Sampling

Technical field

The invention belongs to the optical device field, relate in particular to a kind of electro-optical transducer that can be used for Ultra-high-speed Electro-optic Sampling.

Background technology

Electro optic sampling technology tool in the ultrafast opto-electronics field is of great significance, and this technology proposes in nineteen eighty-two in the people such as Valdmanis by U.S. Rochester university.Electro Optic Sampling System changes by the light intensity of measuring electric signal modulation to be measured take ultrashort light pulse as " sampling gate ", realizes the test to hypervelocity electron device or circuit.This technology has the bandwidth less than the temporal resolution below the 1ps level and THz level, owing to need not to extract electric charge from measured device or circuit, therefore system under test (SUT) is not almost had electromagnetic interference (EMI) simultaneously.These advantages make it be subject to the scientific research personnel and more and more pay attention to.

In the electro optic sampling technology, one of its Primary Component is electro-optic crystal.Usually adopting the thickness of electro-optic crystal is the hundreds of micron, because the electrooptical coefficient of present electro-optic crystal is smaller, thereby will have enough voltage just can make the light of different directions polarization in output face that enough phase differential are arranged.Usually half-wave voltage is wanted several kilovolts, and the electric signal that we need to survey often only has several volts, and is even less, and this has just limited the sensitivity of detectable signal greatly.Significantly reduce the thickness of electro-optic crystal, can reduce half-wave voltage, make the detection difficulty of electric signal greatly reduce, improve the signal to noise ratio (S/N ratio) of surveying.But significantly reduce the thickness of electro-optic crystal, also can bring certain problem.When electro-optic crystal thickness is very little, when light transmits in this crystal, waveguiding effect can be very obvious, at this moment needs to consider waveguiding effect to the impact of detectable signal, so just can make detection more accurate.

Summary of the invention

Technical matters to be solved by this invention is to provide a kind of electro-optical transducer for Ultra-high-speed Electro-optic Sampling, is intended to improve the sensitivity of detection.

The present invention is achieved in that a kind of electro-optical transducer for Ultra-high-speed Electro-optic Sampling, and described electro-optical transducer comprises AlGaAs sandwich layer and the AlGaAs covering that is positioned at described AlGaAs sandwich layer both sides, and the outside of the AlGaAs covering of described both sides is respectively equipped with electrode; The AlGaAs covering of described both sides is identical to the refractive index of the femtosecond laser of input, and lower than the refractive index of described AlGaAs sandwich layer to the femtosecond laser of input.

Further, the AlGaAs covering of described both sides satisfies following relation to refractive index and the described AlGaAs sandwich layer of the femtosecond laser of input to the refractive index of the femtosecond laser inputted:

Wherein, n ₁, n ₂Be respectively the refractive index of waveguide core layer and covering, a is the thickness of waveguide core layer, and λ is the wavelength of the femtosecond laser of incident.

Further, described lambda1-wavelength is 1.064 μ m, and the refractive index of the AlGaAs covering of described both sides is 3.52, and thickness is 8 μ m; The refractive index of described AlGaAs sandwich layer is 3.53, and thickness is 2 μ m.

Further, the field distribution of the light that conducts in described electro-optical transducer is the linear combination of two orthogonal mode fields, and two orthogonal modes phase differential in described electro-optical transducer output cross section is linear change with impressed voltage.

The present invention utilizes the AlGaAs optical waveguide to transmit femtosecond laser as electro-optic crystal, thereby can greatly reduce half-wave voltage, improves the sensitivity of surveying.

Description of drawings

Fig. 1 is the variation schematic diagram of AlGaAs crystal provided by the invention index ellipsoid main shaft when extra electric field;

Fig. 2 is planar waveguide structural representation provided by the invention;

Fig. 3 is TE provided by the invention ₀Mould and TM ₀Mould is exported the phase differential in cross section with the schematic diagram that concerns of on-load voltage variation in waveguide.

Embodiment

In order to make purpose of the present invention, technical scheme and advantage clearer, below in conjunction with drawings and Examples, the present invention is further elaborated.Should be appreciated that specific embodiment described herein only in order to explain the present invention, is not intended to limit the present invention.

The propagation law of light in crystal deferred to electromagnetic theory of light, and the method for geometry of utilizing index ellipsoid can be complete and express easily the refractive index that characterizes the crystal optics characteristic and distribute in the value of space all directions.The impact of extra electric field on the crystal optics characteristic, the variation of size, shape and orientation that can be by index ellipsoid is described.

The AlGaAs crystal belongs to

Crystal class, its three four-fold axis of symmetry are its crystalline axis direction, and three axles (x, y, z) can exchange.

Point group is isotropic crystal, its refractive index n ₀The ellipsoid equation be:

$x^2+{\mathrm{<figure-callout\; id="2"\; label="y"\; filenames="HDA00002851200100013.png"\; state="\{\{state\}\}">y</figure-callout>}}^2+z^2={\mathrm{<figure-callout\; id="0"\; label="n"\; filenames="HDA00002851200100013.png"\; state="\{\{state\}\}">n</figure-callout>}}_0^2---\left(1\right)$

In formula (1), x, y, three coordinates of z are got crystalline axis direction, as shown in Figure 1.At extra electric field E (E _x, E _y, E _zUnder effect, inductive refractive index ellipsoid equation is:

$\frac{x^2+y^2{z}^2}{{\mathrm{<figure-callout\; id="0"\; label="n"\; filenames="HDA00002851200100013.png"\; state="\{\{state\}\}">n</figure-callout>}}_0^2}+{2\mathrm{\&gamma;}}_{41}(E_x\mathrm{yz}+E_y\mathrm{zx}+E_z\mathrm{xy})=1---\left(2\right)$

Wherein, γ ₄₁Be electrooptical coefficient.In practical application, the direction of extra electric field has three kinds of possible situations: electric field is perpendicular to (001) face, perpendicular to (110) face or perpendicular to (111) face.Electric field is discussed in the present invention perpendicular to the situation of (001) face (namely along the z direction of principal axis).This moment E _x=E _y=0, E _z=| E|, so inductive refractive index ellipsoid equation is:

$\frac{x^2+{\mathrm{<figure-callout\; id="2"\; label="y"\; filenames="HDA00002851200100013.png"\; state="\{\{state\}\}">y</figure-callout>}}^2+z^2}{{\mathrm{<figure-callout\; id="0"\; label="n"\; filenames="HDA00002851200100013.png"\; state="\{\{state\}\}">n</figure-callout>}}_0^2}+{2\mathrm{\&gamma;}}_{41}{E}_z\mathrm{xy}=1---\left(3\right)$

The appearance of cross term xy means that the main shaft of index ellipsoid around the z axle, rotation has occured, and is the convenience of computing, can be with its main shaft.If new main shaft has rotated the α angle around the z axle, the pass between new (xyz) old (x ' y ' z ') coordinate is:

x=x′cosα-y′sinα

y=x′sinα+y′cosα

z=z′ (4)

With (4) substitution (3) Shi Kede:

$(\frac{1}{{\mathrm{<figure-callout\; id="0"\; label="n"\; filenames="HDA00002851200100013.png"\; state="\{\{state\}\}">n</figure-callout>}}_0^2}+{2\mathrm{\&gamma;}}_{41}{E}_z\sin \mathrm{\&alpha;}\cos \mathrm{\&alpha;})x^{\&prime;2}$

$+(\frac{1}{{\mathrm{<figure-callout\; id="0"\; label="n"\; filenames="HDA00002851200100013.png"\; state="\{\{state\}\}">n</figure-callout>}}_0^2}-{2\mathrm{\&gamma;}}_{41}{E}_z\sin \mathrm{\&alpha;}\cos \mathrm{\&alpha;})y^{\&prime;2}$

$+\frac{1}{{\mathrm{<figure-callout\; id="0"\; label="n"\; filenames="HDA00002851200100013.png"\; state="\{\{state\}\}">n</figure-callout>}}_0^3}{z}^{\&prime;2}+{2\mathrm{\&gamma;}}_{41}{E}_z{x}^{\&prime;}{y}^{\&prime;}({\mathrm{<figure-callout\; id="2"\; label="cos"\; filenames="HDA00002851200100013.png"\; state="\{\{state\}\}">cos</figure-callout>}}^2\mathrm{\&alpha;}-{\mathrm{<figure-callout\; id="2"\; label="sin"\; filenames="HDA00002851200100013.png"\; state="\{\{state\}\}">sin</figure-callout>}}^2\mathrm{\&alpha;})=1---\left(5\right)$

Due to x ', y ', z' are three major axes orientations of inductive refractive index ellipsoid, and the following formula cross term should be zero, so:

α=±45° (6)

When face applied, inductive refractive index ellipsoid main shaft will be around z axle rotation 45 degree along (001) for this explanation extra electric field, and as shown in Figure 1, this corner and electric field level are irrelevant, but rotation direction is relevant with direction of an electric field.If get ° inductive refractive index ellipsoid main shaft of α=45, its equation is:

$(\frac{1}{{\mathrm{<figure-callout\; id="0"\; label="n"\; filenames="HDA00002851200100013.png"\; state="\{\{state\}\}">n</figure-callout>}}_0^2}+{\mathrm{\&gamma;}}_{41}{E}_z)x^{\&prime;2}+(\frac{1}{{\mathrm{<figure-callout\; id="0"\; label="n"\; filenames="HDA00002851200100013.png"\; state="\{\{state\}\}">n</figure-callout>}}_0^2}-{\mathrm{\&gamma;}}_{41}{E}_z)y^{\&prime;2}+\frac{1}{{\mathrm{<figure-callout\; id="0"\; label="n"\; filenames="HDA00002851200100013.png"\; state="\{\{state\}\}">n</figure-callout>}}_0^2}{z}^{\&prime;2}=1---\left(7\right)$

γ ₄₁The order of magnitude be 10 ^-12M/V, γ usually ₄₁E＜＜1 utilizes binomial theorem, can derive three main shaft inductive refractive indexes and be respectively:

$n_x^{\&prime;}={\mathrm{<figure-callout\; id="0"\; label="n"\; filenames="HDA00002851200100013.png"\; state="\{\{state\}\}">n</figure-callout>}}_0+\frac{1}{2}{\mathrm{<figure-callout\; id="0"\; label="n"\; filenames="HDA00002851200100013.png"\; state="\{\{state\}\}">n</figure-callout>}}_0^3{\mathrm{\&gamma;}}_{41}{E}_z$

$n_y^{\&prime;}=n_0-\frac{1}{2}{\mathrm{<figure-callout\; id="0"\; label="n"\; filenames="HDA00002851200100013.png"\; state="\{\{state\}\}">n</figure-callout>}}_0^3{\mathrm{\&gamma;}}_{41}{E}_z$

$n_z^{\&prime;}=n_0---\left(8\right)$

This moment, crystal became biaxial crystal by isotropy.

Above-described is the electrooptical effect of AlGaAs crystal, and the below further describes the transport property of AlGaAs optical waveguide.The designed AlGaAs optical waveguide of the present invention is symmetrical planar waveguide, and as shown in Figure 2, its sandwich layer and both sides covering are all Al _xGa _1-xThe As material.The ratio of Al is different, the refractive index of crystal is just different, can make the refractive index of both sides covering identical by the ratio of controlling Al, the refractive index of refractive index ratio covering that simultaneously can sandwich layer is slightly high, for example can pass through extra doped with Al in waveguide, and make the doping of Al in covering higher than the doping of Al in sandwich layer.Then the skin at the both sides covering all plates electrode, just can be to AlGaAs crystal on-load voltage.

The transport property of AlGaAs optical waveguide when not powering up

When not powering up, the AlGaAs crystal is isotropic body.Utilizing electromagnetic theory to derive can get, and when light transmits along the y' direction in this waveguide, can be decomposed into two quasi-modes, i.e. TE mould (transverse electric mode) and TM mould (transverse magnetic wave).The TE mould includes only three field component E _x', H _y', H _z', the TM mould also includes only three field component H _x', E _y', E _z', TE mould and TM mould are mutually orthogonal.The field distribution of conducting in this waveguide can be regarded the linear combination of these two kinds of patterns as.The secular equation of TE mould and TM mould is respectively:

$\tan \left(\mathrm{ha}\right)=\frac{2\mathrm{hq}}{h^2-q^2}---\left(9\right)$

$\tan \left(\mathrm{ha}\right)=\frac{2\frac{n_1^2}{{\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="HDA00002851200100013.png"\; state="\{\{state\}\}">n</figure-callout>}}_2^2}\mathrm{hq}}{h^2-\frac{n_1^4}{{\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="HDA00002851200100013.png"\; state="\{\{state\}\}">n</figure-callout>}}_2^4}{q}^2}---\left(10\right)$

Wherein, n ₁, n ₂Be respectively the refractive index of waveguide core layer and covering, a is the thickness of waveguide core layer, $h=\sqrt{n_1^2{\mathrm{<figure-callout\; id="0"\; label="k"\; filenames="HDA00002851200100013.png"\; state="\{\{state\}\}">k</figure-callout>}}_0^2-{\mathrm{\&beta;}}^2},$ $q=\sqrt{{\mathrm{\&beta;}}^2-{\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="HDA00002851200100013.png"\; state="\{\{state\}\}">n</figure-callout>}}_2^2{\mathrm{<figure-callout\; id="0"\; label="k"\; filenames="HDA00002851200100013.png"\; state="\{\{state\}\}">k</figure-callout>}}_0^2},$ ${\mathrm{<figure-callout\; id="0"\; label="k"\; filenames="HDA00002851200100013.png"\; state="\{\{state\}\}">k</figure-callout>}}_0=\frac{2\mathrm{\&pi;}}{\mathrm{\&lambda;}},$ λ is the wavelength of the femtosecond laser of incident, and β is propagation constant.By finding the solution above-mentioned two equations, can obtain respectively the propagation constant β of TE mould and TM mould, and then can find the solution phase place variation, the mould field distribution isotype characteristic of light in waveguide.Phase differential at any m of waveguide output terminal rank TE mould and n rank TM mould is:

Wherein, L is waveguide length.

The TE mould that can exist in waveguide and the number of TM mould are equal, and they are all to be determined by following parameters:

$V=\sqrt{n_1^2-{\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="HDA00002851200100013.png"\; state="\{\{state\}\}">n</figure-callout>}}_2^2}\frac{\mathrm{\&pi;}}{\mathrm{\&lambda;}}a---\left(12\right)$

When

The time, only may there be a TE mould and TM mould, i.e. a TE in waveguide ₀Mould and TM ₀Mould.And TE in waveguide ₀Mould and TM ₀The intensity size of mould is determined by field distribution and the polarization direction of incident light.

The transport property of AlGaAs optical waveguide when powering up

When on-load voltage outside the AlGaAs optical waveguide, according to the electrooptical effect of introducing previously as can be known, the AlGaAs crystal has become biaxial crystal.Wherein, z' direction crystal refractive index n ₀Constant, and x', y' direction crystal refractive index become respectively With

Because the change of refractive index is very little with respect to original crystal refractive index, thereby the still approximate establishment of scalar Helmholtz equation, the pattern in waveguide still can be decomposed into TE mould and TM mould.Wherein the electric field component of TM mould is mainly along the z' direction, and z' direction crystal refractive index n ₀Constant, thereby mode propagation constant β is constant.The electric field component of TE mould is along the x' direction, and change has occured the refractive index of x' direction, thereby mode propagation constant β also changes.The phase differential of any m of waveguide output terminal rank TE mould and n rank TM mould is become by (11):

The change amount of the propagation constant β of TE mould can be obtained by (9).

In Ultra-high-speed Electro-optic Sampling Technology, we can design structure and the index distribution of waveguide, make and only have TE in waveguide ₀Mould and TM ₀Mould.If at voltage of waveguide covering outside loading, can make TE ₀The propagation constant of mould changes, and TM ₀The propagation constant of mould is constant, thereby can change the phase differential of the pattern of two electric field mutually perpendicular direction vibrations of output terminal, and then changes the polarization state of outgoing beam, and we just can utilize this waveguide to survey ultrafast electric signal like this.

The last relation that pattern phase differential and impressed voltage are discussed again.When lambda1-wavelength adopts 1.064 μ m, by controlling Al _xGa _1-xThe ratio of As crystal Al can be so that waveguide core layer and cladding index be respectively 3.53 and 3.52.In the situation that satisfy single mode condition (12), we arrange core layer thickness a is 2 μ m, only has like this TE in waveguide ₀Mould and TM ₀Mould.Waveguide both sides cladding thickness is all 8 μ m, and waveguide length is 1mm.Can obtain TE according to (9), (10) ₀Mould and TM ₀The propagation constant of mould, and then can get according to formula (11), do not powering up in situation, at output cross section TE ₀Mould and TM ₀The phase differential of mould is 0.068.Powering up in situation, on the x' direction that causes due to linear electro-optic effect, the variation of waveguide core layer and clad crystal refractive index is respectively:

${{\mathrm{\&Delta;n}}_{1x}}^{\&prime;}=\frac{1}{2}{n}_1^3{\mathrm{\&gamma;}}_{41}{E}_z---\left(14\right)$

${{\mathrm{\&Delta;n}}_{2x}}^{\&prime;}=\frac{1}{2}{\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="HDA00002851200100013.png"\; state="\{\{state\}\}">n</figure-callout>}}_2^3{\mathrm{\&gamma;}}_{41}{E}_z---\left(15\right)$

According to (9), (10), (13), (14) and (15), we can obtain TE by numerical evaluation ₀Mould and TM ₀Mould is exported the phase differential in cross section and the relation of on-load voltage in waveguide, as shown in Figure 3.As shown in Figure 3, on-load voltage and TE ₀Mould and TM ₀Mould is linear at the phase differential in waveguide output cross section.What particularly point out is, making phase differential change the required impressed voltage of π is 335V, and namely half-wave voltage is 335V, and the half-wave voltage in this voltage ratio tradition Ultra-high-speed Electro-optic Sampling Technology reduces approximately 1 order of magnitude.Like this, by adopting AlGaAs optical waveguide transmission laser, can greatly reduce half-wave voltage, this also means presses the ability of acquisition of signal greatly to strengthen to light current, in the situation that other condition is constant, can make detectivity improve approximately order of magnitude.

The present invention proposes and utilize the AlGaAs optical waveguide to transmit femtosecond laser as electro-optic crystal, thereby can greatly reduce half-wave voltage, improve the sensitivity of surveying.At first studied in literary composition and powering up and do not power up the transport property of laser in the AlGaAs optical waveguide in situation, and then with concrete model, TE mould, the phase differential of TM mould and the relation between impressed voltage have been discussed, result shows, on-load voltage and TE ₀Mould and TM ₀Mould is linear at the phase differential in waveguide output cross section.Especially adopt the AlGaAs electro-optical transducer to transmit femtosecond laser, can make half-wave voltage reduce an order of magnitude than classic method, thereby can greatly improve the detectivity to weak signal.

The above is only preferred embodiment of the present invention, not in order to limiting the present invention, all any modifications of doing within the spirit and principles in the present invention, is equal to and replaces and improvement etc., within all should being included in protection scope of the present invention.

Claims (4)

1. an electro-optical transducer that can be used for Ultra-high-speed Electro-optic Sampling, is characterized in that, described electro-optical transducer comprises AlGaAs sandwich layer and the AlGaAs covering that is positioned at described AlGaAs sandwich layer both sides, and the outside of the AlGaAs covering of described both sides is respectively equipped with electrode; The AlGaAs covering of described both sides is identical to the refractive index of the femtosecond laser of input, and lower than the refractive index of described AlGaAs sandwich layer to the femtosecond laser of input.

2. electro-optical transducer as claimed in claim 1, is characterized in that, the AlGaAs covering of described both sides satisfies following relation to refractive index and the described AlGaAs sandwich layer of the femtosecond laser of input to the refractive index of the femtosecond laser inputted:

Wherein, n ₁, n ₂Be respectively the refractive index of waveguide core layer and covering, a is the thickness of waveguide core layer, and λ is the wavelength of the femtosecond laser of incident.

3. electro-optical transducer as claimed in claim 2, is characterized in that, described lambda1-wavelength is 1.064 μ m, and the refractive index of the AlGaAs covering of described both sides is 3.52, and thickness is 8 μ m; The refractive index of described AlGaAs sandwich layer is 3.53, and thickness is 2 μ m.

4. electro-optical transducer as claimed in claim 3, it is characterized in that, the field distribution of the light that conducts in described electro-optical transducer is the linear combination of two orthogonal mode fields, and two orthogonal modes phase differential in described electro-optical transducer output cross section is linear change with impressed voltage.

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