CN101251627A - Photonic crystal waveguide polarizing beam splitter - Google Patents

Abstract

The invention discloses a photonic crystal waveguide polarization beam splitter. It is a structure in which line defects are introduced into a two-dimensional tellurium dielectric column photonic crystal to form a coupling waveguide, and the splitting of TE light and TM light is realized by using the different waveguide coupling lengths of TE light and TM light. The characteristics of using a two-dimensional photonic crystal waveguide to realize a polarization beam splitter are as follows: First, compared with various existing polarization beam splitting devices, it can realize polarization splitting with high polarization degree and high extinction ratio. Secondly, the size of the device can be made very small, and it is structurally compatible with existing photonic crystal devices, meeting the requirements of integration. Third, the structure is very flexible, and the polarization splitting of any wavelength within the range of 3.5 to 35 μm can be realized by adjusting the length of the coupling region or the lattice constant. The invention also introduces the design idea of the polarization beam splitter, the specific structural design and the optical performance of the polarization beam splitter obtained under the design idea.

Description

Photonic crystal waveguide polarizing beam splitter

technical field

The invention relates to an optical element, in particular to a polarization beam splitter applied in a waveguide based on a photonic crystal structure.

Background technique

A photonic crystal is a microstructure in which periodic dielectric constant modulation is artificially introduced by various methods, and the period of the dielectric constant can be compared with the wavelength of light. The periodic change of the dielectric function can modulate the state mode of photons in the material, so that the photonic band gap appears. When the frequency of light is within the range of the photonic band gap, it will not be able to propagate in any direction in the photonic crystal. Photonic crystals have important application prospects. Due to its characteristics, it is possible to fabricate new principles or high-performance devices that could not be fabricated before. At present, the theoretically and experimentally realized photonic crystal polarization beam splitter mainly utilizes the principle of polarization dependence of the photonic band gap, among which reflected light and refracted light are most utilized. These polarizing beam splitters are not only structurally incompatible with existing devices implemented by waveguide coupling, but also difficult to control the optical path due to the use of reflected light and refracted light.

Contents of the invention

The object of the present invention is to provide a photonic crystal waveguide polarization beam splitter, which solves the problem that the structure of the existing polarization beam splitter cannot be compatible with the existing devices realized by the waveguide coupling method.

The photonic crystal waveguide polarization beam splitter of the present invention realizes the splitting of TE light and TM light according to the principle of an optical waveguide directional coupler. When two waveguides with exactly the same structure are very close together, due to the coupling effect between the waveguides, when light is incident by one waveguide, the light will show a phenomenon of space-period coupling and forward propagation between the two waveguides. If TE light and TM light have different coupling lengths, they can be coupled into different waveguides by using their different coupling lengths, so as to realize the splitting of TE light and TM light. For example, suppose the coupling length of TE light is LE, and the coupling length of TM light is LM. If we choose the length of the coupling region between the two waveguides as L=even number×LE, L=odd number×LM, then TE light will return to the original waveguide after even number of couplings, and TM light will go through odd number of couplings. Propagate into another waveguide, so as to realize the splitting of TE light and TM light. Since the photonic crystal waveguide has many advantages compared with the traditional waveguide: for example, the bending loss is very low, and it is beneficial to integrate with other photonic crystal components, etc., the present invention adopts the photonic crystal waveguide to realize the polarization beam splitter.

Based on the above design idea, the technical solution of the present invention is: introducing a waveguide coupling structure into the photonic crystal of the two-dimensional dielectric columnar square lattice to realize the splitting of TE light and TM light. We adopt a structure that is fully compatible with other photonic crystal devices (eg, wavelength division multiplexers, directional couplers), so as to form an integrated optical circuit with these devices, as shown in Figure 1. The incident light is incident on the port 1 of the waveguide 4. In the coupling area, the TE light returns to the waveguide 4 after two couplings, and is extracted from the port 2; the TM is coupled to the waveguide 5 once, and is extracted from the port 3. The dielectric column material of the photonic crystal is tellurium single crystal. This material is characterized as an anisotropic material. In the wavelength range of 3.5 μm to 35 μm, the refractive index of ordinary light is n _o = 4.8, and the refractive index of abnormal light is n _e = 6.2.

When designing the device, the direction of the abnormal optical axis is parallel to the direction of the dielectric cylinder, and the device structure is determined as follows:

The lattice constant is a and the length of the coupling region is L ₂ , which can be adjusted according to the needs of the splitting wavelength λ, and its magnitude is microns, where:

a＝0.3894×λ (1)

L ₂ = even number x LE = odd number x LM (2)

In the formula: LE, LM can be obtained by the plane wave expansion method shown in Figure 2.

The width of the waveguide (the distance between the two rows of dielectric cylinders above and below the waveguide):

W＝2×a (3)

The length from the entrance port 1 to the edge of the coupling region:

L ₁ ≥ 3a (4)

Diameter of the dielectric cylinder:

D＝0.4×a (5)

Length of transition from coupled region to uncoupled region:

L ₃ =4×a (6)

As shown in Figure 1, a dielectric column is deleted at the waveguide convex right angle 6 at the right-angle bend, and a dielectric column is added at the waveguide concave right angle 7.

The length from the edge of the uncoupling region to the exit port:

L ₄ ≥ 3a (7)

Distance from waveguide 4 to upper boundary:

L ₇ ≥ 3a (8)

Distance from waveguide 4 to 5:

_L6≥3a (9)

Distance from waveguide 5 to lower boundary:

L ₅ ≥ 3a (10)

The advantages of the polarization beam splitter realized by the present invention are concentrated in that it can be directly integrated in the photonic crystal waveguide, which is difficult for all current polarization beam splitters. An indispensable basic device in the miniaturization of integrated systems.

Description of drawings

Fig. 1 is a device structure schematic diagram of the present invention;

In the figure: 1——light incident port;

2——TE light output port;

3——TM light output port;

4 - incident waveguide;

5—coupling waveguide;

6—convex right angle of waveguide corner;

7—The waveguide corner is concave and right-angled.

Fig. 2 is the coupling length of TE light and TM light of the device of the present invention.

Fig. 3 is the FDTD simulation result of the TE light of the present invention (the normalized frequency is 0.3893 (a/λ)).

Fig. 4 is the FDTD simulation result of the TM light of the present invention (the normalized frequency is 0.3893 (a/λ)).

Fig. 5 is the TE light transmission spectrum of the device of the present invention.

Fig. 6 is the TM light transmission spectrum of the device of the present invention.

Fig. 7 is a graph of the extinction ratio of the device of the present invention.

Detailed ways

According to the technical solution of the present invention, we take a polarization beam splitter with an operating wavelength of λ=5.13 μm as an example, and illustrate the implementation method of the device with reference to FIG. 1 .

The main structural parameters of the device are designed as follows:

Lattice constant of the device (distance between two cylindrical dielectric pillars): a=0.3894×λ=2um.

Diameter of the dielectric cylinder: D=0.4×a=0.8um.

The width of the waveguide (the distance between the two rows of dielectric cylinders above and below the waveguide): W=2×a=4um.

The length from the incident port 1 to the edge of the coupling region: L ₁ ≥ 3a In the embodiment, L ₁ =4×a=8um.

The length of the coupling region: L ₂ =23×a=46um.

The transition length from the coupling region to the non-coupling region: L ₃ =4×a=8um. (Note, a dielectric column is deleted at the convex right angle 6 of the waveguide at the right-angle bend, and a dielectric column is added at the concave right angle 7 of the waveguide).

The length from the non-coupling region to the exit port: L ₄ ≥ 3a, in the embodiment, L ₄ =7×a=14um.

The distance from the waveguide 4 to the upper boundary: L ₇ ≥ 3a, in the embodiment, L ₇ =3×a=6um.

The distance between waveguides 4 and 5: L ₆ ≥ 3a, in the embodiment, L ₆ =4×a=8um.

The distance from the waveguide 5 to the lower boundary: L ₅ ≥ 3a, in the embodiment, L ₅ =3×a=6um.

The system performance is as follows:

The transmission spectrum of the device: the transmittances corresponding to TE light and TM light are shown in Figures 5 and 6. It can be seen from the figure that the transmittance of port 2 to TE light is near the wavelength of λ=5.13μm The transmittance of port 3 for TM light is as high as 98%. That is: about 84% of TE light exits from port 2, and 98% of TM light exits from port 3. Figures 3 and 4 are the simulation results of TE light and TM light respectively when the light wavelength is λ=5.13um. It can be seen from the figures that good light splitting is achieved at this frequency. Extinction ratio of the device: at the exit port 2 of TE light, the extinction ratio is defined as:

$\mathrm{<span\; class="notranslate">\; Ratio</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 10</span>}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; log</span>}}_{\mathrm{<span\; class="notranslate">\; 10</span>}}<span\; class="notranslate">\; (</span>\frac{{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; TE</span>}}}{{\mathrm{<span\; class="notranslate">\; p</span>}}_{\mathrm{<span\; class="notranslate">\; tm</span>}}}<span\; class="notranslate">\; )</span>$

Wherein, P _TE is the outgoing light intensity of TE light at port 2; p _TM is the outgoing light intensity of TM light at port 2; the unit of Ratio is decibel. Also at the exit port 3 of TM light, the extinction ratio is defined as:

$\mathrm{<span\; class="notranslate">\; Ratio</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 10</span>}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; log</span>}}_{\mathrm{<span\; class="notranslate">\; 10</span>}}<span\; class="notranslate">\; (</span>\frac{{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; tm</span>}}}{{\mathrm{<span\; class="notranslate">\; p</span>}}_{\mathrm{<span\; class="notranslate">\; TE</span>}}}<span\; class="notranslate">\; )</span>$

Among them, p _TM is the transmitted light intensity of TM light at port 3; P _TE is the transmitted light intensity of TE light at port 3; the unit of Ratio is decibel. The extinction ratio of the device is an important parameter to measure the degree of polarization of the outgoing light. Its physical meaning is how many times the intensity of one polarized light is the intensity of the other polarized light at the light exit port. The extinction ratio spectrum line diagram of the device we obtained is shown in Figure 7. It can be seen from the figure that the extinction ratios of the TE light exit port 2 and the TM light exit port 3 reach 19 dB and 20 dB respectively when the light wavelength is around λ=5.13 μm. This result indicates that the TE light emitted from port 2 and the TM light emitted from port 3 have a high degree of polarization.

We can obtain devices working at other wavelengths by adjusting the value of the lattice constant a of the device. For example, we need to obtain a device working at a wavelength of 8 μm, we can use the conversion formula a=0.3894×λ to obtain the lattice constant a=3.12 μm required in the process. According to this parameter to prepare the device, a polarization beam splitter with a working center wavelength of 8 μm can be obtained.

Claims (2)

1. A photonic crystal waveguide polarization beam splitter, characterized in that: it is a structure in which line defects are introduced into a two-dimensional tellurium dielectric column photonic crystal to form a coupling waveguide, and the waveguide coupling length of TE light and TM light is different to realize TE For the splitting of light and TM light, the structural parameters of the beam splitting device are determined as follows: Lattice constant a and coupling region length L ₂ : a＝0.3894×λ (1) L ₂ = even number x LE = odd number x LM (2) In the formula: LE and LM are the coupling lengths of TE light and TM light respectively, which can be obtained by the plane wave expansion method; The width W of the waveguide: W＝2×a (3) The length L ₁ from the entrance port 1 to the edge of the coupling region: L ₁ ≥ 3a (4) Diameter D of the dielectric cylinder: D＝0.4×a (5) The length L ₃ of the transition from the coupling region to the uncoupling region: L ₃ =4×a (6) And delete a dielectric column at the convex right angle (6) of the waveguide at the right angle bend, and add a dielectric column at the concave right angle (7) of the waveguide; The length L ₄ from the edge of the uncoupling region to the exit port: L ₄ ≥ 3a (7) The distance L ₇ from the waveguide 4 to the upper boundary: L ₇ ≥ 3a (8) Distance _L6 of waveguides 4 to 5: _L6≥3a (9) The distance L ₅ from the waveguide 5 to the lower boundary: L ₅ ≥ 3a (10) 2. A photonic crystal waveguide polarization beam splitter according to claim 1, characterized in that: said polarization beam splitter is made of tellurium single crystal material.

Images