WO2018141084A1

WO2018141084A1 - Apparatus and method for cell calibration of optical switch matrix

Info

Publication number: WO2018141084A1
Application number: PCT/CN2017/072856
Authority: WO
Inventors: Dominic John Goodwill; Mohammad KIAEI
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2017-02-03
Filing date: 2017-02-03
Publication date: 2018-08-09

Abstract

A system and method for cell calibration of a switch matrix are provided. The optical switch matrix includes a plurality of switch cells comprising M input switch cells, N output switch cells, and a plurality of intermediate switch cells. Each one of the plurality of switch cells has at least one input waveguide and at least one output waveguide. A plurality of intermediate photodetectors are provided. Each intermediate photodetector is optically coupled to an output waveguide of one of the plurality of intermediate switch cells such that no more than two optically coupled output waveguides uniquely define a single light path from one of the M input switch cells to one of the N output switch cells. The method includes providing light to a selected one of the M input switch cells, and calibrating the intermediate switch cells located along a first light path extending from the selected M input switch cell. The first light path includes output waveguides of not more than two intermediate switch cells. These output waveguides uniquely define the first light path.

Description

APPARATUS AND METHOD FOR CELL CALIBRATION OF OPTICAL SWITCH MATRIX

FIELD OF THE INVENTION

The present invention relates generally to an optical switch matrix, and in particular embodiments, to an apparatus and method for cell calibration of an optical switch matrix.

BACKGROUND

In optical networks, bandwidth is increasingly becoming a precious resource and expectations from future optical networks are that they should be able to efficiently handle resources as quickly as possible. Optical path switching technology can be used for switching individual optical channels between different paths for routing of information carried by these optical channels.

Optical switching can be performed by an optical switch matrix built using photonic integrated circuit technology. A switch matrix can have M inputs arbitrarily connectable to N outputs. This type of matrix can be constructed by using a plurality of switch cells arranged in multiple stages in a column and row configuration, wherein the total number of columns is C. The number of cells in each column is P (q) , wherein q is a reference to each respective column. N, M, C and P (q) are positive integers. Each matrix input port, matrix output port and switch cell can have one or more input and output waveguides. The state of each cell can be changed, so that the path of light through said cell is changed, thus allowing the optical switch matrix to connect the light from a desired matrix input port to a desired matrix output port, creating a light path. An example switch cell is a Mach-Zehnder interferometer, which can be constructed in 2x2, 1x2, x1 or 1x1 configurations wherein AxB means that the switch cell has A switch cell inputs and B switch cell outputs where A, B are positive integers.

Typically, an optical switch matrix needs to be calibrated to provide consistent performance. Each switch cell needs to be calibrated, which may become time consuming in switch matrices having many tens or even hundreds switch cells. The related cost for calibration depends both upon the number of switch cells and the time required to calibrate an individual cell. Prior art calibration usually requires two photodetectors for calibrating each switch cell. By way of example, a 32x32 switch matrix having 448 cells will require 896 photodetectors to calibrate the switch matrix.

Accordingly, there is a need for improved calibration of a switch matrix.

SUMMARY

The present invention provides an apparatus and method for cell calibration of a switch matrix.

In accordance with embodiments of the present invention, an aspect of an apparatus for calibrating an optical switch matrix is provided. The optical switch matrix has a plurality of switch cells being defined as M input switch cells, N output switch cells, and a plurality of intermediate switch cells. M and N are positive integers and each one of the plurality of switch cells has at least one input waveguide and at least one output waveguide. The matrix further includes a plurality of intermediate photodetectors. Each intermediate photodetector is optically coupled to an output waveguide of one of the plurality of intermediate switch cells such that, depending of the configuration of the optical switch matrix, the output from one, two or more than two optically coupled output waveguides may be required to uniquely define a single light path from one of the M input switch cells to one of the N output switch cells. A uniquely defined single light path is a light path that passes through a set of intermediate switch cells from a given input switch cell to a given output switch cell and cannot pass through any other switch cells. The system further has a plurality of output photodetectors, each of which are optically coupled to an output waveguide of one of the N output switch matrix. The number of the plurality of output photodetectors is equal to the number of N output switches.

The apparatus can also include a plurality of the input monitor photodetectors, each of which are optically coupled to an input switch cell waveguide of one of the M input switch cells, where the number of input monitor photodetectors is equal to the number of M input switch cells. The purpose of these input monitor photodetectors is to track the optical power of the input light due to the optical power of the input light not being perfectly steady. A control system then uses the photocurrent signal from the intermediate photodetectors or the output photodetectors, divided by the signal from the input photodetector to normalize any fluctuations in the optical power at the input.

Further in accordance with embodiments of the present invention, there is provided a method for calibrating an optical switch matrix. In this aspect, the method includes utilizing the above described apparatus of calibrating an optical switch matrix. The method includes providing light to one of the M input switch cells. All intermediate switch cells along all uniquely defined light paths reachable, i.e. can be seen, from the lighted one of the M input switch cells are then calibrated. The calibration of each intermediate switch cell is based on measurements obtained through the photodetectors located along each unique light path from the given input switch cell to the corresponding given output switch cell. Once all the reachable intermediate switch cells have been calibrated, light is then provided to another one of the M input switch cells. All the intermediate switch cells that are reachable along all unique light paths will be calibrated, except for those intermediate switch cells that have been previously calibrated will be skipped. The method repeats until all intermediate switch cells have been calibrated.

BRIEF DESCRIPTION OF THE FIGURES

Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which:

Fig. 1 is a diagram of an 8x8 switch matrix；

Fig. 2 is a diagram of a 64x64 switch matrix；

Fig. 2a is a close up diagram of area A of Fig. 2；

Figs. 3-6 are diagrams of an 8x8 switch matrix showing example light paths that are uniquely identifiable and example light paths that are not uniquely identifiable；

Fig. 7 a diagram of an 8x8 switch matrix showing pairs of intermediate photodetectors required to uniquely identify a light path； and

Fig. 8 is an example table illustrating the paths, photodetectors and calibrated switch cells associated with input 1, according to embodiments of the present invention； and

Fig. 9 is an example table illustrating the paths, photodetectors and calibrated switch cells associated with input 1 and output1, according to embodiments of the present invention.

Fig. 10 illustrates an embodiment of method of the present invention；

DETAILED DESCRIPTION

Figure 1 is an illustrative example of an 8x8 optical switch 100 having 16 switch cells configured in a 4x4 matrix. Switch 100 has a plurality of switch cells being defined as having four input switch cells 110, four output switch cells 120 and a plurality of intermediate switch cells 130. The columns are referred to as Columns A through D. The rows are referred to as Row 1 through 4.

The identification of an input switch cell, output switch cell, or intermediate switch cell is based on the column and row. For example, input switch cell A1 is located at Column A, Row 1； output switch cell D4 is located at Column D, Row 4, and intermediate switch cell B2 is located at Column B, Row 2. Figure 1 further illustrates input switch cell A1 having two input waveguides 112 and two output waveguides 114. This is referred to as a 2x2 switch cell. Optical switch matrices can have any number of input and output ports MxN, such as 4x4, 4x16, 8x8, 32x16, 32x32, 64x64, etc., any internal matrix arrangement such as Banyan, Benes, PILOSS, enhanced dilated Benes, crossbar, etc, as well as other configurations of the input and output waveguides for the individual cells, such as 1x2 or 2x1, are within the scope of this invention.

The path upon which light physically passes through the switch 100 from a given input to a given output is referred to a light path. A light path is thus defined by the switch cells through which the light passes. There can be multiple light paths from a given input to a given output.

By way of example, referring back to Figure 1, light path 140 refers to the path that light physically passes from input A4, through switch cells B4 and C4, to output D4. This light path can also be referred to as light path A4, B4, C4, D4. Light path 150 refers to the path that light physically passes from input A4, through switch cells B1 and C2, to output D4. This light path can also be referred as light path A4, B4, C3, D4. As can be seen in Figure 1, light that originates at input A4 and ends at output D4 can travel through two separate light paths.

The act of calibrating a switch cell involves sending light having a pre-defined optical power through a light path and providing a drive signal, i.e. drive power, drive current or drive voltage, from external control electronics to the switch cells in that path. Photodetectors optically coupled to corresponding switch cells in the light path generate photocurrent proportional to optical power of light passing through each switch cell. The control circuit then can vary the drive signal on the switch cells until the optical power is at a desired level.

Figure 2 illustrates an embodiment of the apparatus of the present invention. In this embodiment, an optical switch matrix 200 has 32 1x2 input switch cells 202, 32 2x1 output switch cells 204, i.e. the optical switch matrix 200 has a 64x64 architecture where M＝32 and N＝32. A plurality of 2x2 intermediate switch cells 206 are disposed between the input switch cells 202 and the output switch cells. The columns are referred to as Columns A through K and the rows are referred to as Rows 1 through 32. Column F is a crosstalk-suppression stage. As illustrated in Fig. 2a, the intermediate switch cells 260 in the Column F comprise two separate columns, Fa and Fb, and two rows each, such as Row 1a and Row 1b, for a total of 64 rows. Columns Fa and Fb are referred to as the middle stage columns.

The input switch cells A1-A32, output switch cells K1-K32 and intermediate switch cells B1-J32 can be optically connected through various light paths. Each switch cell can be in a bar-state or cross-state. For example, B1b and B1x refers to intermediate switch cell B1 in bar-state and in cross-state, respectively.

As illustrated in Fig. 2a, the switch 200 further includes a plurality of intermediate photodetectors PDE. Each PDE is optically coupled to an output waveguide of one of the plurality of intermediate switch cells along Column E such that the location of each of the plurality of intermediate photodetectors PDE uniquely defines a light path from one of the inputs A1-A32 to one of the outputs K1-K32.

A uniquely defined light path is a light path that passes through a set of switch cells from a given input to a given output and cannot pass through any other switch cells. Figures 3-6 illustrate an 8x8 matrix 300 and demonstrates when a light path is, and is not, uniquely defined. Matrix 300 includes eight input switch cells 310, eight output switch cells 320 (where M ＝ 8 and N ＝ 8) and a plurality of intermediate switch cells 330, all arranged in multiple stages in a column and row configuration. The columns are referred to as Columns A through G. The rows are referred to as Row 1 through 8. Each input switch cell 310 is a 1x2 cell having one input waveguide and two output waveguides. Each output switch cell 320 is a 2x1 cell having two input waveguides and one output waveguide. Each intermediate switch cell 330 is a 2x2 cell having two input waveguides and two output waveguides. Also, illustrated are all the light paths from input A3 and end at output G7. These light paths designated by the following

reference numbers

340, 342, 344, 346, 348, 350, 352 and 354.

Figure 4 illustrates the same matrix as in Figure 3 with the addition of eight input photodetectors 360, each of which being optically coupled to an separate input waveguide associated with each input 310, eight output photodetectors 370, each of which being optically coupled to a separate output waveguide associated with each output 320, and sixteen intermediate photodetectors 380, each of which being optically coupled to a separate output waveguide associated with each switch cell in column B. The location of the intermediate photodetectors 380, however, does not uniquely define the light paths. For example,

light paths

340 and 342 both pass through the same output waveguide associated with the B2 switch cell and thus cannot be distinguished by the intermediate photodetector optically connected to that output waveguide.

Figure 5 illustrates the same matrix as in Figure 4 but with the intermediate photodetectors 385 being optically coupled to a separate output waveguide associated with each switch cell in column C. By placing the internal photodetectors along the output waveguides of switch cells in column C allow for the light paths to be uniquely defined. For example,

light paths

340 and 342 pass through different output waveguides in the A3 switch cell.

Figure 6 illustrates the same matrix as in Figure 4 but with the intermediate photodetectors 390 being optically coupled to a separate output waveguide associated with each switch cell in column D. By placing the internal photodetectors along the output waveguides of switch cells in column D allow for the light paths to be uniquely defined. For example,

light paths

340 and 342 pass through different output waveguides in the A4 and B4 switch cell. Further, this illustrates that for certain switch matrices, there may be more than one location at which photodetectors can be placed that uniquely define the light paths.

Referring back to Figure 2, all of the plurality of intermediate photodetectors PDE are located along the outgoing waveguides from the intermediate switch cells in Column E. Thus, the plurality of intermediate photodetectors PDE are referred to as PDE1, PDE2, …, PDE 64. Each of the intermediate photodetectors PDE are utilized in measuring the drive current associated with optical light that can be seen through various light paths that pass from a given input through switch cell upon it is associated.

This embodiment further comprises a plurality of output photodetectors PDK, each of which are optically coupled to an output waveguide of outputs K1-K32. The number of the plurality of output photodetectors PDK is equal to the number of outputs K1-K32. These plurality of output photodetectors PDK can be referred to as PDK1, PDK2, …, PDK32. Each of the output photodetectors PDK are utilized in measuring the drive current associated with optical light that can be seen through various light paths that pass from a switch cell associated with an intermediate photodetector through the given output.

By way of example, referring to the light path that extends across the Row 1, i.e. A1, B1, ..., J1, K1, in Figure 2, the intermediate photodetector PDE1 which is located at the first output waveguide 210 of switch cell E1 is utilized in measuring the photocurrent from input A1 through switch cell E1. Output photodetector PDK1 which is located at the output waveguide 212 of output K1 is utilized in measuring the photocurrent from switch cell E1 through output K1. While Figure 2 illustrates the intermediate photodetectors being located along all of Column E switch cells, this is not meant to be limiting. Those skilled in the art will recognize that due to optical switch matrixes having various configurations, the path for uniquely identifiable single light paths can require the location of the intermediate photodetectors can be placed at various locations within an optical matrix, and is within the scope of the present invention.

As illustrated in Figure 7, an additional embodiment of the apparatus of the present invention comprises the optical switch matrix 700 where intermediate photodetectors located on two separate intermediate switch cells are required to unique define a single light path from one of an input to one of the outputs. Matrix 700 has eight input switch cells 810, eight output switch cells 820 (where M ＝ 8 and N ＝ 8) and a plurality of intermediate switch cells 830, all arranged in multiple stages in a column and row configuration. The columns are referred to as Columns A through G. The rows are referred to as Row 1 through 8. Each input switch cell 810 is a 1x2 cell having one input waveguide and two output waveguides. Each output switch cell 820 is a 2x1 cell having two input waveguides and one output waveguide. Each intermediate switch cell 830 is a 2x2 cell having two input waveguides and two output waveguides. Also, illustrated are all the light paths from input A1 and end at either output G5 or G8. These light paths designated by the following

reference numbers

840, 842, 844, 846 and 848. Further, eight input photodetectors 860 are each optically coupled to an separate input waveguide associated with each input switch cell 810, eight output photodetectors 870 are each optically coupled to a separate output waveguide associated with each output switch cell 820, and 32 intermediate photodetectors 880 are each optically coupled to a separate output waveguide associated with each intermediate switch cell in columns B and C. The location of the intermediate photodetectors 880 illustrate that a single intermediate photodetector cannot uniquely define a light path, but rather two intermediate photodetectors are required. For example, light path 342 requires an intermediate photodetector to be located at an output waveguide associated with the B2 and A3 switch cells.

Referring back to Figure 2, an additional embodiment of the apparatus of the present invention comprises the optical switch matrix 200 with the addition of a plurality of input monitor photodetectors PDA, each of which are placed along each incoming waveguide into each input A1-A32. These photodetectors are referred as PDA1, PDA2, PDA3, …, PDA64. These plurality of input monitor photodetectors PDA can be referred to as PDA1, PDA2, …, PDA32. They are utilized in measuring the photocurrent associated with optical light that is being provided to each input.

Figure 8 is an example table 800 illustrating the paths 804, photodetectors 806 and calibrated switch cells 808 associated with input switch cell A1 from Figure 2, according to embodiments of the present invention. By way of example, table 800 shows that path through B1, C1, D1 and E1 from input 1 will pass through photodetector PDE1 resulting in the calibration of switch cells B1, C1, D1 and E1, all in bar-state. By further example, table 800 shows that path through B1, C9, D9 and E9 from input switch cell A1 will pass through photodetector PDE17 resulting in the calibration of switch cells B1, C1, D1 and E1, where B1 is in cross-state and remainder are in bar-state.

Figure 9 is an example table 900 illustrating the paths 904 and calibrated switch cells 906 associated with input switch cell A1 and output switch cell K1 from Fig. 2, according to embodiments of the present invention. By way of example, table 900 shows that path through B1, C1, D1, E1, Fa1, Fb1, G1, H1, I1 and J1 from input A1 into output A1 will result in the calibration of switch cells Fa1, Fb1, G1, H1, I1, J1 and K1, all in bar-state. By further example, table 900 shows that path through B17, C17, D17, E17, Fa33, Fb33, G17, H17, I17 and J17 from input A1 into output K1 will result in the calibration of switch cells Fa33, Fb33, G17, H17, I17, J17 and K17, where K17 is in cross-state and the remainder are all in bar-state.

According to additional embodiments of the present invention, the optical switch matrix disclosed above further comprises a controller. This controller is configured to calibrate the intermediate switch cells located along the uniquely defined light paths. In one embodiment, the controller calibrates the intermediate switch cells along the uniquely defined light paths sequentially from one of the M input switch cells to one of the N output switch cells.

According to additional embodiments of the present invention, the optical switch matrix disclosed above comprises a plurality of intermediate switch cells and a plurality of intermediate photodetectors. Each of the intermediate photodetectors are optically coupled to an output waveguide of one of intermediate switch cells, wherein the number of optically coupled intermediate switch cells is a subset, i.e. less than all, of the plurality of intermediate switch cells. As explained above, depending of the configuration of the optical switch matrix, the output from a single, two or more than two optically coupled output waveguides may be required to uniquely define a single light path from one of the M input switch cells to one of the N output switch cells.

Figure 10 illustrates an embodiment of the method of the present invention. The switch matrix as illustrated in Figure 3 will be utilized to describe this embodiment. This, however, is not meant to be limiting. Those skilled in the art will recognize that switch matrix with different architecture is within the scope of this present invention.

This embodiment comprises the steps of providing light to one of the M input switch cells 910. All intermediate switch cells along all uniquely defined light paths reachable from the lighted one of the M input switch cells are then calibrated 920. Herein, the term “reachable” means that the cell in question can be optically coupled to the lighted one of the M input switch cells via other cells along the uniquely defined optical path. The calibration of each intermediate switch cell is based on measurements obtained through the intermediate photodetector located along each uniquely defined light path from the given input to the corresponding given output. Once all the reachable switch cells have been calibrated, light is then provided to another one of the M input switch cells 930. All the intermediate switch cells that are reachable along all uniquely defined light paths will be calibrated, except for those switch cells that have been previously calibrated will be skipped 940. The method repeats until all switch cells have been calibrated 950.

The monitoring of the photodetector measurements can be performed on the optical chip, or at a location or component separate from the optical chip. Those skilled in the art will recognize that the photodetectors can be located on the same chip as the inventive switch matrix chip or located off chip. This allows for the monitoring of the photodetectors to be performed either on chip, i.e. on-chip photodetectors, or off chip, i.e. the use of an optical fiber that is positioned to collect the light from an output coupler, such as a grating coupler or out-of-plane mirror.

Although the present invention has been described with reference to specific features and embodiments thereof, it is evident that various modifications and combinations can be made thereto without departing from the invention. The specification and drawings are, accordingly, to be regarded simply as an illustration of the invention as defined by the appended claims, and are contemplated to cover any and all modifications, variations, combinations or equivalents that fall within the scope of the present invention.

Claims

An optical switch matrix comprising:

a plurality of switch cells comprising M input switch cells, N output switch cells, and a plurality of intermediate switch cells, wherein M and N are positive integers and wherein each one of the plurality of switch cells has at least one input waveguide and at least one output waveguide； and

a plurality of intermediate photodetectors, wherein each intermediate photodetector is optically coupled to an output waveguide of one of the plurality of intermediate switch cells such that no more than two optically coupled output waveguides uniquely define a single light path from one of the M input switch cells to one of the N output switch cells.
The optical switch matrix of claim 1, wherein each intermediate photodetector is optically coupled to an output waveguide of one of the plurality of intermediate switch cells such that a single optically coupled output waveguide of a single one of the plurality of intermediate switch cells uniquely defines a single light path from one of the M input switch cells to one of the N output switch cells.
The optical switch matrix of claim 1, further comprising:

a plurality of output photodetectors, wherein each one of the plurality of output photodetectors is optically coupled to an output waveguide of one of the N output switch cells.
The optical switch matrix of claim 3, wherein the number of output photodetectors in the plurality of output photodetectors is equal to N.
The optical switch matrix of claim 1 further comprising:

a plurality of input photodetectors, wherein each one of the plurality of input photodetectors is optically coupled to an input waveguide of one of the M input switch cells.
The optical switch matrix of claim 1, wherein the single light path comprises a unique set of intermediate switch cells of the plurality of intermediate switch cells, and does not comprise any other intermediate switch cell of the plurality of intermediate switch cells.
The optical switch matrix of claim 1, wherein each one of the photodetectors is in connection with a control circuit.
The optical switch matrix of claim 1, wherein the optical switch matrix is located on a switch matrix chip and wherein each one of the plurality of intermediate photodetectors is located on the switch matrix chip.
The optical switch matrix of claim 1, wherein the optical switch matrix is located on a switch matrix chip and wherein each one of the plurality of intermediate photodetectors is physically located off of the switch matrix chip.
The optical switch matrix of claim 1, further comprising a controller configured to calibrate the intermediate switch cells located along the uniquely defined light paths.
The optical switch matrix of claim 10, wherein the controller is configured to calibrate the intermediate switch cells located along the uniquely defined light paths sequentially from one of the M input switch cells to one of the N output switch cells.
An optical switch matrix comprising:

a plurality of switch cells comprising M input switch cells, N output switch cells, and a plurality of intermediate switch cells, wherein M and N are positive integers and wherein each of the plurality of switch cells have at least one input waveguide and at least one output waveguide； and

a plurality of intermediate photodetectors, wherein each intermediate photodetector is optically coupled to an output waveguide of one of the plurality of intermediate switch cells of a subset of the plurality of intermediate switch cells such that optically coupled output waveguides of the subset uniquely define a single light path from one of the M input switch cells to one of the N output switch cells.
The optical switch matrix of claim 12, wherein each intermediate photodetector is optically coupled to an output waveguide of one of the plurality of intermediate switch cells of the subset of the plurality of intermediate switch cells such that a single optically coupled output waveguide of a single one of the plurality of intermediate switch cells of the subset uniquely defines a single light path from one of the M input switch cells to one of the N output switch cells.
A method for calibrating an optical switch matrix, the optical switch matrix having a plurality of switch cells comprising M input switch cells, N output switch cells, and a plurality of intermediate switch cells, wherein M and N are positive integers and wherein each one of the plurality of switch cells has at least one input waveguide and at least one output waveguide； and a plurality of intermediate photodetectors, wherein each intermediate photodetector is optically coupled to an output waveguide of one of the plurality of intermediate switch cells such that no more than two output waveguides uniquely define a single light path from one of the M input switch cells to one of the N output switch cells.

the method comprising:

providing light to a selected one of the M input switch cells；

calibrating the intermediate switch cells located along a first light path extending from the selected M input switch cell, wherein the first light path comprises output waveguides of not more than two intermediate switch cells which uniquely define the first light path.
The method of claim 14, wherein the step of calibrating comprises:

sending light having a pre-defined optical power through the first light path；

providing a drive signal to the intermediate switch cells located along the first light path；

generating photocurrent, by the optically coupled intermediate photodetectors, dependent upon optical power of light passing through the intermediate switch cells； and

varying the drive signal on the intermediate switch cells until the optical power is at a desired level.
The method of claim 15, wherein the calibrating comprises sequentially calibrating intermediate switch cells along the first light path.
The method of claim 14, further comprising calibrating intermediate switch cells along a second light path extending from the selected input switch cell and different from the first light path, wherein the second light path comprises at least one intermediate switch cells which uniquely define the second light path.
The method of claim 17, further comprising repeating the calibration of all remaining intermediate switch cells for all remaining input switch cells while skipping those intermediate switch cells that have been previously calibrated.
A method for calibrating an optical switch matrix, the optical switch matrix having a plurality of switch cells being defined as M input switch cells, N output switch cells, and a plurality of intermediate switch cells, wherein M and N are positive integers and wherein each of the plurality of switch cells have at least one input waveguide and at least one output waveguide； and a plurality of intermediate photodetectors, wherein each intermediate photodetector is optically coupled to an output waveguide of one of the plurality of intermediate switch cells such that each output waveguide uniquely defines a single light path from one of the M input switch cells to one of the N output switch cells.

the method comprising:

providing light to a selected one of the M input switch cells；

calibrating the intermediate switch cells located along a first light path extending from the selected M input switch cell, wherein the first light path comprises at least one intermediate switch cell which uniquely defines the first light path；

calibrating intermediate switch cells along a second light path extending from the selected input switch cell and different from the first light path, wherein the second light path comprises at least one intermediate switch cells which uniquely define the second light path； and

repeating the calibration of all remaining intermediate switch cells for all remaining input switch cells while skipping those intermediate switch cells that have been previously calibrated.
The method of claim 19, wherein the calibrating comprises sequentially calibrating intermediate switch cells along the first light path.