Introduction

The present invention relates to an integrated optical polarization rotator component, comprising three passive unitary polarization rotator cascaded and alternated with two phase shifters, which are adjusted to accurately and robustly provide a given polarization rotation, in order to compensate for manufacturing deviations from a nominal geometry and/or tuning the central wavelength of component to a given value.

Background

Numerous fields of information technologies use luminous signals in many applications, such as in high bit rate and long distance digital communications or environmentally robust systems. While independent optic fibers are still employed as waveguides between remote devices or systems, many work is done now in the field of integrated photonic circuits, were several photonic waveguides and/or photonic components are produced on a single small sized substrate, thus designing an electro-optical chip or a purely photonic chip.

Polarization management is becoming of fundamental importance in integrated optical devices. The high index contrast of the Indium Phosphide and Silicon-on-Insulator fabrication platforms enables the design of densely integrated, high performance devices, which exhibit, however, a strong polarization dependence. To achieve polarization independent operation polarization diversity approaches are required, for which polarization rotators and splitters are key components. Additionally, in high speed coherent communication systems polarization multiplexing is used to enhance transmission efficiency, so that integrated transmitter and receiver modules have to provide polarization rotation and polarization splitting/combining functionalities.

For polarization rotation a large number of implementation alternatives have been described . These include designs based on slanted sidewalls such as in US 7 204 932 or in A. Velasco et al. : "Ultracompact polarization converter with a dual subwavelength trench built in a silicon- on-insulator waveguide," Opt. Lett. 37, 365-367 (2012), two-step dry etches, cross-polarization coupling, curved waveguides, waveguides with transversal slots and modal evolution, or waveguides with longitudinal trenches such as the publication in "A Waveguide Polarization Toolset Design Based on Mode Beating" from Hutchings et al. in IEEE Photonics Journal vol.3 pp.450-461 June 2011, or with multi quantum wells layers superposed upon a waveguide as in US 7 141 843.

A significant effort has been made to achieve ultra-short devices, and conversion lengths of only a few tens of microns have been demonstrated. However, it is well known that polarization rotators exhibit stringent fabrication tolerances, which severely limit fabrication yield and thus hinder their application in practical devices. This also limits performance reliability and mass production regularity, and could constitute a kind of technological bottle-neck in the improvement of integrated optical circuits.

It may be noted that The Hutchings et al . document includes a short passage which seems to suggest that cascading three tuneable phase shifters(PS) alternated with two polarisation mode converters (PMC) could lead to a circuit (i.e. PS-PMC-PS-PMC-PS) providing any kind of polarization transformation. This approach requires to use TE to TM polarization mode converters with a perfect conversion ratio of 50%. This ratio is very sensitive to waveguide dimensions and thus the proposed circuit exhibits stringent tolerance requirements. Furthermore, the document gives no data that could enable to realize such a circuit, or that could enable to assess the width of the transformation that could be obtained .

Thus, if the circuit proposed in Hutchings et al . uses several tuneable phase shifters to provide additional functionalities (based on adjustable rotation) to conventional rotators, its implementations still maintains the same stringent tolerance constraints which makes practical implementation very difficult.

An object of the invention is to relax the current fabrication tolerances and enable to obtain a polarization rotation function in integrated optic devices or circuits with more reliability, robustness, stability and/or with less complexity, while maintaining all or part of the advantages of integrating several waveguides and/or functional components on a same substrate or chip.

Another object is to widen the functional possibilities of integrated devices that include polarization rotation, and enable a more simple and fluent process for designing or tuning such devices or circuits.

Summary of the invention

Accordingly, the present invention proposes an ooptical component for providing, within an integrated photonic circuit, an accurate and robust rotation of an input light signal with a certain polarization, possibly a coherent polarization, into an input to another output light signal with another polarization state. According to the invention, said component comprises:

- a first integrated polarization rotator, receiving the input signal and transforming it along a first length toward a first polarization state through a first polarization rotation for a given central wavelength;

- a first integrated tunable phase shifter receiving the signal issued from the first rotator and transforming it along a second length toward a second polarization state through a tuned first phase shifting for said given central wavelength;

- a second polarization rotator, receiving the signal from the first phase shifter and transforming it along a third length toward a third polarization state through a second polarization rotation for said given central wavelength;

- a second integrated tunable phase shifter receiving the signal issued from the second rotator and transforming it along a fourth length toward a third polarization state through a tuned second phase shifting for said given central wavelength;

- a third polarization rotator, receiving the signal from the second phase shifter and transforming it along a fifth length toward a fifth polarization state through a third polarization rotation for said given central wavelength, thus providing between said input polarization state and said fifth polarization state a combined transformation; and said first and second phase shifters are provided with means for tuning their respective phase shifting in order to adjust said combined transformation.

Furthermore, the means for tuning respective phase shifting of first and second phase shifters may be commanded so as to provide a combined transformation corresponding to a given desired polarization rotation value.

Thus, the invention proposes to build an integrated combination of several unitary components: three polarization mode converters are cascaded alternated with two tuneable phase shifters (i.e. PMC-PS-PMC-PS- PMC). All of these unitary components may be realized according to known methods.

This circuit provides any kind of polarization transformation using polarization mode converters with any conversion ratio between 20%-80%. Thus optimal performance is ensured even for large deviations from the nominal dimensions.

The circuit proposed by the inventors thus uses two tuneable phase shifters, and possibly only two, not only to provide adjustable rotation but also to compensate possible deviations in the polarization mode converters produced in the fabrication process. Thus increased functionality is provided together with improved fabrication tolerances, possibly through implementing only known and well tested waveguide cross-section geometries and manufacturing processes.

In another aspect of the invention, it is proposed a method for controlling such a component, comprising :

- controlling the means for adjusting the respective phase shifters so as to adjust the combined transformation in order to compensate a deviation between a polarization rotation value measured for said component and a given polarization rotation memorized for said component as a nominal value; or

- controlling the means for adjusting the respective phase shifters so as to adjust the combined transformation in order to re-tune the central wavelength of the extinction ration of said component to a given value; or

- a combination of both. Still in another aspect of the invention, it is proposed a method for manufacturing such a component, or an integrated circuit comprising such a component.

This and other aspects and features of the present invention will now become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention and the accompanying drawings.

Brief description of the drawings

A detailed description of examples of implementation of the present invention is provided herein below with reference to the following drawings, in which :

- FIGURE 1 schematically illustrates an integrated polarization rotator according to a prior art;

- FIGURE 2 schematically illustrates the rotator of FIGURE 1, in a cross section of respectively:

o in FIGURE 2a : in the polarization rotator segment, and o in FIGURE 2b : in the input and output waveguide segment;

- FIGURE 3 is a graph showing the extinction ration of the polarization, with nominal dimensions, and with a + 10% and -10% deviation of the dimensions of the trenches, in the rotator of FIGURE 1 ;

- FIGURE 4 schematically illustrates an integrated polarization rotator according to en embodiment of the invention;

- FIGURE 5 schematically illustrates the rotator of FIGURE 4, in a cross section of respectively:

o in FIGURE 5a : in one of the unitary polarization rotator segments, and

o in FIGURE 5b: in one of the input or output segments, o in FIGURE 5c: in one of the phase shifter segments;

- FIGURE 6 is a graph showing the extinction ration of the polarization, with nominal dimensions, and with a + 10% and -10% deviation of the dimensions of the trenches, in the rotator of FIGURE 4;

- FIGURE 7 is a representation on a Poincare-sphere of a perfect polarization rotation from a TE to TM mode in the rotator of FIGURE 4; - FIGURE 8 is a graphical representation of the angle/phase condition where a perfect polarization rotation can be achieved with the rotator of FIGURE 4. Detailed description of the drawings

Description of a Prior Art component

In FIGURE 1, FIGURE 2 and FIGURE 3 is illustrated an example of a polarization rotator PR1 with a nominal polarization axis angle of 45°, according to a prior art as disclosed in A. Velasco et al.

FIGURE 2a and FIGURE 2b illustrate the cross-section geometry and materials of this rotator, respectively in the polarization rotator segment PR1 and in the input 10 and output 19 waveguide segments.

The device PR1 consists of an input waveguide 10 and an output waveguide 19 that support a fundamental horizontally polarized mode (TE) and a fundamental vertically polarized mode (TM). Between the input 10 and output 19 waveguides, the rotator waveguide PRAA supports two orthogonal hybrid modes, H _+45° and H_ _45° , which are polarized at exactly Θ=45. The angle Θ of the polarization axis is defined as 0=arctan(JJQ| E _y | ² dQ/JJQ| E _x | ² dQ), with Ω the waveguide cross-section. The length L _R of the rotator waveguide PRAA is exactly the half beat length of the two hybrid modes, i.e. Ι_ _π =π/| β- _45° - β _+45° | .

Light S10 traveling in the TE polarization of the input waveguides 10 equally excites the two hybrid modes of the rotator waveguide PRAA, as shown by the arrows of the two schematics at bottom left of FIGURE 1. At the end of the rotator waveguides PRAA, the hybrid modes have accumulated a n phase shift between H _+45° and H _-45° , so that they cause a TM polarized light S19 to couple to the output waveguides 19, as shown by the arrows of the two schematics at bottom right of the same figure. Operation starting with a TM polarized input is analogous and results in a TE polarized light output.

The Polarization Conversion Efficiency (PCE), i.e. the percentage of power transferred from a given input polarization S10 to the orthogonal output polarization S19, is given by PCE=sin ² (20) sin ² (n/2 L _R / ), where L _R is the actual physical length of the rotator waveguide. From this expression it is clear that the PCE is very sensitive to variations in the polarization axis angle Θ, and to variations in the beat length of the hybrid modes L _n , both of which strongly depend on the waveguide geometry. Any fabrication fluctuation will thus negatively impact the performance of the rotator.

For this prior art rotator, extinction ratio of the polarization were computed through 3D full-vectorial simulations of the complete rotator structure, using the commercial "Fimmwave" package, with the following cross-sections dimensions and materials.

The waveguides are 1-1 = 260 nm high and W=415 nm wide. The trenches of different depths are defined using the etch-lag effect, and their dimensions are Gl = 60 nm, Dl = 210 nm and G2=85 nm, D2 = 238 nm. The trenches are separated by Wb=40 nm, and located at Wa = 200 nm and Wc=30 nm from the waveguide edges. The refractive indices of the bottom oxide, silicon waveguide core and SU8 cladding at λ= 1.55 prn are nSi0 ₂ = 1.444, nSi=3.476 and nSU8= 1.58 respectively. The corresponding thermo-optical coefficients are: dnSiO ₂ /dT= l x l0 ^"5 . K ^"1 , dnSi/dT= 1.8x lO ^"4 . K ^_1 and dnSU8/dT=- l . l x lO ^"4 . K ^_1 , as disclosed in J. Schmid et al ., "Temperature-independent silicon subwavelength grating waveguides" Opt. Lett. 36, 2110-2112 (2011).

In FIGURE 3 is shown the extinction ration of the polarization obtained :

- for the continuous line curve: with its nominal dimensions,

- for the dashed line curve : with a + 10% deviation of the dimensions of the trenches, and

- for the mixed line curve : with a -10% deviation of the dimensions of the trenches.

In this prior art design the rotator waveguide PR1 is 13 prn long and is connected to the input 10 and output 19 with low loss transitions. The simulated extinction ratio of this device, defined as ER[dB] = 10.logio( PTM,out/PTE,out) when TE polarized light S10 is launched into the device, is shown by the continuous line curve in FIGURE 3. An extinction ratio in excess of 20 dB is achieved over ~40 nm of bandwidth when all device dimensions are nominal.

However, as shown by the dashed line and mixed line curves in FIGURE 3, variations of ±10 % in the size of the trenches can reduce the extinction ratio to below 5 dB, because the polarization axis deviates by more than 15° from the nominal 0=45° and the beat length changes more than 10 %. Similar performance deteriorations are generally observed in other rotators. Description of an exemplary embodiment

The invention thus proposes to build an integrated combination of several unitary components: three polarization mode converters are cascaded alternated with two tuneable phase shifters (i.e. PMC-PS-PMC-PS- PMC). An exemplary embodiment of the invention is proposed here, designed on a SOI substrate, which is illustrated in FIGURE 4 and FIGURE 5.

The cross-section geometry and materials of the unitary rotator waveguides segments PRa, PRb and PRc are illustrated in FIGURE 5a. The nSi waveguide 201 with longitudinal trenches 204 and a SU8 cladding 203 is lying on the nSi0 ₂ layer 200 of a SOI substrate.

In this embodiment, input 20 and output 29 waveguides, as well as first PRa, second PRb and third PRc unitary rotators with longitudinal trenches are realized with the same materials and cross-section dimensions as used for the prior art rotator of FIGURE 1.

Input and output waveguides 20 and 29 are shown in FIGURE 5b, and cross-sections of the unitary phase shifter waveguides segments PSa and PSb are illustrated in FIGURE 5c.

Unitary phase shifter waveguides segments PSa and PSb are realized by adding a heating electrode 204 on a waveguide 201, possibly identical to the input and output waveguides.

Transition between the input 20 and output 29 waveguides and the rotator section PR2 can be through direct butt-coupling or through a smooth taper to reduce losses.

In this embodiment of the invention, the polarization rotator comprises an input waveguide 20 and an output waveguide 29 supporting a fundamental TE and TM mode, three arbitrary rotators PRa, PRb and PRc with a polarization axis angle Θ, and two tunable polarization phase shifters PSa and PSb that introduce a phase delay p, between the fundamental TE and TM mode. The concept that underlies this approach is the following. The three arbitrary rotators are realized with waveguides that all produce a polarization axis angle Θ which may deviate significantly from the nominal angle, which is typically 45°. The invention enables to compensate this deviation the tunable polarization phase delays pi and p2, which can be implemented by a known type, for example with simple waveguide heaters.

The behavior of each of the arbitrary polarization rotators PRa, PRb and PRc can be described via their Jones matrix R=A(0) D(cp) - Α(-Θ), where and φ= | βΘ-βΘ-90° | L _R

with β _Θ and ββ-9ο° the propagation constants of the two hybrid modes and

L _R the physical length of the rotator waveguide segment.

The polarization phase shifters PSa and PSb are modeled as D(p,), with D as defined in equation (1) and ρϊ = ( β _Τ Ε- βτΜ)ί _Ρ , where L _P is the physical length of the polarization phase shifter waveguide segment. The

Jones matrix of the complete polarization rotator scheme PR2 is thus

R- D(p2)- R- D(pl)- R.

In this simulation, all arbitrary unitary rotators have been assumed to be equal, since they will be closely spaced on the chip, and will thus be affected by the same fabrication fluctuations. However, it has to be highlighted that this principle is general and does not require rotators to have the same deviation. Under the assumption of physically equal rotators, an analytical condition for perfect polarization conversion can be derived with basic algebra yielding

sin ² (20)sin ² (cp/2)> 1/4 (2)

This condition, which is an important result of the invention, establishes which conditions the arbitrary unitary rotators PRa, PRb and PRc have to fulfill so that perfect polarization conversion is possible with some polarization phase shifts pi and p2.

This condition (equation 2) is plotted in FIGURE 8, showing that perfect rotation is feasible (in the grey area) for polarization axis angles as low as 0=15° (mixed line) provided that cp~180°, i.e. provided that the length L _R of the unitary rotator waveguide is chosen adequately.

By way of example, FIGURE 7 shows the transition from TE to TM polarization with unitary rotators PRa, PRb and PRc with 0=20° and cp=110°, alternated with phase shifters respectively of pi=132.2° for PSa and p ₂ =58.8° for PSb. An input light S20 in a polarization P0 is thus rotated by PRa to a first polarization state PI, then delayed by PSa to a second polarization state P2, then rotated by PRb to a third polarization state P3, then delayed by PSb to a fourth polarization state P4, then rotated by PRc to a fifth polarization state P5. The phase shifters PSa and PSb are here adjusted, possibly in real-time, to adjust this fifth polarization state P5 to coincide with the TM polarization requested for the output light S29.

Thus, the invention enable to can compensate for the largest deviation in polarization axis angle 0, when φ= |β _Θ -βΘ-9ο _° Ι i8o° as seen in FIGURE 8.

Ideally, it is preferable to choose the length L _R of the rotator waveguides segments such that cp~180°. However, when the exact propagation constants β _Θ and ββ-9ο° that will result from the fabricated device are not known, it is preferable to choose L _R as the half-beat length of the device with nominal dimensions, i.e. LR~13 prn.

Typically, the polarization phase shifters PSa and PSb are realized by heating of waveguides segments that interconnect the different unitary rotators. The TE mode is mainly confined within the silicon waveguide core, which has a positive thermo-optical coefficient. Hence, its effective index appreciably increases with temperature. In this simulation, the rate is The TM mode strongly senses the SU8 cladding, which has a negative thermo-optical coefficient, thus reducing the rate to The temperature dependent polarization phase shift is then given by

+ Lp.2n/A.(dneff,TE/dT-dn _e ff,TM/dT)AT. The length of the shifter is set to LP=334 pm so that p can be varied over 360° with temperature increments ΔΤ<75 K.

To compensate a AG= + 10 % deviation in the width Gi and G ₂ of the trenches, the temperature in the polarization phase shifters PSa and PSb is increased by ΔΤι= 13.9 K and respectively ΔΤ ₂ =66 K. These temperature values can be found through iteration or using an optimization algorithm. Similarly, a AG = -10% deviation is compensated with temperature increments ΔΤι=53.5 K and ΔΤ ₂ =7.5 K.

As shown in FIGURE 6, in both cases this yields more than 30 dB extinction ratio at the compensated wavelength λ= 1.55 prn.

It has to be highlighted that the prior art component, with the same materials and cross-section geometry, provide for such deviations only impractically low extinction ratios of less than 15 db or even less than 5 dB.

In FIGURE 6 is shown the extinction ration of the polarization obtained :

- for the dashed line curve : with a + 10% deviation of the dimensions of the trenches,

- for the mixed line curve : with a -10% deviation of the dimensions of the trenches, and

- for the dotted line curve : with a + 10% deviation of the dimensions of the trenches, and re-tuned to a different central wavelength.

As illustrated in FIGURE 6, it may also be seen that the component PRl according to the invention may be tuned, or re-tuned, to a different central wavelength. In FIGURE 6, the dotted line curve may be seen as a re-tuning to a central wavelength of 1.52 pm, of a component with a AG = + 10% deviation. This re-tuning is achieved with temperature increments ΔΤι=37.7 K and ΔΤ ₂ =68.7 K.

Such a tuning to a given wavelength may be obtained as long as the condition of equation (2) remain respected for this particular wavelength.

It may be noted that the bandwidth of the compensated device PR2 of the invention is reduced when compared to the prior art device PRl with nominal dimensions. This is attributed to the fact that the compensated device uses an overall longer rotator waveguide, as well as to the limited bandwidth of the polarization phase shifters. However, this drawback may be compensated or avoided by adjusting the global component PR2 for re-tuning it or fine-tuning it more easily to a given wavelength. Although various embodiments have been illustrated, this was for the purpose of describing, but not limiting, the invention. Various modifications will become apparent to those skilled in the art and are within the scope of this invention.

As an example, in other embodiments, unitary rotator segments PRa, PRb and PRc may also be chosen with nominal characteristics different from each other, in angle and/or length, or with a nominal or controlled total polarization rotation different from 90°, such as to provide different compensating ranges or functional features.