CA2368161A1 - Low loss and low polarization dependence waveguide variable optical attenuator - Google Patents
Low loss and low polarization dependence waveguide variable optical attenuator Download PDFInfo
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- CA2368161A1 CA2368161A1 CA 2368161 CA2368161A CA2368161A1 CA 2368161 A1 CA2368161 A1 CA 2368161A1 CA 2368161 CA2368161 CA 2368161 CA 2368161 A CA2368161 A CA 2368161A CA 2368161 A1 CA2368161 A1 CA 2368161A1
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- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
- G02F1/21—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour by interference
- G02F1/225—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour by interference in an optical waveguide structure
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- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F2203/00—Function characteristic
- G02F2203/48—Variable attenuator
Abstract
A waveguide variable optical attenuator using a pair of waveguide 3dB couplers configuration and having a polarization dependence compensator is proposed in this invention. This pair of 3dB couplers forms a Mach-Zehnder interferometer where a pair of cross-state input port and output port is used. Because the access loss of a 3dB coupler is much less than that of a Y junction, the system loss of the variable optical attenuator based on this invention is much less than that of the other typical structure of the variable optical attenuators based on a pair of Y junctions. Generally, the polarization dependent loss is a vital issue for a variable optical attenuator with either the 3dB
couplers structure or the Y junctions structure during it is being attenuated. In this invention, a polarization dependence compensator is made on this variable optical attenuator to correct any polarization dependent loss to the acceptable level. Therefore, the waveguide variable optical attenuator based on this invention can have low system loss and low polarization dependent loss.
couplers structure or the Y junctions structure during it is being attenuated. In this invention, a polarization dependence compensator is made on this variable optical attenuator to correct any polarization dependent loss to the acceptable level. Therefore, the waveguide variable optical attenuator based on this invention can have low system loss and low polarization dependent loss.
Description
Low Loss and Low Polarization Dependence Waveguide Variable Optical Attenuator Technical Field The present invention is a variable optical attenuator using a Mach-Zehnder interferometers configuration and polarization dependence compensator. It relates to a variable optical attenuator with low insertion loss and low polarization dependence for optical communication systems and simultaneous testing systems of multiple parameters.
Background of the Invention Development of fiber-optic telecommunication systems has exactly passed a whole process of the dramatic growing and the rapid falling. This whole process not only has stimulated new microstructure optoelectronic technologies instead of mechanical individual devices, but also given us how to focus on feasible new products with reliable technologies. Among various microstructure optoelectronic technologies, integrated optics represents a promising strategy in these advanced oprical information areas. One implementation of this strategy relies on the waveguide technology. The thermo-optic (TO) waveguide devices using PECVD-based silica-on-silicon have shown an exciting advantage over the currently used mechanical and bulk optic devices in fiber-optic telecommunications because of their great flexibility in fabrication and processing as well as speedy operations than the mechanical ones. The electro-optic (E0) waveguide devices using diffused LiNb03-based waveguides have also presented a promising application in the future with its high-speed operation, Iow loss and mature manufacturing technology. But, the fabrication of LiNb03-based electro-optic waveguide devices is really has its own limitation. Polymer, as a new kind of EO film material, always receives much research aimed at solving its stability and manufacturability.
Recently, research on practical EO polymers has really had some significant progresses.
Thus, developing new high-performance EO waveguide devices also gives a new hope to industry. Among all the active devices in both optical communication systems and simultaneous testing of multiple parameters, the optical space switches are certainly key components. But, in these two typical cases, variable optical attenuators are indispensable to protect the detecting equipments from damaging. Especially, in these two cases, variable optical attenuator arrays are strongly requested for the signal protection of optical multiple channel systems. Thus, the arrayed variable optical attenuators based on plannar waveguides technology will play an increasingly critical role in emerging multichannel and reconfigurable photonic networks such as the dense wavelength division multiplexing (DWDM) and the simultaneous testing systems of multiple parameters together with optical switches.
Most of both variable optical attenuators and optical switches in production today use an opto-mechanical means to implement optical attenuating and steering. This is accomplished through the separation, or the alignment by an opto-mechanically driven optical parts. These designs offer good optical performance, but have two main drawbacks. One is slow speed. The typical settling times for operating from 10 ms to 100 ms. And the other drawbacks includes the noise and size. In an era when the use of electronics is considered an intrusion in the all-optical networks, mechanically based devices seem out of place. Especially, this design is really hard to meet the marketing needs for the arrayed variable optical attenuators. To overcome some of these limitations, non-mechanical and no-moving-part variable optical attenuators and optical switches based on the integrated optical technology are paid much research and development in the past a few years. But, the main critical obstacles blocking these efforts from challenging the conventional products based on the opto-mechanical designs are system loss and polarization dependence. But, both the EO and the TO waveguides for these two main active components have shown a huge potential of applications not only in the operation speed, but also in compatibility with integrated optic circuits.
Totally there are two typical designs of Mach-Zehnder interferometer (MZI) configuration for waveguide variable optical attenuators. One uses two 3dB
couplers and its operation is based on the controlling of optical coupling process between two waveguide channels. This design is the same as the 2x2 optical switches by using a pair of cross-state input/output ports. The other one uses two Y junctions and its operation is based on the splitting and interfering of optical beams with waveguides. These two designs of waveguide variable optical attenuators have some similar properties and some different optical characteristics at both the unattenuated state and the attenuated state. For example, they have similar attenuating process with the applied power for the thermal modulating (or the electric voltage for the electrical modulating) and the same power consumption for the same attenuated level with the same waveguides structure.
But, they have different system losses at the unattenuated state and different polarization dependent losses at the same attenuated level. The design based on 3dB couplers generally has lower system loss at the unattenuated state and higher polarization dependent loss at the attenuated state than the design based on Y junctions.
Summary of the Invention A waveguide variable optical attenuator using a pair of waveguide 3dB couplers configuration and having a polarization dependent loss compensator is proposed in this invention. This pair of 3dB couplers forms a Mach-Zehnder interferometer where a pair of cross-state input port and output port is used. One modulating electrode is made on one arm of the Mach-Zehnder interferometer and used to change the optical phase of the modulated arm. The modulating form can be either thermal-optic or electro-optic. This structure has some advantages over the other typical one that uses a pair of Y
junctions.
Generally the access loss of a 3dB coupler is much less than that of a Y
junction, so the system loss of the waveguide variable optical attenuator based on this invention is much less than that of the other typical structure of the variable optical attenuators based on a pair of Y junctions. Generally, the polarization dependent loss is a vital issue for a variable optical attenuator with either the 3dB couplers structure or the Y
junctions structure during it is being attenuated. Even at the attenuated states, the polarization dependent loss of the waveguide variable optical attenuator with the 3dB
couplers is higher than that of the waveguide variable optical attenuator with the Y
junctions. In this invention, a polarization dependence compensator is introduced to correct any polarization dependent loss to the acceptable level. The operation principle of this polarization dependence compensator is to rotate the polarization of the optical beam by 90 degree with an efficiency of 50% and let the optical beam have a same amount in two polarization directions. This polarization dependence compensator can have several different structures and integrated together with the Mach-Zehnder interferometer.
Therefore, the waveguide optical attenuator based on this invention can have low system loss and low polarization dependent loss.
In a desirable embodiment according to the present invention, the Mach-Zehnder interferometer composed of two 3dB couplers is typically a 2x2 switch structure with a modulating electrode, then a pair of cross-state ends as input port and output port of variable optical attenuator. But, for the variable optical attenuator, the modulating electrode is not used to only produce an optical phase change ~, it is needed to produce many different optical phase changes to attenuate the optical output signal to different levels according to the requirements of applications. What is more important is a polarization dependence compensator is made on the output end to correct the polarization dependent loss induced when the variable optical attenuator is being operated.
Brief Description of the Drawing FIG. 1 Configuration of a waveguide variable optical attenuator using the Mach-Zehnder interferometer configuration and polarization dependence compensator, where FIG. 1(a) is the top view, FIG. 1(b) is the cross section along the axis A-A, and FIG. 1(c) is the detailed schematic and operation principle of the Mach-Zehnder interferometers configuration based on two 3dB couplers.
FIG. 2 Two different connection forms of polarization dependence compensator for the waveguide variable optical attenuator, where FIG. 2(a) is based on the bending waveguides structure and FIG. 2(b) is based on the asymmetric periodic waveguides structure.
FIG. 3 Schematic of the Mach-Zehnder interferometer configuration based on two 3dB
Y junctions, other possible option for the variable optical attenuator based on the current invention.
FIG. 4 Schematic of the Mach-Zehnder interferometer configuration based on electro-optic modulation, the other possible modulation for the variable optical attenuator based on the current invention.
Detailed Descriution of the Invention In this invention, the waveguide Mach-Zehnder interferometer (MZI) configuration is used and it contains two 3dB directional couplers connected by two waveguide arms.
This configuration basically exploits the phase property of the light. The input light is split and sent to two separate waveguide arms by the first 3dB directional coupler, then combined and split one last time by the second 3dB directional coupler. One or two of the waveguide arms are modulated to produce a difference of optical path length between these two waveguide arms. The modulating means can be either thermo-optic (TO) or electro-optic (E0). If these two optical paths are the same length, light chooses one exit, and if they have a difference it chooses the other. As a 2x2 switch, two input ports and two output ports are needed and this phase difference is ~, so that the optical signals can have two exits and each exit can have two output states of high and low. But, as a variable optical attenuator, only the input end where the light is launched and the output exit that the light chooses are needed and this phase change can be any value with respect to the desirable attenuated degree. As either 2x2 switches or variable optical attenuators, the isolation between two output ports is important because it directly determines the ON/OFF extinction ratio of one output port as 2x2 switches or attenuation dynamics as variable optical attenuators. Meanwhile, the isolation is strongly dependent of the coupling ratio of the two 3dB directional couplers. Namely, the closer to 50°Io the coupling ratio of the 3dB directional coupler is, the higher the isolation between two outputs at the two exits is, and further the higher ON/OFF extinction ratio the 2x2 switch has at each output port. In theory, if the coupling ratio of the 3dB coupler is exactly 50%
(i.e., -3dB), the isolation between two output ports should be inFnity. In fact, no perfect 3dB directional coupler exists because the errors in both design and fabrication, especially in fabrication, are not avoidable.
As shown in FIG. 1, this waveguide variable optical attenuator comprises a substrate 20, cladding 22, waveguide of input port 24, two waveguide 3dB couplers 26a, 26b, two waveguide arms 28a and 28b connecting the two 3dB couplers, and one modulating electrode 30 (it is also called heater for thermal modulation), a waveguide channel 32 connecting the exit that the light chooses when no optical phase change between two waveguide arms, one polarization dependence compensator (PDC) 34, and a waveguide output port 36 connecting the PDC. The MZI configuration is composed of two 3dB
directional couplers 26a and 26b, and two waveguide arms 28a and 28b. The modulating electrode 30 is made on one waveguide arm 28a of the MZI configuration to produce an optical phase change. In fact, the MZI configuration based on two 3dB couplers should have two input ports and two output ports as shown in FIG. 1 (c), so it is more popularly used to form a 2x2 optical switch as mentioned above. In this invention, it is used as a variable optical attenuator and only one input port 24 where the optical signal 38 is launched and one output port 32 that the optical signal 40 chooses are used.
The other input port 24a is at the idle state or probably useful for reducing the return loss of the system and the other output port 32a is used to split the undesirable optical beam away during this system is being operated to attenuate the output optical signal 40.
For simplicity, the thermal modulation is taken as an example to describe the operation process and the difference between the variable optical attenuator based on the present invention and the conventional structure having no polarization dependence compensator.
As shown in Fig. 1, if an optical signal 38 is launched into the input port 24, it is split into two parts at 50% coupling ratio by the 3dB directional coupler 26a, then these two parts pass through two waveguide arms with the same length 28a and 28b, finally they are combined into one optical signal again by the 3dB directional coupler 26b. If the electrode 30 is not activated by a modulating signal (at the OFF-state), the optical signal is sent into the cross-state waveguide path 32 as an output optical signal of the MZI
configuration. This optical signal has to pass through a PDC 34 before it is coming out at the output port of waveguide 36, so the output optical signal 40 is at the high output state for both the TE-mode and the TM-mode. Research shows the polarization of the optical beam can be changed when it passes through the two 3dB couplers and these two 3dB
couplers have different coupling efficiencies for the TE-mode and the TM-mode, so the output optical signal 40 should have different values for the TE-mode and TM-mode.
Namely, the output optical signal 411 has polarization dependence, the experimental results show the polarization dependence of output optical signals induced by MZI
configuration is not much if the birefringence of the waveguide material can be controlled at a small value. For example, the polarization dependence of MZI
configuration based on silica-on-silicon waveguides is averagely less than 0.3dB, which basically can be acceptable in the fiber-optical communications industry. For the same optical signal 38 launched into the 3dB directional coupler 26a, when the electrode 30 is activated by a modulating signal (at the attenuated-state), an optical phase difference is produced by this modulating process, the two parts of the optical signal coming from two waveguide arms can not be completely combined into one optical signal again and only some of optical beam is sent to the waveguide path 32 and the left pqrt of optical beam is sent to the other exit 32a of the MZI configuration as shown in Fig. 1(c), then the output signal 40 is attenuated some. Namely, the value of output optical signal 40 depends on the optical phase change induced by the applied modulating process. When the optical phase change induced by the modulating process is exactly ~ or the odd times of ~, the optical beam will be 100% sent to the other exit 32a and no optical beam can be sent to the expected waveguide channel 32, so the value of output optical signal 40 is theoretically zero. But, in practice the absolute zero output never exists because the optical phase change cannot exactly be controlled at the value of ~ and the coupling efficiency of two 3dB couplers also has some errors. The attenuated part sent to the expected waveguide channel 32 can be practically attenuated to much less than 20dB, which is referred as attenuation dynamics. What is more important is the optical phase change is different for the TE-mode from for the TM-mode at the same modulating process, thus the attenuated degree is certainly different for the TE-mode from for the TM-mode, and even the difference in the deeply attenuated output of the MZI
configuration based on two 3dB couplers between the TE-mode and the TM-mode is much higher. Namely, the polarization dependent loss (PDL) of the variable optical attenuator with MZI configuration is always a very critical issue in the product development. Therefore, in this invention, a PDC 34 is introduced into the variable optical attenuator with the MZI configuration. The operation principle of the PDC 34 is to produce a polarization rotation of 90° with a ratio of 50% for the optical beams passing through it. Namely, the original beam of TE-mode will become a half of TE-mode and a half of TM-mode after the optical beam passes through this PDC. This polarization rotation effect is the same to the original beam of TM-mode. Therefore, the output optical signal 40 will theoretically have the same amount between the TE-mode and the TM-mode. Namely, the PDL will be corrected to a much lower value in theory. The used in this invention can have several different designs including the curved waveguide channels, the periodic changed waveguide channels, the asymmetric poling and so on. In this invention, two typical designs for the PDC 34: the curved waveguide channels and the asymmetric periodic changed waveguide channels are provided for choices as shown in Fig. 2(a) and Fig. 2(b), respectively.
As mentioned above, the directional couplers with a coupling ratio of 50%, called 3dB
directional coupler, are the most useful optical function elements in not only the 2x2 optical switch, but also the variable optical attenuator based on the current invention. As shown in Fig. 1, the MZI configuration consists of two 3d8 directional couplers and two waveguide arms of the same length. One of the waveguide arms is deposited with the metal electrode (it is also called heater for the thermal modulation, while for the electrical modulation, it is called as electrode and two electrodes have to be used). The PDL comes from the coupling process of two 3dB couplers during the MZI configuration is attenuated by a modulation, so we start the analysis with one 3dB coupler at two different mode states: the TE and the TM modes. For a 3d8 directional coupler, assuming the input optical power Po exactly has O.SPo TE-mode and O.SPo TM-mode, and the output powers of the 3dB directional coupler at the TE-mode and the TM-mode are P,TE
and P,"" , respectively, at the bar-state port and are PTE and PT"' , respectively, at the cross-state port, then the coupling ratio at the two polarization modes kTE and k""
are defined by PTE
k~ - PTE + PTE (la) z PTM
TM __ ) k P,~" + P~" 1b In the same manner, the coupling losses at the two polarization modes L~E and Lc"' of the 3dB directional coupler are defined by L~E = l O logio ( l,~ ~+PprE ) (2a) L~ =101og1o ( P"o.+PP~ ) (2b) i As well known, the same thermal (or electrical) modulation can produce different change of refractive index. Assuming the changes of refractive index of waveguide produced by the modulation are OreTE and tlnTM for the TE-mode and the TM-mode, respectively, and the corresponding phase differences between two waveguide arms of the MZI
configuration for the two polarization modes should be TE
e~TE _ 2~.~1 Y' ~ (3a) ~~Ti" ' 2~L~rc (3b) where L is the length of the modulated waveguide (i.e., the length of the electrode) and ~, is wavelength. For the TO modulation, OrcTE and OnTM are related to the temperature change OT by the TO coefficients dn~ l dT and dn"" l dT of the waveguide material as TE dnTE
dT ~T (4a) OrcTM - daT OT (4b) and for the EO modulation, ~rzTE and ~rcTM are related to the applied electrical field E
by the EO coefficient r33 of the waveguide material as OnTE - - ~ r33nTE E (5a) ~~ - - 2 rs3nM E (5b) where n,.E and n,.~ are the refractive indices of the EO waveguide material for the TE-mode and the TM-mode, respectively. Then two output efficiencies of the MZI
configuration for the variable optical attenuator based on the current invention at the TE-mode and the TM-mode are ~TE
ATE = 4kTE (1- kTE ) cos z ( 2 ) (6a) Q ~TM
ATM = 4kTM (1- kTM )cost( Y'2 ) (6b) In terms of the definition of the PDL of the communication components, without the PDC, the PDL of the variable optical attenuator based on the current invention at any state can be defined by TE
PDL =1 loglo ( ~~ ) (7) Because kTE and kT"'' indicate the coupling ratios of the 3d8 couplers in this regime at the TE-mode and the TM-mode, respectively, and ATE and ATM indicate the optical phase changes between two waveguide arms in this regime for the TE-mode and the TM-mode, respectively, the PDL can be existing in both the unattenuated and the attenuated states, and it is also a function of the optical phase changes (i.e., the attenuated depth) even when the polarization dependence of the 3dB couplers is reduced to be zero (i.e., kTE -kT"' ). As described above, a PDC, which typically has two designs as shown in Fig. 2(a) and Fig. 2(b), respectively, is introduced in this variable optical attenuator. It can make the polarization state of the optical beams rotate 90° with 50% ratio.
Thus, the polarization states of the two output efficiencies of the MZI configuration for the variable optical attenuator based on the current invention defined by the set of equations (6a) and (6b) become new states ~N and ~N' as defined by TE TM
rIN =2(rITE'f'~7~)=2 kTE(1-kTE)cos2(~~ )+kTM(1-kTM)cos2(~~ ) (8a) TE TM
~7N - 2(~1TE+~7TM)-2 kTE(1 kTE)cOSz(~~ )+kTM(1-kTM)COS2(~~ ) 8b Thus, with the PDC, the new PDL of the variable optical attenuator based on the current invention should be defined by TE
PDLN =1 loglo ( ~ M ) = 0 (9) rIN
The result defined by Eq. (9) is based on the theoretical state, but the polarization rotation ratio of a practical PDC cannot be exactly 50%, some designed and fabricated errors are not avoidable, so the practical value of PDLN could not be exactly zero like Eq. (9). But, it can be reduced to an acceptable value with the introduction of the PDC.
Therefore, two paramount parameters, the PDL and the system loss of the variable optical attenuators based on the current invention can be directly improved much better than any conventional structure, which is exactly the main goals of the current invention. In this invention, the MZI configuration is based on two 3dB couplers in order to obtain a low system loss. It, however, can be also based on two Y junctions as shown in Fig. 3 where the system loss will be relatively high. In addition, the schematic structure shown in Fig.
1 is based on the thermo-optic modulation. In fact, as mentioned in the context, this device is also based on the electro-optic modulation as shown in Fig. 4 where two electrodes are used as cathode and anode and labeled as 30a and 30b, respectively.
Finally some useful papers for understanding the operation principle of "the PDC"
based on the polarization rotation are the following:
~ Polarization rotation in semiconductor bending waveguides: a coupled-mode theory by Liu, et al., Journal of Lightwave Technology, Vol. 16, No. 5, May 1998, pp. 929-936;
~ Novel compact polarization converters based on ultra short bends by Dam, et al., IEEE Photonics Technology Letters, Vol. 8, No. 10, October 1996, pp. 1346-1348.
~ First realixed polarization converter based on hybrid supermades by Mertens, et al., IEEE Fhotonics Technology Letters, Vol. 10, No. 3, March 1998, pp. 388-390.
~ Polarization rotation in asymmetric periodic loaded rib waveguides by Shani, et al., Applied Physics Letters, Vol. 59, No. 11, September 1991, pp. 1278-1280.
Background of the Invention Development of fiber-optic telecommunication systems has exactly passed a whole process of the dramatic growing and the rapid falling. This whole process not only has stimulated new microstructure optoelectronic technologies instead of mechanical individual devices, but also given us how to focus on feasible new products with reliable technologies. Among various microstructure optoelectronic technologies, integrated optics represents a promising strategy in these advanced oprical information areas. One implementation of this strategy relies on the waveguide technology. The thermo-optic (TO) waveguide devices using PECVD-based silica-on-silicon have shown an exciting advantage over the currently used mechanical and bulk optic devices in fiber-optic telecommunications because of their great flexibility in fabrication and processing as well as speedy operations than the mechanical ones. The electro-optic (E0) waveguide devices using diffused LiNb03-based waveguides have also presented a promising application in the future with its high-speed operation, Iow loss and mature manufacturing technology. But, the fabrication of LiNb03-based electro-optic waveguide devices is really has its own limitation. Polymer, as a new kind of EO film material, always receives much research aimed at solving its stability and manufacturability.
Recently, research on practical EO polymers has really had some significant progresses.
Thus, developing new high-performance EO waveguide devices also gives a new hope to industry. Among all the active devices in both optical communication systems and simultaneous testing of multiple parameters, the optical space switches are certainly key components. But, in these two typical cases, variable optical attenuators are indispensable to protect the detecting equipments from damaging. Especially, in these two cases, variable optical attenuator arrays are strongly requested for the signal protection of optical multiple channel systems. Thus, the arrayed variable optical attenuators based on plannar waveguides technology will play an increasingly critical role in emerging multichannel and reconfigurable photonic networks such as the dense wavelength division multiplexing (DWDM) and the simultaneous testing systems of multiple parameters together with optical switches.
Most of both variable optical attenuators and optical switches in production today use an opto-mechanical means to implement optical attenuating and steering. This is accomplished through the separation, or the alignment by an opto-mechanically driven optical parts. These designs offer good optical performance, but have two main drawbacks. One is slow speed. The typical settling times for operating from 10 ms to 100 ms. And the other drawbacks includes the noise and size. In an era when the use of electronics is considered an intrusion in the all-optical networks, mechanically based devices seem out of place. Especially, this design is really hard to meet the marketing needs for the arrayed variable optical attenuators. To overcome some of these limitations, non-mechanical and no-moving-part variable optical attenuators and optical switches based on the integrated optical technology are paid much research and development in the past a few years. But, the main critical obstacles blocking these efforts from challenging the conventional products based on the opto-mechanical designs are system loss and polarization dependence. But, both the EO and the TO waveguides for these two main active components have shown a huge potential of applications not only in the operation speed, but also in compatibility with integrated optic circuits.
Totally there are two typical designs of Mach-Zehnder interferometer (MZI) configuration for waveguide variable optical attenuators. One uses two 3dB
couplers and its operation is based on the controlling of optical coupling process between two waveguide channels. This design is the same as the 2x2 optical switches by using a pair of cross-state input/output ports. The other one uses two Y junctions and its operation is based on the splitting and interfering of optical beams with waveguides. These two designs of waveguide variable optical attenuators have some similar properties and some different optical characteristics at both the unattenuated state and the attenuated state. For example, they have similar attenuating process with the applied power for the thermal modulating (or the electric voltage for the electrical modulating) and the same power consumption for the same attenuated level with the same waveguides structure.
But, they have different system losses at the unattenuated state and different polarization dependent losses at the same attenuated level. The design based on 3dB couplers generally has lower system loss at the unattenuated state and higher polarization dependent loss at the attenuated state than the design based on Y junctions.
Summary of the Invention A waveguide variable optical attenuator using a pair of waveguide 3dB couplers configuration and having a polarization dependent loss compensator is proposed in this invention. This pair of 3dB couplers forms a Mach-Zehnder interferometer where a pair of cross-state input port and output port is used. One modulating electrode is made on one arm of the Mach-Zehnder interferometer and used to change the optical phase of the modulated arm. The modulating form can be either thermal-optic or electro-optic. This structure has some advantages over the other typical one that uses a pair of Y
junctions.
Generally the access loss of a 3dB coupler is much less than that of a Y
junction, so the system loss of the waveguide variable optical attenuator based on this invention is much less than that of the other typical structure of the variable optical attenuators based on a pair of Y junctions. Generally, the polarization dependent loss is a vital issue for a variable optical attenuator with either the 3dB couplers structure or the Y
junctions structure during it is being attenuated. Even at the attenuated states, the polarization dependent loss of the waveguide variable optical attenuator with the 3dB
couplers is higher than that of the waveguide variable optical attenuator with the Y
junctions. In this invention, a polarization dependence compensator is introduced to correct any polarization dependent loss to the acceptable level. The operation principle of this polarization dependence compensator is to rotate the polarization of the optical beam by 90 degree with an efficiency of 50% and let the optical beam have a same amount in two polarization directions. This polarization dependence compensator can have several different structures and integrated together with the Mach-Zehnder interferometer.
Therefore, the waveguide optical attenuator based on this invention can have low system loss and low polarization dependent loss.
In a desirable embodiment according to the present invention, the Mach-Zehnder interferometer composed of two 3dB couplers is typically a 2x2 switch structure with a modulating electrode, then a pair of cross-state ends as input port and output port of variable optical attenuator. But, for the variable optical attenuator, the modulating electrode is not used to only produce an optical phase change ~, it is needed to produce many different optical phase changes to attenuate the optical output signal to different levels according to the requirements of applications. What is more important is a polarization dependence compensator is made on the output end to correct the polarization dependent loss induced when the variable optical attenuator is being operated.
Brief Description of the Drawing FIG. 1 Configuration of a waveguide variable optical attenuator using the Mach-Zehnder interferometer configuration and polarization dependence compensator, where FIG. 1(a) is the top view, FIG. 1(b) is the cross section along the axis A-A, and FIG. 1(c) is the detailed schematic and operation principle of the Mach-Zehnder interferometers configuration based on two 3dB couplers.
FIG. 2 Two different connection forms of polarization dependence compensator for the waveguide variable optical attenuator, where FIG. 2(a) is based on the bending waveguides structure and FIG. 2(b) is based on the asymmetric periodic waveguides structure.
FIG. 3 Schematic of the Mach-Zehnder interferometer configuration based on two 3dB
Y junctions, other possible option for the variable optical attenuator based on the current invention.
FIG. 4 Schematic of the Mach-Zehnder interferometer configuration based on electro-optic modulation, the other possible modulation for the variable optical attenuator based on the current invention.
Detailed Descriution of the Invention In this invention, the waveguide Mach-Zehnder interferometer (MZI) configuration is used and it contains two 3dB directional couplers connected by two waveguide arms.
This configuration basically exploits the phase property of the light. The input light is split and sent to two separate waveguide arms by the first 3dB directional coupler, then combined and split one last time by the second 3dB directional coupler. One or two of the waveguide arms are modulated to produce a difference of optical path length between these two waveguide arms. The modulating means can be either thermo-optic (TO) or electro-optic (E0). If these two optical paths are the same length, light chooses one exit, and if they have a difference it chooses the other. As a 2x2 switch, two input ports and two output ports are needed and this phase difference is ~, so that the optical signals can have two exits and each exit can have two output states of high and low. But, as a variable optical attenuator, only the input end where the light is launched and the output exit that the light chooses are needed and this phase change can be any value with respect to the desirable attenuated degree. As either 2x2 switches or variable optical attenuators, the isolation between two output ports is important because it directly determines the ON/OFF extinction ratio of one output port as 2x2 switches or attenuation dynamics as variable optical attenuators. Meanwhile, the isolation is strongly dependent of the coupling ratio of the two 3dB directional couplers. Namely, the closer to 50°Io the coupling ratio of the 3dB directional coupler is, the higher the isolation between two outputs at the two exits is, and further the higher ON/OFF extinction ratio the 2x2 switch has at each output port. In theory, if the coupling ratio of the 3dB coupler is exactly 50%
(i.e., -3dB), the isolation between two output ports should be inFnity. In fact, no perfect 3dB directional coupler exists because the errors in both design and fabrication, especially in fabrication, are not avoidable.
As shown in FIG. 1, this waveguide variable optical attenuator comprises a substrate 20, cladding 22, waveguide of input port 24, two waveguide 3dB couplers 26a, 26b, two waveguide arms 28a and 28b connecting the two 3dB couplers, and one modulating electrode 30 (it is also called heater for thermal modulation), a waveguide channel 32 connecting the exit that the light chooses when no optical phase change between two waveguide arms, one polarization dependence compensator (PDC) 34, and a waveguide output port 36 connecting the PDC. The MZI configuration is composed of two 3dB
directional couplers 26a and 26b, and two waveguide arms 28a and 28b. The modulating electrode 30 is made on one waveguide arm 28a of the MZI configuration to produce an optical phase change. In fact, the MZI configuration based on two 3dB couplers should have two input ports and two output ports as shown in FIG. 1 (c), so it is more popularly used to form a 2x2 optical switch as mentioned above. In this invention, it is used as a variable optical attenuator and only one input port 24 where the optical signal 38 is launched and one output port 32 that the optical signal 40 chooses are used.
The other input port 24a is at the idle state or probably useful for reducing the return loss of the system and the other output port 32a is used to split the undesirable optical beam away during this system is being operated to attenuate the output optical signal 40.
For simplicity, the thermal modulation is taken as an example to describe the operation process and the difference between the variable optical attenuator based on the present invention and the conventional structure having no polarization dependence compensator.
As shown in Fig. 1, if an optical signal 38 is launched into the input port 24, it is split into two parts at 50% coupling ratio by the 3dB directional coupler 26a, then these two parts pass through two waveguide arms with the same length 28a and 28b, finally they are combined into one optical signal again by the 3dB directional coupler 26b. If the electrode 30 is not activated by a modulating signal (at the OFF-state), the optical signal is sent into the cross-state waveguide path 32 as an output optical signal of the MZI
configuration. This optical signal has to pass through a PDC 34 before it is coming out at the output port of waveguide 36, so the output optical signal 40 is at the high output state for both the TE-mode and the TM-mode. Research shows the polarization of the optical beam can be changed when it passes through the two 3dB couplers and these two 3dB
couplers have different coupling efficiencies for the TE-mode and the TM-mode, so the output optical signal 40 should have different values for the TE-mode and TM-mode.
Namely, the output optical signal 411 has polarization dependence, the experimental results show the polarization dependence of output optical signals induced by MZI
configuration is not much if the birefringence of the waveguide material can be controlled at a small value. For example, the polarization dependence of MZI
configuration based on silica-on-silicon waveguides is averagely less than 0.3dB, which basically can be acceptable in the fiber-optical communications industry. For the same optical signal 38 launched into the 3dB directional coupler 26a, when the electrode 30 is activated by a modulating signal (at the attenuated-state), an optical phase difference is produced by this modulating process, the two parts of the optical signal coming from two waveguide arms can not be completely combined into one optical signal again and only some of optical beam is sent to the waveguide path 32 and the left pqrt of optical beam is sent to the other exit 32a of the MZI configuration as shown in Fig. 1(c), then the output signal 40 is attenuated some. Namely, the value of output optical signal 40 depends on the optical phase change induced by the applied modulating process. When the optical phase change induced by the modulating process is exactly ~ or the odd times of ~, the optical beam will be 100% sent to the other exit 32a and no optical beam can be sent to the expected waveguide channel 32, so the value of output optical signal 40 is theoretically zero. But, in practice the absolute zero output never exists because the optical phase change cannot exactly be controlled at the value of ~ and the coupling efficiency of two 3dB couplers also has some errors. The attenuated part sent to the expected waveguide channel 32 can be practically attenuated to much less than 20dB, which is referred as attenuation dynamics. What is more important is the optical phase change is different for the TE-mode from for the TM-mode at the same modulating process, thus the attenuated degree is certainly different for the TE-mode from for the TM-mode, and even the difference in the deeply attenuated output of the MZI
configuration based on two 3dB couplers between the TE-mode and the TM-mode is much higher. Namely, the polarization dependent loss (PDL) of the variable optical attenuator with MZI configuration is always a very critical issue in the product development. Therefore, in this invention, a PDC 34 is introduced into the variable optical attenuator with the MZI configuration. The operation principle of the PDC 34 is to produce a polarization rotation of 90° with a ratio of 50% for the optical beams passing through it. Namely, the original beam of TE-mode will become a half of TE-mode and a half of TM-mode after the optical beam passes through this PDC. This polarization rotation effect is the same to the original beam of TM-mode. Therefore, the output optical signal 40 will theoretically have the same amount between the TE-mode and the TM-mode. Namely, the PDL will be corrected to a much lower value in theory. The used in this invention can have several different designs including the curved waveguide channels, the periodic changed waveguide channels, the asymmetric poling and so on. In this invention, two typical designs for the PDC 34: the curved waveguide channels and the asymmetric periodic changed waveguide channels are provided for choices as shown in Fig. 2(a) and Fig. 2(b), respectively.
As mentioned above, the directional couplers with a coupling ratio of 50%, called 3dB
directional coupler, are the most useful optical function elements in not only the 2x2 optical switch, but also the variable optical attenuator based on the current invention. As shown in Fig. 1, the MZI configuration consists of two 3d8 directional couplers and two waveguide arms of the same length. One of the waveguide arms is deposited with the metal electrode (it is also called heater for the thermal modulation, while for the electrical modulation, it is called as electrode and two electrodes have to be used). The PDL comes from the coupling process of two 3dB couplers during the MZI configuration is attenuated by a modulation, so we start the analysis with one 3dB coupler at two different mode states: the TE and the TM modes. For a 3d8 directional coupler, assuming the input optical power Po exactly has O.SPo TE-mode and O.SPo TM-mode, and the output powers of the 3dB directional coupler at the TE-mode and the TM-mode are P,TE
and P,"" , respectively, at the bar-state port and are PTE and PT"' , respectively, at the cross-state port, then the coupling ratio at the two polarization modes kTE and k""
are defined by PTE
k~ - PTE + PTE (la) z PTM
TM __ ) k P,~" + P~" 1b In the same manner, the coupling losses at the two polarization modes L~E and Lc"' of the 3dB directional coupler are defined by L~E = l O logio ( l,~ ~+PprE ) (2a) L~ =101og1o ( P"o.+PP~ ) (2b) i As well known, the same thermal (or electrical) modulation can produce different change of refractive index. Assuming the changes of refractive index of waveguide produced by the modulation are OreTE and tlnTM for the TE-mode and the TM-mode, respectively, and the corresponding phase differences between two waveguide arms of the MZI
configuration for the two polarization modes should be TE
e~TE _ 2~.~1 Y' ~ (3a) ~~Ti" ' 2~L~rc (3b) where L is the length of the modulated waveguide (i.e., the length of the electrode) and ~, is wavelength. For the TO modulation, OrcTE and OnTM are related to the temperature change OT by the TO coefficients dn~ l dT and dn"" l dT of the waveguide material as TE dnTE
dT ~T (4a) OrcTM - daT OT (4b) and for the EO modulation, ~rzTE and ~rcTM are related to the applied electrical field E
by the EO coefficient r33 of the waveguide material as OnTE - - ~ r33nTE E (5a) ~~ - - 2 rs3nM E (5b) where n,.E and n,.~ are the refractive indices of the EO waveguide material for the TE-mode and the TM-mode, respectively. Then two output efficiencies of the MZI
configuration for the variable optical attenuator based on the current invention at the TE-mode and the TM-mode are ~TE
ATE = 4kTE (1- kTE ) cos z ( 2 ) (6a) Q ~TM
ATM = 4kTM (1- kTM )cost( Y'2 ) (6b) In terms of the definition of the PDL of the communication components, without the PDC, the PDL of the variable optical attenuator based on the current invention at any state can be defined by TE
PDL =1 loglo ( ~~ ) (7) Because kTE and kT"'' indicate the coupling ratios of the 3d8 couplers in this regime at the TE-mode and the TM-mode, respectively, and ATE and ATM indicate the optical phase changes between two waveguide arms in this regime for the TE-mode and the TM-mode, respectively, the PDL can be existing in both the unattenuated and the attenuated states, and it is also a function of the optical phase changes (i.e., the attenuated depth) even when the polarization dependence of the 3dB couplers is reduced to be zero (i.e., kTE -kT"' ). As described above, a PDC, which typically has two designs as shown in Fig. 2(a) and Fig. 2(b), respectively, is introduced in this variable optical attenuator. It can make the polarization state of the optical beams rotate 90° with 50% ratio.
Thus, the polarization states of the two output efficiencies of the MZI configuration for the variable optical attenuator based on the current invention defined by the set of equations (6a) and (6b) become new states ~N and ~N' as defined by TE TM
rIN =2(rITE'f'~7~)=2 kTE(1-kTE)cos2(~~ )+kTM(1-kTM)cos2(~~ ) (8a) TE TM
~7N - 2(~1TE+~7TM)-2 kTE(1 kTE)cOSz(~~ )+kTM(1-kTM)COS2(~~ ) 8b Thus, with the PDC, the new PDL of the variable optical attenuator based on the current invention should be defined by TE
PDLN =1 loglo ( ~ M ) = 0 (9) rIN
The result defined by Eq. (9) is based on the theoretical state, but the polarization rotation ratio of a practical PDC cannot be exactly 50%, some designed and fabricated errors are not avoidable, so the practical value of PDLN could not be exactly zero like Eq. (9). But, it can be reduced to an acceptable value with the introduction of the PDC.
Therefore, two paramount parameters, the PDL and the system loss of the variable optical attenuators based on the current invention can be directly improved much better than any conventional structure, which is exactly the main goals of the current invention. In this invention, the MZI configuration is based on two 3dB couplers in order to obtain a low system loss. It, however, can be also based on two Y junctions as shown in Fig. 3 where the system loss will be relatively high. In addition, the schematic structure shown in Fig.
1 is based on the thermo-optic modulation. In fact, as mentioned in the context, this device is also based on the electro-optic modulation as shown in Fig. 4 where two electrodes are used as cathode and anode and labeled as 30a and 30b, respectively.
Finally some useful papers for understanding the operation principle of "the PDC"
based on the polarization rotation are the following:
~ Polarization rotation in semiconductor bending waveguides: a coupled-mode theory by Liu, et al., Journal of Lightwave Technology, Vol. 16, No. 5, May 1998, pp. 929-936;
~ Novel compact polarization converters based on ultra short bends by Dam, et al., IEEE Photonics Technology Letters, Vol. 8, No. 10, October 1996, pp. 1346-1348.
~ First realixed polarization converter based on hybrid supermades by Mertens, et al., IEEE Fhotonics Technology Letters, Vol. 10, No. 3, March 1998, pp. 388-390.
~ Polarization rotation in asymmetric periodic loaded rib waveguides by Shani, et al., Applied Physics Letters, Vol. 59, No. 11, September 1991, pp. 1278-1280.
Claims (4)
1. An optical waveguide device comprising:
a substrate;
on said substrate, two 3dB directional couplers are connected by two waveguide arms to form a Mach-Zehnder interferometer (MZI) configuration, a modulating electrode (or heater) for the MZI configuration, and a polarization dependence compensator (PDC) is connected to the MZI configuration to reduce the polarization dependent loss (PDL) induced by both the MZI configuration and the modulating process;
a lower cladding layer and an upper cladding layer surrounding all the waveguides;
a substrate;
on said substrate, two 3dB directional couplers are connected by two waveguide arms to form a Mach-Zehnder interferometer (MZI) configuration, a modulating electrode (or heater) for the MZI configuration, and a polarization dependence compensator (PDC) is connected to the MZI configuration to reduce the polarization dependent loss (PDL) induced by both the MZI configuration and the modulating process;
a lower cladding layer and an upper cladding layer surrounding all the waveguides;
2. Based on claim 1, the waveguide variable optical attenuators based on the present invention are intendly thermo-optically modulated by applying an electric power.
3. These two 3dB directional couplers can be either narrow band or broadband.
If the broadband 3dB directional couplers are used, this variable optical attenuator can be insensitive to wavelength.
If the broadband 3dB directional couplers are used, this variable optical attenuator can be insensitive to wavelength.
4. For the PDC, the design based on the serial bending waveguides and the design based on asymmetric periodic waveguides are preferred to be used, but any other design can be used if it can have a function of TE-TM conversion with 50% ratio and have any other advantages in the design, the fabrication or the operation.
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|---|---|---|---|
| CA 2368161 CA2368161A1 (en) | 2002-01-16 | 2002-01-16 | Low loss and low polarization dependence waveguide variable optical attenuator |
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| Application Number | Priority Date | Filing Date | Title |
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| CA 2368161 CA2368161A1 (en) | 2002-01-16 | 2002-01-16 | Low loss and low polarization dependence waveguide variable optical attenuator |
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN108227084A (en) * | 2018-01-16 | 2018-06-29 | 上海理工大学 | Unrelated integrated optical switch of a kind of polarization based on silicon nitride waveguides and preparation method thereof |
| CN110646882A (en) * | 2018-06-19 | 2020-01-03 | 上海交通大学 | Polarization independent broadband waveguide beam splitter |
-
2002
- 2002-01-16 CA CA 2368161 patent/CA2368161A1/en not_active Abandoned
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN108227084A (en) * | 2018-01-16 | 2018-06-29 | 上海理工大学 | Unrelated integrated optical switch of a kind of polarization based on silicon nitride waveguides and preparation method thereof |
| CN110646882A (en) * | 2018-06-19 | 2020-01-03 | 上海交通大学 | Polarization independent broadband waveguide beam splitter |
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