CN1282892C - Method and apparatus for polarizing partial wave based on optical interference and dispersion principle - Google Patents

Abstract

The present invention relates to a polarization wave dispersion device based on principles of optical interference and chromatic dispersion, which is composed of a polaroid, a birefringent crystal wave plate and a wide band polarization beam splitter, wherein incident light generates a beam of incident polarized light through a polarizer, and incident polarized light makes the vibration direction of the incident polarized light and an optical axis of the wave plate form normal incidence at a pi /4 included angle through the birefringent crystal wave plate. The incident polarized light in the wave plate is divided into a beam of extraordinary light and a beam of ordinary light which have different propagation rates. Electric displacement vectors De and D o of the extraordinary light and the ordinary light vibrate in two mutually perpendicular directions, refractive indexes ne and no of the extraordinary light and the ordinary light are different in the propagation of the wave plate, and after penetrating through the wave plate, the ordinary light and the extraordinary light from a phase difference delta=2 pi (ne-no) d/lambda. The phase difference is a function of wavelength lambda, a passage penetrates through a waveband of light wave, and the other passage perpendicular to the passage penetrates through the other waveband of light wave which is perpendicular to the vibration direction of the light wave of the first passage. The wavelength and the bandwidth of dispersive beams can be optionally designed, and structures of the dispersive beams are compact.

Description

Method and device for polarization splitting based on the principle of optical interference and dispersion

1. Technical field

The invention relates to a polarization splitting method and device, in particular to a polarization splitting method and device based on the principles of optical interference and dispersion.

2. Background technology

Wavelength splitting devices play an important role in optics, optical devices, and fiber optic communications, and are generally used as filters and wavelength division multiplexing devices. Traditional wave splitting devices consist of gratings (refractive and reflective gratings, or fiber gratings) and optical interferometers, including Fabry-Perot interferometers, Mach-Zehnder interferometers, and multilayer optical thin film systems. The split beams emerging from these devices have the same polarization state as the incident light. However, in interferometry and some optical sensors, such as fiber optic gyroscopes, linearly polarized light is often required. The traditional method is to insert a polarizer on the optical path between the incident light and the splitting device, but this method inevitably reduces the reliability of the system and increases the loss of light intensity. Therefore, it is very important to design a new type of polarization splitting device suitable for multi-channel interferometers and other related optical systems.

Typically, polarization beamsplitting can be achieved by passing the incident light through birefringent crystals such as calcite and α-quartz. However, the two perpendicularly polarized light waves emerging from the birefringent crystal have the same spectrum as the incident light.

3. Contents of the invention

The object of the present invention is to construct a polarized beam splitting method and device based on the principle of crystal optics and dispersion, so that the outgoing two mutually perpendicularly polarized light waves have different central wavelengths; the present invention also provides a precise Polarization beam splitting method and device. For example, when two light waves with center wavelengths λ ₁ and λ ₂ with a certain bandwidth enter the device, two beams of polarized light vibrating perpendicularly to each other are decomposed, and the center wavelengths are λ ₁ and λ ₂ respectively. The two outgoing light waves are two wave bands whose vibration directions are perpendicular to each other. Provides a polarizing filter with two channels as beam splitting in a certain wavelength band. That is, when the incident light wave band covers the range from ultraviolet to visible light, one channel transmits polarized ultraviolet light waves, and the other channel transmits polarized visible light waves. Or when the incident light waves cover the visible to infrared range, one channel transmits polarized visible light waves, and the other channel transmits polarized infrared light waves.

The scheme of the present invention is achieved like this: a kind of polarization beam splitting method and device based on crystal optics and dispersion principle, utilize polarized light in waveplate, due to ordinary light (o-light) and extraordinary light (e- The refractive index _ne , n _o of light) is not the same, and the incident light is divided into two beams of light with different propagation speeds. The electric displacement vectors D _e and D _o of e-light and o-light vibrate in two mutually orthogonal directions and are at right angles to the normal of the wave plate, as shown in Fig. 1, Fig. 1. Incident light field vector Schematic projections on the fast and slow axes of the waveplate and axial projections of the polarizer and analyzer. θ is the angle between polarizer and analyzer axis. φ is the angle between the electric displacement vector D _o in the crystal and the axial direction of the polarizer.

In Figure 1, the incident light is vertically incident on the wave plate, and the vibration directions of the vectors D _e and D _o in the crystal are orthogonal. In order to study the interference produced by the optical system composed of polarizer, wave plate and analyzer, we let φ be the angle between the vector D _o and the axis of the polarizer, and let θ be the axis of the polarizer and the analyzer angle between the shafts. The components of the electric field E of the incident light in the direction of vector D _e and D _o are Esin(φ) and Ecos(φ) respectively. The e-light and o-light pass through a wave plate with a thickness of d to produce a phase difference δ=2π( _ne -n _o )d/λ, where λ is the wavelength of the incident light in vacuum. In this system, only incident light waves vibrating parallel to the analyzer axis can pass through. In Figure 1, the components OG and OF are the projections of the optical fields OC and OB on the analyzer axis, which are represented as Esin(φ)sin(φ-θ) and Ecos(φ)cos(φ-θ) respectively. When the axes of the polarizer and the analyzer are parallel to each other (θ=0), the transmittance T (T=I _t /I _i , where I _t and I _i are the intensity of the outgoing light and the incident light respectively) can be expressed as T=1-sin ² (2φ)sin ² (π(n _e -n _o )d/λ). For a given phase difference δ, when φ=π/4, the transmittance has a maximum value of cos ² (π(ne-no)d/λ). In the case that the axes of the polarizer and the analyzer are orthogonal (θ=π/2), the transmittance also has a maximum value sin ² (π(n _e -n _o )d/λ) at φ=π/4 . Therefore, in both cases, when φ = π/4, the optical interference contrast is maximized due to equal-amplitude interference, and the optical interference shows complementarity, i.e., destructive interference when the polarizer is axially parallel to the analyzer Corresponds to enhanced interference when the two axes are perpendicular, and vice versa. These characteristics are the physical basis for making a novel polarized beam splitter.

For a given wave plate thickness d, since the birefringence is a function of wavelength, that is, n _e -n _o = f(λ), the phase difference δ (δ = 2π(n _e -n _o )d/λ) is also the wavelength function of lambda. According to the principle of optical interference, when δ=2mπ, the light wave produces enhanced interference transmittance has a maximum value, and when δ=(2m+1)π, light wave produces destructive interference transmittance has a minimum value, where m=0,1, 2, 3, ..., called the interference series. In order to separate light waves with wavelengths λ ₁ and λ ₂ (λ ₁ > λ ₂ ), the thickness of the wave plate must meet the following conditions:

d(n _e -n _o )λ ₁ =mλ ₁ (m=0, 1, 2, 3...), (1)

and

$\mathrm{<span\; class="notranslate">\; d</span>}{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}<span\; class="notranslate">\; )</span>}_{{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; .</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

For given two wavelengths λ ₁ and λ ₂ , after the thickness d of the wave plate is determined by formula (1) and formula (2), the optical path difference between the ordinary light and the extraordinary light passing through the wave plate is relative to the wavelength λ ₁ is mλ ₁ and (m+1/2)λ ₂ for wavelength λ ₂ . Therefore, when the optical axis of the birefringent crystal is π/4 to the axial direction of the polarizer and the analyzer respectively, if the two mutually perpendicular polarized lights can be separated in space, then the separation wavelength can be realized to be λ ₁ and λ ₂ two beams of polarized light waves, and the optical interference contrast of the transmitted light is the largest. The value of the interference order m is high, that is, the device has a beam splitting capability with high wavelength resolution when the wave plate is thick.

The present invention is also used to realize the separation of two beams of polarized light waves whose wavelengths are two broadbands centered on _λ1 and _λ2 . Same as above parameter selection.

If this method is used continuously, two beams of polarized light waves centered on λ ₁ and λ ₂ with narrow bandwidth are split.

The device for polarization splitting consists of a polarizer, a wave plate, and a polarization beam splitter, as shown in Figure 2(a). The angle between the axial direction of the polarizer and the optical axis of the wave plate is π/4, and the parameters of the wave plate are as mentioned above. In this case, the two components of the electric field E along the D _o and D _o directions are equal. The optical axis of the square wave plate is along the direction of the diagonal, as shown in Fig. 2(b). The transmitted light through the wave plate is composed of ordinary light and extraordinary light, the polarization directions of which are perpendicular to each other, and the vibration azimuth angles are π/4 and 3π/4 respectively. The transmitted e-light and o-light are transmitted and reflected in two mutually perpendicular directions in the polarizing beam splitter. Under the action of the polarizing beam splitter, the polarization direction of the transmitted beam is parallel to the incident plane, and the polarization direction of the reflected beam is perpendicular to the incident plane. Two optical channels are formed, and these two channels can be used as polarization filters of a certain wavelength band.

In this case, channel I is equivalent to the case where the axes of the two polarizers are parallel to each other, while channel II corresponds to the case where the two polarizers are perpendicular to each other. The o-light and e-light have a phase difference of 2π(n _e -n _o )d/λ, and interfere in two mutually perpendicular vibration directions. The transmittances for these two cases are equal to cos ² (π(n _e -n _o )d/λ) and sin ² (π(n _e -n _o )d/λ), respectively. Thus, the transmission channel I is used to pass the p-wave with wavelength _λ1 , while the reflection channel II is used to pass the s-wave with wavelength _λ2 . When the incident light wave band covers the range from ultraviolet to visible light, one channel transmits polarized ultraviolet light waves, and the other channel transmits polarized visible light waves. Or when the incident light waves cover the visible to infrared range, one channel transmits polarized visible light waves, and the other channel transmits polarized infrared light waves.

The present invention can constitute ultraviolet or infrared and visible light spectroscopic device, promptly selects λ ₁ and λ ₂ to be respectively visible light wavelength center and ultraviolet or infrared wavelength center, when incident light wave wave band covers ultraviolet to visible light range, then a channel transmits polarized ultraviolet or infrared light wave , and the other channel transmits polarized visible light waves. The following device can be made: it is composed of two or more of the above-mentioned basic devices in succession, and is used to realize wavelength subdivision in broadband and beam splitting with high wavelength resolution.

Features and effects of the present invention: beam splitting in various situations can be performed when the above conditions are met or partly met, and a new type of beam splitting device with simple structure and convenient preparation is provided, especially for precise beam splitting. Wavelength beam splitting control, which cannot be accomplished by existing methods and devices. The selection and application of wave plate materials are very extensive, such as α-quartz, sapphire, calcite, etc., and the polarizing beam splitter is an existing device, which generates polarized light of different frequencies after passing through the polarizer and the polarizing beam splitter. In the present invention, after the incident light wave enters the device, two beams of polarized light vibrating perpendicularly to each other are decomposed, and the center wavelengths of the two beams of split light waves are λ ₁ and λ ₂ . At the same time, it is used as a polarization filter for beam splitting in a certain band, that is, when the incident light wave band covers the range from ultraviolet to visible light, one channel transmits polarized ultraviolet light waves, and the other channel transmits polarized visible light waves. Or when the incident light waves cover the visible to infrared range, one channel transmits polarized visible light waves, and the other channel transmits polarized infrared light waves. The invention can split beams of light waves with arbitrary wavelengths and band ranges. The method and device of the present invention can be used in multiple stages for precise wavelength splitting. Therefore, the present invention can be made into a two-channel or multi-channel bandwidth filter. The invention has a wide range of uses, and can be used in industries or fields such as optical communication, various optical detection instruments, laser application, infrared detection and medical treatment.

3. Description of drawings

Fig. 1 is the schematic diagram of the present invention, the schematic diagram of the axial projection of the incident light field vector on the fast axis and slow axis of the wave plate and the polarizer and analyzer. θ is the angle between polarizer and analyzer axis. φ is the angle between the electric displacement vector D _o in the crystal and the axial direction of the polarizer.

Fig. 2 is a schematic diagram of the structure of the present invention, Fig. 2 (a) a schematic diagram of a polarized beam splitting device; Fig. 2 (b) a schematic diagram of a wave plate structure, the optical axis of the wave plate is π/4 with the x/y axis, and is in a diagonal position , or the x/y axis is in a diagonal position.

Fig. 3 is the embodiment curve of the dispersion relation of e-light and o-light in a-quartz and the embodiment curve that channel sees through of the present invention, the dispersion relation of Fig. 3 (a) e-light and o-light in α-quartz ( The dots and square dots are the experimental results, and the solid line is the fitting value of the formula in the figure); Figure 3(b) shows the transmittance (channel 1) and Transmission between two perpendicular polarizers (Channel 2).

Fig. 4 is an infrared spectrum diagram of an example of the present invention, the infrared spectrum of light passing through a wave plate (542.0 μm) sandwiched between two parallel (channel I) and perpendicular (channel II) polarizers.

Fig. 5 shows that the incident light wave band of the present invention covers the range from visible light to infrared light, one channel transmits polarized infrared light waves, and the other channel transmits polarized visible light waves. The thickness of the α-quartz wave plate is 20 μm.

4. Specific implementation

Example one

As shown in Figures 3 and 4, in order to illustrate this polarization splitting device, we use an α-quartz wave plate to separate two light waves with wavelengths of 1310nm and 1550nm commonly used in optical communications. In α-quartz, the dispersion relation of o-light and e-light is shown in Fig. 3(a). According to the formula (1) and formula (2), the thickness of the wave plate is determined to be 540.4 μm, corresponding to the interference order m=3. The calculation results of light wave transmittance at different wavelengths are shown in Fig. 3(b). In order to verify this calculation result, we placed a quartz wave plate with a thickness of 542.0±0.5 μm between the polarizer and the polarizing beam splitter. The bandwidth of the polarization beam splitter is 400nm (1200~1600nm), and the resolution of the infrared transmission measurement is 4cm ^-1 . The measurement results of the two channels are shown in Fig. 4 . The transmittance maxima of the two channels appear at wavelengths of 1310nm and 1550nm respectively. The measured results shown in Figure 4 are in good agreement with the calculated results shown in Figure 3. These results show that the polarization beam splitting function can be realized by the optical system shown in Fig. 2. The linewidth of the split light wave narrows as the order of interference increases. For the broadband polarization beam splitting function, this device uses a wave plate with low interference series or even zero interference series. If the device is used to separate two incident light waves with very narrow linewidths, a wave plate with a high interference order is selected.

The main feature of this polarized light splitting method is that two mutually orthogonal polarized lights (e-light and o-light) pass through a wave plate to generate a phase difference. For given two wavelengths λ ₁ and λ ₂ , the thickness of the wave plate is determined by the optical dispersion relation of the birefringent crystal. The polarized light with the wavelengths λ ₁ and λ ₂ and perpendicular to each other passes through the wave plate to generate the optical path difference of mλ ₁ and (m+0.5)λ ₂ respectively, and passes through the polarization beam splitter to vibrate in two mutually perpendicular directions and interference (s-wave and p-wave). The function of polarized beam splitting is realized by spatially separating s-wave and p-wave. The transmitted light with the wavelengths λ ₁ and λ ₂ separated by the two channels is mutually perpendicularly polarized light. Therefore, this method is different from traditional split-wave technology.

In addition to the α-quartz wave plate, the wave plate material can also be made of single crystal materials such as sapphire (Al ₂ O ₃ ), calcite, BBO (BaB ₂ O ₄ ), LiNbO ₃ , TiO ₂ , YVO ₄ , etc. These materials are also birefringent material. The birefringence and dispersion relations of these single crystal materials are shown in Table 1.

Different birefringent single crystal materials have different dispersion relations and transmission ranges, but for given wavelengths λ ₁ and λ ₂ and the required bandwidth, the thickness of the wave plate is determined by formula (1) and formula (2). The orientation and optical axis direction of the wave plate are the same as above.

Polarizing beam splitters are commonly used optical devices, and typical ones are: cubic multilayer film polarizing beam splitters, Nicol prisms, Rochon prisms, etc. But the cubic multilayer polarizing beam splitter is more practical.

Example two

The α-quartz wave plate provides a light wave with a wide frequency band that enters the device and decomposes two beams of visible and infrared polarized light that vibrate perpendicularly to each other. Provides a polarizing filter with two channels as beam splitting in a certain wavelength band. That is, when the incident light wave band covers the range from visible light to infrared light, one channel transmits polarized infrared light waves, and the other channel transmits polarized visible light waves.

Table 1 Birefringence and dispersion relations of several typical birefringent single crystal materials

Claims (4)

1. A method of polarization splitting based on optical interference and dispersion, characterized in that the incident light passes through a polarizer to generate a bunch of incident polarized light, and the incident polarized light passes through a birefringent crystal wave plate, so that the vibration direction of the incident polarized light is the same as that of the wave plate The optical axis is normal incident at an angle of π/4, and the incident polarized light in the wave plate is divided into two beams with different propagation velocities: extraordinary light e-light and ordinary light o-light; the two beams of light are in the propagation of the wave plate The refractive index n _e and n _o are different. After passing through the wave plate, the o-light and e-light form a phase difference δ=2π(n _e -n _o )d/λ, where λ is the wavelength of the incident light in vacuum ; For a given wave plate thickness d, since the birefringence is a function of wavelength, i.e. n _e -n _o = f (λ), the phase difference δ is a function of wavelength λ, in order to be the wavelength of λ ₁ and λ ₂ The light wave is separated, λ ₁ > λ ₂ , and the thickness of the wave plate satisfies the following conditions: $\mathrm{<span\; class="notranslate">\; d</span>}{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}<span\; class="notranslate">\; )</span>}_{{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; m</span>}{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0,1,2,3</span>}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$ and $\mathrm{<span\; class="notranslate">\; d</span>}{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}<span\; class="notranslate">\; )</span>}_{{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; .</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$ For given two wavelengths λ ₁ and λ ₂ , after the thickness d of the wave plate is determined by formula (1) and formula (2), the optical path difference between the ordinary light and the extraordinary light passing through the wave plate is relative to the wavelength λ ₁ is mλ ₁ , and for wavelength λ ₂ is (m+0.5)λ ₂ ; when the optical axis of the birefringent crystal wave plate forms an angle of π/4 with the two vertical vibration directions of the polarizer and the polarization beam splitter respectively , the o-light and e-light passing through the wave plate interfere with equal amplitude in the two vibration directions of the polarization beam splitter. When δ=2mπ, the transmittance has a maximum value. When δ=(2m+1)π , the transmittance has a minimum value, here m=0, 1, 2, 3, ..., called the interference series; use a polarizing beam splitter to separate two kinds of mutually perpendicular polarized light, and then realize the split wavelength as λ ₁ and λ ₂ two beams of polarized light waves, and the optical interference contrast of the transmitted light is the largest. 2. The polarization demultiplexing method based on optical interference and dispersion according to claim 1 is characterized in that it produces polarized ultraviolet and visible light splitting or produces polarized visible light and infrared splitting, that is, selecting λ ₁ and λ ₂ to be the visible light wavelength center and ultraviolet light respectively The wavelength center or the wavelength center of visible light and infrared. When the incident light wave band covers the range from ultraviolet to visible light, one channel transmits polarized ultraviolet light waves, and the other channel transmits polarized visible light waves; when the incident light wave covers the visible to infrared range, one channel transmits Visible light waves are polarized, and another channel transmits polarized infrared light waves. 3. A polarization splitting device based on optical interference and dispersion, characterized in that the polarization splitting device consists of a polarizer, The birefringent crystal wave plate is composed of a polarizing beam splitter. The optical axis of the birefringent crystal wave plate forms an angle of π/4 with the optical axis of the polarizer and the optical axis of the polarizing beam splitter respectively. In the propagation of the wave plate, o-light and e - The refractive index ne and _{n o} of light are different. After passing through the wave plate, o-light and e-light will form a phase difference δ=2π(n _e -n _o )d/λ, where λ is the incident light in vacuum The wavelength; the thickness of the wave plate satisfies the following conditions: $d{({\mathrm{no}}_e-{\mathrm{no}}_o)}_{{\mathrm{\&lambda;}}_1}=m{\mathrm{\&lambda;}}_1$ m=0,1,2,3,..., and $d{({\mathrm{no}}_e-{\mathrm{no}}_o)}_{{\mathrm{\&lambda;}}_2}=(m+\frac{1}{2}){\mathrm{\&lambda;}}_2.$ m=0, 1, 2, 3, . . . 4. The polarization splitting device based on optical interference and dispersion according to claim 3 is characterized in that the wave plate material that is divided into extraordinary light e-light and ordinary light o-light in the splitting wave plate is in α-quartz, Choose from sapphire, calcite, BBO, LiNbO ₃ , TiO ₂ or YVO ₄ birefringent crystals.

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