Multi-channel tunable optical filter with AWG optical filter

Abstract

The subject of the present invention is a multi-channel tunable optical filter with arrayed waveguide gratings (AWG) and with waveguide structures comprising switching elements. The filter comprises at least 2 filter modules, the first (M1) being connected in series with the second (M2). The first optical filter module comprises an input filter (AWG1in) whose output ports are connected by waveguide channels comprising switching elements with inputs of the output coupling structure (SP1out) to one waveguide constituting the output of the module. The second module (M2) has a similar structure: it contains an input filter (AWG2in), whose output ports are connected by waveguide channels containing switching elements with inputs of the output coupling structure (SP2out) to one waveguide constituting the output of the module. The role of the output coupling structures may be performed by AWG filters with a structure symmetrical to the input filters (AWG1in and AWG2out, respectively).

Description

[0001] Multi-channel tunable optical filter with AWG optical filter

[0002] The subject of the invention is a multi-channel tunable optical filter with an arrayed waveguide grating (AWG), which is used, among others, in optical coherence tomography (OCT) scanners for ophthalmic applications.

[0003] A popular optical filtering element is the AWG grating, which is a standard solution used as an optical multiplexer / demultiplexer system in optical telecommunications using wavelength division multiplexing (WDM). A functional diagram of the AWG grating is shown in Fig. 7. The operation of the AWG grating is described in many textbooks, including the textbook titled "Podstawy Komunikacji Optofalowej” by Bogdan Galwas ISBN 978-83-952678-0-2, as well as the textbook "Principles of Photonic Integrated Circuits" by Richard Osgood Jr. and Xiang Meng ISBN 978-3-030-65192-3. The AWG grating is an integrated waveguide structure (dielectric or semiconductor) and has a symmetrical structure. It consists of one or more input waveguides (IWg), an input power splitter (ISp), a waveguide grating, an output power splitter (OSp), and one or more output waveguides (OWg). The input power splitter (ISp) is a two-dimensional planar waveguide structure limited by two sections of a circle with properly selected radii r and half of the radius r / 2, it should be emphasized that the side walls of the ISp and OSp splitters are located at a sufficiently large distance (and are often absent altogether) so that they do not affect the propagation of the optical signal in the splitter. A matrix (typically from several to several dozen) of waveguides is attached to the output edge of the splitter (OSp). The lengths of the waveguides in the matrix are selected in such a way that the difference in length (d) between two adjacent waveguides is constant. The output power splitter (OSp) is attached to the outputs of the waveguide array; its design is similar to that of the input power splitter (ISp), being a mirror image of it. Due to the constant difference in the optical path lengths of the waveguides depending on the wavelength, the optical signal is focused at a different point along the output edge. This means that if output optical fibres are attached at selected points along the output edge, it is possible to filter the optical signal. It should be noted that the operation of the AWG multiplexer is reversible and an optical signal with a wavelength corresponding to a given output optical fibre introduced into it will be directed to the corresponding AWG input optical fibre. A key feature of the AWG multiplexer is its cyclical nature. In practical terms, this means that optical signals with a selected central wavelength, and whose wavelengths differ by a certain characteristic value (or a multiple thereof) known as the free spectral range (FSR), are directed into each of the output waveguides.

[0004] From the publication "Monolithically integrated eight -channel WDM modulators with cyclic AWGs for multichannel -block operation over C-band" by Y. Suzaki, H. Yasaka, Y. Kawaguchi, R. Iga and H. Okamoto from ELECTRONICS LETTERS of April 28, 2005, volume 41, no. 9, a solutionshowing a 1 -stage tunable filter based on the same AWG gratings is known. The system has a single optical input (In), then in AWG’ the signal is split into n channels (Out. 0 - Out.n): wavelength ranges, each of these channels is connected to the modulator / switching element (SW. 0 - SW.n), allowing selected wavelength ranges to be modulated or switched off. The signal is then recoupled back into a single output optical fibre using an identical AWG’ ’ . A schematic diagram of the solution is shown in Fig. 5.

[0005] From the paper entitled "100-Channel WDM Rx-Type PIC on InP for Use of Low-Cost and Low Power Consumption Electronics" by M. Baier, R. Broeke, F. Soares, M. Gruner, A. Seeger, M. Moehrle, N. Grote, M. Schell published in 2014 in connection with the European Conference on Optical Communication (ECOC), a multi-channel filter (n*m channels) based on the cascade of AWG gratings is known. In this filter, the spectral characteristics of AWG 1 and AWG 2 are selected such that the FSR of the AWG 2 filter is exactly equal to the spectral width (full -width at half-maximum, FWHM) of a single AWG1 channel. As a result, the signal is split into n channels in the first stage, each of which is then split into m channels in the second stage. The result is an n*m channel filter. In practical applications, 10x10 channel filters have been reported. A schematic diagram of the solution is shown in Fig. 6.

[0006] The technical challenge lies in developing an optical filter capable of rapidly switching the wavelength of the transmitted optical signal across a wide range and in an arbitrary manner (i.e. not necessarily sequentially).

[0007] The aim of the invention was therefore to develop a system that contains no moving elements, thereby enabling significantly faster scanning and ensuring an even distribution of the signal across the spectrum during scanning.

[0008] The invention relates to a multi-channel tunable optical filter comprising at least one filter module, which consists of an AWG optical filter, waveguides and switching elements. The filter according to the invention comprises at least two filter modules such that the first filter module is connected in series with the second filter module. In addition, the first filter module comprises n switches, a first AWG input filter comprising n waveguide channels, and a first AWG output filter comprising n waveguide channels such that the first AWG input filter comprises n outputs connected via n waveguide channels through n switches to the n inputs of the first output filter. The second filter module comprises m switches, a second AWG input filter comprising m waveguide channels, and a second AWG output filter comprising m waveguide channels such that the second AWG input filter comprises m outputs connected by m waveguide channels through m switches to m inputs of the second output filter. Moreover, n or m is an integer greater than or equal to 2, and the input waveguide of the first AWG input filter is a waveguide input of the first filter module, and the output waveguide of the second output filter is a waveguide output of the second filter module.Preferably, the number n of optical fibre channels of the first filter module is equal to the number m of optical fibre channels of the second filter module.

[0009] Preferably, the number n of optical fibre channels of the first filter module or the number m of optical fibre channels of the second filter module is equal to 10.

[0010] Preferably, the number n of optical fibre channels of the first filter module is different than the number m of optical fibre channels of the second filter module.

[0011] Preferably, the switches are optical amplifiers (SOAs) or modulators.

[0012] Preferably, in at least one of the filter modules, or the photonic structure coupling the optical signals from the outputs of the AWG filter to one waveguide constituting the optical output of that module is implemented in the form of a filter having the same parameters as the AWG input filter of that optical module.

[0013] Preferably, in at least one of the filter modules, or the photonic structure coupling the optical signals from the outputs of the AWG filter to one waveguide constituting the optical output of the module comprises multimode interference (MMI) couplers, wherein the multimode interference (MMI) couplers form a connection tree that has an input number corresponding to the number of switches of the filter module or / and one output, and wherein one multimode interference coupler with at least two inputs and one output is arranged in each node of the tree.

[0014] Preferably, in at least one of the first or second filter modules, the photonic structure that couples optical signals from the outputs of the AWG filter into a single optical waveguide constituting the optical output of that module takes the form of a single multimode interferential (MMI) coupler, which has a number of inputs corresponding to the number of switches in the optical module and a single output.

[0015] Preferably, the filter according to the present invention comprises at least three filter modules such that at least one additional filter module is connected in series to the output of the second filter module. Preferably, the at least one additional filter module has a number of optical fibre channels equal to the first filter module, the second filter module, or any other of the additional filter modules.

[0016] Preferably, the at least one additional filter module has a different number of optical fibre channels than any other of the filter modules.

[0017] Preferably, the spectral width (FWHM) of a single channel of the first filter module is equal to the free spectral range (FSR) of the second filter module.

[0018] Preferably, the spectral width (FWHM) of a single channel of the second filter module is equal to the free spectral range (FSR) of the first filter module.Preferably, the spectral width (FWHM) of a single channel of the optical module having the largest spectral width is equal to the free spectral range (FSR) of the fdter module having the second largest spectral width (FWHM).

[0019] Preferably, the free spectral range (FSR) of the fdter module having the smallest spectral width (FWHM) is equal to the spectral width (FWHM) of the fdter module having the second smallest spectral width.

[0020] The advantage of this invention is that it enables both a wide spectral range of the scan and a very narrow spectral response of a single step during scanning. A further advantage of the invention is that it enables the construction of a tunable light source operating in any part of the VIS-NIR spectral range, and due to the use of integrated photonics and the absence of moving elements, the reliability of the system according to the invention is significantly increased.

[0021] The subject of the present invention in an embodiment is shown in the drawings, in which: Fig. 1 shows a fdter system composed of two sequentially connected fdter modules with AWG gratings, wherein in both modules the role of the output passive waveguide structure coupling the optical signals from the outputs of the AWG fdter to one waveguide constituting the optical output of the module is fulfilled by an AWG fdter identical to the input fdter;

[0022] Fig. 2 shows a diagram of a tunable fdter in an exemplary configuration with 512 channels;

[0023] Fig. 3a shows the fdter system used as a switching element in a classic fibre laser configuration;

[0024] Fig. 3b shows the fdter system used as a switching element in a configuration with a MOPA laser; Fig. 4 shows the fdter system consisting of three sequentially connected fdter modules with AWG filters, wherein in each of the modules the role of the output passive waveguide structure coupling the optical signals from the outputs of the AWG fdter to one waveguide constituting the optical output of this module is fulfilled by the multimode interference (MMI) coupler tree.

[0025] The present invention is based on the use of elements without moving parts in the construction of rapidly tunable light sources. In this type of solution, the mechanically tuned laser source cavity of the VCSEL type has been replaced by a hybrid light source containing a fibre laser and a photonic integrated circuit, PIC, acting as a narrow-band filter, allowing for a very accurate selection of the desired wavelength. The spectrum of the output signal is distributed homogeneously across the scanning range. The use of an additional fdter can also provide the same power level for each of the emitted wavelengths.

[0026] Fig. 1 shows a functional diagram of a two-stage fdter with n x m channels. The fdter consists of two fdter modules: the first fdter module Ml and the second fdter module M2. The functional assumption of the present invention is that at a given time only one of the switches SW.O, ... , SW.n can be enabledin each of the Ml or M2 modules. The multi-channel tunable optical filter comprises a first optical module Ml connected in series to the second optical module M2.

[0027] The first optical module Ml comprises a first AWGlin input filter, the outputs of which are connected by optical fibre channels through switches SW.O, ..., SW.n with inputs In.O, ..., In.n of the first AWGlout output filter, and the second optical module M2 comprises a second AWG2in input filter, the outputs of which are connected by optic fibre channels through switches SW.O, ... , SW.m with inputs In.O, ... , In.n of the second AWG2out output filter.

[0028] The first optical module Ml comprises n channels, thus the first AWGlin input filter comprises n outputs Out.O, ... , Out.n connected by n switches SW.O, ... , SW.n with n inputs In.O, ... , In.n of the first AWGlout. output filter.

[0029] The second optical module M2 comprises m channels, so the second AWG2in input filter comprises m outputs Out.O,..., Out.m connected by m switches SW.O, ..., SW.m with m inputs In.O, ..., In.m of the second AWG2out output filter.

[0030] In another implementation, it is possible to expand the filter architecture by attaching further modules. Optical amplifiers (SOA) or modulators can be used in the device as switches SW.

[0031] For the tunable filter to function properly, it is necessary that the free spectral range (FSR) of the AWG gratings in one of the filter modules is equal to the channel width of the AWG grating in the other module. This solution allows the serial connection of the modules to divide the usable transmission spectrum into channels in a manner known from the prior art, as shown in Fig. 6 whilst simultaneously coupling the signal to a single output, as in the prior art solution shown in Fig. 5.

[0032] In an exemplary embodiment, a filter with n x m channels was obtained using n + m switching elements. If the filter is expanded with additional modules, it is possible to achieve an even more favourable ratio of optical channels to switching elements. For example, in a 3-stage filter system, where AWG gratings, each with 10 channels, are used in each stage, a 1,000-channel tunable optical filter with 30 switches was obtained.

[0033] In the classic solutions known from the prior art (shown in Fig. 5 and Fig. 6) the filter would require the use of at least 1000 switching elements, which clearly illustrates the advantages of the present invention. In addition, all the above-mentioned elements are integrated in 1 optical integrated circuit (photonic integrated circuit: PIC).

[0034] Fig. 2 shows a functional diagram of the 512-channel F3 filter operating in the 1550 nm telecommunications band. Such a filter can be produced using integrated photonics technology based on indium phosphide (InP). In the embodiment, the filter is composed of three filter modules Ml, M2, M3. All modules utilise AWG gratings, both as input and output coupling elements, for which the central wavelength is 1550 nm, each grating has 8 spectral channels of equal spectral width, which completelyfill the grating’s FSR. The AWG gratings in the Ml filter module have an FSR of 51.2 nm, those in the M2 module have an FSR of 6.4 nm, and those in the M3 module have an FSR of 0.8 nm. At any given time, only one of the switches can be activated in each of the Ml, M2 and M3 modules. Each of the Ml, M2 and M3 modules is equipped with eight switches, each of which allows the filter transmission to be enabled or disabled within the range corresponding to its respective AWG grating channel. In the example given, it is possible to control 512 spectral channels of the filter transmission (each with a full width at half maximum of 0.1 nm) using 24 switching elements. The number of channels (and thus the number of available filtered wavelengths) can be easily increased (e.g. to 1024 channels) by altering the characteristics of the AWG grating (by appropriately altering its geometry) in one of the sections from a 1:8 grating to a 1:16 grating and appropriately modifying the FSR and FWHM parameters, or by connecting a further optical module to the output of the M3 module.

[0035] The photonic integrated circuit described herein is used as an external tunable mirror for a fibre laser or as a filter for a broadband source acting as a power element in a fibre laser in a MOPA configuration ( Master Oscillator Power Amplifier). Both configurations are shown in Fig. 3. In both configurations, the tunable optical filter is used to select the appropriate wavelength of a seed signal. The most significant difference in the configuration of the integrated photonic system in both configurations is the presence (or absence) of a reflector connected to the output port on the right side of the system containing a multi-channel tunable optical filter. In the configuration shown in Fig. 3a, the pumping signal is fed into the active optical fibre, then the generated broadband signal is filtered through a tunable filter and reflected in the optical reflector (REF), then it passes again through the tunable filter and is fed into the active optical fibre as the seed signal.

[0036] In the configuration shown in Fig. 3b, the signal from the broadband source (BRS) is filtered through a tunable filter, and then light of a selected length is introduced into the active optical fibre, optically pumped as the seed signal.

[0037] The proposed solution for a rapidly tunable, wide-spectral-range coherent light source is used in optical coherence tomography (OCT) scanners for ophthalmic applications. By using an integrated photonic circuit, the wavelength is tuned without the need to use moving parts (e.g. MOEMS mirrors). The developed solution is not based on electromechanical resonance and allows for even distribution of wavelength shifts during spectral scanning. This solution not only reduces production costs and increases scanning speed, but also enables a more accurate reproduction of the objects being scanned. In each of the filter modules, the output coupling structure can be implemented using an AWG output grating or using broadband components: multimode interference (MMI) couplers, and both solutions can be used interchangeably. This is the case in the example shown in Fig. 4.

[0038] According to the example shown in Fig. 4, a device was constructed with spectral characteristics corresponding to those of the device shown in Fig. 2, with the output AWG filters of each modulereplaced by cascades of broadband MMI couplers. The device avoids the need to match AWG gratings within the module (which requires a very precise control of the technological process and is a limitation in the implementation of the device) at the price of asymmetry of the spectral properties of the device (correct operation only in the case of entering the signal from the side of the first AWG grating). The device eliminates the need to align the AWG gratings within the module (which requires very precise control of the manufacturing process and constitutes a limitation in the device’s design) at the cost of asymmetrical spectral characteristics (it operates correctly only when the signal is input from the first AWG grating).

[0039] The output coupling structures comprise multimode interference MMI couplers that form a connection tree. The connection tree has a number of inputs corresponding to the number of switches of the optical module and one output, and one multimode interference MMI coupler with two inputs and one output is placed in each tree node.List of reference symbols:

[0040] AWGlin - first input filter

[0041] AWGlout - first output filter

[0042] AWG2in - second input filter

[0043] AWG2out - second output filter

[0044] d - length difference between two adjacent waveguides F3 - filter containing modules Ml, M2, M3

[0045] IWg - Input Waveguide

[0046] ISp - Input power Splitter

[0047] Ml - first filter module

[0048] M2 - second filter module

[0049] Mn - additional filter module

[0050] MMI - interference coupler

[0051] OSp - Output power Splitter

[0052] OWg - Output Waveguide

[0053] SW - switch

[0054] r - radius

[0055] In. - input

[0056] Out. - output

Claims

Claims1. A multi-channel tunable optical filter comprising at least one filter module, which consists of an AWG optical filter, waveguides and switching elements, characterized in thatit comprises at least two filter modules such that the first filter module (Ml) is connected in series with the second filter module (M2), whereinthe first filter module (Ml) comprisesn switches (SW.O, ..., SW.n),a first AWG input filter (AWGlin) comprising n waveguide channels,and a first AWG output filter (AWGlout) comprising n waveguide channels such that the first AWG input filter (AWGlin) comprises n outputs (Out.O, ..., Out.n) connected by n- waveguide channels via n switches (SW.O, ..., SW.n) with n-inputs (In.O, ..., In.n) of the first output filter (AWGlout),and the second filter module (M2) comprisesm switches (SW.O, ..., SW.m),a second AWG input filter (AWG2in) comprising m waveguide channels,and a second AWG output filter (AWG2out) comprising m waveguide channels such that the second AWG input filter (AWG2in) includes m outputs (Out.O,..., Out.m) connected by m waveguide channels viam switches (SW.O,..., SW.m) withm inputs (In.O, ..., In.m) ofthe second output filter (AWG2out),wherein n or m is an integer greater than or equal to 2, and the input waveguide (In.) of the first AWG input filter (AWGlin) is a waveguide input of the first filter module (Ml), and the output waveguide (Out.) of the second output filter (AWG2out) is a waveguide output of the second filter module (M2).

2. The multi-channel tunable optical filter according to claim 1, characterized in that the number n of optical fibre channels of the first filter module (Ml) is equal to the number m of optical fibre channels of the second filter module (M2).

3. The multi-channel tunable optical filter according to claim 2, characterized in that the number n of optical fibre channels of the first filter module (Ml) or the number m of optical fibre channels of the second filter module (M2) is equal to 10.

4. The multi-channel tunable optical filter according to claim 1, characterized in that the number n of optical fibre channels of the first filter module (Ml) is different from the number m of optical fibre channels ofthe second filter module (M2).

5. The multi-channel tunable optical filter according to any one of claims 1 to 3, characterized in that the switches are optical amplifiers (SOA) or modulators.

6. The multi-channel tunable optical filter according to claim 1, characterized in that in at least one of the filter modules (Ml) or (M2), the photonic structure coupling the optical signals from the outputs of the AWG filter to one waveguide constituting the optical output of that module is implemented in the form of a filter having the same parameters as the input AWG filter of such optical module.

7. The multi-channel tunable optical filter according to claim 1, characterized in that in at least one of the filter modules (Ml) or (M2) the photonic structure coupling the optical signals from the outputs of the AWG filter to one waveguide constituting the optical output of this module comprises multimode interference (MMI) couplers, wherein the multimode interference (MMI) couplers form a connection tree that has an input number corresponding to the number of switches of the filter module (Ml) or (M2) and one output, and in each node of the tree there is provided one multimode interference (MMI) coupler with at least two inputs and one output.

8. The multi-channel tunable optical filter according to claim 1, characterized in that in at least one of the filter modules (Ml) or (M2), the photonic structure coupling the optical signals from the outputs of the AWG filter to one waveguide constituting the optical output of this module is in the form of a single multimode interference (MMI) coupler, which has a number of inputs corresponding to the number of switches of the optical module and one output.

9. The multi-channel tunable optical filter according to any one of claims 1 - 7, characterized in that it comprises at least three filter modules such that at least one additional filter module (Mn) is connected in series to the output of the second filter module (M2).

10. The multi-channel tunable optical filter according to claim 8 characterized in that the at least one additional filter module (Mn) has a number of optical fibre channels equal to the first filter module (Ml), the second filter module (M2) or any other of the additional filter modules (Mn).

11. The multi-channel tunable optical filter according to claim 8 or 9 characterized in that the at least one additional filter module (Mn) has a different number of optical fibre channels than any other of the filter modules.

12. The multichannel tunable optical filter according to any one of claims 1 to 10, characterized in that the spectral width (FWHM) of the single channel of the first filter module (Ml) is equal to the free spectral range (FSR) of the second filter module (M2).

13. The multichannel tunable optical filter according to any one of claims 1 to 11, characterized in that the spectral width (FWHM) of the single channel of the second filter module (M2) is equal to the free spectral range (FSR) of the first filter module (Ml).

14. The multichannel tunable optical filter according to any one of claims 1 to 12, characterized in that the spectral width (FWHM) of the single channel of the optical module having the largestspectral width is equal to the free spectral range (FSR) of the filter module having the second largest spectral width (FWHM).

15. The multi-channel tunable optical filter according to any one of claims 1 to 13, characterized in that the free spectral range (FSR) of the filter module having the smallest spectral width (FWHM) is equal to the spectral width (FWHM) of the filter module having the second smallest spectral width.

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