CN101706617A - Acousto-optic filter - Google Patents

Abstract

The invention discloses an acousto-optic filter. The acousto-optic filter comprises two lenses, two acousto-optic deflectors and a liquid crystal spatial light modulator. The focal distances of the two lenses are equal, and the two lenses are arranged in a confocal way; the two acousto-optic deflectors are completely the same and are respectively arranged at the front focal points and the back focal points of the two lenses; and the liquid crystal spatial light modulator is arranged at the common focal point of the lenses and provides an adjustable phase for carrying out color dispersion effect. The invention can realize microsecond stage quick switching between a plurality of wavelength lasers. Furthermore, the collinearity of emergent light beams and incident light beams can be kept, the adjustment of a light path is convenient, and the color dispersion effect of femtosecond lasers is completely eliminated. The invention has compact structure and easy adjustment and can be widely applied to a plurality of research fields of femtosecond laser biologic imaging, optical storage, micro-machining and the like.

Description

Acousto-optic filter

[technical field]

The present invention relates to a kind of acousto-optic filter.

[background technology]

Femtosecond laser has very important application because its duration of pulse is short, the peak power advantages of higher in fields such as bio-imaging, optical storage and microfabrication, is one of focus of domestic and international laser application in recent years.The matter of utmost importance that acousto-optical device is used to control the required solution of femtosecond laser is exactly an effect of dispersion.Effect of dispersion comprises spacial dispersion effect and time dispersive effect.

The acousto-optic crsytal that comprises in the conventional acousto-optical device is a kind of high dispersive medium, and femtosecond laser will be by serious broadening through the acousto-optic crsytal afterpulse, and this effect is called time dispersive (as shown in Figure 1).Simultaneously, have wavelength dependence through the beam deflection angle behind the acousto-optical device (for example acoustooptic deflector), cause the beam divergence distortion, this effect is called spatial dispersion (as shown in Figure 2).These two kinds of effect of dispersions make the advantage of femtosecond laser not exist, and therefore, the matter of utmost importance that acousto-optical device is used to control the required solution of femtosecond laser is exactly an effect of dispersion.

[summary of the invention]

In view of this, be necessary the effect of dispersion problem that exists when being used to control femtosecond laser to provide a kind of acousto-optic filter that can solve the effect of dispersion problem at traditional acousto-optical device.

A kind of acousto-optic filter comprises first lens, second lens, first acoustooptic deflector, second sound light deflector, the LCD space light modulator that are positioned on the same light path; The focal length of first lens and second lens equates, and the back focus of first lens overlaps with the front focus of second lens; First acoustooptic deflector and second sound light deflector are placed on the front focus of first lens and the back focus of second lens respectively, and first acoustooptic deflector and second sound light deflector are identical, and are operated under the identical frequency; LCD space light modulator is placed on the common focus place of first lens and second lens, and the phase place Φ (ω of LCD space light modulator _j) satisfy formula I:

$\mathrm{\Φ}\left({\mathrm{\ω}}_j\right)=-\frac{1}{2}{\mathrm{GDD}}_m{({\mathrm{\ω}}_j-{\mathrm{\ω}}_0)}^2$ j∈[-N，N] (I)

GDD wherein _mBe the material dispersion that first acoustooptic deflector and second sound light deflector are introduced, ω ₀Be the centre frequency of incident laser, ω _jBe the spectral frequency component at j liquid crystal cells place on the LCD space light modulator, N is a natural number.

Preferably, the clear aperature of described first acoustooptic deflector or second sound light deflector is 4.2mm, and crystalline material is TeO ₂

Preferably, the design wavelength of described first acoustooptic deflector or second sound light deflector is λ _c=800nm, centre frequency is f _c=96MHz.

Above-mentioned acousto-optic filter can keep outgoing beam and incident beam conllinear, thereby eliminates the effect of dispersion problem of femtosecond laser fully.And compact conformation is easy to regulate.

[description of drawings]

Fig. 1 is a time dispersive stretched pulse synoptic diagram.

Fig. 2 disperses synoptic diagram for spatial dispersion makes hot spot.

Fig. 3 is the synoptic diagram of acousto-optic filter.

Fig. 4 is the synoptic diagram of acoustooptic deflector transit time.

Fig. 5 is the synoptic diagram of the relation curve of the frequency of operation of first acoustooptic deflector or second sound light deflector and laser wavelength of incidence.

Fig. 6 is the synoptic diagram of each liquid crystal pixel polishing wax frequency distribution of LCD space light modulator.

[embodiment]

The present invention is described in further detail below in conjunction with accompanying drawing and example.

Fig. 3 is the synoptic diagram of acousto-optic filter.Acousto-optic filter comprises first lens 5, second lens 6, first acoustooptic deflector 7, second sound light deflector 8, the LCD space light modulator 9 that is positioned on the same light path.

The focal length (representing with F among the figure) of first lens 5 and second lens 6 equates, and the back focus of first lens 5 overlaps with the front focus of second lens 6.First acoustooptic deflector 7 and second sound light deflector 8 are placed on the front focus of first lens 5 and the back focus of second lens 6 respectively, and first acoustooptic deflector 7 and second sound light deflector 8 are identical, and are operated under the identical frequency.LCD space light modulator 9 is placed on the common focus place of first lens 5 and second lens 6, and the phase place Φ (ω of LCD space light modulator 9 _j) satisfied following formula I:

$\mathrm{\Φ}\left({\mathrm{\ω}}_j\right)=-\frac{1}{2}{\mathrm{GDD}}_m{({\mathrm{\ω}}_j-{\mathrm{\ω}}_0)}^2$ j∈[-N，N] (I)

GDD wherein _mBe the material dispersion that first acoustooptic deflector 7 and second sound light deflector 8 are introduced, ω ₀Be the centre frequency of incident laser, ω _jBe the spectral frequency component at j liquid crystal cells place on the LCD space light modulator 9, N is a natural number.

Existing commercialization acousto-optical device can be less than the acoustic optic interaction time of 10 microseconds easily, therefore, selects acoustooptic deflector to be used to realize the quick switching of a plurality of wavelength lasers, and its speed can reach the microsecond magnitude fully.

The clear aperature of first acoustooptic deflector 7 or second sound light deflector 8 is 4.2mm, and crystalline material is TeO ₂, being operated under unusual Bragg diffraction (Bragg diffraction) pattern, the velocity of propagation of ultrasound wave in crystal is v=650m/s, its maximum transit time is τ _Max(the maximum transit time is meant that sound wave passes through the required time of overall optical beam diameter D to=6.5 μ s, and computing formula is τ _Max=D/v, as shown in Figure 4).When beam sizes reduced, its transit time can also be littler.Therefore, adopt acoustooptic deflector can realize fully the microsecond level between a plurality of wavelength lasers is switched fast.

When acoustooptic deflector is operated in a certain frequency, after different wavelength of laser enters acoustooptic deflector, will be along different directions outgoing (as shown in Figure 2).The emergent light deflection angle theta that is operated under the unusual Bragg diffraction pattern satisfies formula II:

$\mathrm{\θ}=\frac{\mathrm{\λf}}{v}---\left(\mathrm{II}\right)$

Wherein f is the frequency of operation of acoustooptic deflector, and λ is an optical maser wavelength, and v is hyperacoustic velocity of propagation in the acousto-optic crsytal.

For the ease of using, usually acoustooptic deflector is designed to following situation: when laser wavelength of incidence be the design wavelength of acoustooptic deflector just, and acoustooptic deflector is when being operated in centre frequency, outgoing beam and incident beam maintenance conllinear.In the present invention, the design wavelength of first acoustooptic deflector 7 or second sound light deflector 8 is λ _c=800nm, centre frequency is f _c=96MHz.Therefore, when laser wavelength of incidence is 800nm, when acoustooptic deflector was operated in 96MHz, outgoing beam and incident beam kept conllinear.

From formula II as can be seen, when laser wavelength of incidence λ changes,, make λ f=λ if change the frequency of operation f of acoustooptic deflector simultaneously _cf _c, then still can keep beam deflection angle θ constant.Therefore, when the frequency of operation of acoustooptic deflector satisfied formula III, outgoing beam and incident beam can keep conllinear:

$f=\frac{{\mathrm{\λ}}_c{f}_c}{\mathrm{\λ}}---\left(\mathrm{III}\right)$

What Fig. 5 illustrated is the frequency of operation of first acoustooptic deflector 7 or second sound light deflector 8 and the relation curve of laser wavelength of incidence.

Based on above-mentioned principle, when the laser to a plurality of wavelength switches, at first select necessary wavelength, and the frequency of operation of regulating acoustooptic deflector makes it satisfy formula III, thereby make the laser of required wavelength still transmit along former incident direction, the laser of other wavelength then departs from incident direction, can not enter follow-up system.Therefore, this device is actually the acousto-optic filter of a fast tunable.

Femtosecond laser is through behind the acoustooptic deflector, and by formula II as can be known, the spectral component of different wave length will be propagated along different directions, thereby make light beam have certain angle of divergence Δ θ, and this effect is called as spatial dispersion (as shown in Figure 2).The parameter of describing this effect is:

$\frac{\mathrm{d\θ}}{\mathrm{d\λ}}=\frac{f}{v}---\left(\mathrm{IV}\right)$

In acousto-optic filter of the present invention (as shown in Figure 3), first acoustooptic deflector 7 passes through the Relay lens inverse imaging of first confocal lens 5 and second lens, 6 compositions on second sound light deflector 8, and two focal lengths of lens equate, do not amplify spatial dispersion.Therefore, femtosecond laser through two acoustooptic deflectors after, the total space dispersion parameters is:

$\frac{\mathrm{d\θ}}{\mathrm{d\λ}}=\frac{f_1-f_2}{v}---\left(V\right)$

F wherein ₁And f ₂Be respectively the frequency of operation of first acoustooptic deflector 7 and second sound light deflector 8.When two acoustooptic deflectors are operated under the same frequency, i.e. f ₁=f ₂The time, the spatial dispersion of femtosecond laser is eliminated fully.

Femtosecond laser through first acoustooptic deflector 7 and second sound light deflector 8 after, because the velocity of propagation difference of different spectral components in acousto-optic crsytal, have time delay each other, thereby cause pulse strenching, this effect is called as time dispersive (as shown in Figure 1).In the present invention, main time dispersive derives from the acousto-optic crsytal in the acoustooptic deflector.Acousto-optic crsytal material commonly used is TeO ₂, be a kind of high dispersive medium, its group delay chromatic dispersion expression formula is:

${\mathrm{GDD}}_m=\frac{{\mathrm{\λ}}^3}{2{\mathrm{\πc}}^2}\·\frac{d^{2}n}{{\mathrm{d\λ}}^2}\·l---\left(\mathrm{VI}\right)$

Wherein c is the light velocity in the vacuum, and n is the material refractive index, and l is a crystal thickness.

The group delay dispersion measure that single acoustooptic deflector is introduced is about 7000fs ², the group delay dispersion measure of two acoustooptic deflector introducings is about 14000fs ², this dispersion measure will be the laser pulse stretching of 100fs to 400fs, and seriously reduces the peak power of femtosecond laser.

Acousto-optic filter of the present invention adopts the time dispersive of 9 pairs of femtosecond lasers of LCD space light modulator to compensate.The elementary cell of LCD space light modulator 9 is liquid crystal pixels, and each pixel is under the effect of extra electric field, and its refractive index will change, thereby to modulating through femto-second laser pulse phase place wherein.In actual applications, can be according to the phase function of the group delay dispersion measure design LCD space light modulator of introducing in the system.

First acoustooptic deflector 7 at the space horizontal spreading, on j liquid crystal pixel, has corresponding spectral component ω with the spectrum of femto-second laser pulse _j(as shown in Figure 6). in fact, the group delay chromatic dispersion is the second derivative of spectrum phase about spectral frequency, therefore, for the make-up time chromatic dispersion, only need on LCD space light modulator 9, to add certain phase place, the size of its group delay dispersion measure that provides and the material dispersion equal and opposite in direction of two acoustooptic deflectors are provided, opposite in sign get final product (promptly-GDD _m), therefore, added phase function must satisfy formula I on the LCD space light modulator 9.

The number of the liquid crystal pixel on the LCD space light modulator 9 is 2N+1.In actual applications, the liquid crystal cells of LCD space light modulator 9 is meticulous more, and is good more to the compensation effect of time dispersive.

Above-mentioned LCD space light modulator 9 can add phase function arbitrarily by the method for programming, thereby can realize the effect of dispersion amount of size arbitrarily in theory.Based on this principle, can effectively dwindle the bulk of acousto-optic filter of the present invention, make its structure more compact.For example, selecting focal length is the lens of 5cm, and the size of total system can narrow down to about 20cm.In addition, in the process of selecting different wave length, only need the phase function on the corresponding change LCD space light modulator 9, can realize time dispersive effect to the different wave length femtosecond laser, and the structure of modifier again, this will bring great convenience to practical application.

Can see that by above-mentioned analysis the present invention can realize that the microsecond level between a plurality of wavelength lasers switches fast, can keep outgoing beam and incident beam conllinear, is convenient to the adjusting of light path, and eliminates the room and time effect of dispersion of femtosecond laser fully.Its compact conformation is easy to regulate, and can be widely used in numerous research fields such as femtosecond laser bio-imaging, optical storage and microfabrication.

It is to be noted at last, though what the device among the present invention was primarily aimed at is Application of Spectral Laser, but in fact also can be applied to continuous laser fully: because the monochromaticity of continuous laser is better, the influence of spatial dispersion and time dispersive is all less, therefore, not needing to carry out effect of dispersion can normally use.

The above embodiment has only expressed several embodiment of the present invention, and it describes comparatively concrete and detailed, but can not therefore be interpreted as the restriction to claim of the present invention.Should be pointed out that for the person of ordinary skill of the art without departing from the inventive concept of the premise, can also make some distortion and improvement, these all belong to protection scope of the present invention.Therefore, the protection domain of patent of the present invention should be as the criterion with claims.

Claims (3)

1. an acousto-optic filter is characterized in that: comprise first lens, second lens, first acoustooptic deflector, second sound light deflector, the LCD space light modulator that are positioned on the same light path; The focal length of first lens and second lens equates, and the back focus of first lens overlaps with the front focus of second lens; First acoustooptic deflector and second sound light deflector are placed on the front focus of first lens and the back focus of second lens respectively, and first acoustooptic deflector and second sound light deflector are identical, and are operated under the identical frequency; LCD space light modulator is placed on the common focus place of first lens and second lens, and the phase place Φ (ω of LCD space light modulator _j) satisfy formula I:

$\mathrm{\Φ}\left({\mathrm{\ω}}_j\right)=-\frac{1}{2}G{\mathrm{DD}}_m{({\mathrm{\ω}}_j-{\mathrm{\ω}}_0)}^2,j\∈[-N,N]---\left(I\right)$

GDD wherein _mBe the material dispersion that first acoustooptic deflector and second sound light deflector are introduced, ω ₀Be the centre frequency of incident laser, ω _jBe the spectral frequency component at j liquid crystal cells place on the LCD space light modulator, N is a natural number.

2. acousto-optic filter according to claim 1 is characterized in that: the clear aperature of described first acoustooptic deflector or second sound light deflector is 4.2mm, and crystalline material is TeO ₂

3. acousto-optic filter according to claim 1 is characterized in that: the design wavelength of described first acoustooptic deflector or second sound light deflector is λ _c=800nm, centre frequency is f _c=96MHz.

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