WO2012080714A2

WO2012080714A2 - Mach-zehnder interferometers

Info

Publication number: WO2012080714A2
Application number: PCT/GB2011/052423
Authority: WO
Inventors: Lloyd Nicholas Langley
Original assignee: Oclaro Technology Limited
Priority date: 2010-12-16
Filing date: 2011-12-07
Publication date: 2012-06-21
Also published as: GB201021375D0; WO2012080714A3; GB2486478A

Abstract

A Mach-Zehnder interferometer 200 comprising an optical splitter 201 having at least one input 203R and at least two outputs 204L, 204R, an optical recombiner 202 having at least two inputs 205L, 205R and at least one output 206R and a pair of interferometer arms 207L, 207R. Each of the arms couples an output of the splitter to an input of the recombiner. The arms are configured such that each of the outputs of the splitter is coupled to an opposite side input of the recombiner so as to compensate for phase imbalance of light propagating through the arms. The interferometer is configured such that the splitter and the recombiner operate in cross-state to compensate for split ratio offset errors in the splitter and recombiner.

Description

Mach-Zehnder Interferometers

Field of the Invention

The present invention relates to Mach-Zehnder interferometers.

Background of the Invention

In this specification the term "light" will be used in the sense that it is used in optical systems to mean not just visible light, but also electromagnetic radiation having a wavelength outside that of the visible range.

The extinction ratio of an interferometer or modulator is the ratio of the maximum optical power output level to the minimum power output level, and is often used as a parameter for measuring the performance of a Mach-Zehnder interferometer (MZI). An imbalance in the phase of light transmitted by the two arms of an MZI can result in a sub-optimal extinction ratio.

Achieving a zero phase imbalance is considered to be difficult. If the effective refractive indices of each MZI arm, or the effective optical path length of each MZI arm, are not equal, then a perfect phase balance is not achieved. Typically, phase imbalance is dependent on the length of the arms . Shorter arms produce less imbalance than longer arms. Because the wavelength of light used in typical MZIs is so short, a tiny difference in the optical path length can have a large effect on the phase imbalance. It is important to correct the phase imbalance in MZIs. One approach for effecting a correction is to apply forward bias or reverse bias to the MZI arms. Normally a different bias is applied to separate arms. However, this causes differential loss in the arms and degradation in the effective split ratio (SR) of the MZI, resulting in a degraded extinction ratio. The differential loss in the arms can also be caused by random waveguide imperfections (e.g. roughness scattering) in both arms.

The extinction ratio of an MZI also depends on the split ratios of the multimode interference (MMI) couplers (e.g. splitter and recombiner) at either end of the MZI . Offset errors in split ratio can arise in various ways. For example, an offset error may arise from a variation across a wafer from which couplers are manufactured. This variation might be caused by a change in material index across the wafer, or by variations in the geometry of the MMI couplers caused by lithography or ridge etch variations. Additionally, the split ratio variation could be caused by wafer to wafer variation for the same parameters. Dielectric stress induced index changes in the MMI coupler material could also change the split ratio.

Since the extinction ratio is dependent on the difference in magnitude of the interfered signals at the MZ recombination function, it is determined by the splitter split ratio, the recombiner split ratio, and any differential losses in the two MZI arms. For zero chirp (ZC) system applications, the ideal extinction ratio is infinite.

Conventional indium phosphide (InP) MZIs use either 1x2 or 2x2 MMI couplers as the split and recombine functions, i.e. a 1x2 splitter and a 2x2 recombiner (so that a complementary output waveguide can be used to monitor MZI performance). If there are design errors or process variations in these couplers then a perfect 50.50 split ratio may not be achieved. The extinction ratio therefore will be non-infinite.

The differential loss from the phase imbalance correction and the split ratio offset errors at the splitter and recombiner both degrade the extinction ratio and thus the MZI performance.

Summary of the Invention

According to one aspect of the present invention there is provided a Mach-Zehnder interferometer which comprises an optical splitter having at least one input and at least two outputs, an optical recombiner having at least two inputs and at least one output, and a pair of interferometer arms. Each of the arms couples an output of the splitter to an input of the recombiner. such that each of the outputs of the splitter is coupled to an opposite side input of the recombiner so as to compensate for phase imbalance of light propagating through the arms.

The interferometer may further comprise a waveguide cross-over for coupling the outputs of the splitter to the opposite side inputs of the recombiner. The waveguide cross-over may be located about half-way between the splitter and recombiner. The splltteir may have at least two inputs and the recombiner may have at least two outputs. Light may be propagated into one of the inputs of the splitter and out from one of the outputs of the recombiner such that the splitter and the recombiner operate in cross-state. The splitter and the recombiner may operate in cross-state to compensate for split ratio offset errors in the splitter and recombiner. Light may propagate through one of the inputs of the splitter and one of the outputs of the recombiner on the same side as each other.

According to another aspect of the present invention, there is provided a Mach- Zehnder interferometer which comprises an optical splitter having at least two inputs and at least two outputs, an optical recombiner having at least two inputs and at least two outputs and a pair of interferometer arms. Each of the arms couples an output of the splitter to an input of the recombiner. Light is propagated into one of the inputs of the splitter and out from the opposite side output of the recombiner such that the splitter and the recombiner operate in cross-state to compensate for split ratio offset errors in the splitter and recombiner.

The split ratio offset errors of the splitter and the recombiner may arise from design error and/or process variation. The splitter and the recombiner may be manufactured to comprise identical design parameters and have theoretically equal split ratios. Alternatively, the splitter and the recombiner may be configured such that they do not comprise equal split ratios. The interferometer may be a non zero chirp interferometer.

According to another aspect of the present invention, there is provided a method of compensating for phase imbalance of light propagating through a Mach-Zehnder interferometer comprising an optical splitter having at least one input and at least two outputs, an optical recombiner having at least two inputs and at least one output, and a pair of interferometer arms, each coupling an output of the splitter to an input of the recombiner. The method comprises coupling at least one of the output waveguides of the splitter to an opposite side input waveguide of the recombiner.

According to another aspect of the present invention there is provided a method of compensating for split ratio offset errors in a Mach-Zehnder interferometer. The interferometer comprises an optical splitter having at least two inputs and at least two outputs, an optical recombiner having at least two inputs and at least two outputs, a pair of interferometer arms, each coupling an output of the splitter to an input of the recombiner. The method comprises propagating light into one of the inputs of the splitter and out from the opposite side output of the recombiner such that the splitter and recombiner operate in cross-state so as to compensate a split ratio offset error of the splitter by an opposite split ratio offset error of the recombiner so that a combined split ratio of the interferometer is not affected.

Brief Description of the Drawings

In order that the invention may be more fully understood, a number of embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 a is a schematic representation of an MZI comprising a 2x2 splitter and a 2x2 recombiner; Fig. 1 b shows the variation of the split ratio of the arrangement of Fig. 1 a as a function of the width of the splitter and the recombiner;

Fig. 1 c is a schematic representation of the widths of the splitter/recombiner of Fig 1 a; Fig. 2a is a schematic representation of an alternative MZI 200 comprising a 2x2 splitter and a 2x2 recombiner;

Fig. 2b is a schematic illustration of the MZI arms of the arrangement of Fig. 2a having the RF electrodes;

Fig. 2c is an alternative schematic illustration of the MZI arms of the arrangement of Fig. 2a having the RF electrodes;

Fig. 2d shows the variation of the split ratio of the arrangement of Fig. 2a as a function of the width of the splitter and the recombiner;

Fig. 3a is a schematic illustration of an alternative MZI comprising a 2x2 splitter and a 2x2 recombiner; Fig. 3b shows the variation of the split ratio of the arrangement of Fig. 3a as a function of the width of the splitter and the recombiner; Figs. 3c and 3d are schematic representations of the widths of the splitter and the recombiner of Fig. 3a, respectively;

Fig. 4a is a schematic illustration of an alternative MZI comprising a 2x2 splitter and a 2x2 recombiner;

Fig. 4b shows the variation of the split ratio of the arrangement of Fig. 4a as a function of the width of the splitter and the recombiner; Detailed Description of Preferred Embodiments

Consider the case where an MZI is made using two couplers which are theoretically identical. If these a re m an ufactu red i n the same batch an d the batch has manufacturing offset errors, the couplers may well have the same manufacturing offset error as each other. When the couplers are used as splitter and recombiner, these errors can add together to double the offset error.

Fig. 1 a is a schematic representation of an MZI 100 designed to alleviate this problem. The MZI 100 comprises a 2x2 splitter 101 and a 2x2 recombiner 102. The splitter 101 has two input waveguides 103_L, 1 03_R and two output waveguides 104_L, 104_R. Similarly, the recombiner 102 has two input waveguides 105_L, 105_R and two output waveguides 106_L, 106_R. The MZI 100 also has two arms 107_L, 107_R. One arm 107_L couples the splitter output 104_L to the recombiner input 105_L. The other arm 107_R couples the splitter output 104_R to the recombiner input 105_R. RF electrodes 108|_,108_R,109|_ ,109_R are provided on top of the arms 107_L , 107_R. It will be appreciated that the splitter 101 and the recombiner 1 02 can be for example barrelled and/or butterfly MMI couplers.

In one embodiment, the arrangement of Fig. 1 is applicable to zero chirp applications in which the extinction ratio is infinite. Both the splitter 101 and the recombiner 102 have identical design parameters, i.e. equal split ratios. If they are theoretically identical they may well have identical split ratio offset errors arising from the design error and/or process variations, especially if they are manufactured in the same batch. In order to prevent the split ratio offset error from taking effect, the splitter and the recombiner are arranged in cross-state. For example, light 1 10 launches into one splitter input 103_R and propagates out from the opposite side recombiner output 106_L . Light 1 10 can also propagate in cross-state through the other splitter input 103_L and the opposite side recombiner output 106_R.

Since the splitter 101 and the recombiner 102 have identical design parameters, any split ratio offset errors in the splitter 101 should be cancelled out by similar split ratio offset errors in the recombiner 102. This self-cancelling technique is resultant from the cross-state light propagation. It will be appreciated that any design parameters can be used for the splitter 101 and the recombiner 102 as long as the design parameters (and thus offset errors) for both the splitter 101 and the recombiner 102 are the same.

Fig. 1 b shows the variation of the split ratio as a function of the width, W1 , of the splitter 1 01 and the recombiner 102 of the arrangement of Fig. 1 a. For the zero chirp applications, the ideal split ratio for both the splitter 101 and the recombiner 102 is one. This is shown by point 1 15 on curve 1 16. If both the splitter 101 and the recombiner 1 02 have the same design , the self-cancelling technique ensu res that a target combined split ratio (CSR) is also one (i.e. CSR=SR_splitter x 1/SR_recombiner = 1 ) and being intolerant to the split ratio changes induced from the design errors or process variations. The self-cancelling technique also enables the split ratio to be anywhere on curve 1 15 of Fig. 1 b.

Fig. 1 c is a schematic representation of the widths of the splitter 101/recombiner 102 of Fig 1 a. As can be seen, W1 (which is used in Fig. 1 b) is the width at the centre of the splitter 101 or the recombiner 102. Although the self-cancelling technique reduces the split ratio offset errors in the splitter 101 and the recombiner 102, a phase imbalance effect can still exist in the MZI arms 107|_ _, 107_R. If the phase imbalance effect is corrected by biasing any electrodes 108_L, 108_R, 109L,109R on the MZ arms,, it may cause differential loss in the MZI arms 107_{L ,} 107_R , which degrades the extinction ratio.

Fig. 2a is a schematic representation of an alternative MZI 200 comprising a 2x2 splitter 201 and a 2x2 recombiner 202. Many features of the arrangement of Fig. 2a are similar to those of the Fig. 1 a, except for the configuration of the MZI arms 207_{L ,} 207_R . In this arrangement, waveguide cross-overs 221 , 222 are used for swapping the MZI arms 207_{L ,} 207_R. One cross-over 221 couples one of the splitter output 204_L with the opposite side recombiner input 205_R to form one MZI arm 207_L. The other cross- over 222 couples the other splitter output 204_R with the opposite side recombiner input 205_L to form the other MZI arm 207_R.

The cross-overs 221 , 222 are placed about mid-way down the MZI arms' length. Since the arms 207_L , 207_R are swapped over, any phase offset accumulated in the first half of the MZI arms 207_L , 207_R is cancelled out by the opposite and equal imbalance contribution from the second half of the MZI arms 207_{L ,} 207_R. Light 220 is propagated into one splitter input 203_R and out from the same side recombiner output 206_R. Light 220 can also propagate through the other splitter input 203_L and the same side recombiner output 206_L. For example, if the wafer has a variation in refractive index across it, then the refractive index of the lower portion of Fig. 2a may be different to (e.g. lower than) that of the higher portion. The cross-over ensures that each arm traverses both the lower and higher refractive index regions, and thus both arms should have the same optical path length. This arrangement prevents the need for imbalance correction by biasing the RF electrodes or including imbalance (phase) electrodes. Therefore, the extinction ratio is not degraded by the differential loss induced by the RF electrodes.

The use of the cross-overs 221 , 222 for swapping the MZI arms 207_{L ,} 207_R generally requires some corresponding RF circuit adjustments. Fig. 2b is a schematic illustration of the MZI arms 207_{L ,} 207_R having the RF electrodes 208_{L ,} 208_{R ,} 209_{L ,} 209_R of the arrangement of Fig. 2a. RF electrode cross-overs (not shown) are placed on top of or near to the waveguide cross-overs 221 , 222. One RF electrode cross-over electrically connects the RF electrodes 208_L, 209_R on each half of one MZI arm 207_L, and another RF electrode cross-over electrically connects the RF electrodes 208_R, 209_L on each half of the other MZI arm 207_R. In this figure, V+ and V- represent the two outputs and the complementary output of an electrical differential driver (not shown). Since RF electrode cross-overs are used, the MZI arms 207_{L ,} 207_R can be driven by a single electrical driver.

Fig. 2c is an alternative schematic illustration of the MZI arms 207_{L ,} 207_R having the RF electrodes 208_{L ,} 208_{R ,} 209_{L ,} 209_R of the arrangement of Fig. 2a. Many features of the arrangements of Figs. 2b and 2c are the same and carry the same reference numerals. However, the arrangement of Fig. 2c does not have any RF electrode cross-overs. Therefore the RF electrodes 208_{L ,} 208_{R ,} 209_{L ,} 209_R in each half of the MZI arms 207_{L ,} 207_R are driven by two separate electrical differential drivers. In such an arrangement, the electrical drivers are driven in anti-phase. Th is provides further performance improvements because any drive asymmetries (e.g. RF rise & fall times) may also be self-cancelling in this configuration. Alternatively, if the electrodes 208_L and 208_R are connected in a series "push pull" configuration with electrodes 209_L and 209_R, then a single differential driver is sufficient to drive the MZ arms.

In the arrangements of Figs. 2a to 2c, the cross-state light propagation still applies although light propagates through the same side splitter input and recombiner output. This is because the MZI arms 207_{L ,} 207_R are swapped over by the cross-overs 221 , 222. As a result, the inputs of the recombiner 202 are swapped, so the self-cancelling technique for cancelling ou t th e sp l it rati o offset errors is applicable to the arrangements of Fig. 2a to 2c. Fig. 2d shows the variation of the split ratio as a function of the width , W 1 , of the splitter 201 and the recombiner 202 of the arrangement of Fig. 2a. As with the results of Fig. 1 b, for the infinite extinction ration (zero chirp) applications, the ideal split ratio is one which is shown by point 215 on curve 216. The self-cancelling technique ensures that any changes induced to the split ratios of the splitter 201 and the recombiner 202 (due to the design error and/or process variation) are cancelled out and do not affect the target CSR being one. Fig. 3a is a schematic illustration of an alternative MZI 300 comprising a 2x2 splitter 301 and a 2x2 recombiner 302. This arrangement is suitable for negative chirp applications for which the extinction ratio is finite. The design parameters of the splitter 301 and the recombiner 302 are not the same. For example, the splitter 301 split ratio can be greater than one and the recombiner split ratio can be less than one. As with the arrangement of Fig. 1 a, light 310 propagates in cross-state, e.g. light 310 launches into one splitter input 303_R and propagates out from the opposite side recombiner output 306_L. Light 310 can also propagate in cross-state through the other splitter input 303_L and the opposite side recombiner output 106_R. Therefore the self- cancelling technique as described with reference to Figs. 1 a and 2a is also applicable to this arrangement.

Fig. 3b shows the variation of the split ratio as a function of the width, W1 , of the splitter 301 and the recombiner 302 of the arrangement of Fig. 3a. For negative chirp applications, the ideal split ratio is: SR=VCSR, where

The ideal split ratio for the splitter 301 is shown by point 319 and for the recombiner 302 is shown by point 321 on curve 322 of Fig. 3b. Since the split ratios of the splitter 301 and the recombiner 302 are different, the target CSR (which is equal to SR_splitter x 1/SR_recombiner) is no longer one. If the splitter 301 and the recombiner 302 are designed such that the width, W1 , or the SR is correctly spaced , the CSR reaches its target. When the split ratio is changed from the design error or process variation, the split ratio for both the splitter 301 and the recombiner 302 move in the same direction (as shown by point 318 for splitter 301 and by point 320 for recombiner 302). The self- cancelling technique ensures that the target CSR is not seriously affected by these changes. If the design of the splitter 301 and the recombiner 302 are correctly different, the self-cancelling technique ensures that the split ratios of the splitter 301 and the recombiner 302 are anywhere on the curve 322 (e.g. at point 316 for the splitter 301 and at 317 for the recombiner 302).

Figs. 3c and 3d are schematic representations of the widths of the splitter 301 and the recombiner 302 of Fig. 3a, respectively. As with the arrangements of Figs. 1 c and 2e, these show that W1 (as used in Fig. 3b) is the width at the centre of the splitter 301 and the recombiner 302.

Fig. 4a is a schematic illustration of an alternative MZI 400 comprising a 2x2 splitter 401 and a 2x2 recombiner 402. Many features of the arrangement of Fig. 4a are similar to those of the Fig. 2a, except that the design parameters for the splitter 401 and the recombiner 402 are not the same. This arrangement is applicable to negative chirp applications. Since the MZI arms 407_L, 407_R are swapped over, light 420 launches into one splitter input 403_R and propagates out in cross-state from the same side recombiner output 406_R. As with the arrangement of Fig. 2a, any imbalance accumulated in first halves of the MZI arms 407_L, 407_R are cancelled out by the opposite and equal imbalance contribution from second halves of the MZI arms 407_{L ,} 407_R. This arrangement also requires the same RF circuit adjustments as shown in the arrangements of Figs. 2b and 2c.

As with the arrangement of Fig. 2a, the self-cancelling technique for cancelling out the split ratio offset errors is still applicable to the arrangement of Fig. 4a. Fig. 4b shows the variation of the split ratio as a function of the width, W1 , of the splitter 301 and the recombiner 302 of the arrangement of Fig. 4a. The results shown in this Fig. 4b are th e sa m e as th ose sh own i n F ig . 3 b a s th e self-cancelling technique in this arrangement works in the same way as that of explained with reference to Figs. 3a and 3b.

Although the invention has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.

Claims

CLAIMS:

1 . A Mach-Zehnder interferometer comprising:

an optical splitter having at least one input and at least two outputs;

an optical recombiner having at least two inputs and at least one output;

a pair of interferometer arms, each coupling an output of the splitter to an input of the recombiner;

wherein the arms are configured such that each of the outputs of the splitter is coupled to an opposite side input of the recombiner so as to compensate for phase imbalance of light propagating through the arms.

2. An interferometer according to claim 1 , wherein a phase imbalance between the arms in an upstream half of the interferometer is substantially cancelled by an opposite phase imbalance between the arms in a downstream half of the interferometer.

3. An interferometer according to claim 1 or 2, wherein the splitter has at least two inputs and the recombiner has at least two outputs, and light is propagated into one of the inputs of the splitter and out from one of the outputs of the recombiner such that the splitter and the recombiner operate in cross-state.

4. An interferometer according to 1 , 2 or 3, wherein light propagates through one of the inputs of the splitter and one of the outputs of the recombiner on the same side as each other.

5. An interferometer according to any preceding claim, further comprising a waveguide cross-over for coupling the outputs of the splitter to the opposite side inputs of the recombiner.

6. An interferometer according to claim 5, wherein the waveguide cross-over is located about half-way between the splitter and recombiner.

7. An interferometer according to claim 5 or 6, wherein an RF electrode is located on top of each arm upstream of the cross-over and an RF electrode is located on top of each arm downstream of the cross-over.

8. An interferometer according to claim 7, wherein an RF electrode cross-over is located on top of the waveguide cross-over so that the RF electrodes of each arm are connected to each other, and a single electrical differential driver is configured to drive the RF electrodes.

9. An interferometer according to claim 7, wherein the RF electrodes on top of each arm are driven by two electrical differential drivers operating in anti-phase.

10. An interferometer according to any preceding claim, wherein the splitter has at least two inputs and the recombiner has at least two outputs, the interferometer being configured so that the splitter and the recombiner operate in cross-state to compensate for split ratio offset errors in the splitter and recombiner.

1 1 . A Mach-Zehnder interferometer comprising:

an optical splitter having at least two inputs and at least two outputs;

an optical recombiner having at least two inputs and at least two outputs;

wherein light is propagated into one of the inputs of the splitter and out from the opposite side output of the recombiner such that the splitter and the recombiner operate in cross-state to compensate for split ratio offset errors in the splitter and recombiner.

12. An interferometer according to any of claims 1 to 1 1 , wherein the splitter and the recombiner are configured such that they do not comprise equal split ratios.

13. An interferometer according to claim 12, wherein the interferometer is a nonzero chirp interferometer.

14. An interferometer according to any of claims 1 to 1 1 , wherein the splitter and the recombiner are manufactured to comprise identical design parameters and have theoretically equal split ratios.

15. An interferometer according to claim 14, wherein the interferometer is a zero chirp interferometer.

16. A method of compensating for phase imbalance of light propagating through a Mach-Zehnder interferometer, the interferometer comprising:

an optical splitter having at least one input and at least two outputs;

an optical recombiner having at least two inputs and at least one output;

the method comprising:

coupling at least one of the output waveguides of the splitter to an opposite side input waveguide of the recombiner.

17. A method of compensating for split ratio offset errors in a Mach-Zehnder interferometer, the interferometer comprising:

an optical splitter having at least two inputs and at least two outputs;

an optical recombiner having at least two inputs and at least two outputs;

the method comprising:

propagating light into one of the inputs of the splitter and out from the opposite side output of the recombiner such that the splitter and recombiner operate in cross-state so as to compensate a split ratio offset error of the splitter by an opposite split ratio offset error of the recombiner so that a combined split ratio of the interferometer is not affected.