CN1176393C - Talbot Beam Splitter - Google Patents

Abstract

A wavelength-splitting beam splitter with Talbot effect is suitable for optical communication, optical interconnection, optical calculation, data storage, image processing and optical calculation. The method comprises the steps of outputting optical signals with different wavelengths from an input optical fiber, collimating the optical signals into parallel beams through a collimator, irradiating the parallel beams onto an amplitude or phase grating, and self-imaging the diffracted beams on an output optical fiber array with a condenser at a position which is N times of Talbot distance away from the grating. Large-scale arrays of spatially separated optical signals of different wavelengths are produced. And leading out by using an output optical fiber array. The invention realizes the double functions of coarse wave division and large-scale beam division by using a simple structure.

Description

Talbot Beam Splitter

Technical field:

The invention is a wavelength splitter of Taber effect. It is suitable for optical communication, optical interconnection, optical computing, data storage, image processing and optical computing, etc.

Background technique:

Optical fiber communication system has developed rapidly with large capacity and high speed information transmission. Among them, the wave splitter/beam splitter plays a very important role, and they are usually composed of different devices. At present, splitting (combining) wave devices/beam splitters have been effectively used in multi-user systems. The wavelength division multiplexing system can be divided into dense wavelength division multiplexing (DWDM) and coarse wavelength division multiplexing (CWDM) according to the multiplexing light wave interval, but the cost of the DWDM system is still relatively high. Comparatively speaking, due to the wide channel interval of the CWDM system , relatively cheap transceiver devices and optical components can be used, and the cost of the entire communication system is greatly reduced.

Beam splitting components also play a very important role in optical fiber communication. Commonly used beam splitters are: fused tapered, volume hologram, waveguide [see Fardad, et al., Sol-Gel multimode interference powersplitters, IEEE Photon.Tech.Lett., 11, 1999, p.697-699] wait. For example, 1×2 beam splitting can be achieved by using fused tapered technology, and larger-scale beam splitting can be achieved by multi-level cascading, but this increases loss and makes the system too complex. Judging from the existing research results, it is rare to find devices with both beam splitting and wave splitting functions. They are characterized by having a beam splitting function but not a wave splitting function, or having a wave splitting function but not a very large-scale beam splitting function. At present, the optical array generator or illuminator designed and manufactured based on the Talbot effect principle has been applied in many research fields, such as optical communication, optical interconnection, optical computing, data storage, image processing, and optical metrology, etc. [See ChangheZhou (Zhou Changhe) et al., Analytic phase-factor equations for Talbot array illuminations, Applied Optics, 38, 1999, p.284-290].

Invention content:

The present invention provides a device based on the Talbot effect that combines large-scale beam splitting and wave splitting functions into one, and simultaneously has the functions of a coarse wave splitter/ultra-large beam splitter.

The structure of the wavelength splitter of the present invention is shown in FIG. 1 . It includes an input optical fiber 1, a collimator 2 composed of a beam expander lens 201 and a collimator lens 202, a grating 3 and an output optical fiber array with a concentrator 401 located at the incident end of the distance from the grating 3N≥1 times the Taber distance z . The input fiber 1 is capable of simultaneously transmitting two wavelengths of light λ ₁ and λ ₂ . After the light beam emerges from the input optical fiber 1 , it is firstly collimated by the collimator 2 , and the collimator 2 is composed of a beam expander lens 201 and a collimator lens 202 . The collimator 2 is connected to the output end of the input optical fiber 1 . The parallel light beam emitted by the collimator 2 hits the grating 3, and the grating 3 can be an amplitude grating or a phase grating. From the perspective of improving energy utilization, the phase grating is better. The grating 3 is placed at the beam output end of the collimator 2 . In the light beam output direction of the grating 3 , at a distance z of N≥1 times the Taber distance, there is an output optical fiber array 4 with a concentrator 401 at the input end. The head of each fiber array element of the output fiber array 4 has a concentrator 401 for the purpose of coupling the light spot in free space into the output fiber array 4 .

Optical signals λ ₁ and λ ₂ of different wavelengths transmitted in the input fiber 1 , the light beams emerge from the output end of the input fiber 1 , pass through the collimator 2 , are collimated into parallel light, and irradiate on the grating 3 . The light passing through the grating 3 will be diffracted, self-imaging at the Talbot distance z will generate a large-scale array of spatially separated optical signals of different wavelengths, and use the output fiber array 4 to lead out a large-scale array of spatially cross-separated optical signals of different wavelengths spot array. Therefore, the device of the invention realizes the dual functions of coarse wave splitting and large-scale beam splitting.

In the present invention, the N times Taber distance z between the grating 3 and the output fiber array 4 satisfies the self-imaging condition of the grating. Considering the simplest case, let u ₀ (x) be the one-dimensional periodic transmittance function of the grating 3 on the z=0 plane, which can be obtained by Fourier series expansion:

u ₀ (x)=∑c _k exp(i2π xk/d) (1) where k is an integer, the physical meaning of k is the order of the Fourier spectrum, c _k is the coefficient of the Fourier spectrum, x is the coordinate value perpendicular to the one-dimensional direction of the groove of the grating 3, d is the period of the grating 3 along the x direction, exp is an exponential function, and Σ is the multilevel Fourier spectrum summation symbol. Under the coherent illumination of a plane wave with a wavelength of λ, the Fresnel diffraction field propagating along the z direction is:

u(x, z)=∑c _k exp[-i2π zλk ² /(2d ² )] exp(i2πxk/d) (2) The definition of Talbot distance is Z _T =(2d ² )/λ. Substituting the value of the Taber distance Z _T into (2), the form of (1) shows that the grating 3 is self-imaging at the position of the Taber distance Z _T. For the incident light of wavelength λ ₁ and λ ₂ , the Talbot distances are Z _Tλ1 =(2d ² )/λ ₁ and Z _Tλ2 =(2d ² )/λ ₂ respectively. It can be seen that the corresponding The Taber distance of is different. The condition at a uniform distance z is:

z=N ₁ ·Z _Tλ1 =N ₂ ·Z _Tλ2 +N ₃ ·Z _Tλ2 , (3) where N ₁ and N ₂ are both positive integers, and N ₃ =1/2. For incident light with a wavelength of λ1, u(x, z) has the same distribution as u ₀ (x), and z at this time is N ₁ times the Taber distance Z _Tλ1 . At this time, for the light wave with wavelength λ ₂ , z is (N ₂ +1/2) times the Talbot distance Z _Tλ2 , and self-imaging is also generated, and its period is still d, but the optical field distribution is misaligned along x Shifting by d/2 is called half-period displacement self-imaging. The essence of this phenomenon is that when the diffraction components on the Taber distance are superimposed, the mutual phase relationship is the same as the phase relationship between the spectral components of the object, but for the grating self-imaging with different wavelengths λ ₁ and λ ₂ , it is exactly at The space difference is half a cycle, so wavelength multiple division and large-scale beam splitting can be realized simultaneously.

By changing parameters such as grating period and fractional Talbot distance, the function of splitting wavelength and beam splitting can be realized. Through the ratio of the two wavelengths, the wavelength splitter and large-scale beam splitter functions of different λ ₁ and λ ₂ such as 1300nm and 1550nm (or 1350/1500nm) optical communication window wavelengths can be realized.

For the description of the above Talbot self-imaging effect, N ₁ (2d ² /λ ₁ )=(N ₂ +1/2)(2d ² /λ ₂ ), λ ₂ =1300nm, λ ₂ =1550nm (or λ ₁ = 1500nm, λ ₂ = 1350nm), which are the two incident light wavelengths, and the values are substituted into: N ₁ /(N ₂ +1/2) = λ ₁ /λ ₂ = 1300/1550 = 13/15.5, then N ₁ = 13 , N ₂ =15. When considering different grating periods, there are many situations: for example, when d=100μm, z≈200mm; when d=10μm, z≈2mm; when d=20μm, z≈8mm; when d=30μm, z≈18mm; when d =40μm, z≈32mm.

For the case of coarse wavelength division, when N ₁ =15, N ₂ =13, λ ₁ =1500nm, λ ₂ =1350nm, then when d=100μm, z=200mm; when d=10μm, z=2mm and d= In the case of 20 μm, z=8mm, etc., it is generally considered that choosing d=20 μm, d=30 μm, or d=40 μm is more suitable for using the output fiber array to extract optical signals with spatially separated wavelengths.

For ease of understanding and clarity, the above is the case of using the amplitude grating 3 . In fact, the use of the phase grating 3 can further improve the light efficiency. For the amplitude grating under coherent plane wave illumination, a Fresnel diffraction light field with pure phase distribution is produced on a certain plane. When the pure phase distribution of the phase grating is illuminated by a coherent plane wave, an amplitude modulation structure, that is, an array spot, will be generated on a certain fractional Talbot distance plane to facilitate coupling from the output fiber array.

Assuming a grating 3 with a rectangular opening and a duty ratio d/M is illuminated by a coherent plane wave, the transmittance function of the grating is:

${\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; \&Sigma;rect</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; kd</span>}}{\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

Where rect(x) is a rectangle function, M is a positive integer greater than 1, M=2, 3, . . . The diffraction field u(x, z) of the grating at the fractional Talbot distance (z=(p/2M)Z _T , p=1, 2, 3,...) can be written as:

$\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; z</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\sqrt{\mathrm{<span\; class="notranslate">\; MP</span>}}}\underset{\mathrm{<span\; class="notranslate">\; j</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{\mathrm{<span\; class="notranslate">\; p</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; i\&pi;</span>}\frac{{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; jM</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; pM</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 5</span>}<span\; class="notranslate">\; )</span>$

When p and M are mutually prime, a pure phase diffraction field will appear, and the size of the pure phase distribution can be calculated from formula (5). It can be seen from equations (4) and (5) that for the phase grating, when N=p/2M in N times the Taber distance z, that is, when N is a fraction, the phase grating also produces self-imaging at the z distance position.

Discussing the case of the second-order phase grating, the compression ratio of the array spot that the second-order phase grating can achieve is 1/2 or 1/3, where d/M is the compression ratio of the array spot. The values of N ₁ , N ₂ , and N ₃ can be further determined by selecting phase distributions of different phase gratings and the obtained array light spot compression ratio. Since the compression ratio of the second-order phase grating is limited to 1/2 and 1/3, in order to obtain a larger compression ratio and improve the light intensity of the spot, a multi-level step phase grating must be considered. There are different situations when phase gratings with different phase steps are used as Talbot array illuminator. For example, a second-order phase grating can have six positions, and a multi-level step phase grating will have more situations [see Changhe Zhou et al., Number of phase levels of a Talbot array illuminator, Applied Optics, 40(5) , 2001, p.606-613].

Compared with the prior art, the present invention is cheap and easy to manufacture. By adopting multi-order phase grating, a high-efficiency large-scale optical fiber output array will be obtained. The more array elements in the output array, the higher the efficiency. For larger Arrays, the closer to 100% efficiency. Among them, the manufacturing process of the phase grating (or phase plate) is: coating the photoresist on the glass substrate, after uniform coating, exposure and development, the pattern on the template can be transferred to the photoresist, using wet chemical etching process and high-density plasma etching process to carve the required phase grating on the glass substrate. Of course, the reticle can be drawn by a plotter and then compacted, or it can be directly exposed to optical or electron beams to generate graphics.

The wavelength demultiplexing beam splitter of the present invention fully embodies the advantages of parallel and fast optical information processing, realizes large-scale optical power distribution, and has the function of coarse demultiplexing. The invention has the advantages of low cost and the like, and its new practical value will be fully reflected in the near future.

Description of drawings:

FIG. 1 is a schematic structural diagram of wavelength division and beam division in the present invention.

Detailed ways:

With the structure shown in Fig. 1, for the most commonly used coarse wavelength division channel, the input wavelengths of the two beams of light are λ ₁ =1300nm and λ ₂ =1550nm, and the grating 3 adopts a phase grating. A set of structural parameters obtained from the above is d=20 μm, z=8 mm. Since the device of the present invention is based on the Taber self-imaging effect, a super-large-scale high-efficiency equal-intensity beam splitting array can be formed. An array containing a very large number of array elements can be formed for each wavelength channel, for example, 100*100=10000 beams. The feature of the device of the present invention is that with a very simple structure, it can decompose and multiplex different wavelength coarse waves at the same time, and realize a large-scale high-efficiency, equal-intensity beam splitting array, which has important uses for practical applications such as fiber-to-the-home. value.

Claims (1)

1. A kind of demultiplexing beam splitter of Talbot effect, comprises input optical fiber (1), is made of collimator (2) by beam expander lens (201) and collimator lens (202), grating (3), light beam After exiting from the input optical fiber (1), first collimated by the collimator (2), the outgoing parallel light beam is incident on the grating (3), which is characterized in that the said grating (3) is an amplitude grating, or a phase The grating, in the beam output direction of the grating (3) and at the N times Taber distance Z from the grating (3), is equipped with an output optical fiber array (4) with a concentrator (401) at the incident end, Here Z=2Nd ² /λ, where 2d ² /λ is the Taber distance d is the period of the grating (3), λ is the wavelength of the incident light, N is a positive integer greater than or equal to 1.

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