FIELD OF THE INVENTION

The present invention relates to optical filters, and particularly to optical filters comprising multimode interferometers.

BACKGROUND ART

Multimode interferometers (MMIs) have been used for a large variety of photonic devices, including power splitters/combiners, couplers, switches, and multiplexers. MMI- based devices are very suitable for practical use, largely due to their relative ease of fabrication, low sensitivity to fabrication error, and low dependence on temperature, wavelength and polarization.

Figure la is a plan-view of a lxl MMI 10 with one single-mode input 12 and one single-mode output waveguide 16 joining the multimode waveguide 14 at the mid-point across the width of the single-mode/multimode waveguide interface, i.e. the optical axes of the single mode input and output waveguides and the multimode waveguide are coincident. Only the raised, thicker parts of the device are shown for clarity. Figures lb and lc are cross-sections through the single-mode and multimode parts of the device, respectively. In this example, the MMI is formed using silicon-on-insulator (SOI) methods, with a layer of silicon (in which the core of the waveguide is formed) adjacent to a layer of insulator 18 (for example silicon dioxide, Si0 ₂ ) which acts as the cladding; however, other material systems are also applicable. Cladding (e.g. air, Si0 ₂ , etc) may also be provided around and above the silicon layer although, again, this is not illustrated for clarity. In this MMI 10, the single-mode waveguide has a width of w¾ _M and the multimode waveguide has a width of W _mi and a length of ZMMI- The device has a refractive index of n _r in the core and n _c in the cladding for the operating wavelength, λ. Under the geometric limitations of the configuration considered, only symmetric modes are excited in the multimode waveguide with the result that the two-dimensional approximation of the self-image distance , for the fundamental TE mode input is given by:

2

(1) where p represents the self-image order and is a positive integer, λ is the operating wavelength, n _r is the refractive index of waveguide core, and H _ff is the effective multimode waveguide width given by:

^ = ^ΜΜ! + , ₂ - (2)

Where both symmetric and anti-symmetric modes are excited in the multimode waveguide (for example because the input and/or output is not located at the mid-point of the multimode waveguide), the two-dimensional approximation of the self-image distance for the fundamental TE mode input is given by:

L, = (la)

It is clear from equations (1) and (la) that the self-image distance U is linearly dependent on operating wavelength λ, and is more sensitive to wavelength change at higher self-image order, i.e. larger value of p. Hence, for a fixed multimode waveguide length ZMMI, the wavelength corresponding to different self-image order p appears periodic. Noting that the device's output has almost full transmission when the multimode waveguide has a self- image length, the transmittance of the device can be modulated by varying the input wavelength, with the transmission maxima appearing periodically as a function of wavelength. The M I 10 may therefore work as a coarse wavelength filter, and indeed great effort has been made to design devices based on single MMIs. However, their performance in this role is limited.

EP-A-1705504 discloses an interferometer comprising two MMI devices coupled in series and located adjacent to or between reflectors that couple the light diffused from the first MMI device into the second MMI device after a travelling over a different path length. The reflected and diffused light interferes with light in the second MMI device. A complex and fine-tuned arrangement of reflectors is however unnecessary as disclosed herein.

SUMMARY OF THE INVENTION The present inventors have realised there is significant potential to combine a plurality of MMIs within a single device for the realization of improved device performance, e.g. higher extinction ratio, narrower wavelength transmission linewidth, and larger free spectral range (FSR).

In one aspect therefore, there is provided an optical wavelength filter based on multiple cascaded MMIs. The optical filter comprises an optical waveguide, which itself comprises an input for receiving photons; a first multimode interferometer coupled to the input; a second multimode interferometer, connected in series to the first multimode interferometer; and an output, coupled to the second multimode interferometer, for providing a filtered output. The first multimode interferometer and the second multimode interferometer can be connected via a suitable waveguide structure, and are preferably optically separate apart from that waveguide structure.

The first and second multimode interferometers may have the same or different physical characteristics. For example, in an embodiment the second multimode interferometer may have a different effective refractive index, width, length, height, or etch depth, or be constructed from a different material.

In a further embodiment, the second multimode interferometer has a length, width or height which is an integer multiple of the length, width or height of the first multimode interferometer (or vice versa). That is, the length of the second multimode interferometer is an integer multiple of the length of the first multimode interferometer (or vice versa); the width of the second multimode interferometer is an integer multiple of the width of the first multimode interferometer (or vice versa); or the height of the second multimode interferometer is an integer multiple of the height of the first multimode interferometer (or vice versa). For a device in which the multimode interferometers have a constant height, the length and width of the multimode interferometers may be related to one another according to equations (1) or (la), and (2) above, where λ is the design wavelength.

In embodiments of the present invention, each multimode interferometer comprises a portion of the waveguide having at least one relatively larger dimension, relative to the corresponding at least one dimension of portions between the multimode interferometers. For example, the multimode interferometers may be wider than the portions interconnecting them, as well as the input and output. In an embodiment, one or more of the input, the output, and the portions connecting the multimode interferometers are fundamental-mode waveguides, i.e. single-mode. The optical filter may comprise multiple groups of one or more multimode interferometers, known as "stages". Each multimode interferometer of a particular stage has the same physical characteristics. Each stage may have the same number of multimode interferometers.

The waveguide can be arranged linearly (i.e. in a straight line), or can comprise one or more curved portions. In the latter arrangement, the waveguide can be folded back upon itself, to decrease the footprint of the overall device.

The optical filter may be manufactured using a range of materials and methodologies. In one embodiment, the filter comprises a layer of light-carrying material, wherein the waveguide comprises a relatively thicker portion of the light-carrying layer. This type of waveguide is generally known as a "rib waveguide". However, the present invention is equally applicable to strip waveguides or a planar waveguide with alternative light-guiding structures. The light-carrying material may be any optical material, such as silicon, Si0 ₂ , germanium, UNb0 ₃ , GaAs, InP, etc. BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described by way of example, with reference to the accompanying figures in which;

Figures la, lb and lc show alternative views of a conventional single multimode optical filter;

Figures 2a and 2b show alternative designs of an optical filter according to embodiments of the present invention;

Figure 3 is a graph of the simulated transmission spectra of devices according to Figures la, 2a and 2b; Figures 4a, 4b and 4c show optical filters according to further embodiments of the present invention;

Figure 5 is a graph of the simulated transmission spectra of devices according to Figures la, 4a, 4b and 4c;

Figures 6a and 6b show optical filters according to yet further embodiments of the invention, including a filter in a folded arrangement; and

Figure 7 is a graph of the simulated transmission spectra of devices according to Figures 4c and 6a.

DETAILED DESCRIPTION OF THE EMBODIMENTS

High-performance wavelength filters require larger free spectral range, narrower spectral bandwidth, and a high extinction ratio. These properties are difficult to achieve using a single lx l MMI device. According to embodiments of the present invention, an improved spectral response can be achieved by connecting multiple MMIs in series. The spectral response S of such a filter with /V lxl MMIs (where /V is a positive integer greater than or equal to 1) is simply given by:

N

(3) (=1 where, 5 _/ is the spectral response of the /th MMI. Of course, those skilled in the art will appreciate that the present invention is equally applicable to non lx l MMIs (such as 2x 1 MMIs, etc).

Figure 2a is a plan view of an optical filter 100 according to an embodiment of the present invention. The filter 100 is similar to the filter 10 described with respect to Figure 1, but with two multimode filters connected in series, i.e. N = 2. Thus, the filter 100 comprises a single-mode input waveguide 102, connected to the first multimode waveguide 112. A further single-mode waveguide 103 couples the first multimode waveguide 112 to a second multimode waveguide 112', and this second multimode waveguide is coupled to a single- mode output 104 which outputs a wavelength-filtered optical signal.

As will be appreciated by those skilled in the art, the filter has a similar silicon-on- insulator (SOI) construction to those shown in Figures la to lc (although other optical methods and materials could also be used). Thus, a layered structure is used, comprising a layer of silicon on a layer of insulator (e.g. Si0 ₂ ). The waveguide shown in Figure 2a comprises a relatively thicker portion of the silicon layer, and this may be covered in an insulating cladding (e.g. air, Si0 ₂ etc). For clarity, all of the examples illustrated hereinafter are shown in this way.

Figure 2b shows an optical filter 100' according to a further embodiment of the present invention where N = 3. Thus, a third multimode waveguide 112" is coupled in series between the second multimode waveguide 112' and the output 104. In fact, any number of waveguides may be connected in this way. In these embodiments, each of the multimode waveguides 14, 112, 112', 112" has the same length as well as other physical characteristics, i.e. the same material, same width, same height. The number of multimode waveguides having the same length is referred to as the "order" of the filter: thus, filter 10 is first order; filter 100 is second order; and filter 100' is third order.

A commercial simulation tool, FIMMWAVE6, based on the 3-D eigenmode expansion method, was used to model these devices (and other devices described hereinafter). The modelled device had a width of Μ/ _ΜΜΊ = 1.5 m and a length of ZMMI = 98.3 μητι, which corresponds to the 21st order self-image distance at an input wavelength of λ = 1.55 m. As the multimode waveguide 14 has a small width, it supports only very few (3-4) guided modes and the first-order (p = 1) self-image distance is very short. The fidelity of self- imaging varies with the self-image order. Indeed, such a change in fidelity corresponds to a beating of the modulation depth in the curve of transmittance versus Z! _MMI . Here, for a fixed wavelength, self-imaging with the best fidelity appears at a period of L _p along the MMI length. In embodiments of the present invention, therefore, MMIs with lengths of . _MMI = rnL _p (where m is a positive integer) are chosen in order to achieve an improved performance (as measured from the curve of transmittance versus - _MMI )-

Figure 3 is a graph showing the simulated transmission spectra where N = 1 (i.e. filter 10, Figure la), N = 2 (filter 100, Figure 2a) and N = 3 (filter 100', Figure 2b). The length of each multimode waveguide was / _MM I = 98.3 pm = Lp. As the order number N increases (i.e. the number of multimode waveguides), the extinction ratio increases efficiently (6-7 dB per order) and the spectral bandwidth is reduced, whilst the free spectral range is not affected.

Each of the filters described above has multimode waveguides with the same physical characteristics, i.e. the same material, same height, same width and same length. However, multimode waveguides with different physical characteristics may be combined intelligently to create an optical filter with a desired overall transmission characteristic. In the examples hereinafter, only one of these characteristics (the length) is varied for simplicity; however, one or more of many alternative characteristics may be varied in order to change the effective index of one multimode waveguide as compared to another. For example, the effective index of the modes of a particular multimode waveguide depends on the length, width, height, etch depth and material (i.e. refractive index) of the multimode waveguide. Therefore any one or more of those characteristics may be changed in order to alter the effective index. Each multimode waveguide or group of multimode waveguides having the same physical characteristics is referred to as a "stage" of the filter. For example, Figures 4a, 4b and 4c show further optical filters according to embodiments of the present invention in which multimode waveguides with different lengths have been combined.

Figure 4a shows an optical filter 200 comprising a single-mode input waveguide 202, connected to a first multimode waveguide 212, having a length Lp. A further single-mode waveguide couples the first multimode waveguide 212 to a second multimode waveguide 214 having a length 2L _P , and this second multimode waveguide is coupled to a single-mode output 204.

Figure 4b shows a similar optical filter 200' where the second multimode waveguide 216 has a length 3L _P ; and Figure 4c shows a further optical filter 200" which combines three multimode waveguides 212, 214, 216, having lengths L _P , 2L _Pl and 3L _P , respectively.

The simulated transmission spectra for these devices, together with the conventional filter 10 shown in Figure la, are shown in the graph of Figure 5 as follows: filter 10 (dash- dot line); filter 200 (dash line); filter 200' (dot line); and filter 200" (solid line).

Such a multiple-stage structure gives substantial reduction of spectral bandwidth, while keeping the free spectral range the same as a single lx l MI. The increase of extinction ratio is also shown, although it is not as efficient as the multiple-order structure and some side peaks are shown. The three-stage structure (Fig. 4c) has the optimum spectral response (solid curve in Fig. 5) with a -3 dB spectral bandwidth of 10 nm, an extinction ratio of 10 dB, a side peak suppression ratio of 7 dB, and a free spectral range of 60 nm. By combining multimode waveguides with different lengths, where those lengths are both integer multiples of a common divisor, it can be seen that fewer wavelengths are preferentially transmitted than if all the multimode waveguides have the same length. The filter becomes more selective and has an improved performance.

This performance can be improved still further by combining the two principles described above into a multi-stage, multi-order filter. Such a filter 300 is shown in Figure 6a.

The optical filter 300 comprises a single-mode input waveguide 302, connected to a first stage of three multimode waveguides 312, each having a length L _P , connected in series via respective single-mode waveguides. These are in turn connected to a second stage of three multimode waveguides 314, each having a length 2L _Pf connected in series via respective single-mode waveguides; and then to a third stage of three multimode waveguides 316, each having a length 3L _P This final stage of multimode waveguides is coupled to a single-mode output 304.

This device achieves excellent filter characteristics, as can be seen in Figure 7. This graph shows the progression from a three-stage, first-order filter as shown in Figure 4c (solid curve), to a three-stage, second-order filter (dashed curve), to the three-stage, third- order filter 300 shown in Figure 6a (dotted curve). Clearly, the latter filter achieves the best separation of wavelengths.

In all the embodiments described above the filters have been arranged linearly, that is to say, in a straight line. However, they may also be folded back on themselves to conveniently reduce the overall length of the device. Such a "folded" design of filter is shown in Figure 6b, where a three-stage, third-order filter 400 has been folded back on itself several times by virtue of several curved portions 420. The turning radius of the curved portion r can be chosen to reduce optical losses as a result of the curve. Clearly, a straight line achieves the minimum optical loss; however, simulation has shown that a turning radius rof 5 pm results in acceptable losses while allowing a tight curve to be used.

The devices described above have employed a silicon-on-insulator construction; that is to say, a layer of silicon on an insulator (typically Si0 ₂ , although others can be used), and a thicker portion of the silicon layer used as a waveguide. However, those skilled in the art will appreciate that alternative constructions and materials may be employed without departing from the scope of the invention as defined in the claims. For example, germanium may be employed instead of silicon. The waveguide may comprise a planar waveguide with substantially any light-guiding structure, including a strip waveguide.

The multimode interferometers disclosed herein have a multimode nature which provides wavelength dependent self-imaging of the fundamental mode input field. This means the multimode interferometers support an appropriate spectrum of modes— including relevant higher order modes. The wavelength dependence of the phase coefficients of the modes in the mode spectrum excited in the particular multimode interferometer leads to the wavelength dependence of the axial distance along the multimode interferometer at which self-imaging occurs. In embodiments of the present invention, therefore, the length and width of each multimode interferometer in the optical filter are dictated according to equation (1), where p may take different values for each multimode interferometer in the filter. The value of the design wavelength λ (i.e. the wavelength intended to be passed by each multimode interferometer) for each multimode interferometer in these embodiments may be the same, to the extent that a consistent filtering operation is achieved by the combination of multimode interferometers. For example, depending on the application, the value of design wavelength λ may be the same for each multimode interferometer to within 5 nm. In some applications, the value of design wavelength λ may be the same for each multimode interferometer to within 3 nm. The single-mode waveguides disclosed herein are arranged so that only the single, or fundamental mode propagates therein. The present invention thus provides an optical filter, for use in a range of devices, in which two or more multimode waveguides are combined into a single filter. The filter can be designed to achieve many different transmission characteristics by adapting the physical characteristics and/or number of multimode waveguides.

It will of course be understood that many variations may be made to the above- described embodiment without departing from the scope of the present invention.