Polarization beam splitter with large manufacturing tolerance and high polarization extinction ratio
阅读说明：本技术 一种具有大制作容差高偏振消光比的偏振分束器 (Polarization beam splitter with large manufacturing tolerance and high polarization extinction ratio ) 是由 陆巧银 戴向阳 陈泉安 马向 国伟华 于 2019-07-09 设计创作，主要内容包括：本发明公开了一种具有大制作容差高偏振消光比的偏振分束器,偏振分束器在传播方向上,包括输入波导、多模干涉耦合器、两条相移波导、2×2多模干涉耦合器、输出波导；多模干涉耦合器把输入波导的光强等分,分别进入上下两条相移波导,然后通过2×2多模干涉耦合器干涉输出。波导的朝向与晶体的<Image he="87" wi="139" file="DDA0002123368990000011.GIF" imgContent="drawing" imgFormat="GIF" orientation="portrait" inline="no"></Image>方向的具有45～135度的夹角。通过在两条相移波导上施加电压,分别改变TE和TM偏振光的有效折射率,使TE和TM偏振光的相位分别满足2×2MMI的干涉相长相消条件,实现偏振分束。本发明通过改变TE和TM偏振光的有效折射率来补偿制作工艺偏差造成的偏振光额外的相位差,因此具有大的制作容差；另外由于2×2MMI能实现很高的消光比,因此本发明的偏振分束器具有高的偏振消光比。(The invention discloses a polarization beam splitter with large manufacturing tolerance and high polarization extinction ratio, which comprises an input waveguide, a multi-mode interference coupler, two phase-shifting waveguides, a 2 multiplied by 2 multi-mode interference coupler and an output waveguide in the propagation direction; the multimode interference coupler equally divides the light intensity of the input waveguide, respectively enters the upper phase shift waveguide and the lower phase shift waveguide, and then is interfered and output through the 2 multiplied by 2 multimode interference coupler. Orientation of the waveguide and of the crystal The included angle of the direction is 45-135 degrees. By applying voltage on the two phase-shifting waveguides, the effective refractive indexes of the TE polarized light and the TM polarized light are respectively changed, so that the phases of the TE polarized light and the TM polarized light respectively meet the interference constructive cancellation condition of 2 multiplied by 2MMI, and polarization beam splitting is realized. The invention compensates the extra phase difference of the polarized light caused by the deviation of the manufacturing process by changing the effective refractive indexes of the TE polarized light and the TM polarized light, thereby having large manufacturing tolerance; in addition, the polarization beam splitter of the invention has high polarization extinction ratio because 2 × 2MMI can realize high extinction ratio.)
1. A polarization beam splitter with large manufacturing tolerance and high polarization extinction ratio is characterized in that the polarization beam splitter comprises an input waveguide, a first multimode interference coupler, an upper arm phase shift waveguide, a lower arm phase shift waveguide, a 2 x 2 multimode interference coupler and two output waveguides in the propagation direction; the input waveguide is connected with the input end of a first multimode interference coupler, two output ends of the first multimode interference coupler are respectively connected with an upper arm phase shift waveguide and a lower arm phase shift waveguide, the first multimode interference coupler equally divides the light intensity of the input waveguide and respectively enters the upper arm phase shift waveguide and the lower arm phase shift waveguide, the upper arm phase shift waveguide and the lower arm phase shift waveguide are respectively connected with two input ends of the 2 x 2 multimode interference coupler, and two output ends of the 2 x 2 multimode interference coupler are connected with two output waveguides;
the orientation of the input, phase-shifting, output waveguides and the crystal
One of the phase shift waveguides is applied with forward bias voltage or covered with a hot electrode for changing the effective refractive index of the TE polarized light on the phase shift waveguide, and the other phase shift waveguide arm is applied with reverse bias voltage for changing the effective refractive index of the TM polarized light on the phase shift waveguide, so that the phases of the TE polarized light and the TM polarized light respectively meet the interference constructive cancellation condition of the 2 x 2 multimode interference coupler, and polarization beam splitting is realized.
2. The polarization beam splitter with large manufacturing tolerance and high polarization extinction ratio of claim 1, wherein adjusting the angle between the waveguide orientation and the crystal direction affects the level of the first-order electro-optic effect, thereby affecting the effective refractive index variation of the TE polarized light under reverse bias.
3. The polarization beam splitter with large manufacturing tolerance and high polarization extinction ratio of claim 1, wherein the thickness of the waveguide core layer directly affects the electric field strength under the same voltage, and further affects the strength of the first-order electro-optic effect and the second-order electro-optic effect, and the selection of the appropriate core layer thickness affects the effective refractive index variation of the TE polarized light under the reverse bias.
4. The polarization beam splitter with large manufacturing tolerance and high polarization extinction ratio of claim 1, wherein the doping degree of the waveguide core layer affects the intensity of the carrier dispersion effect and the energy band filling effect, thereby affecting the effective refractive index variation of the TE polarized light under the reverse bias.
5. The polarization beam splitter with large manufacturing tolerance and high polarization extinction ratio of claim 1, wherein the input and output waveguides and the two arm phase shifting waveguides of the polarization beam splitter are single mode waveguides, and the connection parts of the single mode waveguides and the multimode waveguide regions of the first multimode interference coupler and the 2 x 2 multimode interference coupler are tapered waveguides to reduce the connection loss.
6. The polarization beam splitter with large manufacturing tolerance and high polarization extinction ratio of claim 1 wherein an undoped low index spacer layer is introduced near the waveguide core layer to reduce waveguide loss from waveguide cladding and substrate doping.
7. The polarization beam splitter with large manufacturing tolerance and high polarization extinction ratio of claim 1, wherein the structure of the waveguide cross section is of the P-I-N, N-I-P, N-P-I-N or N-I-P-N type.
8. The polarization beam splitter with large manufacturing tolerance and high polarization extinction ratio of claim 1, wherein the widths and lengths of the upper and lower arm phase shifting waveguides are selected such that the accumulated phase difference of TE polarized light and TM polarized light of the two arms is different by pi to reduce the magnitude of the reverse voltage required for realizing polarization beam splitting and the length of the two arm phase shifting waveguides.
Technical Field
The present invention relates to optical devices, and particularly to a polarization beam splitter.
Background
With the continuous improvement of the requirement of the transmission rate of optical communication, the traditional amplitude modulation mode can not meet the upgrading requirement of a communication network, the orthogonal phase shift modulation format of polarization multiplexing is widely adopted, and the traditional direct detection is upgraded into coherent detection in a signal demodulation mode. Meanwhile, the trend of miniaturization, low power consumption and low cost of optical communication devices requires optical devices with different functions to be integrated together. Therefore, a monolithically integrated dual-polarization quadrature phase shift keying receiver becomes crucial, and a waveguide-type polarization beam splitter which cannot realize high performance is a main reason for hindering monolithic integration of a coherent receiver. The polarization beam splitter of the commercial coherent receiver is based on a free light space microlens set, which greatly increases the packaging cost and size of the chip.
The working principle of waveguide-type polarization beam splitters is mainly based on mode interference, for example: multimode interference couplers (MMIs), Directional Couplers (DCs), and Mach-Zehnder interferometers (MZIs). The polarization beam splitting is realized by utilizing the difference of the propagation constants of the two polarization modes and the constructive or destructive interference at different output ports. The polarization beam splitting technology based on the multimode interference coupler requires the cascade connection of a plurality of MMIs, which greatly increases the size of the device and reduces the manufacturing tolerance. The polarization beam splitter based on the directional coupler has small size, but the space between waveguides needs to be accurately controlled to realize phase matching, so that the difficulty of process manufacturing is greatly improved. The mach-zehnder interferometer also faces the problem of small manufacturing tolerance by utilizing the birefringence of the waveguide with different widths of two arms to realize polarization beam splitting. However, the two-arm waveguide of the mach-zehnder interferometer is easy to introduce a phase shifter to adjust the phase difference, so that the manufacturing tolerance is increased. It has been proposed to introduce a thermal phase shifter on the two-arm waveguide to overcome the manufacturing errors of the two-arm waveguide, thereby increasing the manufacturing tolerances (d.x.dai, z.wang, j.e.powers, Considerations for the design of the systematic Mach-Zehnder interferometer used as a polarization beam-on-insulator platform, j.lightwave technology, 2011,29(12): 1808-1817.). In fact, this approach is very limited because temperature can affect the effective refractive index of both TE and TM polarized light (TE polarized light is the dominant electric field perpendicular to the epitaxial growth direction and the propagation direction of the waveguide, and TM polarized light is the dominant electric field parallel to the epitaxial growth direction and perpendicular to the propagation direction of the waveguide.) it is very difficult to achieve simultaneous interference constructive growth of TE and TM polarized light at the respective output ports. Polarization splitting can be easily achieved if the phase shifters on the two-arm waveguides can change the effective refractive indices of the TE and TM polarized light, respectively. To this end, a combination of the polarization-independent Quantum Confinement Stark Effect (QCSE) and the polarization-dependent first-order electro-optic effect (PE) has been used to achieve individual phase control of TE polarized light without affecting the phase of TM polarized light, and a 14dB polarization extinction ratio can be obtained at 1550nm (k. Watanabe, y. nasu, y. ohioso, r. lga, Easy adjustment structure and method for realzing InP based polarization beam splitter effect depletion on crystallization, jpn. j. appl. phys.,2016,55 RB, 08RB 04). However, the complicated device structure increases the manufacturing difficulty, and the wavelength dependence of the quantum confinement stark effect makes the polarization extinction ratio of the scheme unable to meet the application requirement in the C wave band.
In summary, the polarization extinction ratio of the polarization beam splitter of today does not meet the application requirements, and the manufacturing tolerance is small, which hinders the large-scale application of the monolithic coherent receiver.
Disclosure of Invention
The invention aims to solve the technical problem that the manufacturing tolerance of the polarization beam splitter is improved, and the polarization extinction ratio of the device meets the application requirement.
In order to solve the technical problem, the invention provides a polarization beam splitter with large manufacturing tolerance and high polarization extinction ratio. In the propagation direction, the polarization beam splitter comprises an input waveguide, a first multimode interference coupler, two phase shifting waveguides, a 2 × 2 multimode interference coupler, and an output waveguide. Two phase shifting waveguides are used as phase shifters, one waveguide can be applied with reverse bias, the other waveguide can be applied with forward bias or is covered with a hot electrode to apply current.
Waveguide orientation and crystal
Further, when one of the phase-shifting waveguide arms is forward biased VforwardThe injected carriers can change the effective refractive index of TE and TM polarized light, and the other phase-shifting waveguide arm is applied with a reverse bias voltage VreverseThe effective refractive index of TM polarized light can only be changed, and the polarization beam splitter realizes polarization beam splitting, so that the phase difference between TE polarized light and TM polarized light output from the two arms respectively meets the following formula:
in the above formula, λ is the wavelength of incident light of the polarization beam splitter, m is any positive integer, and L is the length of the waveguide arm phase shift region.
Or when the hot electrode covered on one phase-shifting waveguide arm is electrified to generate heat, the thermo-optic effect can change the effective refractive indexes of TE and TM polarized light, and the other phase-shifting waveguide arm applies a reverse bias VreverseThe effective refractive index of TM polarized light can only be changed, and the polarization beam splitter realizes polarization beam splitting, so that the phase difference between TE polarized light and TM polarized light output from the two arms respectively meets the following formula:
in the above formula, PthermalIs the thermal power generated by the thermode.
Preferably, the carrier dispersion effect caused by applying a forward bias on the waveguide arm or the thermo-optic effect caused by hot electrode energization on the waveguide arm can change the phase of both TE and TM polarized light.
Preferably, when a reverse bias is applied to the waveguide arm, a first-order electro-optical effect, a second-order electro-optical effect, a carrier dispersion effect, and a band filling effect can be caused, wherein the first-order electro-optical effect only acts on the TE polarized light. For TM polarized light, the second order electro-optic effect, carrier dispersion effect, and band filling effect all increase the effective refractive index. For TE polarized light, a proper waveguide orientation is selected, the effective refractive index can be reduced by the first-order electro-optical effect, the effective refractive index can be increased by the other three electro-optical effects, and the refractive index change caused by the first-order electro-optical effect can be approximately offset by the other three effects. Thus, the total effective refractive index of TE polarized light changes very little under reverse bias.
Preferably, the strength of the first-order electro-optic effect can be influenced by adjusting the angle between the orientation of the waveguide and the crystal direction, so that the effective refractive index of the TE polarized light under reverse bias is changed minimally.
Preferably, the thickness of the waveguide core layer directly affects the electric field strength under the same voltage, and further affects the strength of the first-order electro-optical effect and the second-order electro-optical effect, and the appropriate core layer thickness can be selected to minimize the effective refractive index change of the TE polarized light under the reverse bias.
Preferably, the doping degree of the waveguide core layer can be adjusted to influence the strength of the carrier dispersion effect and the energy band filling effect, so that the effective refractive index of the TE polarized light under reverse bias is changed minimally.
Preferably, for the waveguide core layer of the quaternary compound, the strength of the first-order electro-optical effect, the second-order electro-optical effect, the carrier dispersion effect and the energy band filling effect can be influenced by adjusting the material composition of the waveguide core layer, so that the effective refractive index change of the TE polarized light under reverse bias is minimum.
Preferably, the input/output waveguide and the two-arm phase-shift waveguide of the polarization beam splitter are both single-mode waveguides, and the connection part of the single-mode waveguides and the multi-mode waveguide region of the MMI can adopt tapered graded waveguides, so that the connection loss is reduced.
Preferably, an undoped low-refractive-index spacer layer can be introduced near the waveguide core layer, so that waveguide loss caused by doping of the waveguide cover layer and the substrate is reduced.
Preferably, the structure of the waveguide cross section may be of the P-I-N type, N-I-P, N-P-I-N type, N-I-P-N type or other P-I-N or N-I-P like structure under forward and reverse bias.
It should be noted that, for the material grown in the [100] direction, the reverse voltage corresponding to the P-I-N type waveguide structure is along the positive direction of [100 ]; and the reverse voltage corresponding to the N-I-P type waveguide structure is along the reverse direction of [100 ].
Preferably, the width and length of the upper and lower arm birefringent region waveguides are selected to make the accumulated phase difference of TE polarized light and the phase difference of TM polarized light differ by pi, so as to reduce the magnitude of reverse voltage required for realizing polarization beam splitting and the length of the two arm phase shift waveguides.
The invention changes the effective refractive index of TE polarized light through forward bias injection carriers or thermo-optic effect caused by hot electrode electrification heating, so that the TE polarized light meets the phase condition of interference phase length at one output port of a 2 multiplied by 2 multimode interference coupler; the appropriate waveguide orientation is selected so that the effective refractive index of the TE polarized light is not affected by the reverse bias effect, so that only the effective refractive index of the TM polarized light is changed by the reverse bias to satisfy the phase condition of constructive interference at the other port of the 2 × 2 multimode interference coupler, thereby realizing polarization beam splitting. The phase of the polarized light is changed by adjusting forward and reverse bias or the heating power of the thermode, so that the extra phase difference introduced by the process manufacturing errors of the two phase-shifting waveguide arms to the TE and TM polarized light can be compensated, polarization beam splitting can still be realized, and the manufacturing tolerance of the device is greatly improved.
Drawings
The technical solution of the present invention will be further specifically described with reference to the accompanying drawings and the detailed description.
Fig. 1 is a schematic diagram of a polarization beam splitter according to the present invention.
Fig. 2 is a schematic diagram of a polarization beam splitter according to a first embodiment of the present invention.
Fig. 3 is a schematic cross-sectional structure of a waveguide according to a first embodiment of the present invention.
FIG. 4 is a diagram illustrating the calculation results of the phase change of TE polarized light caused by the electro-optic effect when different reverse bias voltages are applied to the phase shifting waveguide according to the first embodiment of the present invention.
FIG. 5 is a diagram illustrating the calculation results of the phase change caused by the electro-optic effect when different reverse bias voltages are applied to the phase shifting waveguide for TM polarized light in the first embodiment of the present invention.
Fig. 6 shows the calculation result of the output light intensity of the waveguide 10 when different reverse bias voltages are applied to the electrode 8 after the electrode 7 applies a proper forward bias voltage to output TE polarized light from the waveguide 10 in the first embodiment of the present invention.
Fig. 7 shows the calculation result of the output light intensity of the waveguide 11 when different reverse bias voltages are applied to the electrode 8 after the electrode 7 applies a proper forward bias voltage to output TE polarized light from the waveguide 10 in the first embodiment of the present invention.
Fig. 8 shows the calculation results of the polarization extinction ratios of the waveguide 10 and the waveguide 11 when different reverse biases are applied to the electrode 8 after the electrode 7 is applied with a proper forward bias so that the TE polarized light is output from the waveguide 10 in the first embodiment of the present invention.
Fig. 9 is a schematic diagram of a polarization beam splitter according to a second embodiment of the present invention.
FIG. 10 is a schematic cross-sectional view of a waveguide according to a second embodiment of the present invention.
Detailed Description
As shown in FIG. 1, the polarization beam splitter of the present invention comprises, from left to right, an input waveguide 1, a first multimode interference coupler 2, an upper waveguide arm 3, a lower waveguide arm 4, an upper arm birefringent region waveguide 5,A lower arm birefringent region waveguide 6, an upper arm electrode 7, a lower arm electrode 8, a 2 × 2 multimode interference coupler 9, an output waveguide 10 and an output waveguide 11. It is clear to those skilled in the art that the upper waveguide arm 3 includes an upper arm birefringent region waveguide 5 and a straight waveguide, and the upper arm electrode 7 is disposed in the straight waveguide region. The lower waveguide arm 4 includes a lower arm birefringent region waveguide 6 and a straight waveguide, and a lower arm electrode 8 is disposed in the straight waveguide region. For the present invention, the waveguide is oriented and
After entering the input waveguide 1, the incident light is divided into two beams by the first multimode interference coupler 2 and enters the upper and lower phase shift waveguides respectively. The two beams are then interferometrically output in a 2 x 2 multimode interference coupler. If the width of the birefringent region waveguide of the upper and lower arms is consistent with that of the straight waveguide, ideally, the accumulated phases of the upper and lower arm waveguides for the TE and TM polarized light are completely the same. If the width and length of the birefringent region waveguides of the upper and lower arms are selected appropriately, the accumulated phase difference of the TE and TM polarized light of the upper and lower arm waveguides can be made to be different by pi. When the upper arm electrode is heated by applying a suitable forward bias or energization of the hot electrode, the carrier dispersion effect or the thermo-optical effect changes the effective refractive index of the TE polarized light so that the TE polarized light is completely output from the waveguide 10. Due to manufacturing errors in the birefringent regions of the upper and lower arm waveguides, TM polarized light cannot be completely output from the waveguide 10 or the waveguide 11. At this time, a reverse bias is applied to the lower arm electrode, and the effective refractive index of TM polarized light is changed to be completely output from the waveguide 11. Since the refractive index change caused by the first-order electro-optical effect of the TE polarized light is offset by the second-order electro-optical effect, the carrier dispersion effect, and the band filling effect, the reverse voltage does not affect the phase of the TE polarized light, and it can still be output from the waveguide 10.
In order to further explain the scheme, the invention also provides two application examples of the polarization beam splitter.
Example A selected waveguide core thickness was 0.4 μm, the material was InGaAsP, the width was 2 μm, and the waveThe conducting substrate and the cap layer were InP and the phase-shifted waveguide length was 2.5mm at an incident wavelength of 1550 nm. Waveguide orientation and crystal
The waveguide adopts a P-I-N type structure. Fig. 3 is a cross-sectional view of a P-I-N waveguide structure, from bottom to top, of an N-doped waveguide substrate 12, an undoped waveguide core layer 13 and a P-doped waveguide cap layer 14.
First, we specifically analyze the first-order electro-optic effect, the second-order electro-optic effect, the carrier dispersion effect and the band filling effect.
The first order electro-optic effect refers to the linear relationship between the change of effective refractive index and the intensity of applied electric field. For group III-V compounds with a zincblende crystal structure, the linear electro-optic tensor is left with only r41This term, and thus the first order electro-optic effect, does not work for TM polarized light, and the effect on TE polarized light will also depend on its electric field polarization direction. If the waveguide orientation and crystal are definedAngle of direction is beta, voltage direction is [100]]The direction, the first order electro-optic effect, satisfies the following formula:
from the above formula, the strength of the first-order electro-optical effect is affected by adjusting the size of beta, when the beta is 45-135 degrees, the first-order electro-optical effect can reduce the effective refractive index of the TE polarized light, and the reduction amount of the effective refractive index is the largest when the beta is 90 degrees.
The second order electro-optic effect refers to the fact that the change of the effective refractive index is linear with the square of an applied electric field. Also for group III-V compounds having a zincblende crystal structure, if the direction of the applied voltage is along [100]]Directional, second order electro-optic tensor is R only11And R12Capable of changing TM and TE biasAn effective refractive index of the vibrating light, and satisfies the following formula:
the carrier dispersion effect means that the real part and the imaginary part of the dielectric constant of a carrier injected or extracted material can be changed. According to the classical Drude model:
the change in effective refractive index and the loss of light can be calculated. Where e is the electronic charge,. epsilon0Is the dielectric constant of free space, Δ NeIs the change in electron concentration, Δ NhIs the change in hole concentration, m* ceAnd m* chRespectively an electron effective mass and a hole effective mass, mueAnd muhRespectively electron mobility and hole mobility. It should be noted here that in this example, it can be estimated that only 5mA of current is required for the phase shift where the carrier dispersion effect changes pi when forward bias is applied, which results in optical losses of less than 0.15dB, and that the extinction ratio of the 2 x 2MMI can still be greater than 30dB (input light intensity imbalance of less than 0.3 dB).
The band filling effect means that doping of a material or injection of carriers causes holes with low energy in the conduction band to be occupied, which in turn causes blue shift of the absorption peak. Even if the energy of the incident light is lower than the material band width, the effective refractive index of the incident polarized light is still reduced due to the Kramers-Kronig relationship. It should be noted that the application of reverse bias in the present example actually reduces the Carrier density to form depletion region, resulting in red shift of the absorption peak, which can increase the effective refractive index, and can be estimated according to the linear model derived from Brian et al (B.R.Bennett, R.A.Soref, J.A.D.Alamo, Carrier-induced change in reactive index of InP, GaAs, and InGaAsP, J.Lightwave Technol.,1990,26(1):113-122.)
Δn＝-1.6×10-12×(ΔNe+ΔNh)
Based on the above theoretical analysis, the phase shift changes of the TE and TM polarized light under reverse bias in the present example can be calculated.
The distribution conditions of the electric field intensity and the carrier density of the phase-shift waveguide under forward and reverse bias are calculated by a finite element method, and the distribution conditions of the electric field amplitude of the TE polarized light and the TM polarized light of the phase-shift waveguide are calculated by a light beam transmission method. Therefore, the change of the effective refractive index under reverse bias can be calculated through the electro-optic effect, and the phase shift change of TE and TM polarized light under different reverse biases in the phase shift waveguide can be further calculated. The simulation results of example one are given below:
fig. 4 respectively calculates the phase shift of the first-order electro-optical effect, the second-order electro-optical effect, the carrier dispersion effect and the energy band filling effect of the TE polarized light under different reverse biases. The results show that the phase shift due to the first order electro-optic effect is negative and the value is maximum. The phase shift for the other three effects is positive. When the reverse bias is less than 10V, the total phase shift of the TE polarized light is small.
Fig. 5 respectively calculates the phase shift of the second-order electro-optic effect, the carrier dispersion effect and the band filling effect of the TM polarized light under different reverse biases. Since the phase shifts of the three effects are all positive, the total phase shift of TM polarized light is large.
When the incident light is TE polarized light, the electrode 7 of the phase shift waveguide applies a forward bias voltage to allow the TE polarized light to be completely output from the waveguide 10.
Considering the manufacturing error of the two-phase-shift waveguide, which may bring a phase difference to the TE polarized light in the waveguide, a small forward bias may be selectively applied to the electrode 7 or the electrode 8 to allow the TE polarized light to be completely output from the waveguide 10 or the waveguide 11. For convenience of description, the selection electrode 7 applies a forward bias and the waveguide 10 outputs TE polarized light.
When the incident light is switched to TM polarized light, the electrodes 8 of the waveguide arms apply a reverse bias, allowing the TM polarized light to be completely output from the waveguide 11.
Fig. 6 shows the optical power of the output waveguide 10 as a function of the voltage of the electrode 8 when TE and TM polarized light is input. The different effects of carrier dispersion effects on the effective refractive index changes of TM and TE polarized light under forward bias of the electrode 7 are ignored here. Both TE and TM polarized light will be output from the waveguide 10 when the reverse bias is 0. It can be seen that the reverse bias has little effect on TE polarized light, V of TM polarized lightπAround 7V.
Fig. 7 shows the optical power of the output waveguide 11 as a function of the voltage of the electrode 8 when TE and TM polarized light is input. When the reverse bias is 0, the waveguide 11 has almost no light intensity output. When the reverse bias is gradually increased, the TM polarized light of the waveguide 11 is quickly adjusted to the maximum intensity, while the TE polarized light of the port is still weak.
Fig. 8 shows the polarization extinction ratios of the two output ports under different reverse biases. When the reverse bias of the electrode 8 is 7V, the waveguide 10 mainly outputs TE polarized light, the waveguide 11 mainly outputs TM polarized light, and the polarization extinction ratios of both ports are greater than 25 dB.
As can be seen from the above description, the first application example of the present invention provides a polarization beam splitter, which uses the carrier dispersion effect to change the effective refractive index of the TE polarized light, adjusts the phase of the TE polarized light to completely output from the waveguide 10, and uses the adjusted electro-optical effect to change only the effective refractive index of the TM polarized light to output from the waveguide 11, thereby implementing polarization beam splitting. The design has the characteristics of large manufacturing tolerance and high extinction ratio, and can realize the monolithic integration of the coherent receiver.
In example one above, when the voltage is along [100]]In direction, the effective refractive index change caused by the first-order electro-optic effect is
In addition, on the basis of the first example, the doping degree of the waveguide core layer can change the strength of the carrier dispersion effect and the energy band filling effect in reverse bias, so that the effective refractive index change caused by the total electro-optic effect of the TE polarized light is minimum.
On the basis of the first example, the thickness of the waveguide core layer directly affects the electric field strength under the same voltage, and further affects the strength of the first-order electro-optical effect and the second-order electro-optical effect, and the appropriate thickness of the core layer can be selected to minimize the change of the effective refractive index caused by the total electro-optical effect of the TE polarized light.
On the basis of the first example, the effective refractive index change caused by the total electro-optic effect of the TE polarized light is minimized by adjusting the components of the waveguide core layer and further adjusting the strength of the first-order electro-optic effect, the second-order electro-optic effect, the carrier dispersion effect and the energy band filling effect.
In the first embodiment, the waveguides are all single-mode waveguides, the interference region of the multimode interference coupler is actually multimode waveguides, and the two waveguides are connected by adopting the tapered graded waveguides, so that the loss during connection can be greatly reduced.
On the basis of the first example, an undoped low-refractive-index spacer layer can be introduced near the waveguide core layer, so that the waveguide loss is reduced.
On the basis of the first example, the structure of the cross section of the waveguide can be a P-I-N, N-I-P, N-P-I-N, N-I-P-N type or other structures similar to the P-I-N, N-I-P type under forward and reverse bias.
Example two the waveguide core layer chosen was 0.4 μm thick, the material was InGaAsP, the width was 2 μm, the waveguide substrate and cap layer were InP, a 1.5um thick InGaAs spacer layer was placed 2 μm below the waveguide core layer, the phase-shifted waveguide length was 2.5mm, the incident wavelength was 1550nm, and the upper arm waveguide covered the hot electrode 15 and the hot electrode 15A lower arm waveguide electrode 16. Waveguide orientation and crystal
The waveguide adopts a P-I-N type structure. Fig. 10 is a cross-sectional view of a P-I-N waveguide structure, from bottom to top, of an N-doped waveguide substrate 17, an N-doped spacer layer 18, an undoped waveguide core layer 19 and a P-doped waveguide cap layer 20.
The hot electrode 15 of the upper arm waveguide is electrically heated to change the effective refractive index of the TE polarized light so that the TE polarized light is completely output from the waveguide 10.
The lower arm waveguide electrode 16 applies a reverse voltage to change the effective refractive index of the TM polarized light so that the TM polarized light is completely output from the waveguide 11.
On the basis of the second example, the reverse voltage of the second example has the same effect on TE and TM polarized light.
It can be seen that, the first and second examples above can respectively design the inclination angle β of the waveguide towards the crystal direction according to the specific structural parameters of the actual waveguide, and by adjusting the strength of the first-order electro-optic effect of the TE polarized light, the total effective refractive index under the reverse bias of the TE polarized light is minimized, while the total effective refractive index under the reverse bias of the TM polarized light is greatly changed. Therefore, TE polarized light is adjusted by adjusting the magnitude of forward bias (or current on a thermoelectric electrode) on one arm, TM polarized light is adjusted by adjusting the magnitude of reverse bias, and the phases of the TE polarized light and the TM polarized light respectively meet interference constructive and destructive conditions of the 2 x 2 multimode interference coupler at different output ports, so that the polarization beam splitter realizes TE and TM polarization beam splitting.
Finally, it should be noted that the above-mentioned embodiments are only for illustrating the technical solutions of the present invention and not for limiting, and although the present invention has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that modifications or equivalent substitutions may be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, which should be covered by the claims of the present invention.
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