CN201018186Y

CN201018186Y - Acoustooptic modulator for femtosecond laser

Info

Publication number: CN201018186Y
Application number: CNU2006201633661U
Authority: CN
Inventors: 毕昆; 曾绍群; 骆清铭; 李德荣
Original assignee: Huazhong University of Science and Technology
Current assignee: Huazhong University of Science and Technology
Priority date: 2006-12-01
Filing date: 2006-12-01
Publication date: 2008-02-06
Anticipated expiration: 2016-12-01

Abstract

The utility model discloses an acousto-optic modulator applied for femtosecond pulse consisted of two parallel acoustic-optical deflectors in reversed direction with the same acoustic frequency. The distance between the acousto-optic deflectors is L plus or minus 10 per cent L. The value of L meets the requirements of formula I, in which v represents the acoustic speed and f represents the acoustic frequency in the acoustic-optical deflector; GDD m represents the material dispersion brought by the acoustic-optical deflector; and Lambada represents the laser wave length. The utility model can realize coaxial adjustment while the acoustic-optical deflector adjusts the laser intensity of different wave length, and at the same time the utility model redeems the time and spatial dispersion caused by the acoustic-optical crystal to the super short pulse. Having the advantages of flexible structure and easy adjustment, the utility model is applicable in the fields such as multi-photon scanning, imaging and laser micro-processing. L is equal to GDD m 2 Pi <2> / Lambada <3> (v/f) <2>.

Description

Acousto-optic modulator for femtosecond laser

Technical Field

The utility model belongs to femto second laser modulation technique, concretely relates to an acousto-optic modulator (AOM) for femto second laser, it is especially suitable for fields such as many photon laser scanning measurement, formation of image, laser micromachining.

Background

The rapid modulation of laser power by acousto-optic modulators is a very widely used technique, especially in laser measurement, imaging and micromachining. And the acousto-optic modulator has a large market at home and abroad. However, these acousto-optic modulators are directed to continuous light sources and cannot be applied to ultrashort pulse lasers. The ultrashort pulse high instantaneous peak power and high resolution capability have very important application in the laser imaging and micromachining field, and are hot spots of application and research at home and abroad in recent years.

In U.S. Pat. No.6,804,000B2, a single prism 1 is used to compensate the spatial dispersion of a single acousto-optic deflector 2 (as shown in fig. 1), the invention cannot compensate the temporal dispersion of the acousto-optic deflector, and when the wavelength of incident light changes, the light beam emitted from the prism deviates from the optical axis, so that the subsequent optical path is no longer aligned or even cannot be used. Since the optical path length in the optical path is fixed for a laser light of a certain wavelength, modulation of the phase cannot be achieved.

In the article "Double-pass optical modulator system" (Review of Scientific Instruments, 2005) by e.a. donley, a configuration of an acousto-optic deflector 2, a convex lens 3 and a mirror 4 is used (see fig. 2), the acousto-optic deflector 2 and the mirror 4 are located at the front focal plane and the back focal plane of the convex lens 3, respectively, and the mirror 4 returns the beam in its original path. The structure determines that the time dispersion brought by the acousto-optic modulator cannot be compensated at all, but the pulse width can be widened secondarily, so that the pulse width of emergent light is widened a lot. Also, the optical path length of the light beam in this structure is constant, and modulation of the phase cannot be achieved. In addition, due to the structure, the laser beams with different wavelengths can return in the original path, so that the lasers with the different wavelengths are mixed together, and the lasers with the different wavelengths cannot be selectively and rapidly switched.

The spectral width of the femtosecond laser is very wide compared with that of monochromatic light, and the acousto-optic deflector is a high-dispersion optical element, and the femtosecond laser generates spatial and temporal dispersion after passing through the acousto-optic deflector, so that pulse broadening and facula divergence are caused. As shown in fig. 3, the incident light is incident at a bragg angle i. The zero-pole light propagates in a straight line. The included angle between the first-order light and the zero-pole light is theta. When the device is used for femtosecond laser modulation, the divergence angle of the primary light is delta theta due to the bandwidth of the spectrum, so that the original circular light spot is expanded into an ellipse in the scanning direction and is also expanded in the pulse time width due to material dispersion. The device reduces the instantaneous power and the excitation efficiency of the femtosecond laser, and loses the advantages of the femtosecond laser. Compression of spatial and temporal dispersion is typically achieved in existing systems using grating pairs or prism pairs. The acousto-optic modulator is more and more widely applied to the laser intensity modulation in recent years, but is mainly applied to the field of continuous light, and if the acousto-optic modulator is applied to femtosecond laser, serious spatial and temporal dispersion can be caused.

Disclosure of Invention

An object of the utility model is to provide an acousto-optic modulator for femto second laser, the device can compensate time and spatial dispersion that acousto-optic modulator brought well.

The utility model provides an acousto-optic modulator for femtosecond pulse, including two acousto-optic deflectors, two acousto-optic deflector antiparallel are placed, and the sound wave frequency equals, and the interval of two acousto-optic deflectors is L, and the L value satisfies (I) formula requirement:

where v and f are the speed and frequency of sound in the acousto-optic deflector, respectively, GDD _m The group delay for the two acoustic-optical deflectors is given by λ, which is the laser wavelength.

The utility model discloses the device can be with the coaxial output of the incident laser of different wavelength, and the emergent light still is according to former direction outgoing when incident light wavelength changes promptly. And the time and space dispersion brought by the acousto-optic modulator can be completely compensated. The utility model discloses not only can be used for femto second laser also can directly be used for the modulation of continuous light, it can carry out quick switching between certain wavelength spaced continuous light or femto second laser. Laser light of a certain fixed wavelength can realize rapid modulation of phase by changing the optical path. Because the utility model discloses can realize acousto-optic modulator's space and time dispersion, can also realize the fast modulation to the phase place, consequently be applicable to many photon systems, including many photon laser scanning measurement formation of image, laser field such as little processing.

Drawings

FIG. 1 is a schematic diagram illustrating the principle of prism compensation for spatial dispersion of a single acousto-optic modulator.

FIG. 2 is a schematic diagram of the principle of compensating spatial dispersion with an acousto-optic modulator using convex lenses and mirrors.

FIG. 3 is a schematic diagram of the spatial and temporal dispersion of the acousto-optic modulator to the femtosecond laser.

FIG. 4 is an optical diagram of the wavelength of incident light and the operating frequency of an acousto-optic modulator to achieve coaxial output.

Fig. 5 is a schematic diagram of the device of the present invention.

FIG. 6 is a graph showing the relationship between the spacing between two acousto-optic modulators and the pulse width of the emitted laser.

FIG. 7 is a graph showing the relationship between the wavelength of the incident laser and the pulse width of the outgoing laser when the distance between two acousto-optic modulators is 65 cm.

Fig. 8 is a schematic diagram illustrating the principle of the present invention for realizing phase modulation.

Fig. 9 is a graph of frequency difference versus phase difference.

Fig. 10 is a spectrum diagram of the present invention when switching the femtosecond laser incident and emergent systems with five wavelengths of 740nm, 780nm, 820nm, 860nm and 900 nm. There is no loss of spectral content.

Fig. 11 is a diagram of an application example of the device of the present invention.

FIG. 12 is a graph of wavelength versus system transmittance at maximum modulation power.

Fig. 13 is a diagram of another embodiment of the present invention.

FIG. 14a is the pulse width of the original femtosecond laser; FIG. 14b shows the pulse width of the femtosecond laser beam broadened as it passed through an acousto-optic modulator; FIG. 14c shows the pulse width of the femtosecond laser after the time dispersion compensation by the present apparatus.

FIG. 15 is a graph comparing signal intensities when a compensated femtosecond laser and an uncompensated femtosecond laser are used to Z-scan a fluorescent layer.

Detailed Description

The present invention will be described in further detail with reference to the accompanying drawings and examples.

In order to realize that emergent light does not deviate from an optical axis after different incident lasers enter a system, the sound wave frequency in the sound-light modulator needs to be correspondingly modulated. The formula of the exit angle theta of the laser after passing through the acousto-optic modulator is as follows:

wherein lambda is the wavelength of the incident laser, f is the emission frequency of the sound in the acousto-optic modulator, and v is the velocity of the sound wave in the acousto-optic crystal.

Coaxial modulation can be achieved if the product of the wavelength of the incident laser light and the acoustic frequency in the acousto-optic modulator is a constant. Assuming that the light path is adjusted at a certain wavelength lambda _m Laser and acousto-optic modulator operating at a frequency f _m Going on for different wavesLong lambda _i Frequency f of the incident light acousto-optic modulator _i The calculation method is as follows:

the relationship between the operating frequency of the acousto-optic modulator and the wavelength λ is shown in FIG. 4.

As shown in fig. 5, the present invention includes a first acousto-optic deflector 5 and a second acousto-optic deflector 6, where the two acousto-optic deflectors are placed in opposite directions so that the propagation directions of acoustic waves are opposite, the distance is L, and the L value calculation method is as follows:

where c is the speed of light, GDD _m Is the group delay caused by the crystal materials of the two acousto-optic deflectors, if the time dispersion caused by other dispersion elements in the optical path needs to be compensated, at the moment, the GDD _m The sum of the group delay amounts of all the elements in the optical path should be taken to simultaneously compensate for the time dispersion of other dispersive elements in the optical path.

Fig. 6 is a graph of pulse width versus distance L. Under the conditions that the femtosecond laser wavelength is 800nm, the initial pulse width is 141fs and the frequency applied by the acousto-optic deflectors is 96MHz, the distance L between the two acousto-optic deflectors is changed, and the pulse width of emergent light is changed. It can be seen from the figure that as L increases, the amount of compensation increases and the pulse width decreases, completely compensating for the system time dispersion at a distance of about 65 cm. When L > 65cm enters the overcompensation region, the pulse bandwidth begins to increase.

As shown in FIG. 7, when the wavelength λ is determined to change after L, the clock width still has a compressed effect, which is less than 160fs. In the case of femtosecond laser wavelength of 800nm, the frequency applied by the acousto-optic deflector is 96MHZ, and the distance L between the two acousto-optic deflectors is 65 cm. When the femtosecond laser with different wavelengths enters, the pulse width of the laser is emitted.

The utility model discloses a this kind of structure still possesses following function except having as the basic function that acousto-optic modulator should possess modulation laser intensity: the coaxial output can be realized for the incident light with different wavelengths; not only can the space and time dispersion of the acousto-optic device be compensated, but also the time dispersion brought by other dispersion elements in the optical path can be compensated; the phase can be rapidly modulated; but also can be directly applied to a continuous laser system or a mixed system of continuous laser and femtosecond laser.

The space dispersion amount of the femtosecond laser passing through two acousto-optic deflectors arranged in an antiparallel manner is as follows:

wherein f is ₁ ，f ₂ Respectively, the frequency of the acoustic wave in the first and second acousto-optic deflectors, and v is the propagation velocity in the acousto-optic crystal. So when f ₁ And f ₂ When equal, the spatial dispersion in the system can be eliminated.

As shown in fig. 8, for an incident light with a certain wavelength, assuming that two acousto-optic deflectors operate at a frequency f, when the operating frequencies of the two acousto-optic deflectors are changed to f + Δ f, the difference Δ ω is:

wherein L is the distance between the two acousto-optic deflectors, the distance satisfies formula (III), and theta is the Bragg angle of the acousto-optic crystal. Δ f is in the range of 0.75MHz, providing a phase difference in the range of 9921rad and no spatial and temporal dispersion. Fig. 9 is a graph of frequency difference versus phase difference.

Because the clear aperture of the second acousto-optic deflector is limited and the two acousto-optic deflectors are separated by L and Bragg angles, the invention has the function of tunable filtering, and as mentioned above, the incident light lambda of each wavelength corresponds to the frequency f matched with the incident light lambda, and if the value of f is not matched with lambda, the light coming out of the first acousto-optic deflector deviates from the optical axis, so that the diffraction efficiency is very low even the light cannot pass through the clear aperture when passing through the second acousto-optic deflector, therefore, the function of rapidly switching certain interval wavelengths can be realized, which cannot be realized by the structure shown in FIG. 2. Since the continuous laser does not have the problem of time dispersion, the present invention can be directly applied to the continuous laser. For the femtosecond laser, the wavelength resolution is 30nm (as shown in fig. 10), so that all spectral components of the femtosecond laser pass through and the femtosecond or continuous laser with the interval of 30nm can be effectively distinguished, and thus, the rapid switching between the lasers with multiple wavelengths (including the femtosecond laser and the continuous laser) can be realized.

Finally, the modulation of the emergent laser intensity can be realized by modulating the power of the acousto-optic deflector.

If the acousto-optic deflectors can change the polarization state, a 1/2 wave plate is arranged between the two acousto-optic deflectors; if the acousto-optic deflectors do not change the polarization state, a 1/2 wave plate can be omitted between the two acousto-optic deflectors.

Example 1:

as shown in fig. 11, collimated laser light (800 nm) emitted from a laser 7 passes through first and second acoustic-

optical deflectors

5 and 6 each having an operating frequency of 96MHz and then strikes a white screen 9, and the position is observed and recorded by a CCD 8, and the wavelength (710 nm) of the laser is changed so that the laser does not pass through the system, no light spot is present on the white screen 9, the operating frequencies of the two acoustic-optical deflectors are changed to 108.2MHz, and the CCD 8 observes that the laser strikes the same position on the white screen 9. The wavelength of the laser 7 is changed to 990nm, no light spot is on the white screen at this time, the working frequency of the two acousto-optic deflectors is changed to 77.6MHz, and the CCD 8 observes that the laser still hits the same position on the white screen 9. Varying the power of either or both of the acousto-optic deflectors achieves modulation of the diffraction efficiency in the range of 0-89%, with the maximum transmission of the entire system at 77-89% for different wavelengths λ (as shown in figure 12). This device not only can be used for femto second laser, can directly be applied to continuous laser moreover, because continuous light does not have the problem of time dispersion, so the utility model discloses can be applied to in the hybrid system of continuous laser and femto second laser to can come to carry out fast switch over (10 MHz) to specific wavelength through the settlement to acousto-optic deflector frequency.

Example 2:

according to the schematic diagram shown in fig. 13, a beam of femtosecond laser (800 nm) with a pulse width of 141fs (see fig. 14 a) passes through the first acousto-optic deflector 5, the frequency of the acousto-optic deflector is set to 96MHz, the femtosecond laser is broadened to 345fs (see fig. 14 b), and has spatial dispersion, and after the femtosecond laser transmits a distance L between the two acousto-optic deflectors, the temporal dispersion is over-compensated by an amount that can just offset the temporal dispersion brought by the second acousto-optic deflector 6. After the femtosecond laser passes through the second acousto-optic deflector 6, the spatial dispersion is also completely compensated, the emergent light has no temporal dispersion, and the pulse width is still 141fs (as shown in fig. 14 c). The green fluorescent sample layer 13 was scanned in the Z direction with a 40-fold objective lens 12 using a laser beam (2 mW) of the same power of 141fs after complete dispersion compensation and 345fs without dispersion compensation, respectively, and the dichroic mirror 10 was used to separate excitation light and fluorescence and the PMT 11 was used to collect fluorescence signals. The femtosecond pulse of 141fs can excite the fluorescent protein, and the light beam of 345fs can not excite the fluorescent protein to generate a fluorescent signal under the power (as shown in figure 15). In the figure, 141fs laser after complete dispersion compensation (the device of the present invention) and 345fs laser without dispersion compensation through a single acousto-optic deflector excite the signal intensity obtained by the green fluorescence sample under the condition that the laser output power is the same, and the intensity ratio is about 5.8: 1.

Claims

1. An acousto-optic modulator for a femtosecond laser, characterized by: including two reputation deflectors, two reputation deflectors are antiparallel to be placed, and the sound wave frequency equals, and the interval of two reputation deflectors is L, and the L value satisfies formula (I) requirement:

where v and f are the speed and frequency of sound in the acousto-optic deflector, respectively, GDD _m The group delay brought by the two acousto-optic deflectors is lambda, which is the laser wavelength.

2. An acousto-optic modulator for a femtosecond laser, characterized by: a1/2 wave plate is arranged between the two acousto-optic deflectors.