Patent Publication Number: US-2023136742-A1

Title: Optoelectronic chip and method for testing photonic circuits of such chip

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 16/249,546, filed on Jan. 16, 2019, application claims the priority benefit of French patent application number 18/51202, filed on Feb. 13, 2018, which applications are hereby incorporated herein by their reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure generally relates to optoelectronic chips, and more particularly to the testing of photonic (that is, optical or optoelectronic) circuits of such chips. 
     BACKGROUND 
     To make sure that photonic circuits of a chip are functional, the latter are tested or characterized. For this purpose, an optical signal injected at the level of an optical input of the chip is supplied to a circuit to be tested. An output signal of the circuit is then observed to determine whether the circuit is functional. 
     SUMMARY 
     In accordance with an embodiment of the present invention, an optoelectronic chip comprises optical inputs having different passbands, a photonic circuit to be tested, and an optical coupling device configured to couple the optical inputs to the photonic circuit to be tested. 
     In accordance with an embodiment of the present invention, an optoelectronic chip comprises a first pair of optical inputs comprising a first input and a second input, where the first and the second inputs are each adapted to a different wavelength. The chip also includes a photonic circuit to be tested; and a first optical coupling device configured to couple the first pair of optical inputs to the photonic circuit to be tested. 
     In accordance with an embodiment of the present invention, a method of testing an optoelectronic chip comprises providing an optoelectronic chip comprising optical inputs having different passbands, a photonic circuit to be tested, and an optical coupling device configured to couple the optical inputs to the photonic circuit to be tested. At a first input of the optical inputs, a first signal is supplied at a first wavelength in the passband of the first input, and coupling the first signal to the photonic circuit to be tested through the optical coupling device. At a second input of the optical inputs, a second signal is supplied at a different second wavelength in the passband of the second input, and coupling the second signal to the photonic circuit to be tested through the optical coupling device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a partial simplified top view illustrating an embodiment of an optoelectronic chip according to a first aspect, the chip comprising photonic circuits to be tested; 
         FIG.  2    is a partial simplified top view illustrating another embodiment of the chip of  FIG.  1   ; 
         FIG.  4    is a partial simplified top view illustrating another alternative embodiment of the chip of  FIG.  1   ; 
         FIG.  3    is a partial simplified top view illustrating an alternative embodiment of the chip of  FIG.  1   ; 
         FIG.  5    is a flowchart illustrating an embodiment of a method of testing photonic circuits of a chip of the type in  FIGS.  1  to  4   ; and 
         FIG.  6    is a partial simplified top view illustrating an embodiment of an optoelectronic chip according to a second aspect, the chip comprising one or a plurality of integrated photonic circuits to be tested; 
         FIG.  7    is a partial simplified top view illustrating another embodiment of the chip of  FIG.  6   ; and 
         FIG.  8    is a partial simplified top view illustrating an alternative embodiment of the chip of  FIG.  6   . 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     It would be desirable to have a device for testing circuits of an optoelectronic chip which overcomes at least some of the disadvantages of known test devices, particularly which enables to test one or a plurality of circuits of the chip at different wavelengths, in particular over a wide wavelength range, and/or for different polarization modes, that is, electric transverse and magnetic transverse. 
     It would also be desirable to have a method of testing photonic circuits of an optoelectronic chip which overcomes at least certain disadvantages of known test methods, particularly which enables to test such circuits at different wavelengths, for example, over a wide wavelength range, and/or for different polarization modes. 
     Thus, according to a first aspect, an embodiment provides an optoelectronic chip comprising optical inputs having different passbands, at least one photonic circuit to be tested, and an optical coupling device configured to couple said inputs to the circuit to be tested. 
     According to an embodiment, the device is configured to provide, when an input of the chip receives a signal at a wavelength in the passband of this input, a signal of same wavelength to the circuit to be tested. 
     According to an embodiment, the device comprises at least one optical coupler. 
     According to an embodiment, each optical coupler is of multimode interferometer type. 
     According to an embodiment, each optical coupler is an evanescent coupler. 
     According to an embodiment, the device comprises at least one optical coupler of multimode interferometer type, and at least one evanescent coupler. 
     According to an embodiment, each evanescent coupler has two inputs and two outputs. 
     According to an embodiment, the device comprises at least one optical splitter having an input coupled to the inputs of the chip and having an output coupled to the photonic circuit to be tested. 
     According to an embodiment, the different passbands are adjacent. 
     According to an embodiment, the chip comprises, for each of said different passbands, two optical inputs having this passband and being each adapted to a different polarization, the coupling device being configured to couple said two optical inputs to the circuit to be tested. 
     According to an embodiment, the device is configured to provide, when an input of the chip receives a polarization signal adapted to this input, at a wavelength in the passband of this input, a signal of same wavelength and of same polarization to the circuit to be tested. 
     Another embodiment according to the first aspect provides a method of testing a chip such as defined hereabove, comprising, for each input of the chip, supplying to said input a signal at a wavelength in the passband of said input of the chip. 
     According to an embodiment, the signal supplied to the input of the chip is at the polarization to which said input of the chip is adapted. 
     According to an embodiment, the signal is supplied to said input of the chip by means of an optical fiber, the method comprising a step of aligning one end of said optical fiber with said input of the chip. 
     According to an embodiment, said fiber belongs to an array of optical fibers held in place in a support and, on alignment of said end of said optical fiber, ends of other optical fibers of the array are aligned with other inputs of the chip. 
     According to a second aspect, an embodiment provides an optoelectronic chip comprising a pair of optical inputs having a same passband and each of which is adapted to a different polarization, at least one photonic circuit to be tested, and an optical coupling device configured to couple the two inputs to the circuit to be tested. 
     According to an embodiment, the device is configured to provide, when an input of the chip receives a polarization signal adapted to this input, at a wavelength in the passband of this input, a signal of same wavelength and of same polarization to the circuit to be tested. 
     According to an embodiment, each optical coupler is of multimode interferometer type. 
     According to an embodiment, the device comprises at least one optical coupler. 
     According to an embodiment, each optical coupler is an evanescent coupler. 
     According to an embodiment, the device comprises at least one optical coupler of multimode interferometer type, and at least one evanescent coupler. 
     According to an embodiment, each evanescent coupler has two inputs and two outputs. 
     According to an embodiment, the device comprises at least one optical splitter having an input coupled to the inputs of the chip and having an output coupled to the photonic circuit to be tested. 
     According to an embodiment, the different passbands are adjacent. 
     According to an embodiment, the chip comprises a plurality of optical input pairs, the optical inputs of each pair being each adapted to a different polarization and having a same passband different from those of the other pairs of optical inputs. 
     Another embodiment according to the second aspect provides a method of testing a chip such as defined hereabove, comprising, for each input of the chip, supplying to said input of the chip a polarization signal adapted to said input of the chip, at a wavelength in the passband of said input of the chip. 
     According to an embodiment, the signal is supplied to said input of the chip by means of an optical fiber, the method comprising a step of aligning one end of said optical fiber with said input of the chip. 
     According to an embodiment, said fiber belongs to an array of optical fibers held in place in a support and, on alignment of said end of said optical fiber, ends of other optical fibers of the array are aligned with other ends of the chip. 
     The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
     The same elements have been designated with the same reference numerals in the various drawings and, further, the various drawings are not to scale. For clarity, only those steps and elements which are useful to the understanding of the described embodiments have been shown and are detailed. In particular, the photonic circuits to be tested and their operation have not been detailed, the described embodiments being compatible with usual photonic circuits. Further, the usual components or photonic elements of optoelectronic chips (waveguides, photodiodes, grating couplers, couplers, splitters, . . . ) and the optical fibers currently used to test an optoelectronic chip have not been detailed, the described embodiments being compatible with such elements and such optical fibers. 
     The terms “approximately”, “substantially”, and “in the order of” are used herein to designate a tolerance of plus or minus 10%, preferably of plus or minus 5%, of the value in question. 
     Unless otherwise specified, when reference is made to two photonic elements connected together, this means that the elements are directly connected with no intermediate element other than waveguides, and when reference is made to two photonic elements coupled together, this means that the two elements may be directly coupled (connected) or coupled via one or a plurality of other elements. 
       FIGS.  1  to  5    illustrate a first aspect of the present description corresponding to the case where the circuits of an optoelectronic chip are tested for different wavelengths. 
       FIG.  1    is a partial simplified top view illustrating an embodiment of an optoelectronic chip  5  comprising one or a plurality of integrated photonic circuits to be tested. 
     Chip  5  comprises an optical coupling device  1 , here an evanescent coupler using the near field radiation properties of the waveguides to achieve a coupling between two waveguides, part of the power of an optical signal in one of the two waveguides being transmitted, by radiation, into the other one of the waveguides at the level of parallel portions sufficiently close to the two waveguides. Here, evanescent coupler  1  has two inputs  11  and  12  and two outputs  13  and  14 , and is currently called X coupler. Inputs  11  and  12  of coupler  1  are connected to respective grating couplers E 1  and E 2 , each corresponding to an optical input of chip  5 . Grating couplers E 1  and E 2  are adapted to respective wavelengths λ 1  and λ 2  different from each other. In practice, a grating coupler is designed to receive an optical signal having a given wavelength and typically has a narrow bandwidth around this wavelength, for example, a bandwidth at −3 dB in the range from 20 to 25 nm. In the present description, an optical input of chip  5  is said to be adapted to a wavelength when the passband of this input is centered on this single wavelength, a wavelength being said to be adapted to an input when it is within the passband of this input. 
     One of the outputs (for example,  13 ) of coupler  1  is coupled to an input of a photonic circuit to be tested DUT 1  of chip  5 , in this example, via an optical splitter S 1 . In the shown example, the other output of coupler  1 , here output  14 , is neither connected, nor coupled to a circuit to be tested. As a variation, each output of the coupler may be coupled to a different circuit to be tested. 
     In the example of  FIG.  1   , circuit DUT 1  is an optical circuit with one input and one output. Optical splitter S 1  having its input coupled, preferably connected, to output  13  of coupler  1 , comprises an output coupled, preferably connected, to the input of circuit DUT 1 , and an output coupled, preferably connected, to a photodiode PDref 1 . Splitter S 1  is configured to supply, at each of its outputs, a signal at the wavelength of the signal received on its input. Optical splitter S 1  is calibrated, that is, the percentage of the power of the signal received on its input which is transmitted to each of its outputs is known, for example, to within 5%, preferably to within 1%. Preferably, this percentage is the same, for example, to within 5%, preferably to within 1%, for each output of optical splitter S 1 , which is then said to be balanced. A photodiode PD 1 , preferably identical to photodiode PDref 1 , is coupled, preferably connected, to the output of circuit DUT 1 . Each of photodiodes PD 1  and PDref 1  is configured to convert the optical signal that it receives into an electric signal supplied at a terminal, respectively B 1  or Bref 1 . The assembly of optical splitter S 1 , of circuit DUT 1 , of photodiodes PD 1  and PDref 1 , and of terminals B 1  and Bref 1  here forms a test chain (delimited by dotted lines in  FIG.  1   ). 
     In a first phase of testing circuit DUT 1 , a signal of wavelength λ 1  adapted to input E 1  is applied thereto, typically by means of an optical fiber having one end aligned with input E 1 , and having its other end coupled to a source of a light signal of wavelength λ 1 . A signal of wavelength λ 1  is then transmitted to the circuit to be tested DUT 1  via coupler  1 . The output signal of the circuit is then observed to determine whether the circuit is functional at wavelength λ 1 . 
     More particularly, in the shown example, coupler  1  supplies a signal of wavelength λ 1  to optical splitter S 1 , which then supplies a first signal of wavelength λ 1  to circuit DUT 1 , and a second signal of wavelength λ 1  to photodiode PDref 1 . The electric signals available on terminals B 1  and Bref 1 , for example the currents measured at the level of these terminals, are then compared to characterize the operation of circuit DUT 1  at wavelength λ 1 , for example, to determine losses in circuit DUT 1  at wavelength λ 1 . 
     During a second test phase, similar to the first one, a signal of wavelength λ 2  adapted to input E 2  is applied thereto. A test signal of wavelength λ 2  is then transmitted to the circuit to be tested DUT 1  via coupler  1 , the output signal of the circuit being observed to determine whether the circuit is functional at wavelength λ 2 . 
     More particularly, in the shown example, coupler  1  supplies a signal of wavelength λ 2  to optical splitter S 1 , which supplies a first signal of wavelength λ 2  to circuit DUT 1 , and a second signal of wavelength λ 2  to photodiode PDref 1 . The electric signals available on terminals B 1  and Bref 1  are then compared to characterize the operation of circuit DUT 1  at wavelength λ 2 . 
     At the end of the two test phases, circuit DUT 1  has been tested or characterized for wavelengths λ 1  and λ 2 . 
     It could then have been devised to couple, without coupler  1 , input E 1  to a first test chain to test a circuit DUT 1  at wavelength λ 1 , and input E 2  to a second test chain to test a circuit DUT 1  at wavelength λ 2 . An advantage of the solution of  FIG.  1    however is that the necessary surface area of chip  5  is decreased. 
     Further, the provision of a plurality of circuits DUT 1  would not have enabled to take into account possible manufacturing dispersions between circuits DUT 1 , and/or between the test chains comprising such circuits DUT 1 . 
       FIG.  2    is a partial simplified top view illustrating another embodiment of optoelectronic chip  5  described in relation with  FIG.  1   . 
     Unlike coupling device  1  of  FIG.  1   , optical coupling device  2  of  FIG.  2    comprises an evanescent X coupler  10  having its inputs  11  and  12  respectively connected to inputs E 1  and E 2  of chip  5  and having at least one of its outputs, in the shown example, each output of coupler  10 , connected to an optical splitter  21  having one input and a plurality of outputs, for example, three outputs. Optical splitters  21  need neither be calibrated, nor balanced, and may comprise a different number of outputs. Each output of each optical splitter  21  then corresponds to an output  25  of device  2  and supplies a signal at wavelength λ 1  or λ 2  when the input, respectively E 1  or E 2 , receives a signal at this wavelength. Device  2 , due to the splitters  21  that it comprises, has more outputs than device  1  of  FIG.  1    and thus enables to simultaneously test or characterize a larger number of photonic circuits of chip  5 . 
       FIG.  3    is a partial simplified top view illustrating an alternative embodiment of optoelectronic chip  5  described in relation with  FIG.  1   . In this variation, photonic circuits of chip  5  are desired to be tested for different wavelengths λi, i being an integer in the range from 1 to N, N being equal to 4 in this example. Chip  5  then comprises N grating couplers defining N inputs Ei of chip  5 , each of which is adapted to a different wavelength λi. 
     Unlike coupling devices  1  and  2  of  FIGS.  1  and  2   , optical coupling device  3  of  FIG.  3    comprises N inputs  30   i  connected to respective inputs Ei. Further, device  3  comprises M outputs  31   j,  j being an integer in the range from 1 to M, M being equal to four in this example. Outputs  31   j  are coupled, for example connected, to different circuits to be tested (not shown) and/or to different test chains (not shown). 
     Device  3  comprises at least one optical coupler coupling each input  30   i  to each of outputs  31   j.  Thus, by successively applying to each input Ei of the chip an optical signal of adapted wavelength λi, a circuit coupled, for example, connected, to an output  31   j  of device  3  successively receives a signal at each of wavelengths λi. Observing an output signal of the circuit, for each signal of wavelength λi received by the circuit, enables to determine whether the circuit is functional at each wavelength λi. 
     In the shown example, device  3  comprises four evanescent X couplers  33 . The inputs of a first coupler  33  are respectively coupled, preferably connected, to inputs  301  and  302 , the inputs of a second coupler  33  being respectively coupled, preferably connected, to inputs  303  and  304 . The two outputs of first coupler  33  are respectively coupled, preferably connected, to an input of a third coupler  33  and to an input of a fourth coupler  33 . Similarly, the two outputs of second coupler  33  are respectively coupled, preferably connected, to the other input of third coupler  33  and to the other input of fourth coupler  33 . Each of the outputs of the third and fourth couplers  33  is coupled, for example, connected, to a different output  31   j.    
     As shown in  FIG.  3   , waveguides of device  3  may cross. Preferably, such a crossing is implemented with the device described in article “Ultralow loss single layer submicron silicon waveguide crossing for SOI optical interconnect” of Yangjin Ma, published in 2016 in Optic Express. This enables to limit losses and interference due to the crossing of the waveguides, while limiting the necessary surface area of chip  5 . 
     Device  3  may be adapted to the case where chip  5  comprises less than four inputs and/or outputs, for example, by not connecting some of the inputs/outputs. The alternative embodiment of  FIG.  3    further applies to the embodiment of  FIG.  2   . 
       FIG.  4    is a partial simplified top view illustrating another alternative embodiment of optoelectronic chip  5  described in relation with  FIG.  1   . In this variation, photonic circuits are desired to be tested for different wavelengths λi, i being an integer in the range from 1 to N, N being equal to eight in this example. Chip  5  then comprises N inputs Ei, for example defined by grating couplers, each of which is adapted to a different wavelength λi. 
     Unlike coupling devices  1 ,  2 , and  3  of  FIGS.  1 ,  2 , and  3   , optical coupling device  4  of  FIG.  4    comprises N inputs  40   i  connected to respective inputs Ei. Further, device  3  comprises M outputs  41   j , j being an integer in the range from 1 to M, M being equal to eight in this example. Outputs  41   j  are coupled, for example, connected, to different circuits to be tested (not shown) and/or to different test chains (not shown). 
     Device  4  comprises at least one optical coupler coupling each input  40   i  to each of outputs  41   j . The operation of device  4  is similar to that of previously-described devices  1 ,  2 , and  3 . 
     In the shown example, device  4  comprises twelve evanescent X couplers distributed in three groups of four couplers, that is, one group of four couplers  421 ,  422 ,  423 , and  424  having its inputs respectively coupled, preferably connected, to inputs Ei, one group of four couplers  441 ,  442 ,  443 , and  444  having its outputs respectively coupled, preferably connected, to outputs  40   j,  and one group of four couplers  431 ,  432 ,  433 , and  434  coupling the outputs of couplers  421 ,  422 ,  423 , and  424  to the inputs of couplers  441 ,  442 ,  443 , and  444 . The respective outputs of each coupler are here designated with A and B. Couplers  421  and  422  have their A outputs coupled, preferably connected, to the inputs of coupler  431 , their B outputs being coupled, preferably connected, to the inputs of coupler  432 . Couplers  423  and  424  have their A outputs coupled, preferably connected, to the inputs of coupler  433 , their B outputs being coupled, preferably connected, to the inputs of coupler  434 . Couplers  431  and  433  have their A outputs coupled, preferably connected, to the inputs of coupler  443 , their B outputs being coupled, preferably connected, to the inputs of coupler  444 . 
     Device  4  may comprise, like device  3  of  FIG.  3   , waveguides crossing one another. Such waveguide crossings are for example implemented identically to what is described in relation with  FIG.  3   . 
     Device  4  may be adapted to the case where chip  5  comprises less than eight inputs and/or outputs. The alternative embodiment of  FIG.  4    further applies to the embodiment of  FIG.  2   . 
       FIG.  5    is a flowchart illustrating an embodiment of a method of testing an optoelectronic chip  5  for N different wavelengths. Chip  5  then comprises N inputs Ei and one coupling device of the type previously described, with at least N inputs. 
     At a state  51  (block i=1), for example, the initial state of the method, a loop variable i is initialized, for example, to 1. 
     At a next state  52  (block ALIGN FIBER/Ei), the end of an optical fiber, having its other end coupled to a light source of wavelength λi, is aligned with the input Ei adapted to wavelength λi. 
     At a next state  53  (block INJECT λi/Ei), a signal of wavelength λi is applied to input Ei. Each circuit to be tested, coupled or connected to an output of the coupling device, then receives a signal at wavelength λi and can thus be tested or characterized at this wavelength. 
     At a next state  54  (block i=N?), it is tested whether variable i is or not equal to N. This amounts to verifying whether the circuits have been tested for each of wavelengths λi, with i ranging from 1 to N. In other words, it is verified whether each input Ei of the chip has received a signal at the wavelength λi adapted to this input. It has if variable i is equal to N (output YES of block  54 ) and the next state  55  (block END) then corresponds to the end of the process. At state  55 , the data collected during the test, for example, currents measured from terminals of test chains, may be compiled. However, if variable i is different from N (output NO of block  54 ), at a next state  56  (block i=i+1), loop variable i is incremented by one, the method carrying on at state  52 . 
     The above method is for example implemented when a single optical fiber is selectively coupled, for example, with an optical switch, to one or a plurality of light sources supplying signals of different wavelengths λi, for example, to N light sources each supplying a different wavelength λi. The end of the fiber intended to be aligned with inputs Ei is then offset to be aligned with a different input Ei for each loop  52 ,  53 ,  54 , and  56 . This fiber may for example belong to an array of optical fibers held in place in a support block. In this case, the entire block is displaced at each loop. 
     In an alternative embodiment, not shown, an array of optical fibers held in place in a support block is provided, the spacing between fibers being the same as that between optical inputs Ei of the chip so that a plurality of fibers are simultaneously aligned with a plurality of inputs Ei. In each fiber of the array aligned with an input Ei, a signal having a wavelength adapted to this input Ei is injected, for example, by means of one or a plurality of light sources and, possibly, of an optical switch. The optical signals are successively injected into the fibers of the array so that a single input Ei at a time receives an optical signal at a wavelength adapted to this input. 
     For a same number N of inputs Ei, this alternative embodiment comprises less steps of alignment of fiber ends than the method of  FIG.  5   . 
     It has been considered up to now that inputs Ei of chip  5  are each adapted to a different wavelength λi. In practice, each optical input Ei of chip  5  may also be designed to receive a signal having a specific polarization mode, that is, an electric transverse or a magnetic transverse mode, the input being then said to be adapted to this polarization. For example, all the inputs Ei described hereabove may be adapted to a given polarization, for example identical for all inputs, preferably electric transverse. 
     As an example, the optical fibers used to provide the signals to the inputs of chip  5  are monomode fibers. 
       FIGS.  6  to  8    illustrate a second aspect of the present description corresponding to the case where the circuits of an optoelectronic chip are tested for different polarization modes. In the following description, E TEi  designates an input of chip  5  adapted to a given wavelength λi and to an electric transverse polarization, and E TMi  designates an input of chip  5  adapted to wavelength λi and to a magnetic transverse polarization. 
       FIG.  6    is a partial simplified top view illustrating an embodiment of an optoelectronic chip  5  comprising one or a plurality of integrated photonic circuits to be tested. 
     In this embodiment, chip  5  comprises the same elements as chip  5  of  FIG.  1   , designated with the same reference numerals and coupled or connected together in the same way as in  FIG.  1   , with the difference that inputs E 1  and E 2  of chip  5  are here replaced with respective inputs E TE1  and E TM1 , for example of the grating couplers. Further, coupling device  1  and optical splitter S 1  are polarization-maintaining. In other words, when coupler  1  receives on one of its inputs  11  or  12  an optical signal of given polarization (electric transverse or magnetic transverse) and at a given wavelength, each of its outputs  13  and  14  provides an optical signal having the same polarization and the same wavelength as the received signal. Similarly, splitter S 1  is configured to provide, at each of its outputs, a signal of same wavelength and of same polarization (electric transverse or magnetic transverse) as those of the signal received by its input. As for the embodiment of  FIG.  1   , optical splitter S 1  is calibrated, preferably balanced. 
     During a first phase of testing circuit DUT 1 , a signal adapted to input E TE1 , here a signal of electric transverse polarization and at wavelength λ 1 , is applied to input E TE1 . This signal is typically provided to input E TE1  by means of a polarization-maintaining optical fiber having one end aligned with input E TE1 , and having its other end coupled to a source of an electric transverse polarization light signal of wavelength λ 1 . A corresponding test signal having the same polarization and the same wavelength is then transmitted to the circuit to be tested DUT 1  via coupling device  1 . The output signal of the circuit is then observed to determine whether the circuit is functional for the electric transverse polarization, at wavelength λ 1 . 
     During a second test phase, similar to the first one, a magnetic transverse polarization signal at wavelength λ 1  is supplied to input E TM1 . A corresponding test signal is then transmitted to the circuit to be tested DUT 1  via coupling device  1 , the output signal of the circuit being observed to determine whether the circuit is functional for the magnetic transverse polarization, at wavelength λ 1 . 
     At the end of the two test phases, circuit DUT 1  has been tested and characterized for each of the electric transverse and magnetic transverse polarizations, at wavelength λ 1 . 
       FIG.  7    is a partial simplified top view illustrating another embodiment of the optoelectronic chip  5  described in relation with  FIG.  6   . 
     Chip  5  comprises the same elements as chip  5  of  FIG.  2   , designated with the same reference numerals and coupled or connected together in the same way as in  FIG.  2   , with the difference that inputs E 1  and E 2  of chip  5  are here replaced with respective inputs E TE1  and E TM1 , for example, grating couplers. Further, coupling device is a polarization-maintaining device, in other words, coupler  10  and optical splitters  21  are polarization maintaining. 
     Thus, each output  25  of device  2  provides an electric transverse or magnetic transverse polarization signal, at wavelength λ 1 , the input, respectively E TE1  or E TM1 , receives a signal at this polarization and at this wavelength. 
       FIG.  8    is a partial simplified top view illustrating another alternative embodiment of the optoelectronic chip  5  described in relation with  FIG.  6   . 
     This embodiment benefits from the advantages previously described in relation with  FIG.  2   . 
     Chip  5  comprises the same elements as chip  5  of  FIG.  3   , designated with the same reference numerals and coupled or connected together in the same way as in  FIG.  3   , with the difference that inputs E 1 , E 2 , E 3 , and E 4  of chip  5  are here replaced with respective inputs E TE1 , E TE2 , E TM1 , and E TM2 , for example, grating couplers. Further, coupling device  3  is a polarization-maintaining device, in other words, couplers  33  and the guide crossing device are polarization-maintaining. 
     By successively applying to each input E TE1 , E TE2 , E TM1  et E TM2  an optical signal having a polarization and a wavelength adapted to this input, a circuit coupled, for example, connected, to an output  31   j  of device  3  successively receives an electric transverse polarization signal at wavelength λ 1  and then λ 2 , and a magnetic transverse polarization signal at wavelength λ 1  and then λ 2 . Observing an output signal of the circuit, for each signal received by the circuit, enables to determine whether the circuit is functional for each of the electric transverse and magnetic transverse polarizations, at wavelengths λ 1  and λ 2 . 
     Device  3  may be adapted to the case where chip  5  comprises less than four outputs. The alternative embodiment described in relation with  FIG.  8    further applies to the embodiment of  FIG.  7   . 
     According to an alternative embodiment, not shown, similarly to what has been described in relation with  FIG.  8   , chip  5  of  FIG.  4    may be adapted to the case where chip circuits are desired to be tested for each of the electric transverse and magnetic transverse polarizations, for N different wavelengths, N being an integer in the range from 1 to 4. Such an alternative embodiment applies to the embodiment described in relation with  FIG.  7   . 
     The method described in relation with  FIG.  5    may be adapted to the case where circuits of chip  5  are tested for N different wavelengths, for each of the electric transverse and magnetic transverse polarizations. For this purpose, states  52  and  53  are replaced with first, second, third, and fourth successive states. The first state comprises aligning the end of an optical fiber, having its other end coupled to a source of an electric transverse polarization signal at wavelength λi, with the input E TEi  adapted to this signal. The second state comprises applying this signal to input E TEi . The third state comprises aligning the end of an optical fiber, having its other end coupled to a source of a magnetic transverse polarization signal at wavelength λi, with the input E TMi  adapted to this signal. The fourth state comprises applying this signal to input E TMi . 
     As described in relation with  FIG.  5   , this method may be implemented by means of a single fiber or of a fiber array to decrease the number of alignment steps. For example, when a fiber of the array is aligned with an input E TEi , another fiber of the array may be simultaneously aligned with input E TMi . 
     As a variation, the first and second states may be exchanged with the third and fourth states, respectively. 
     In the embodiments and variations described hereabove in relation with  FIGS.  1  to  8   , when chip circuits are tested for at least two different wavelengths, it is preferably provided for the inputs of chip  5  adapted to these wavelengths to have adjacent passbands. In other words, it is provided for the maximum wavelength in the passband of an input to be equal to the minimum wavelength of the passband of another input, for example, to within 10%, preferably 5%, of the passband of one of these inputs. Thus, the circuits of chip  5  will be tested for a wide range of wavelengths. For example, in the implementation illustrated in  FIG.  3   , if inputs E 1 , E 2 , E 3 , and E 4  have a bandwidth of approximately 20 nm, wavelengths λ 2 , λ 3  and λ 4  are substantially equal respectively to λ 1  plus 20 nm, to λ 1  plus 40 nm, and to λ 1  plus 60 nm. The circuits may then be tested or characterized over a wavelength range having an extension substantially equal to 80 nm. 
     As a variation, the passbands of the chip inputs may partially overlap. 
     Further, the signal received by a given input of chip  5  may be at any wavelength selected from the passband of this input. Particularly, during the test at a given wavelength λi, without changing input, signals at other wavelengths within the passband of this input may be applied thereto. This results in a more accurate test. 
     As an example, wavelengths λi are in the range from 1,250 to 1,360 nm. The waveguides formed on chip  5 , in particular those of previously-described devices  1 ,  2 ,  3 , and  4 , are for example provided to transmit a signal having a wavelength substantially equal to 1,310 or 1,350 nm, such waveguides also enabling to transmit any other wavelength in the range from approximately 1,250 nm to approximately 1,360 nm, for example, with losses smaller than 3 dB, preferably 1 dB. 
     Preferably, as shown in  FIGS.  1 ,  3 ,  4 ,  6 , and  8   , each of coupling devices  1 ,  3 , and  4  is configured to have, between each of its outputs and each input of the chip to which it is connected, optical paths having substantially equal lengths crossing a same number of evanescent couplers. This enables the device to provide test signals having substantially equal powers. When such a device is implemented according to the embodiment of  FIG.  2  or  7   , it is preferably provided for all the outputs of evanescent coupler  1  ( FIGS.  1  and  6   ), of the concerned evanescent couplers  33  (the two couplers  33  on the right-hand side of  FIGS.  3  and  8   ), or of evanescent couplers  441 ,  442 ,  443 , and  444  ( FIG.  4   ) to be connected to identical optical splitters having their outputs then forming the outputs of the coupling device. In this case, each output of the coupling device provides a power signal substantially identical to that of the signal supplied by the other outputs of the device. 
     Although an example of a test chain comprising a single circuit to be tested with one optical input and one optical output has been described, the embodiments and their variations described hereabove also apply to different test chains. For example, a test chain comprising more than one calibrated or balanced optical splitter, possibly with more than two outputs, where the output of a first optical splitter may be coupled, preferably connected, to the input of a second optical splitter, may be provided. Further, a plurality of different circuits to be tested may be provided in a same test chain. Further, a circuit to be tested of a test chain may comprise a plurality of optical inputs and/or a plurality of optical outputs. A circuit to be tested of a test chain may be an optical circuit such as circuit DUT 1 , or an optoelectronic circuit, then comprising at least one input or one output adapted to an electronic signal. More generally, it will be within the abilities of those skilled in the art, based on the above indications, to provide any test chain where the ratio of the power of one or a plurality of optical signals provided to a circuit to be tested to that of a reference optical signal is known, so that the comparison of the output signal(s) of the circuit with the reference signal enables to determine whether the circuit is functional. 
     Specific embodiments have been described. Various alterations, modifications, and improvements will occur to those skilled in the art. In particular, although embodiments and alternative embodiments where the coupling devices comprise evanescent X couplers have been described, each of these evanescent couplers may be replaced with a coupler of multimode interferometer type (MMI) with two inputs and two outputs. More generally, an assembly of a plurality of evanescent X couplers and possible of one or a plurality of optical splitters  21  of a coupling device may be replaced with a MMI-type coupler, for example comprising more than two inputs and/or more than two outputs. For example, device  3  may be only formed with a MMI-type coupler with four inputs and four outputs, and device  2  such as shown in  FIG.  2    may be implemented with only one MMI-type coupler with two inputs and six outputs. In the case where a MMI-type coupler replaces polarization-maintaining evanescent couplers and possibly one or a plurality of polarization-maintaining optical splitters, the MMI-type coupler is also polarization-maintaining. 
     Further, the above-described embodiments and variations thereof may be combined. 
     Further, the above-described embodiments and variations thereof also apply when the inputs of chip  5  are not grating couplers, but inputs having narrow bandwidths, for example, smaller than 50 nm, or even 25 nm, or even more 20 nm and/or adapted to a given polarization. As an example, these embodiments and their variations apply to the case where the chip inputs are arranged in a plane corresponding to the edge of the chip (edge coupling). 
     Although this has not been specified, the various elements, circuits, and devices of chip  5  may be formed from a portion of an SOI-type semiconductor layer, for example, made of silicon, resting on an insulating layer, itself resting on a support such as a semiconductor substrate. 
     It will be within the abilities of those skilled in the art based on the above description to form coupling devices of the type previously described, with any number of inputs greater than or equal to two and/or with any number of outputs. 
     Various embodiments with different variations have been described hereabove. It should be noted that those skilled in the art may combine various elements of these various embodiments and variations without showing any inventive step. 
     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.