CN1249425C - Femtosecond interferometer for testing optical characteristics of scatterer and use method thereof - Google Patents

Abstract

A femtosecond interferometer for testing the optical properties of a scatterer and a method for using the same. The femtosecond interferometer comprises a femtosecond laser, a beam splitter, an optical delay line, a first total reflection mirror, a second total reflection mirror, a third total reflection mirror, a semi-transparent semi-reflecting mirror, a sample chamber, a detector and a calculator. The positional relationship of each component is as follows: a beam splitter is arranged on the laser output optical path of a femtosecond titanium sapphire laser, and an A beam and a B beam are output from the generator. The A beam enters the detector via the second reflection mirror, the optical delay line, the third reflection mirror and the semi-transparent semi-reflecting mirror; while the B beam is reflected by the first reflection mirror into the sample, and then reflected by the semi-transparent semi-reflecting mirror into the detector, where it overlaps and interferes with the A beam to form a hologram. The hologram is digitized by the detector and sent to the computer. The present invention can accurately and high-resolutionly give the optical properties of a strong scatterer, and will provide a powerful tool and research method for biomedicine.

Description

Femtosecond interferometer for testing optical properties of scatterers and method of use thereof

Technical field:

The invention belongs to physical testing instruments, in particular to a femtosecond interferometer for testing the optical properties of scatterers and a method for using the same, which is suitable for testing the optical properties of various strong scatterers.

Background technique:

Since biological tissue is a highly scattering medium, light not only absorbs but also scatters when propagating in the biological medium. The degree of absorption and scattering is related to the incident wavelength. For red light and near-infrared light with a wavelength between 600-1000nm, the absorption of tissue is the smallest, and the attenuation of light is mainly due to scattering. Experimental measurements on human tissue have shown that the mean free path of near-infrared light propagating in tissue is generally tens to hundreds of microns, depending on the incident wavelength.

The mechanism of optical imaging of biological tissue is the interaction between near-infrared light and biological tissue. Different biological tissues have different absorption coefficients and scattering coefficients for near-infrared light.

Studies have shown that the transmitted light after passing through the tissue can be divided into three types: ballistic light, snake light and diffuse light, and the reflected light also follows the same physical mechanism. Ballistic light is unscattered light that still travels in the direction of the incident light. The intensity attenuation of ballistic light in tissue satisfies Beer's law:

I(x)＝I ₀ exp[-(μ _a +μ _s )x]

where μ _a is the absorption coefficient of the tissue and μ _s is the scattering coefficient of the tissue. Obviously, ballistic light can be used for transmission projection imaging similar to X-CT, and its theoretical resolution can reach the diffraction limit. However, due to the extremely weak ballistic light, when the thickness of the tissue reaches tens of mean free paths, the ballistic light may not be detected at all, so its imaging thickness can only reach a few millimeters.

Snaking light is light with relatively few scattering times, and each scattering is a small-angle scattering, so its propagation path can be considered as a near-linear propagation after averaging. Experiments show that the attenuation law of snaking light in biological tissues can be approximately described by Beer's law: I(x)∝exp(-μ _s ′x)

Among them, μ _s ′=(1-g) μs is the transmission and scattering coefficient of the tissue, g is the average cosine value of the photon scattering angle, the typical value is 0.7-0.95, and the snake light also carries part of the information inside the biological tissue, so it can also be used It is suitable for biological tissue imaging, but its resolution is much lower than that of ballistic light, and the detection depth of several centimeters can be obtained with snake light.

Diffuse light consists of light that has been scattered multiple times, and its scattered paths are random. In principle, these photons cannot be used for transmission imaging. When imaging, the diffuse light becomes the background noise and covers the image. Since the intensity of the diffuse light is much greater than that of the ballistic light and the snaking light, the image quality is greatly reduced.

Ballistic light, snaking light and diffuse light propagate in biological tissues at different speeds, different spatial frequencies, and the propagation paths are also different. People's understanding of them is also limited to conceptual nature, so it is particularly important to test the characteristics of these lights .

In recent years, the rapid development of ultrafast femtosecond lasers, especially the commercialization of femtosecond lasers, has brought new opportunities to femtosecond holography and femtosecond interferometry. Using femtosecond laser pulses to record holograms or interferograms to study various ultrafast It is a fast process and some meaningful results have been obtained. However, the linewidth of femtosecond lasers is usually above 5nm, and the shorter the pulse, the wider the spectral line, which brings obstacles to many femtosecond holography research.

Invention content:

The technical problem to be solved by the present invention is to provide a femtosecond interferometer for testing the optical properties of a scatterer and a method of using the same for testing the optical properties of a scatterer.

The basic idea of the present invention is to turn the disadvantage of femtosecond laser spectral line width into an advantage, that is, to use its short coherence length to study the transformation of femtosecond pulsed laser into ballistic light, snake light and diffuse light through strong scatterers Later, the optical properties of the scatterers were tested by interfering with femtosecond pulsed laser light passing through an optical delay line without the scatterers, respectively.

Technical solution of the present invention is as follows:

A femtosecond interferometer for testing the optical properties of a scatterer is characterized in that it includes a femtosecond laser, a beam splitter, an optical delay line composed of five mirrors, a first total reflection mirror, and a second total reflection mirror , the third total reflection mirror, half-transmission mirror, a sample chamber, a detector and a calculator, the positional relationship of each component is as follows: a beam splitter is placed on the laser output optical path of the femtosecond Ti:sapphire laser, The A beam and the B beam are output from the beam splitter, and the A beam enters the detector through the second mirror, the optical delay line, the third mirror and the half mirror; while the B beam enters the sample after being reflected by the first mirror , and then reflected by the semi-transparent mirror into the detector, where it overlaps and interferes with the A beam to form a hologram, which is digitized by the detector and sent to the computer.

The femtosecond laser is a titanium sapphire laser with an output pulse width of 5-150 femtoseconds, a radiation wavelength of 780-830 nm, a line width of 5-10 nm, and a single pulse energy of 5-1000 nanojoules. available in the market.

The sample in question is a scatterer sample.

The detector is a charge-coupled device CCD with a receiving card.

The computer is a computer capable of storing, filtering and reconstructing the digital signal output by the detector.

The method for using the femtosecond interferometer of the present invention to test the optical properties of scatterers is characterized in that it comprises the following steps:

First, by adjusting the distance between the optical delay line mirrors, the optical paths of the A beam and the B beam are adjusted to be equal, so that the ballistic light generated by the B beam passing through the sample interferes with the A beam, and the visibility of the ballistic light interference fringes is measured to obtain the ballistic beam. coherent properties of light;

Then adjust the optical delay line so that the serpentine light produced by the B beam passing through the sample overlaps with the A beam, then measure the visibility of the interference fringes, and adjust the optical delay line repeatedly until the visibility of the interference fringes drops to close to that of ballistic light interference fringes 0.1 or lower to obtain the coherent characteristics of the snaking light.

Finally, use the shading plate to block the A beam, and then observe whether the scattered light of the B beam is appearing on the CCD, and then open the shading plate, so that the A beam and the B beam overlap, and then check the visibility of the interference fringes to obtain the coherence of the scattered light.

Compared with the prior art, the femtosecond interferometer for testing the optical properties of scatterers in the present invention skillfully utilizes the broadband spectral properties of Ti:sapphire lasers, and can provide the optical properties of strong scatterers accurately and with high resolution, which will provide Biomedicine provides a powerful tool and research method.

Description of drawings:

Fig. 1 is a structural block diagram of a femtosecond interferometer for testing the optical properties of a scatterer according to the present invention.

Detailed ways

Please refer to Fig. 1 earlier, Fig. 1 is the structural block diagram of the femtosecond interferometer of testing scatterer optical characteristic of the present invention, as seen from the figure, its composition is that it comprises femtosecond laser 1, a beam splitter 2, a by five An optical delay line composed of mirrors 6, 7, 8, 9, 10, a first total reflection mirror 3, a second total reflection mirror 5, a third total reflection mirror 11, a half mirror 4, a sample chamber, a Detector 12 and a calculator 13, the positional relationship of each element part is as follows: arrange beam splitter 2 on the laser output optical path of femtosecond Ti:Sapphire laser 1, output A light beam and B light beam from this beam splitter 2, A The light beam enters the detector 12 through the second mirror 5, the optical delay line, the third mirror 11 and the half mirror 4, and the B beam is reflected by the first mirror 3 and enters the sample 14, and then is half-reflected Mirror 4 enters the detector 12 through reflection, overlaps and interferes with beam A to form a hologram, and the hologram is digitized by the detector 12 and sent to the computer 13.

The femtosecond laser 1 is a titanium sapphire laser with an output pulse width of 5-150 femtoseconds, a radiation wavelength of 780-830 nm, a line width of 5-10 nm, and a single pulse energy of 5-1000 nanojoules.

The detector 12 is a charge coupled device CCD with a receiving card.

The computer 13 is a computer capable of storing, filtering and reconstructing the digital signal output by the detector.

The method for using the femtosecond interferometer for testing the optical properties of scatterers in the present invention comprises the following steps:

First, by adjusting the distance between the optical delay line mirrors, the optical paths of the A beam and the B beam are adjusted to be equal, so that the ballistic light generated by the B beam passing through the sample 14 interferes with the A beam, and the visibility of the ballistic light interference fringe is measured to obtain the ballistic coherent properties of light;

Then adjust the optical delay line so that the serpentine light produced by the B beam passing through the sample overlaps with the A beam, then measure the visibility of the interference fringes, and adjust the optical delay line repeatedly until the visibility of the interference fringes drops to close to that of ballistic light interference fringes 0.1 or lower to obtain the coherent characteristics of snake light;

Finally, use the shading plate to block the A beam, and then observe whether the scattered light of the B beam is appearing on the CCD, and then open the shading plate, so that the A beam and the B beam overlap, and then check the visibility of the interference fringes to obtain the coherence of the scattered light.

In the embodiment of the present invention:

The said femtosecond laser 1 is a titanium sapphire laser with an output pulse width of 100 femtoseconds, a radiation wavelength of 800 nm, a line width of 10 nm, and a single pulse energy of 50 nanojoules, which can be purchased in the market.

Said beam splitter 2 and half-mirror 4 are actually a kind of half-reflection film, which is a kind of film with a reflectivity of 50% and a transmission rate of 50% when the angle of incidence is 45°. Dielectric diaphragm.

Said total reflection mirrors 3, 5, 6, 7, 8, 9, 10, 11 are dielectric film plates with a reflectivity of 100% at an incident angle of 45°.

Said detector 12 is a charge coupled device CCD with a receiving card.

Said calculator 13 is a display system for displaying interference patterns in real time.

Said sample 14 is a scatterer sample.

The operating principle and basic process of the optical characteristic device of the pulse interferometer test scatterer of the present invention are:

After the femtosecond laser is running, it is incident on the 45° semi-transparent and semi-reflective film plate 2, and is divided into A beam and B beam with equal intensity. After the A light beam passes through the 45° total reflection film plate 5, it enters into the optical retardation formed by the total reflection film plate 6, 7, 8, 9, 10, and then is incident on the 45° total reflection film plate 11. Reflective film 11 reaches detector 12 . The beam B passes through the first total reflection film 3 at 45°, enters the sample 14, passes through the semi-transparent and semi-reflective film 4 at 45°, meets the beam A on the detector 12, and interferes. The interference fringe spacing on the interferogram is related to the angle between the A beam and the B beam.

First, adjust the A beam and the B beam so that their optical paths are equal, and this purpose is achieved by adjusting the distance between the mirrors of the optical delay lines 6, 7, 8, 9, and 10. In this way, the ballistic light produced by the B beam passing through the sample 14 interferes with the A beam, and the visibility of the interference fringes is measured. Use this method to test the coherent properties of ballistic light.

Then adjust the optical delay line so that the serpentine light generated by the B beam passing through the sample overlaps with the A beam, and then measure the visibility of the interference fringes. Repeatedly adjust the optical delay line until the visibility of interference fringes drops to close to 0.1 or lower. Use this method to test the coherent properties of snaking light.

Finally, use the shading plate to block the A beam, and then observe whether the scattered light of the B beam is appearing on the CCD, and then open the shading plate, so that the A beam and the B beam overlap, and then check the visibility of the interference fringes. Use this method to test the coherence properties of scattered light.

Claims (5)

1. A femtosecond interferometer for testing the optical properties of a scatterer is characterized in that it comprises a femtosecond laser (1), a beam splitter (2), a five reflectors (6, 7, 8, 9, 10) An optical delay line composed of a first total reflection mirror (3) and a second total reflection mirror (5), a third total reflection mirror (11), a half mirror (4), a sample chamber, and a detection device (12) and a calculator (13), the positional relationship of the above-mentioned components is as follows: a beam splitter (2) is arranged on the laser output optical path of the femtosecond Ti:sapphire laser (1), and a beam splitter (2) is arranged from the beam splitter (2 ) output A light beam and B light beam, A light beam enters the detector (12) through the second reflector (5), optical delay line, the third reflector (11) and half-reflector (4); and B The beam is reflected by the first mirror (3) into the sample (14), and then reflected by the half mirror (4) and enters the detector (12), where it overlaps and interferes with the A beam to form a hologram. The figure is digitized by the detector (12) and sent to the computer (13). 2. The femtosecond interferometer for testing the optical properties of scatterers according to claim 1, characterized in that said femtosecond laser (1) is a titanium sapphire laser with an output pulse width of 5-150 femtoseconds and a radiation The wavelength is 780-830nm, the line width is 5-10nm, and the single pulse energy is 5-1000 nanojoules. 3. The femtosecond interferometer for testing the optical properties of scatterers according to claim 1, characterized in that said detector (12) is a charge-coupled device (CCD) with a receiving card. 4. The femtosecond interferometer for testing the optical properties of scatterers according to claim 1, characterized in that said computer (13) is a digital signal capable of storing, filtering and reconstructing the digital signal output by the detector. computer. 5. The method for using a femtosecond interferometer for testing the optical properties of a scatterer according to claim 1, characterized in that it comprises the following steps: First, by adjusting the distance between the optical delay line reflectors, the optical paths of the A beam and the B beam are adjusted to be equal, so that the ballistic light produced by the B beam passing through the sample (14) interferes with the A beam, and the visibility of the ballistic light interference fringe is measured, so that Obtain the coherent properties of ballistic light; Then adjust the optical delay line so that the serpentine light produced by the B beam passing through the sample overlaps with the A beam, then measure the visibility of the interference fringes, and adjust the optical delay line repeatedly until the visibility of the interference fringes drops to close to that of ballistic light interference fringes 0.1 or lower to obtain the coherent characteristics of snake light; Finally, use the shading plate to block the A beam, and then observe whether the scattered light of the B beam is appearing on the CCD, and then open the shading plate, so that the A beam and the B beam overlap, and then check the visibility of the interference fringes to obtain the coherence of the scattered light.