Optical waveguide and method of manufacturing the same

An optical waveguide which can suppress adjacent crosstalk even when wavelength intervals to be multiplexed/demultiplexed are narrow. A lower clad film and a core film are deposited and formed on a substrate ( 11 ) by flame hydrolysis deposition, and they are consolidated, whereupon the core film is processed into a waveguide pattern. The waveguide pattern is formed by successively connecting at least one optical input waveguide ( 12 ), a first slab waveguide ( 13 ), an arrayed waveguide ( 14 ) consisting of a plurality of channel waveguides ( 14 a ) arranged side by side and having lengths different from one another, a second slab waveguide ( 15 ), and a plurality of light output waveguides ( 16 ) arranged side by side. The waveguides arranged side by side are at intervals from one another. An upper clad film covering the waveguide pattern is deposited and formed by flame hydrolysis deposition, and it is thereafter consolidated. Herein, a sintering rate in a temperature rise from a temperature at which the density change of the glass particles of the upper clad film starts, to a temperature at which the density change ends, is set at 1.0° C./min or below at the step of consolidating the upper clad film, whereby the arrayal aspect of the channel waveguides ( 14 a ) is brought close to an ideal aspect.

DETAILED DESCRIPTION An arrayed waveguide grating (AWG) is produced by a manufacturing method which employs, for example, flame hydrolysis deposition (FHD) explained below. First, as shown in FIG. 10 A, one or more substrates 11 are arranged as an array at circumferential positions about the center C of rotation on a turntable 5 which is rotated at a constant angular velocity. Subsequently, the turntable 5 is rotated in a direction B by way of example, and a burner 6 is reciprocated in the radial direction of the turntable 5 as indicated by an arrow A so as to be reciprocated on each of the substrates 11 . While the burner 6 is thus being moved, the raw material gas of a glass, oxygen gas and hydrogen gas are caused to flow from the burner 6 as indicated by an arrow D, so as to cause the hydrolysis reaction of the raw material gas in oxyhydrogen flame and to deposit lower clad glass particles on the substrate 11 . A mixed raw material halogen gas which consists of SiCl 4 , BCl 3 and PCl 3 , is applied as the raw material gas of the glass for the clad. Besides, the hydrolysis reaction of the raw material gas is caused in the oxyhydrogen flame, and the glass particles of a lower clad (the lower clad glass particles) are deposited on the substrate 11 and formed into a lower clad film. Thereafter, a mixed raw material halogen gas which consists of SiCl 4 , BCl 3 , PCl 3 and GeCl 4 and which is the raw material gas of a glass for cores is caused to flow from the burner 6 together with oxygen gas and hydrogen gas. Besides, the hydrolysis reaction of the raw material gas is caused in oxyhydrogen flames, and the glass particles of the cores (core glass particles) are deposited and formed into a core film. FIG. 10B shows a state where the film of the lower clad and the film of the cores have been formed on the substrate 11 in the above way. A step shown in FIG. 10C is the step of consolidating the lower clad film and the core film. The lower clad glass particles and the core glass particles deposited and formed as explained above are heat-treated at a high temperature of at least 1300 ° C., thereby to consolidate the lower clad 1 and the cores 2 . Subsequently, as shown in FIG. 10 D, the optical waveguide pattern of the arrayed waveguide grating, that is, the waveguide construction of the cores 2 is formed using photolithography and reactive ion etching. The waveguide construction is the foregoing construction shown in FIG. 9 . The step S 1 of forming the waveguide construction of the cores, from FIG. 10 A through FIG. 10 D, is followed by the step S 2 of forming the film of an upper clad 3 in an aspect where the upper clad film covers the waveguide construction of the cores 2 as shown in FIG. 10E . Incidentally, the upper clad film is formed in such a way that, as in the formation of the lower clad 1 , the hydrolysis reaction of the raw material of the clad glass is caused in oxyhydrogen flame so as to deposit and form the glass particles of the upper clad 3 (upper clad glass particles). Thereafter, the step S 3 of consolidating the upper clad film at a high temperature of, for example, 1200° C. is performed, whereby an optical waveguide is manufactured. Heretofore, in manufacturing an arrayed waveguide grating, a method as explained below has been applied at the step S 3 of consolidating an upper clad film. Letting T 1 denote a temperature at which the density change of the glass particles of the upper clad film starts, and T 2 denote a temperature at which the density change ends, a sintering rate in a temperature rise from the point T 1 to the point T 2 is set at about 2.5° C./min so as to cosolidate the upper clad film and to manufacture the arrayed waveguide grating. Meanwhile, in recent years, it has been required in optical wavelength division multiplexing transmission to increase the number of wavelengths to-be-multiplexed and to narrow wavelength intervals. It has consequently been required to narrow the wavelength intervals of lights which are multiplexed/demultiplexed by an arrayed waveguide grating. Concretely, an arrayed waveguide grating of 40 ch-50 GHz in the band of 1.55 &mgr;m (which has the function of multiplexing/demultiplexing lights of 40 wavelengths different from one another at intervals of 50 GHz) has been demanded. However, when the inventor manufactured the arrayed waveguide grating of 40 ch-50 GHz by employing the prior-art manufacturing method, there has been revealed the problem that the value of adjacent crosstalk degrades. This problem will be concretely explained below. FIGS. 11 and 12 show examples of optical spectrum in the vicinities of the light transmission center wavelengths of arrayed waveguide gratings produced by the prior-art manufacturing method. The optical spectrum shown in FIG. 11 is the optical spectrum example of the arrayed waveguide grating of 40 ch-50 GHz, while the optical spectrum shown in FIG. 12 is the optical spectrum example of the arrayed waveguide grating of 40 ch-100 GHz. Each of the optical spectrum is indicated by the transmittance of the arrayed waveguide grating normalized by the minimum loss. As seen from the figures, with the example of the arrayed waveguide grating of 40 ch-50 GHz (refer to FIG. 11 ), the value of the worst adjacent crosstalk within a range of ±(0.4±0.05) nm with respect to the light transmission center wavelength is estimated to be on the order of −23 dB. On the other hand, with the example of the arrayed waveguide grating of 40 ch-100 GHz (refer to FIG. 12 ), the value of the worst adjacent crosstalk within a range of ±(0.8±0.1) nm with respect to the light transmission center wavelength is estimated to be on the order of −27 dB. Incidentally, the range for determining the adjacent crosstalk has been set with reference to wavelength intervals at which lights are multiplexed/demultiplexed by the corresponding arrayed waveguide grating. More specifically, in the arrayed waveguide grating of 40 ch-50 GHz, the frequency intervals at which the lights are multiplexed/demultiplexed are 50 GHz, and hence, the adjacent crosstalk determining range has been set at the range of ±(0.4 ±0.05) nm with reference to 0.4 nm in terms of the wavelength intervals. On the other hand, in the arrayed waveguide grating of 40 ch-100 GHz, the frequency intervals at which the lights are multiplexed/demultiplexed are 100 GHz, and hence, the range has been set at the range of ±(0.8±0.1) nm with reference to 0.8 nm in terms of the wavelength intervals. Here, in order to compare the shape of the optical spectrum shown in FIG. 11 with that of the optical spectrum shown in FIG. 12 , the scales of the axes of abscissas are normalized on the basis of the wavelength intervals at which the lights are multiplexed/demultiplexed by the respective arrayed waveguide gratings, and the two graphs of FIGS. 11 and 12 are superposed on each other. Then, the example of the optical spectrum shape demonstrated by the arrayed wavelength grating of 40 ch-50 GHz becomes as indicated by a characteristic curve a in FIG. 13 . It is seen that the optical spectrum shape of the characteristic curve a in the wavelength band adjacent to the light transmission center wavelength is wider than in the example of the optical spectrum shape (characteristic curve b) demonstrated by the arrayed wavelength grating of 40ch-100 GHz. As explained before, therefore, the adjacent crosstalk of the arrayed waveguide grating of 40 ch-50 GHz exemplarily studied degrades more than that of the arrayed waveguide grating of 40 ch-100 GHz. The value of the adjacent crosstalk is one of very important parameters which determine a bit error rate in the case of applying the arrayed waveguide grating to a wavelength division multiplexing transmission systems. Accordingly, enhancement in the adjacent crosstalk is an important theme even in the arrayed wavelength grating of 40 ch-50 GHz in which the multiplexing/demultiplexing wavelength intervals are narrowed. That is, it is required also of the arrayed wavelength grating of 40 ch-50 GHz to exhibit good characteristics on the same order as the exemplified adjacent crosstalk of the arrayed waveguide grating of 40 ch-100 GHz. Meanwhile, in the arrayed waveguide grating, the phase &Dgr;&phgr; of light propagated through an arrayed waveguide is indicated by the following equation (1): &Dgr;&phgr;&equals;(2&pgr;/&lgr;)·n eff ·&Dgr;L (1) Here, &lgr; denotes the wavelength of the light, n eff the effective refractive index of the arrayed waveguide, and &Dgr;L the optical path length difference of adjacent channel waveguides constituting the arrayed waveguide. In a case where the values of the phases &Dgr;&phgr; have fluctuated in the individual channel waveguides, a disturbance arises in the phasefront of the whole arrayed waveguide. The disturbance defocuses the condensed image of lights outputted from the arrayed waveguide, and degrades the adjacent crosstalk of the arrayed waveguide grating. When the fluctuations of the values of the phases &Dgr;&phgr; are defined as phase errors, the phase errors can be elucidated from the fluctuation of the effective refractive index of the arrayed waveguide. The effective refractive index of the arrayed waveguide is a function of the refractive index and film thickness of the arrayed waveguide and the line width of the channel waveguides, and the phase errors are ascribable to the delicate fluctuations of the variables. In this regard, the inventor examined the sectional profile of a part indicated by a dot-and-dash line A-A′ in FIG. 9 , in the arrayed waveguide grating of 40 ch-50 GHz produced by the prior-art manufacturing method and made studies on the fluctuations of the refractive index, film thickness and line width of the arrayed waveguide. As a result, it has been revealed that, in the arrayed waveguide grating of 40 ch-50 GHz produced by the prior-art manufacturing method, the individual channel waveguides 14 a of the arrayed waveguide 14 are arrayed as indicated by the cores 2 in a schematic view shown in FIG. 4 , so the channel waveguides 14 a have shapes which incline more toward the central side of the array at positions nearer to the end sides of the array. It is considered that, when the channel waveguides 14 a incline in this manner, a fluctuation will appear in the effective refractive index of the arrayed waveguide 14 , thereby to incur the phase errors. Besides, since the channel waveguides 14 a constituting the arrayed waveguide 14 have the shapes which incline more toward the central side of the array at the positions nearer to the end sides of the array, it is considered that the phase errors of the channel waveguides 14 a will enlarge more toward the end sides of the array of these channel waveguides, so a relationship as shown in FIG. 5 by way of example will appear. Incidentally, the figure shows the relationship between array Nos. and the phase errors in the case where the number of the arrayed channel waveguides 14 a is set at 400 and where the array Nos. of 1, 2, 3, . . . and 400 are successively assigned from one end side of the array. The relationship becomes a phase error distribution in which the phase errors enlarge more from the central position of the array of the channel waveguides of the arrayed waveguide toward the end sides of the array. Hereinbelow, the distribution shall be termed a “correlative phase error”. Further, the optical spectrum of the arrayed waveguide grating in the presence of the correlative phase error shown in FIG. 5 was computed by simulation, and the result is shown at a characteristic curve a in FIG. 6 . Also, a theoretical spectral shape in the absence of the correlative phase error is shown at a characteristic curve b in the figure. As seen from the figure, the shape of the optical spectrum of the arrayed waveguide grating widens due to the presence of the correlative phase error, and the adjacent crosstalk degrades greatly. On the basis of the above studies, the inventor has found out that the adjacent crosstalk can be enhanced in the arrayed waveguide grating in which the frequency intervals of lights to be multiplexed/demultiplexed are narrowed, by suppressing the correlative phase error. Besides, the fluctuations of the phase errors are ascribable to fluctuations in a process for manufacturing the arrayed waveguide grating. Upon various studies, the inventor has found out that the correlative phase error can be suppressed by making appropriate the conditions of the step of consolidating the upper clad. The inventor's studies will be explained below. The glass particles produced by the flame hydrolysis deposition undergo an abrupt density change as indicated by a characteristic curve in FIG. 7 , during sintering. This is because the behavior of the sintering is predominated by viscous flow sintering. By the way, in the figure, symbol S 1 denotes the start temperature of the sintering, and symbol S 2 the end temperature thereof. Besides, the start temperature T 1 and end temperature T 2 of the abrupt density change are determined chiefly by the composition and diameters of the glass particles. As shown in FIG. 8 A, the film of an upper clad 3 is deposited and formed so as to cover the waveguide construction of cores 2 . Therefore, when the abrupt density change mentioned above takes place during the consolidating of the film of the upper clad 3 , gaps appear on both the sides of core channels (the waveguide construction of the cores 2 ) forming an arrayed waveguide, with a temperature rise as shown in FIG. 8B . When the temperature of the consolidating is raised in this state, a glass ought to flow into the gaps gradually until the voids are finally filled up with the glass to complete the sintering. However, the supply of the glass into the gaps fails when a sintering rate is high in a temperature rise from the point T 1 at which the density change of the glass particles forming the film of the upper clad 3 starts, to the point T 2 at which the density change ends. It has accordingly been revealed that, when the sintering rate is high in the temperature rise from the point T 1 to the point T 2 , the sintering ends in a state where the upper clad 3 rolls the arrayed cores 2 in. As a result, in the case of the high sintering rate, as shown in FIG. 8 C, the cores 2 incline more at positions nearer to the end sides of the array thereof, and channel waveguides 14 a come to have shapes which incline more toward the central side of the array at the positions nearer to the end sides of the array. Incidentally, FIG. 8D schematically shows the ideal arrayal aspect of the cores 2 of the channel waveguides 14 a. Accordingly, the inventor conducted an experiment explained below, with the intention of reliably performing the supply of the glass into the gaps and suppressing the inclinations of the shapes of the channel waveguides 14 a by making appropriate the sintering rate in the temperature rise from the point T 1 at which the density change of the glass particles of the upper clad film starts, to the point T 2 at which the density change ends. In manufacturing samples of an arrayed waveguide grating of 40 ch-50 GHz, the sintering rate was variously changed within a range of from 2.5° C./min to 0.1° C./min. Besides, the relationship between the sintering rate and the adjacent crosstalk of the manufactured arrayed waveguide grating was found. As a result, relation data shown in FIG. 3 has been obtained, and it has been revealed that the adjacent crosstalk can be suppressed to or below −27 dB when the sintering rate is set at or below 1° C./min. The adjacent crosstalk value of −27 dB or below is equivalent or superior to the adjacent crosstalk of an arrayed waveguide grating of 40 ch-100 GHz. The present invention has its construction determined on the basis of the above studies. An optical waveguide in one aspect of the present invention, and a manufacturing method therefor are an optical waveguide which can narrow wavelength intervals to-be-multiplexed/demultiplexed and which exhibit good adjacent crosstalk characteristics, and a manufacturing method therefor. Besides, the optical waveguide is, for example, an arrayed waveguide grating. Now, an aspect of performance of the present invention will be described in conjunction with the drawings. By the way, in the ensuing description of embodiments, the same symbols will be assigned to the parts of the prior-art example having identical names and shall not be repeatedly explained. FIG. 1 shows the essential construction of one embodiment of an optical waveguide according to the present invention. The optical waveguide of the embodiment is a 40 ch-50 GHz arrayed waveguide grating, the construction of which is substantially the same as that of the arrayed waveguide grating shown in FIG. 9 . Besides, the embodiment is produced by a manufacturing method which is similar to the prior-art manufacturing method explained before, but it is characterized by setting a sintering rate as follows, at the step S 3 of consolidating an upper clad film in the manufacture of the arrayed waveguide grating: Letting T 1 denote a temperature at which the density change of the glass particles of the upper clad film starts, and T 2 denote a temperature at which the density change ends, the sintering rate in a temperature rise from the point T 1 to the point T 2 is set at 1.0° C./min so as to sinter and transparentize the upper clad film at the step S 3 . Incidentally, the temperatures T 1 , T 2 are appropriately set on the basis of data obtained by experiments or the likes beforehand, as shown in FIG. 7 . In the manufacture of the embodiment, the temperatures T 1 and T 2 are respectively set at 1000° C. and 1125° C. The embodiment is produced by the above manufacturing method, and the sintering rate from the temperature T 1 at which the density change of the glass particles of the upper clad film starts, to the temperature T 2 at which the density change ends, at the step of consolidating the upper clad film is set at 1.0° C./min as explained above. As understood from the studied result shown in FIG. 3 , therefore, the embodiment can be manufactured as an excellent, arrayed waveguide grating which suppresses the correlative phase error of an arrayed waveguide 14 and whose adjacent crosstalk is of small value. FIG. 2 shows a result obtained by measuring a optical spectrum in the vicinity of a light transmission center wavelength as to the arrayed wavelength grating of the embodiment. As seen from the figure, the arrayed wavelength grating of the embodiment can lower the adjacent crosstalk to about −27 dB. From this result, it has been verified that the adjacent crosstalk can be enhanced to the same degree as the adjacent crosstalk of an arrayed waveguide grating of 40 ch-100 GHz by applying the manufacturing method of the embodiment. Incidentally, the present invention is not restricted to the foregoing embodiment, but it can adopt various aspects of performance. By way of example, the sintering rate from the temperature T 1 at which the density change of the glass particles of the upper clad film starts, to the temperature T 2 at which the density change ends, at the step of consolidating the upper clad film, is set at 1.0° C./min in the embodiment, but it can be set at an appropriate value of 1.0° C./min or below in accordance with the composition and diameters of the glass particles of the upper clad film. Likewise, the temperature T 1 at which the density change of the glass particles of the upper clad film starts, and the temperature T 2 at which the density change ends, at the step of consolidating the upper clad film, are set at appropriate values in accordance with the composition and diameters of the glass particles of the upper clad film. Besides, the production of the arrayed waveguide grating by applying the manufacturing method of the embodiment has been exemplified in the above, but the manufacturing method for the optical waveguide according to the present invention as indicated in the embodiment is also applicable to the manufacture of an optical waveguide other than the arrayed waveguide grating. A Mach-Zehnder interference type optical waveguide, a Y-branch optical waveguide, and various optical waveguides having directional couplers are mentioned as examples of the optical waveguide to which the present invention is applied. The same effects as those of the embodiment can be brought forth by applying the present invention to an optical waveguide which includes a waveguide construction having a plurality of waveguides arranged side by side. That is, since the correlative phase error as explained above can be suppressed by producing the optical waveguide by the use of the manufacturing method for the optical waveguide according to the present invention, the optical waveguide which includes the waveguide construction having the plurality of waveguides arranged side by side can be made an optical waveguide of superior adjacent crosstalk characteristics as indicated in the embodiment of the arrayed waveguide grating.