Method and apparatus for reducing temperature-related spectrum shifts in optical devices

<u>Technical Field</u>

0001This invention relates to a method and apparatus for passively reducing the temperature-related spectrum shifts in optical devices, such as waveguide grating routers (WGR) and wavelength division multiplexers (WDM).

<u>Background of the Invention</u>

0002Generally speaking, optical wavelength multiplexing and demultiplexing have been accomplished in the past by using an interconnection apparatus having a plurality of closely spaced input waveguides communicating with the input of a star coupler. The output of the star coupler communicates with an optical grating comprising a series of optical waveguides, each of the waveguides differing in length with respect to its nearest neighbor by a predetermined amount. The grating is connected to the input of a second star coupler, the outputs of which form the outputs of the switching, multiplexing, and demultiplexing apparatus. Examples of such interconnection apparatuses are disclosed in U.S. Patents 5,002,350, 5,136,671 and 5,412,744.

0003The geometry of such an interconnection apparatus may be such that a plurality of separate and distinct wavelengths each launched into a separate and distinct input port of the apparatus will all combine and appear on a predetermined one of the output ports. In this manner, the apparatus performs a multiplexing function. The same apparatus may also perform a demultiplexing function. In this situation, an input wavelength is separated from the others and directed to a predetermined one of the output ports of the apparatus. An appropriate selection of input wavelength also permits switching between any selected input port to any selected output port. Accordingly, these devices are generally referred to as frequency routing devices and more specifically wavelength division multiplexers (WDM).

0004Ideally, the operation of these WGR and WDM optical devices should by predictable and consistent over a wide range of environmental conditions. Unfortunately however, in practice, the operational performance of such devices is significantly affected by variations in the temperature of the environment surrounding the device.

0005More specifically, the wavelength spectrum of existing WGR designs shifts with temperature (T) for at least two reasons. First, where n represents the refractive index of the waveguide material, dn/dT ≠ 0 and secondly, the thermal expansion, i.e. dL/dT, where L represents length, likewise does not equal zero.

0006To date, some of the techniques used to create optical devices that are less sensitive to temperature changes have included using a heater with a temperature controller to stabilize the wavelength spectrum of the WGR. Unfortunately, such a design is expensive and impractical in applications where electrical power is not readily available. In addition, the semiconductor art has demonstrated a temperature insensitive semiconductor WGR that includes a waveguide region with different <i>dn</i>/<i>dT</i>.

0007However, even in light of the technical advances mentioned above, there remains a definite need for a practical design and method for making optical waveguide filtering devices temperature independent. This is particularly true for compensating waveguide grating routers, which are, at present, the multiplexers of choice for dense WDM systems. Furthermore, given that silicon optical bench routers are now components of various Next Generation Lightwave Networks (NGLN) and are planned for use in Fiber-to-the-Home (FTTH) access networks, temperature-compensating optical devices like WGRs are highly desirable for these networks.

0008An athermal silica-based arrayed-waveguide grating (AWG) multiplexer is disclosed by Y. Inoue and others in ECOC 97, 22-25 September 1997, Conference Publication No. 448, pages 33-36. The temperature dependent optical path difference in the silica-based arrayed waveguides is compensated with a triangular groove filled with silicone adhesive which has negative thermal coefficient. The groove can be formed by reactive ion etching, which is the same process used for patterning the core.

<u>Summary of the Invention</u>

0009A device and method of manufacturing according to the invention are as set out in claims 1 and 6. Preferred forms are set out in the dependent claims.

<u>Brief Description of the Drawings</u>

0010Other features of the present invention will be more readily understood from the following detailed description of specific embodiments thereof when read in conjunction with the accompanying drawings, in which: <ul id="ul0001" list-style="none" compact="compact"><li>FIG. 1a discloses a cross-section view of a basic optical device having multiple waveguides;</li><li>FIG. 1b discloses a cross-section view of the length of a waveguide incorporating the aspects of the present invention;</li><li>FIG. 2 is a graph showing a portion of the transmission coefficients for an optical device of Fig. 1 at two different temperatures, T₁ and T₂;</li><li>FIG. 3 illustrates an example of a conventional optical frequency routing device;</li><li>FIG. 4 is a graph showing the transmission coefficients T_ik for the routing device of Fig. 1; and</li><li>FIGS. 5 depicts a design of optical frequency routing devices which employs the novel attributes of the present invention.</li></ul>

<u>Detailed Description</u>

0011The present invention teaches a novel technique for reducing the temperature-related spectrum shifts in optical devices, particularly waveguide grating routers (WGR). In general, the present invention modifies a portion of the length of at least one waveguide within an optical device in a manner that stabilizes the wavelength spectrum passing therethough even when exposed to temperature variations. However, before specifically addressing the novel aspects of the present invention, a brief discussion is provided to set forth the basic structure of a common type of optical device which may be improved by incorporating the aspects of the present invention.

0012The most advanced and technically developed planar waveguides are doped-silica waveguides fabricated with silicon optical bench (SiOB) technology. A doped-silica waveguide is usually preferred because it has a number of attractive properties including low cost, low loss, low birefringence, stability, and compatibility for coupling to fiber. Further, the processing steps are compatible with those in silicon integrated circuit (IC) technology, which are geared for mass production and are readily known.

0013Generally, a doped-silica waveguide is formed by initially depositing a base or lower cladding layer of low index silica on a carrier substrate, which typically comprises silicon or silica. A layer of doped silica with a high refractive index, <i>i.e.,</i> the core layer, is then deposited on top of the lower cladding layer. The core layer is subsequently patterned or sculpted into structures required by the optical circuits using photo-lithographic techniques similar to those used in integrated circuit fabrication. Lastly, a top cladding layer is deposited to cover the patterned waveguide core. This technology is generally described in U.S. Patent No. 4,902,086 issued to C. H. Henry et al., and in an article entitled <i>"Glass Waveguides on Silicon for Hybrid Optical Packaging"</i> at pp. 1530-1539 of the <u>Journal of Lightwave Technology,</u> Vol. 7, No. 10, October 1989 - both of which are hereby incorporated by reference.

0014Critical to the performance of any planar optical waveguide are the waveguide dimensions, <i>i.e</i>., the height and width of the waveguide core, and the refractional difference of the refractive index between the core and the cladding of the waveguide, generally denoted as Δ. The height or thickness of the core is determined by the amount of core material deposited on a carrier substrate; and the width of the core is determined by the photo-lithographic mask and undercut in chemical etching. The Δ of the waveguide is mostly determined by the material system and the fabrication process. In practice, different waveguide structures and systems are used for different types of functions and tradeoffs are made in the core dimensions and Δ to optimize different aspects of optical performance.

0015Illustratively, P-doped waveguides are used in the present invention, each having a core whose thickness is about 7µm, and each resting on a 15µm lower cladding layer. A 15µm upper cladding layer covers the waveguide cores. The dimensions of the waveguide cores are chosen to be as large as possible for strong optical confinement and low propagation loss, but small enough so that the waveguides remain singlemode. Similarly, the silica paths that comprise the transition region have cores that are about 7µm high; but their widths vary from about 18µm (near the slab) to about 2µm (distant from the slab).

0016Referring now to FIG. 1, a cross section of an optical device is shown in order to illustrate its dimensions and materials in the region where waveguides are present. Substrate 10 comprises silicon having a thickness of 500µm. Cladding 12 comprises a 15µm layer of silica having an index of refraction of about 1.445 at a wavelength (λ) of 1.55µm. Waveguide cores 14 are generally square, they have a thickness and width of about 7µm, and they comprise doped-silica having an index of refraction of about 1.454 at λ = 1.55µm. The waveguide cores 14 are separated by a distance of about 2.5µm. The fact that the core material has a higher index of refraction than the cladding material enables the core to guide lightwaves in accordance with Snell's law. Cladding layer 16, which is substantially the same as cladding layer 12 in refractive index, is deposited on top of cores 14 to complete the structure.

0017The present invention sets forth and claims a novel design change in the structure of optical devices which causes these devices to be significantly less temperature sensitive than existing designs. More specifically, the design changes described herein obviate the inherent shifts in the wavelength spectrum of the optical devices which occur with temperature variations. In other words, the present invention is generally directed toward an optical waveguide which has the attribute of being temperature-independent in its ability to transmit light energy.

0018As stated earlier, the wavelength spectrum of optical devices, particularly WGRs and WDMs, shifts with temperature variations (T) for at least two reasons. First, where n represents the refractive index of the waveguide material, dn/dT ≠ 0 and secondly, the thermal expansion, i.e. dUdT, where L represents length, likewise does not equal zero. In other words, the rate at which the refractive index changes with temperature variations is a given characteristic of that material. For example, silica which is commonly used in the manufacture of waveguides within optical devices has a dn/dT value equal to about 1.1 x 10^-5 I/°C.

0019Figure 2 generally illustrates the effect that temperature variations can have on the transmission coefficients of an optical signal. As shown, when the temperature increases from T₁ to T₂ , the transmission coefficient moves by some amount of wavelength λ to the right. Stated differently, an increase in temperature causes a shift of the wavelength spectrum upward to a higher wavelength.

0020Based on the above-stated facts, the present inventors realized that it is the characteristics of the particular material used to create the waveguides, namely silica, that controls or dictates the devices response to temperature variations. More importantly, the present inventors determined that other materials exist that can provide optical performances similar to silica while exhibiting more desirable reactions to temperature variations. To specifically address the problem of temperature-related spectrum shifts in optical devices, the inventors recognized that selected portions of one or more waveguides could be made of a different material than the remaining portion of that waveguide so as to controllably compensate for characteristics of that waveguide which are commonly temperature sensitive.

0021In particular, elastomeric materials, or elastomers, were investigated since they have low optical loss at communications wavelengths and refractive indices quite close to that of silica. However, beyond these factors, another interesting property of elastomers is their generally rapid change of refractive index with temperature and how such relates to the characteristics of silica. One specific elastomer, namely RFX-36HN manufactured by General Electric or Dow Corning, has a refractive index that changes as temperature increases by an amount that is opposite in sign and about 30 times greater than that of silica. Since the refractive index of the elastomer reacts to temperature variations in a direction opposite to that of the typical waveguide material silica, more elastomer should be used in the longer waveguides of the grating in order to ensure a consistent reaction or compensation for temperature variations for each of the waveguides within the optical device.

0022In accordance with the above principles, one embodiment of the present invention describes an optical device, such as a WGR, which instead of having the entire length of each waveguide core region made of one constant material like silica, purposely manufactures some portion of at least one waveguide of a different material. More specifically, the different material is selected so as to have a refractive index which changes with temperature variations at a different rate than the given rate of change for the material of the remaining portion of that grating waveguide. Furthermore, when a material is selected whose refractive index which reacts to temperature variations in an opposite direction than that of the material of the remaining waveguide core, one can use various amounts of the two different materials to effectively compensate for any temperature variations.

0023FIG. 3 shows the pertinent details of a conventional frequency routing device. The frequency routing device contains a plurality of input waveguides 2_<i>i</i>, i = 1, 2, ... , N connected to a free space region 100. A plurality of output waveguides 110 extends from the free space region 100 and is connected to an optical grating 120. The optical grating 120 comprises a plurality of unequal length waveguides which provides a predetermined amount of path length difference to a corresponding plurality of input waveguides 130 connected to another free space region 140. The free space region 140 is connected to a plurality of output waveguides 4_<i>k</i>, k = 1, 2, ... , N.

0024In practice, these frequency routing devices may operate as multiplexers and/or demultiplexers of optical frequencies. For example, if a signal of amplitude A is applied to input waveguide 2₁ then signals of amplitudes AT₁₁, AT₁₂, ... AT₁_<i>N</i> are produced at the output waveguides where T_<i>ik</i> is the value of the transmission coefficient for input waveguide 2_<i>i</i> and output waveguide 4_<i>k</i>. Additional details concerning these routing devices are found in the above-referenced patents which are expressly incorporated by reference herein.

0025The typical behavior of the routing device shown in FIG. 3 is illustrated in FIG. 4. This figure shows the set of transmission coefficients T₁_<i>k</i> for the routing device for the particular input waveguide 2₁ as a function of wavelength λ. The variation of each transmission coefficient as a function of wavelength is essentially periodic with a period X_<i>o</i>. For a symmetric arrangement in which the input and output waveguides are uniformly positioned within the device's field of view (i.e., the Brillouin Zone), X_<i>o</i> is given by<maths id="math0001" num="(1)"><math display="block"><mrow><msub><mrow><mtext>X</mtext></mrow><mrow><mtext mathvariant="italic">o</mtext></mrow></msub><mtext mathvariant="italic"> = N·S</mtext></mrow></math><img file="EP0907091B1_D0001.tif" /></maths> where N is the total number of input (or output) waveguides and S is the channel spacing, which is defined as the wavelength separation between maximum peaks of adjacent transmission coefficients (see FIG. 4). The wavelength λ_o is a wavelength that corresponds to a maximum value for one of the transmission coefficients T_<i>ik</i>. In FIG. 3, λ_o corresponds to a maximum of the coefficient T₁₁. All other wavelength of maximum transmission λ_<i>ik</i> for the various coefficients T_<i>ik</i> essentially differ from λ_<i>o</i> by multiples of S and therefore are given by the expression<maths id="math0002" num="(2)"><math display="block"><mrow><msub><mrow><mtext>λ</mtext></mrow><mrow><mtext mathvariant="italic">ik</mtext></mrow></msub><msub><mrow><mtext> = λ</mtext></mrow><mrow><mtext>o</mtext></mrow></msub><mtext> + (</mtext><mtext mathvariant="italic">i-k</mtext><mtext> + </mtext><mtext mathvariant="italic">q·N</mtext><mtext>)</mtext><mtext mathvariant="italic">S</mtext></mrow></math><img file="EP0907091B1_D0002.tif" /></maths> where the integers i,k = 1, ... , N specify the locations of the input and output waveguides, q is an integer, and λ_o is a wavelength of maximum transmission for the waveguides I = k = 1. The wavelength λ_o will be referred to as the multiplexer center wavelength. The parameter X is defined as follows:<maths id="math0003" num="(3)"><math display="block"><mrow><mtext mathvariant="italic">X</mtext><msub><mrow><mtext> = λ - [λ</mtext></mrow><mrow><mtext>o</mtext></mrow></msub><mtext> + (</mtext><mtext mathvariant="italic">i-k</mtext><mtext> + </mtext><mtext mathvariant="italic">q·N</mtext><mtext>)</mtext><mtext mathvariant="italic">S</mtext><msub><mrow><mtext>/] = λ - λ</mtext></mrow><mrow><mtext mathvariant="italic">ik</mtext></mrow></msub></mrow></math><img file="EP0907091B1_D0003.tif" /></maths> The transmission coefficients have essentially identical behavior and, therefore, they are described by the same function T(X). Thus, all the transmission coefficients corresponding to a particular input waveguide yield an array of equally spaced transmission peaks such as shown in FIG. 4. The spacing of adjacent peaks is equal to the channel spacing S.

0026Figure 5 sets forth a design which illustrates how a temperature-independent optical device may be made in accordance with the present invention.

0027As one example of how the elastomer material may be incorporated into the selected waveguide(s), Figure 5 shows a varying number of relatively small blocks of elastomer material being placed in the selected waveguide path. More specifically, as illustrated, the longest waveguide shown (top) may contain seven such elastomeric blocks, the second longest shown (second from top) may contain six such elastomeric blocks, the third longest shown (third from top) may contain five such elastomeric blocks, and so on until the shortest full waveguide shown (seventh from top) may contain one such elastomeric block.

0028However, it is important to note that the exact number and/or size of elastomeric blocks used in any particular waveguide is a matter of design choice and that Figure 5 is not intended to be an accurate scale drawings. Most importantly, while any numerical sequence of elastomeric blocks, i.e. 1, 2, 3, 4, ... or 2, 4, 6, 8, ... or 3, 6, 9, ..., may be used in accordance with the present invention, the objective is to maintain a linear relationship between the total length of the combined elastomeric blocks and the overall length of the associated waveguide. Stated differently, the percent increase in length from one waveguide to an adjacent waveguide should be accompanied by a similar increase in the amount of elastomeric material used in each of those two waveguides.

0029By designing the device in this manner, the selected number of elastomeric blocks may be controllable positioned into the desired waveguide path. More specifically, one particular method of manufacturing this design would involve each of the steps set forth earlier followed by etching away the cladding 16 and core material 14 from the device down to the substrate layer 12.

0030In order to more clearly illustrate this design, Figure 1 b) shows a cross-sectional view along the length of a waveguide. As shown, a portion of both cladding 16 as well as core material 14 has been replaced with elastomeric material 20. Furthermore, an additional step may be included whereby the portion of the elastomeric material 20 adjacent the upper cladding 16 is etched away and replaced with more cladding material.

0031When implementing the elastomeric block design set forth above, typical sizes for some of the blocks used in conventional optical devices may be about 3 µm. Based on an example structure where there is about 100 grating waveguides (instead of the seven fully illustrated), there would be a total of about 300 µm of elastomeric material within the overall length of the longest waveguide instead of the base waveguide material such as silica. Likewise, according to this example, one of the shorter waveguides would include about 3 µm of elastomeric material along its length. However, even though particular numbers may be given herein, it should be fully understood that depending on the exact design and type of optical device being modified, as well as the expected environment into which the device will be operating, the specific values used can vary significantly but none the less be considered within the scope of the present invention.

0032To actually calculate the amount of wavelength shift achieved, or compensated for, it should first be remembered that the refractive index of the particular elastomer described earlier shifts opposite the direction of that of silica when exposed to temperature variations and with about a 30 times greater magnitude. Furthermore, the total length of the elastomeric material section(s) is given by<maths id="math0004" num=""><img file="EP0907091B1_D0004.tif" /></maths> where L is the increment of the adjacent grading waveguide length, I is the index count of the grating waveguides, I=1 for the shortest waveguide, I=2 for the next waveguide, etc.

0033In closing, the present invention addresses the need to provide a passive optical device whose operation is essentially independent of any temperature variations to which it may be exposed. By knowing how the refractive index of a certain material changes with temperature variations as compared to the way the refractive index of common waveguide materials, such a silica, change with temperature variations, one may employ the teachings of the present invention to precisely modify the nature of the optical path through which a signal travels to fully compensate for any wavelength spectrum shift that may otherwise occur. In other words, provide an optical device with a plurality of waveguides each of which is appropriately modified so that any optical signal passing therethrough has the same wavelength at any two given temperatures.

0034It is to be understood that the above-described arrangements are simply illustrative of the invention. Other arrangements may be devised by those skilled in the art which will embody the invention as claimed. In particular, Mach-Zehnder interferometers, Bragg filters and/or optical Fourier filters may also benefit from the aspects of the present invention.