GB2411486A

GB2411486A - Array waveguide grating with multiple input channels

Info

Publication number: GB2411486A
Application number: GB0404337A
Authority: GB
Inventors: Paola Galli
Original assignee: Agilent Technologies Inc
Current assignee: Agilent Technologies Inc
Priority date: 2004-02-27
Filing date: 2004-02-27
Publication date: 2005-08-31
Also published as: GB0404337D0

Abstract

An AWG module comprises a substrate, the substrate having disposed thereon at least a pair of input channels 1 optically coupled to a first free space couple 2, 23, said first free space coupler being optically coupled to an input facet of an array waveguide grating 3, and a plurality of output channels 5 optically coupled to a second free space coupler 4, said second free space coupler being optically coupled to an output facet of said array waveguide grating 3, said at least one pair of input channels being spaced apart optical waveguides 21,22, and said plurality of output channels being spaced apart output optical waveguides.

Description

241 1 486 Array Waveguide Grating with Improved Frequency Range The

present invention relates to a method for increasing the frequency range of an array waveguide grating (AWG). Furthermore, the present invention relates to an AWG module having a greater tunable frequency range than known AWG modules.

Currently dense wavelength division multiplexed (DWDM) systems are tuned to standard fibre optic transmission frequencies, known as the ITU grid frequencies by changing the temperature of the AWG module. Typically the output channel central frequency of a known AWG module is tuned to a lower frequency at a rate of approximately 1.5 GHz / C. Given the environmental limitations placed by the end user on the allowable temperature variation of an AWG module; typically 70-85 C, known AWG modules are only capable of approximately 22.5 GHz of tuning range.

Furthermore, unavoidable manufacturing tolerances also affect the resulting operating range of an AWG module. For example, an AWG module theoretically designed to have a specific central output channel frequency of 193.3 THz and working temperature range of 72 C may fall outside those specifications due to manufacturing tolerances with the material's refractive index and the substrates thickness. This may result in the manufactured AWG module hitting the theoretical centre ITU frequency at a different temperature from the designed one, which may or may not be compensated for by changing the operating temperature of the module. This can ultimately lead to a drop in manufacturing yield.

Furthermore, many end users are increasingly asking for AWG modules with tuneable ranges greater than is currently possible with known temperature tuning methods.

Thus, it is an object of the present invention to address and overcome the above- mentioned technical problems and challenges. This objective is achieved by the method and module as set forth in the subsequent claims.

Advantageously, the method and module of the present invention offers the customer an AWG module that has an increased frequency tuning range.

Furthermore, the present invention improves the manufacturing yield of AWG modules by either doubling the frequency tuning range for a given working temperature range, or by maintaining the frequency tuning range but halving the While the principle advantages and features of the invention have been described above, a greater understanding and appreciation of the invention may be obtained by referring to the drawings and detailed description of the invention, presented by way of example only, in which; Figure 1 is a plan view of the AWG module; Figure 2 gives a more detailed view of the interface between the input waveguide and the first FSC, Figure 3 is a more detailed view of the interface between the second FSC and the output waveguides; and Figure 4 shows a graph of the frequency range possible with an AWG module according to the present invention.

The operation of AWG's and the free space coupler (FSC), or Star Coupler as it is know in the art, are both well known an as such will not be discussed in detail here. As seen in figure 1, an AWG module general comprises an input section consisting of an input waveguide 1. The input waveguide receives electromagnetic radiation (kin), typical in the infrared region of the spectrum, from an input optical fibre (not shown). One optical fibre is coupled to each input waveguide usually via the well know "pigtail" method.

The input waveguide 1 is coupled to a first FSC 2. The individual input channel waveguides increase in diameter as they approach the first FSC in order to ensure that multimode radiation enters the first FSC. The input waveguide may also have a parabolic taper as it approaches the interface with the first FSC. The use of parabolic tapers is well known and function to shape the bandwidth of the filter response in order to produce either a flat-topped or Gaussian shaped frequency response.

The first FSC 2 is coupled to the array waveguide grating 3, which is then coupled to the second FSC 4. The second FSC is coupled to an array of output waveguides 5. The array, as seen in figure 3, may consist of several channel waveguides 51-61. The output waveguides are finally coupled to output fibres (not shown), from which electromagnetic radiation (Xout) leaves the module. The output waveguides 51-61 may also have parabolic tapers 65 at the interface with the second FSC. As will be appreciated, the number of output channels may vary according to the specific module requirements, for example, 8 channels with 200 GHz spacing, or 32 channels with 100 GHz spacing.

However, as seen in figure 2, the present invention differs from prior art AWG modules in that two or more input waveguides 21, 22 are coupled to the first FSC 23. Two input optical fibres (not shown) are pigtailed to the input waveguides giving the end user a choice of input channels in which to transmit kin. The use of a more than one input waveguide channels enables the user of the module to select the input channel with yields to the desired frequency range at the output of the module for a defined working temperature, typically 72 C.

However, as will be appreciated, the invention is not limited to the use of only two input channels. The present invention envisages the use of a multiple of input channels (M) coupled to the first FSC, despite the module being designed to function as a I x N (with N being the number of output channels) AWG module.

The spacing d between the input waveguide channels determines the central frequency of the light existing each output waveguide channel and can be chosen according to the operating specification of the module. For example in a 1 OOGHz spacing device (i.e. the frequency of each output channel differs by lOOGHz from the adjacent channel) with M=2 and with an operating module temperature of 72 C, light entering input channel 21 will exit the central output channel 56 with a central frequency denoted by f.

However, light entering adjacent input channel 22 will have an output frequency at output channel 56 of f + Af, where Af is determined by the following relationship: An = (Us / Nc) x.

(fs x AL) where: = central frequency of f fs = length of FSC d = distance of input waveguide at FSC 1 da = distance of array waveguide at FSC I AL = path length difference between adjacent array waveguides us = effective index of FSC Nc = group index of array waveguide Thus with an AWG module according to the present invention, the range of frequency the module can be tuned over is increased compared to that achievable with conventional temperature tuning methods. For example, to achieve +/- 40GHz tuning range, a +/-26.6 C temperature range would be required. This is not possible due to packaging requirements of the module. However, by choosing the correct number of input channels and spacing between them, a +/- 40GHz frequency range AWG module is achievable with the present invention. s

Figure 4 shows the frequency range possible with a known temperature tuneable AWG module. Given the limitations placed on the useable operating temperature range of 65-85 C, which is often detennined by the module package, a maximum frequency range of +/- 22.5 GHz is all that is possible around the ITU grid.

Howcvcr, a maximum frequency range of +t- 40GHz is possible in an AWG module according to the present invention with two input channels spaced at 25 microns and with 9 output channels, where the value of d depends strongly on the channel spacing.

For example, if input channel IN1 is selected with output channel OUT2, the central output frequency is approximately 192.48 THz, whereas if input channel IN2 is chosen with output channel OUTI, the output frequency is 192.52 THz.

Thus each output channel has +/- 40 GHz tunability and the overall frequency comb can be moved up or down by +/- 4()GHz around the central frequency.

It is not intended that the present invention be limited to the above embodiments and other modifications and variations are envisaged within the scope of the claims.

Claims

Claims 1. A chip comprising a substrate, the substrate having disposed

thereon a pair of input channels optically coupled to a first free space coupler, said first free space coupler being optically coupled to an input facet of an array waveguide grating, and a plurality of output channels optically coupled to a second tree space coupler, said second free space coupler being optically coupled to an output facet of said array waveguide grating, said pair of input channels being spaced apart optical waveguides, and said plurality of output channels being spaced apart output optical waveguides.
2. A chip as claimed in Claim 1, wherein said pair of input optical waveguides is spaced apart by a predetermined distance.
3. A chip as claimed in Claim 2, wherein said predetermined distance is approximately 25 microns.
4. A chip as claimed in any preceding Claim, wherein said output channel comprises 8 output optical waveguides.
5. A chip as claimed in any preceding Claim, wherein said pair of input channels is further optically coupled to an input pigtail optical timbre.
6. A chip as claimed in any preceding Claim, wherein said plurality of output channels is further optically coupled to an output pigtail optical fbre.
7. A module comprising a chip as claimed in any preceding Claim.
8. A module as claimed in Claim 7, wherein said module further comprises temperature control means.
9. A module as claimed in Claim 8, wherein said temperature control means is a thermal-electric cooler.
10. A module as claimed in Claims 7-8, wherein one of said pair of input channels

is user selectable.