JP2005300679A - Wavelength filter and wavelength variable filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a band variable wavelength filter, in which a filter transmission band is variable in a range of several hundreds of GHz, and to provide flattening wavelength spectrum characteristics. <P>SOLUTION: The wavelength filter is composed of a pair of an input waveguide 51 and an output waveguide 54, optical couplers 52-1 to 52-N and 55-1 to 55-N, arranged on the input waveguide 51 and the output waveguide 54 at regular intervals; and N array waveguides 53-1 to 53-N (N: natural number of 2 or larger) connected between the input and output waveguides via the optical couplers 52-1 to 52-N and 55-1 to 55-N. The length from the incident end to the emission end is composed of a ladder interference structure so as to increase and decrease the array waveguides 53-1 to 53-N, connected to the incident end side with a difference of the same length in order from a path. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a wavelength filter and a wavelength tunable filter, and more particularly to a semiconductor wavelength tunable filter and a transmission band tunable filter that are important optical components of a wavelength division multiplexing optical communication system.

  The wavelength tunable filter and the band tunable filter are important optical components indispensable in a large-capacity optical communication network. A wavelength filter used in an optical multiplexer / demultiplexer for wavelength division multiplexing optical communication includes band filtering of wavelength-multiplexed optical signals, a soliton fiber laser (see, for example, Non-Patent Document 1), and a supercontinuum fiber (eg, non-continuous). It plays an important role in applications such as the generation of ultrashort pulses using Patent Document 2). In particular, in the case of generation of ultrashort pulses extending to sub-picoseconds, the spectral characteristics and pulse waveform of the generated optical pulse are determined by the wavelength spectral characteristics and transmission band of the wavelength filter. It is being

  Up to now, an arrayed waveguide grating (AWG) type band-variable filter (for example, see Non-Patent Document 3), a ring resonance type band-variable filter (for example, see Non-Patent Document 4), and a transversal-type band-variable filter. A filter (for example, see Non-Patent Document 5) has been reported.

  FIG. 24 is an explanatory diagram conceptually showing the structure of a conventional band variable wavelength filter, and shows an AWG type band variable wavelength filter. In the figure, reference numeral 241 denotes an input waveguide, 242 denotes an input side slab waveguide, 243 denotes an array waveguide, 244 denotes an output side slab waveguide, and 245 denotes an output waveguide.

  This AWG type band variable wavelength filter realizes a band variable operation by using a plurality of input / output waveguide pairs 241 and 245 having different waveguide widths. The degree of freedom of the filter band and the variable band is determined by the input / output waveguide width and the number of input / output waveguides, respectively. Since the AWG type band-variable filter has spatially separated input / output waveguides, it is necessary to use an optical multiplexer / demultiplexer before and after the optical filter.

  FIG. 25 is an explanatory diagram conceptually showing the structure of a conventional band tunable wavelength filter, and shows a ring resonance type band tunable wavelength filter. In the figure, reference numeral 251 denotes an input waveguide, 252 denotes a variable coupling ratio 2 × 2 coupler, 253 denotes a ring resonator, and 254 denotes an output waveguide.

  This ring resonance type band tunable wavelength filter includes a ring resonator 253, a coupling rate variable coupler 252, and input / output waveguides 251, 254. When the band is variably operated, a problem that the selected wavelength is changed or variable is possible. The disadvantage is that the filter extinction ratio may vary greatly depending on the range.

  FIG. 26 is an explanatory diagram conceptually showing the structure of a conventional band tunable wavelength filter, and shows a transversal band tunable wavelength filter. In the figure, reference numeral 261 indicates a coupling rate variable coupler, 262 indicates a delay line, 263 indicates a phase shifter, and 264 indicates a multiplexer.

  This transversal type band variable wavelength filter includes a tap (coupling rate variable coupler) 261, a delay line 262, and a phase shifter 263, and the frequency characteristic is obtained by Fourier transform of impulse time response. In addition, although the transversal type tunable wavelength filter has good band tunable characteristics, in terms of wavelength tunable characteristics, the tunable range is about 100 GHz and the element size is relatively large (68 * 71 mm).

K. Tamura et al, IEEE Photon. Technol. Lett., 6, (1994), pp1433-1435 T. Morioka et al, Electron. Lett., 30, (1994), pp1166-1168 K. Okamoto et al, Electron. Lett., 31, (1995), pp1592-1593 T. Kominato et al, IEEE Photon. Technol. Lett., 5, (1993), pp560-562 E. Pawlowski et al, Electron. Lett., 32, (1996), pp113-114 S. Matsuo et al, IEEE Photon. Technol. Lett., 7, (2003), pp1114-1116

  In general, it is desirable that the tunable wavelength filter has wavelength variability in order to greatly expand the application range. This is a very important factor in applications such as ultrashort pulse generation. This is because, in order to generate an optical signal pulse having equal time resolution in a wavelength multiplexed optical signal, the band of the wavelength filter required for each wavelength component is different.

  At present, only the transversal type tunable wavelength filter described with reference to FIG. 26 is a filter element that can simultaneously perform tunable and tunable operations. In this case, the tap and the phase shifter serve to control the complex amplitude and the complex declination of the Fourier coefficient, respectively. Therefore, the band variable operation can be obtained by controlling the coupling ratio of taps, and the wavelength variable operation can be obtained by controlling the phase of the phase shifter. However, the transversal type band tunable wavelength filter has a drawback that it is difficult to control because it is necessary to independently and accurately adjust the phase of the phase shifter shown in FIG. In fact, it has been reported that the crosstalk of the filter wavelength spectrum varies greatly within a relatively narrow wavelength variable range of 0.8 nm (100 GHz) (see, for example, Non-Patent Document 5).

  On the other hand, it is desirable that the band-variable wavelength filter has a flat wavelength characteristic in order to broaden the application range. The flat wavelength spectrum is considered to be a very important characteristic not only for widening the temperature control allowable range of the filter element itself, but also for applications in which wavelength multiplexed optical signals are extracted by changing the filter band depending on the situation. . Usually, a shape factor is used as a parameter for evaluating the flatness of the wavelength spectrum, and is defined by a ratio of a transmission spectral bandwidth of -1 dB and -10 dB. Accordingly, a rectangular wavelength spectrum is shown as the shape factor approaches 1. In the case of a ladder interference type wavelength filter, the shape factor of the filter wavelength characteristic is determined in the vicinity of 0.32 in order to show a Gaussian function type spectrum like the AWG type element, and it is difficult to obtain a flat wavelength spectrum and characteristics. is there.

  As described above, the wavelength filter exhibiting the above-mentioned characteristics has not been reported so far, although the bandwidth and wavelength variability of the wavelength filter and the flatness of the filter spectrum are indispensable elements. is there.

  The present invention has been made in view of such problems, and an object of the present invention is to use a ladder interference type wavelength filter capable of performing a broadband wavelength variable operation, and a band in which the filter transmission band can be varied within a range of several hundred GHz. An object of the present invention is to provide a variable wavelength filter, a wavelength filter having a flat wavelength spectral characteristic, and a variable wavelength filter.

  The present invention has been made to achieve such an object. The invention according to claim 1 is a pair of input waveguides and output waveguides, and a constant distance between the input waveguides and the output waveguides. And N (N; a natural number of 2 or more) array waveguides connecting the input / output waveguides via the optical coupler, and from the input end to the output end The arrayed waveguides whose lengths are connected to the incident end side are configured with a ladder interference structure that increases or decreases in order from the path by the same length difference, and a coupling factor variable coupler is used as the optical coupler. Thus, the filter band can be varied.

  According to a second aspect of the present invention, in the first aspect of the invention, the diffraction order of the ladder interferometer is changed by periodically changing the optical coupling rate using the coupling rate variable coupler. It is characterized by that.

  According to a third aspect of the present invention, in the first or second aspect of the invention, the selection wavelength can be varied by changing the refractive index of the input / output waveguide by heat, voltage application or current injection. In addition, the selected wavelength variable operation and the band variable operation can be performed independently.

  According to a fourth aspect of the present invention, in the first aspect of the present invention, only the optical path length difference generated by the ladder interferometer in the Nth stage array waveguide and the (N-1) th stage array waveguide is obtained. The filter frequency spectrum characteristic has flatness by setting the integral multiple larger or smaller than the optical path length difference up to the (N-1) stage.

  Further, in the invention described in claim 5, in the invention described in claim 4, by using the variable coupling factor coupler, the transmission band of the selected wavelength can be varied while maintaining the flat filter characteristic. It is characterized by that.

  According to a sixth aspect of the present invention, in the wavelength filter according to the fourth or fifth aspect, the selected wavelength is variable by changing the refractive index of the input / output waveguide by heat, voltage application, or current injection. In addition, by adjusting the refractive index change region length in proportion to the optical path length difference between the arrayed waveguides, the flatness spectrum wavelength characteristic can be kept constant during the wavelength variable operation, and the selected wavelength variable operation and the band variable operation. It is a wavelength tunable filter characterized by being able to perform independently.

  According to the present invention, the transmission band of the wavelength filter and the wavelength tunable filter can be varied, which plays an important role in application fields such as generation of ultrashort pulses. Further, according to the present invention, the transmission band of the wavelength filter and the wavelength tunable filter can be flattened, and the strict temperature control of the semiconductor laser used as the light source and the element itself can be relaxed. In addition, the wavelength tunable filter having the band variable characteristic and the flatness spectrum characteristic according to the present invention plays an important role in applications such as band filtering of wavelength-multiplexed optical signals.

  Furthermore, although InP-based compound semiconductors are used in the embodiments of the present invention, the same operation is possible even when using a material composed of PLC, GaAs, or SOI and a polymer material, and further low cost performance. Can be realized.

Embodiments of the present invention will be described below with reference to the drawings.
(Embodiment 1)
FIG. 1 is a block diagram for explaining a ladder interference type wavelength filter according to the present invention. In the figure, reference numeral 11 denotes an input waveguide, 12-1 to 12-N denote optical couplers on the input waveguide side, 13- Reference numerals 1 to 13-N denote array waveguides, reference numeral 14 denotes an output waveguide, and reference numerals 15-1 to 15-N denote optical couplers on the output waveguide side. N indicates the number of array waveguides.

  This ladder interference type wavelength filter includes a pair of input waveguides 11 and output waveguides 14, and optical couplers 12-1 to 12 -N, 15 arranged in the input waveguides 11 and 14 at regular intervals. -1 to 15-N and N (N: a natural number of 2 or more) array conductors connecting the input / output waveguides via optical couplers 12-1 to 12-N and 15-1 to 15-N The waveguides 13-1 to 13-N are configured.

That is, when the array waveguide length is sequentially increased by ΔS from the left, the transmittance is maximized at a wavelength (λ ₀ ) that satisfies the following equation.
λ ₀ = (n _eff ΔS) / m

Here, n _eff and m are the effective refractive index of the optical waveguide and the diffraction order of the ladder interferometer, respectively. As shown in FIG. 1, the light incident from the arrayed waveguides 13-1 to 13-N and the light incident from the output waveguide 14 interfere with the optical couplers 15-1 to 15-N on the output waveguide side. Since wavelengths other than the light having the same phase are subjected to radiation loss, only a desired wavelength satisfying the above equation propagates through the output waveguide 14 and functions as a wavelength filter.

FIG. 2 is a diagram showing frequency spectrum characteristics with respect to the number of arrayed waveguides (N) when the optical coupling rate (k) of the optical coupler of the ladder interference type wavelength filter shown in FIG. 1 is set to 0.85. . Here, m = 120 and λ ₀ = 1550 nm. As shown in FIG. 2, it can be seen that the frequency spectrum band of the ladder interference type wavelength filter changes with respect to the number of arrayed waveguides. In this case, the 3-dB frequency bandwidth is 155 (N = 8), 125 (N = 10), 105 (N = 12), 85 (N = 15) and 62 sequentially from N = 8 to N = 15. [GHz] (N = 20). Therefore, if each optical signal is taken out by the optical couplers 15-1 to 15-N on the output waveguide side, it becomes possible to vary the discrete spectral band corresponding to the number of array waveguides. However, the filter characteristics described above can be obtained only when k of the optical coupler is set to a specific value (k = 0.85 in the example of FIG. 2).

  In other words, the input and output waveguides of the ladder interference type wavelength filter shown in FIG. 1 have a structure shared by all the array waveguides, and the Nn-th stage (n Cannot take out an optical signal having a ladder interference up to an array waveguide of a natural number). In other words, since the wavelength filter shown in FIG. 1 has only one output port, when a multimode interference (MMI) coupler or a directional coupler is used as an optical coupler, k of the optical coupler is used. Is uniformly determined by the waveguide parameter, it is not possible to obtain a band-variable wavelength spectrum as described above.

On the other hand, since the loss caused by k of the optical coupler in the ladder interferometer depends on the number of array waveguides, it is necessary to control k of the optical coupler to an appropriate value for each number of array waveguides. In optical couplers having a uniform k, the loss due to ladder interference differs, and there is a problem that good band variable filter characteristics cannot be obtained. As shown in FIG. 2, when k of the optical coupler is constant at 0.85, the transmittance of λ ₀ of the ladder interference type wavelength filter is −3.87 (N = 20), −2.84 (N = 15), -2.66 (N = 12), -2.83 (N = 10) and 3.35 (N = 8) [dB], and it can be seen that it varies depending on the number of arrayed waveguides. .

  In the ladder interference type wavelength filter according to the present invention, the ladder interferometer using the variable coupling rate optical coupler makes it possible to keep the bandwidth variable filter operation and the transmittance in the transmission band constant. Usually, in order to obtain an arbitrary k, it is necessary to provide a refractive index changing region in the optical coupler, but a Mach-Zehnder Lnterferometer (MZI) using a high-mesa waveguide structure as the optical coupler structure. Is considered to be most effective for achieving the above-mentioned purpose.

  FIG. 3 is a structural diagram of an MZI including two 3 dBMMI couplers. In the figure, reference numeral 31 denotes an optical waveguide, 31a denotes a refractive index change region, and 32 denotes a 3 dB coupler. Normally, in MZI, when the optical path lengths of two arms are equal, all the light waves incident from the input port are output to the cross port (k = 1). However, as shown in FIG. 3, if the refractive index changing region 31a is provided in one arm of the Mach-Zehnder interferometer, the effective refractive index of the waveguide is different, so that the optical path length between the two arms is different. Light is also output to the port according to the optical path length difference. In order to configure the refractive index changing region 31a shown in FIG. 3, a method of controlling the refractive index by providing a heat, voltage application or current injection structure is effective.

  In this case, in the case of a current injection structure by electrode formation, the refractive index change amount and the refractive index change region length (electrode length) are in a trade-off relationship. That is, the longer the electrode length, the desired k can be obtained with a small change in the refractive index, but the optical coupler length becomes longer, and the element size and the insertion loss of the element increase. Therefore, the electrode length may be set within a range in which an arbitrary k can be obtained by a change in refractive index obtained by current injection.

FIG. 4 is a diagram showing a coupling coefficient with respect to the refractive index change amount in the refractive index change region of the MZI optical coupler shown in FIG. 3, and a band gap wavelength (λ _g ) of 1.4 μm as a core layer on the InP substrate. When a high mesa waveguide structure having a waveguide width (W) of 1.6 μm and an electrode length of 100 μm is formed using a wafer using the compound semiconductor GaInAsP, k of the MZI optical coupler with respect to a change in refractive index is shown. When there is no refractive index change, k = 1, but as the refractive index change amount by current injection increases, k decreases, and k changes to 0 with a refractive index change of about 0.23 [%]. That is, an arbitrary k can be obtained according to the refractive index change.

  FIG. 5 is a configuration diagram for explaining the first embodiment of the ladder interference type wavelength filter of the present invention, and is a configuration diagram of a ladder interference type wavelength filter using the MZI shown in FIG. 3 as an optical coupler. It is a block diagram of a ladder interference type wavelength filter using a coupling rate variable MZI as an optical coupler. In the figure, reference numeral 51 is an input waveguide, 52-1 to 52-N are optical couplers on the input waveguide side, 53-1 to 53-N are array waveguides, 54 is an output waveguide, and 55-1 to 55-. N indicates an optical coupler on the output waveguide side.

  This ladder interference type wavelength filter includes a pair of input waveguides 51 and output waveguides 54, and optical couplers 52-1 to 52-N, 55 arranged in the input waveguides 51 and 54 at regular intervals. -1 to 55-N and N (N: a natural number of 2 or more) array conductors connecting the input / output waveguides via optical couplers 52-1 to 52-N and 55-1 to 55-N It consists of waveguides 53-1 to 53-N.

  Moreover, the structure of ladder interference in which the length from the incident end to the output end is increased or decreased in the order of the same length difference in the arrayed waveguides 53-1 to 53-N connected to the incident end side. By using a coupling rate variable coupler as the optical couplers 52-1 to 52-N and 55-1 to 55-N, the filter band can be varied.

  Compared with the ladder interference type wavelength filter structure shown in FIG. 1, k of the optical coupler has a variability and can be controlled to an arbitrary k. In this case, since the k of the optical coupler can be varied, by setting the k of the optical coupler after the Nn-th stage array waveguide to 1 (no current injection), a pseudo-digital variable band operation is possible. Become.

FIG. 6 is a band variable characteristic diagram of the ladder interference type wavelength filter shown in FIG. 5, and k of the optical coupler before the Nn-th stage array waveguide has a transmittance of λ ₀ according to the number of array waveguides. The wavelength spectrum characteristic when k of the optical coupler is controlled to be constant is shown. It was confirmed that by adjusting k of the MZI optical coupler in the number of arrayed waveguides, the transmittance of λ ₀ becomes constant in the vicinity of −2.8 dB. In this case, the appropriate k for the number of arrayed waveguides is k = 0.89 (N = 20), k = 0.85 (N = 15), k = 0.82 (N = 12), k = 0. 85 (N = 10) and k = 0.82 (N = 8).

  The extinction ratio of the wavelength filter characteristic shown in FIG. 6 is about 12 dB, which is relatively small. However, the extinction ratio of the ladder interference type wavelength filter can be increased by changing the k of the MZI optical coupler in a distributed manner (Apodization).

  FIG. 7 is a configuration diagram of a ladder interference type tunable filter in which the MZI optical coupler shown in FIG. 3 is apodized. Reference numeral 71 denotes an input waveguide, and reference numerals 72-1 to 72 -N denote inputs. Side waveguide optical couplers, 73-1 to 73-N are array waveguides, 74 are output waveguides, and 75-1 to 75-N are output side waveguide optical couplers.

Apodization optical coupling rate ladder interferometer (Apodization) reduces the optical coupling ratio of the input waveguide side from any k _a to any _{k b (k a> k b} ), the output waveguide side light setting the coupling ratio to increase from k _b described above to any k _a achieved by. In this case, k _a is related to the transmittance of the lambda _0, the absolute value of lambda ₀ of the transmittance of the k _a -k _b, relating to the extinction ratio and the filter band of the wavelength spectrum characteristics. Generally, the closer to k _a is 1, increases the transmittance of the lambda ₀ is, as the k _a is reduced, the transmittance of the lambda ₀ is also reduced.

On the other hand, k _a -k absolute value of _b is larger, and at the same time the extinction ratio of the wavelength spectrum increases, the transmission bandwidth is also increased.

FIG. 8 is a band variable characteristic diagram of the ladder interference type wavelength filter shown in FIG. 7, and shows frequency spectrum characteristics with respect to the number of arrayed waveguides when linearly changing apodization is applied to k of the MZI optical coupler. Show. In this case, k _a and k _b are controlled to indicate the transmission of a certain lambda _0. In order to obtain a variable band operation, k of the optical coupler after the Nn-th stage array waveguide was controlled to 1. As shown in FIG. 8, the extinction ratio of the frequency spectrum increased to 27 dB or more in any frequency band.

(Embodiment 2)
Embodiment 2 of the present invention will be described below. In the ladder interference type wavelength filter described in the first embodiment described above, the band variable operation corresponding to the number of array waveguides is realized by controlling k of the optical coupler. The bandwidth change is relatively small.

  FIG. 9 is a frequency band characteristic diagram of the number of array waveguides of the ladder interference type wavelength filter according to the second embodiment, and shows a 3-dB frequency band with respect to the number of array waveguides of the ladder interference type wavelength filter shown in FIG. ing. As shown in FIG. 9, the 3-dB frequency band change from 20 to 13 array waveguides is about 5 GHz frequency interval for each array waveguide, and the number of array waveguides is from 12 to 10 Is approximately 10 GHz frequency interval for each array waveguide. However, when the number of arrayed waveguides is reduced from 10, the 3-dB frequency band change rapidly increases, and it becomes difficult to make a fine frequency band change.

  Therefore, the number of array waveguides capable of changing the digital frequency band is limited, and the variable band of the frequency band is limited to about several tens of GHz. In order to increase the variable band, it is effective to decrease the diffraction order of the ladder interference type wavelength filter. In this case, when the diffraction order is decreased, the filter frequency band is also increased, and the variable filter band is also increased. In order to obtain a narrow filter frequency spectrum, if the number of arrayed waveguides is changed in inverse proportion to the change in the diffraction order, the same filter frequency characteristic as in the case of a high diffraction order can be obtained. However, in the case of a ladder interference type wavelength filter in which the number of diffraction waveguides is reduced and the number of array waveguides is increased, as described with reference to FIG. 6, the appropriate number of k for each number of array waveguides also increases. The control of k of the MZI optical coupler shown is complicated. The same applies to the apodization described in FIG.

That, as also arrayed waveguide number control of k _a and k _b for minimizing the increase and diffraction losses ladder interferometer filter extinction ratio is increased, it becomes complicated. This problem can be solved by controlling the k of the MZI optical coupler to change the diffraction order of the ladder interferometer.

  FIG. 10 is a configuration diagram for explaining the second embodiment of the ladder interference type wavelength filter according to the present invention, and is a configuration diagram of a variable bandwidth ladder interference type filter that can vary the filter frequency band from several tens of GHz to several hundreds of GHz. Show. In the figure, reference numeral 101 is an input waveguide, 102-1 to 102-N are optical couplers on the input waveguide side, 103-1 to 103-N are array waveguides, 104 is an output waveguide, and 105-1 to 105-. N indicates an optical coupler on the output waveguide side.

The ladder interference type wavelength filter of Embodiment 2 is a ladder interference type wavelength filter of m = 20. In this case, if the optical coupling ratios k ₂ , k ₄ , k ₆ , k _{8 and the} like corresponding to the 2 × pth (p is an integer) are controlled to 1, the ladder interference wavelength filter is equivalent to an element with m = 40. . If the (3 × p) −1 and 3 × p th optical coupling ratios k ₂ , k ₃ , k ₅ , k _6, etc. are controlled to 1, a ladder interference type wavelength filter equal to m = 60 elements. Become. Here, when the excess optical path length difference increases, p becomes positive, and when the excess optical path length difference decreases, p becomes negative. Therefore, a ladder interferometer corresponding to a multiple of the diffraction order can be formed by controlling the optical coupling rate.

  FIG. 11 is a band variable characteristic diagram of the ladder interference type wavelength filter shown in FIG. 10, and shows the 3-dB frequency band with respect to the number of arrayed waveguides in the diffraction order of the ladder interference type wavelength filter. In any case, although the change is almost linear between the number of array waveguides 10 to 20, the 3-dB frequency band increases rapidly when the number of array waveguides becomes 10 or less. As shown in FIG. 11, the 3-dB frequency bands of m = 20 (N = 20), m = 40 (N = 10), and m = 80 (N = 5) match, and m = 40 (N = The 3-dB frequency bands of 20) and m = 80 (N = 10) also coincide.

  That is, if m × N is constant, it means that the filter frequency band does not change. In this case, since the filter band change between N = 10 and N = 20 is almost linear, a ladder interference type wavelength filter capable of changing the filter frequency band in a quasi-digital manner from several tens GHz to several hundreds GHz is possible. become. In particular, the advantage of the configuration shown in FIG. 10 is that the appropriate optical coupling factor k corresponding to the number of arrayed waveguides does not depend on the diffraction order of the ladder interferometer.

  That is, when the number of arrayed waveguides is the same, the appropriate optical coupling rate k in all diffraction orders is the same. Therefore, the number of optical coupling ratios k to be controlled is greatly reduced as compared with the case of the variable band characteristics using only the number of arrayed waveguides, and good filter characteristics can be expected.

(Embodiment 3)
FIG. 12 is a configuration diagram for explaining the third embodiment of the ladder interference type wavelength filter according to the present invention, and is a structural diagram of a ladder interference type wavelength filter provided with a current injection structure using electrodes for wavelength variable operation. is there. In the figure, reference numeral 121 is an input waveguide, 121a is a wavelength variable control electrode on the input waveguide side, 122-1 to 122-N are optical couplers of the input side waveguide, and 123-1 to 123-N are array waveguides. , 124 is an output waveguide, 124a is a wavelength variable control electrode on the output waveguide side, and 125-1 to 125-N are optical couplers of the output side waveguide.

In the ladder interference type wavelength filter of the third embodiment, the transmission band changes according to the number of array waveguides. Therefore, if k of the optical coupler after the array waveguide from which a desired band is obtained is set to 1, Variable is possible. In addition, if the refractive indexes of L ₁ and L ₂ are changed independently, wavelength tunable operation can also be realized.

FIG. 13 is a diagram showing the layer structure of the epitaxial substrate of the ladder interference type tunable filter shown in FIG. 12, where reference numeral 131 denotes an InP substrate, 132 denotes an n-type InP layer, and 133 denotes a GaInAsP layer (λg = 1. 4 μm), 134 is a p ⁻ -InP layer, 135 is a p ⁺ -InP layer, and 136 is a p ⁺ -InGaAs layer.

On an n-type InP substrate 131, an n-doped InP layer 132, a non-doped GaInAsP layer 133 having a thickness of 0.5 μm, a p-doped InP layer 134 having a thickness of 1.2 μm, and a p ⁺ doped InP having a thickness of 0.3 μm. A layer 135 and a p ⁺ doped InGaAs layer 136 having a thickness of 30 nm are formed. An AuZnNi electrode and an AuGeNi electrode are formed on the p ⁺ doped InGaAs layer 136 and on the back surface of the n-type InP substrate 131, respectively.

The doping concentration of the n-doped InP layer 132 is 1 × 10 ¹⁸ cm ⁻³ , the doping concentration of the p-doped InP layer 134 is 5 × 10 ¹⁷ cm ⁻³ , and the doping concentration of the p ⁺ -doped InP layer 135 is 1 × At 10 ¹⁸ cm ⁻³ , the doping concentration of the p ⁺ doped InGaAs layer 136 is 8 × 10 ¹⁸ cm ⁻³ .

FIG. 14 is a wavelength variable characteristic diagram of the ladder interference type wavelength filter shown in FIG. 12, and shows the frequency spectrum of variable bandwidth and wavelength variable operation of the ladder interference type wavelength filter. In this case, the diffraction order is 120, and λ ₀ is set to 1550 nm. When the electrode lengths L ₁ and L ₂ are set to 100 μm, the wavelength selection operation is 1.2 THz over the short wavelength side and the long wavelength side, respectively, due to the change in the effective refractive index of the optical waveguide of about 0.18 [%]. was gotten. Note that since the band variable operation can be controlled independently of the wavelength variable operation, the band can be varied within the controllable wavelength range.

(Embodiment 4)
Embodiment 4 of the present invention will be described below. In the case of the ladder interference type wavelength filter shown in the first to third embodiments, the band variable characteristic is obtained, but the flatness of the filter spectrum cannot be obtained. Since the frequency response of the ladder interference type wavelength filter is a Gaussian function type like the AWG type filter, it is difficult to obtain a frequency response close to a rectangle. However, in the ladder interference type wavelength filter used in the fourth embodiment, a frequency response close to a rectangle can be achieved by changing an input / output transfer function composed of ladder interferometers connected in cascade via N array waveguides. I have to.

  The optical coupler used in the ladder interference type wavelength filter shown in FIG. 1 is a 2 × 2 coupler in any of the MMI coupler, the directional coupler, and the MZI coupler. Light that is coupled to one output port and not interfering is coupled to the remaining ports. In this case, to which port the interfering optical wavelength is coupled has a close dependence on k of the optical coupler. In the ladder interference type wavelength filter used in the fourth embodiment, the input / output transmission of the ladder interference type wavelength filter having a flat frequency spectrum is performed by changing the output port of the optical coupler connected to the N-th array waveguide. Getting function.

FIGS. 15A and 15B and FIGS. 16A and 16B are characteristic diagrams at the bar port of the optical coupler of the ladder interference type wavelength filter shown in FIG. 1, and FIG. The coupling coefficient k of the optical coupler is 0.85, FIG. 15B is 0.72, k is 0.5 in FIG. 16A, and k is 0.15 in FIG. The transmission characteristics in the case are shown. That is, when k of the optical coupler connected to the N-th array waveguide of the ladder interference type wavelength filter shown in FIG. 1 is set to 0.85, 0.72, 0.5, and 0.15, It represents the transmittance of λ ₀ to the bar port.

For comparison, the wavelength spectrum characteristic indicated by the broken line in the figure is the transmittance of λ ₀ to the cross port when k of the optical coupler is set to 0.85, that is, the wavelength characteristic of a normal ladder interference type wavelength filter. It is. The transmittance of λ ₀ into the bar port increases as k decreases. In this case, as shown in FIG. 16A, it can be seen that the filter spectral characteristic has flatness by appropriately setting k of the optical coupler. In this case, the shape factor is 0.52, which is 1.6 times better than 0.32 of the existing ladder interference filter. However, although the filter spectrum shown in FIG. 16 (a) has a flatness, the transmittance of lambda _0, from the transmittance of the lambda ₀ of the existing ladder interferometric wavelength filter shown by dotted lines 5.2dB lower addition, the filter Crosstalk also increases and the extinction ratio is relatively small at about 6 dB. Further, with the flattening of the spectrum, there is a problem that the transition region from the transmission band to the stop band is slightly widened and the steepness of the roll-off is lowered.

  The reason why the loss increased in the transmission band of the selected wavelength is that the optical component k is large and the optical component coupled to the output port (bar port) is reduced. In this case, in order to minimize the loss in the transmission band, it is effective to decrease the optical coupler k and increase the optical component coupled to the bar port, but as shown in FIG. The problem of losing spectral flatness arises. These problems are that only the optical path length difference generated in the ladder interferometer between the N-th stage array waveguide and the N-1th stage array waveguide is an integer multiple larger than the optical path length difference to the N-1th stage array waveguide. Alternatively, it can be improved by reducing the size.

  FIG. 17 is a configuration diagram for explaining a fourth embodiment of the ladder interference type wavelength filter according to the present invention. In the figure, reference numeral 171 denotes an input waveguide, and 172-1 to 172-N denote optical couplings of the input side waveguide. 173-1 to 173-N are array waveguides, 174 is an output waveguide, and 175-1 to 175-N are optical couplers of output side waveguides.

  An excessive optical path length difference corresponding to p × ΔS (p is an integer) is provided only in the Nth stage array waveguide, and only the Nth stage and N−1th stage ladder interferometers have ladder interference up to the N−2th stage. An excess optical path length difference p times larger than the total optical path length difference occurs. In other words, this configuration is a ladder interferometer whose diffraction order is only p times higher only in the rightmost ladder. When the excess optical path length difference increases, p becomes positive, and when the excess optical path length difference decreases, p becomes negative.

FIG. 18 is a diagram of flatness wavelength spectrum characteristics of the ladder interference type wavelength filter shown in FIG. Here, the diffraction order is 120, the number of array waveguides is 15 (N = 15), λ ₀ = 1.55 μm, and the optical path length difference between the 14th and 15th array waveguides is 8 × ΔS. K of the optical coupler of the output waveguide is set to 0.15, and is a wavelength spectrum characteristic output from the bar port. The wavelength characteristics shown by the thin dotted line and the thick dotted line are the wavelength filter and the wavelength spectrum shown in FIG. Compared with the wavelength spectrum (thin dotted line) of the conventional ladder interference type wavelength filter, the transmittance of λ ₀ is reduced by about 1 dB, but the flatness spectrum and the roll-off are improved. The shape factor is estimated to be 0.57, which is about 1.8 times larger than 0.32 of the conventional ladder interference type wavelength filter.

On the other hand, compared with the case where the optical path length difference of the array waveguide is not optimized (thick dotted line), the transmittance of λ ₀ is increased by 4 dB, and the roll-off is also steep, improving the characteristics as a wavelength filter. It was confirmed that

(Embodiment 5)
FIG. 19 is a configuration diagram for explaining the fifth embodiment of the ladder interference wavelength tunable filter according to the present invention, and shows the structure of a band variable ladder interference wavelength filter having a flat wavelength spectrum. In the figure, reference numeral 191 is an input waveguide, 192-1 to 192-N are optical couplers of the input side waveguide, 193-1 to 193-N are array waveguides, 194 is an output waveguide, and 195-1 to 195- N indicates an optical coupler of the output side waveguide.

  In the case of the ladder interference type wavelength filter described in the first embodiment, a band variable operation can be obtained by controlling k of the optical coupler, but the flatness of the wavelength spectrum cannot be obtained. The ladder interference type wavelength tunable filter shown in the fifth embodiment enables a band variable operation while maintaining a flat wavelength spectrum. As described in the fourth embodiment, the output waveguide of the ladder interference type wavelength filter having a flat wavelength spectrum is a structure that is not shared across all the arrayed waveguides.

  That is, the output waveguide of the ladder interference type tunable filter shown in FIG. 19 has a structure that is spatially separated from the waveguide through which the light wave that interferes with the next array waveguide propagates. Wavelength spectra equivalent to the number of n-th array waveguides can be taken out individually. In this case, k of the optical coupler connected to the arrayed waveguide after the output of the wavelength spectrum needs to be set to 1 (no current injection). The number of output ports of the band variable ladder interference type wavelength filter having a flatness spectrum is (N−2) at the maximum, and a coupler (T × 1 multiplexer: T is an output) that combines the respective optical outputs. Port number) is required. At this time, the optical output intensity is reduced to 1 / T times, and there is a drawback that the optical output intensity of the ladder interference type wavelength filter is greatly reduced and the element size is increased as the variation of the variable band is increased.

  FIG. 20 is a configuration diagram of a ladder interference type wavelength filter that solves the problem of the ladder interference type wavelength tunable filter shown in FIG. 19. Light waves that propagate through the array waveguide and interfere with the MZI optical coupler are N × 2 The light is emitted through the MZI optical couplers. In this case, a wavelength spectrum having a transmission band corresponding to the number of (Nn) -th array waveguides is emitted from the output port by controlling k of the MZI optical coupler. That is, if k of the first MZI optical coupler on the output port side shown in FIG. 20 is controlled to 0 and k after that is set to 1, a flat wavelength spectrum equivalent to the number of arrayed waveguides can be obtained. This ladder interference type tunable filter is characterized in that the output light intensity does not decrease regardless of the variation of the variable band. In addition, in the wavelength spectrum for any number of arrayed waveguides, since the same path is propagated, the excess loss generated in the optical coupler or the like is also constant.

  FIG. 21 is a flatness wavelength spectrum band variable characteristic diagram of the ladder interference type wavelength variable filter shown in FIG. 20, and shows frequency spectrum characteristics with respect to the number of arrayed waveguides. By controlling the k of the MZI optical coupler, the filter frequency spectrum has flatness simultaneously with the variable band operation for the number of arrayed waveguides. In this case, the 3 dB frequency bandwidth is 245 (N = 8), 190 (N = 10), 155 (N = 12), 125 (N = 15) and 92 [GHz] sequentially from N = 8 to N = 20. (N = 20). It was confirmed that the shape factor exhibiting the flatness spectrum characteristic was constant at 0.54 for any number of arrayed waveguides due to the variable band operation. The extinction ratio of the wavelength filter characteristic shown in FIG. 20 is about 12 dB, which is relatively small. However, as described in the first embodiment, a ladder interference type wavelength filter is obtained by applying apodization to k of the MZI optical coupler. Needless to say, the extinction ratio can be increased.

  On the other hand, what has been described in the second embodiment is also applicable to the fifth embodiment shown in FIG. Accordingly, a flat frequency spectrum filter having a variable band range over several hundred GHz range is also possible.

(Embodiment 6)
FIG. 22 is a block diagram for explaining the sixth embodiment of the ladder interference type wavelength tunable filter according to the present invention, and shows the structure of a ladder interference type wavelength filter provided with a current injection structure using electrodes for wavelength tunable operation. is there. In the figure, reference numeral 221 denotes an input waveguide, 221a denotes a wavelength variable control electrode on the input waveguide side, 222-1 to 222-N denote optical couplers of the input side waveguide, and 223-1 to 223-N denote array waveguides. Reference numeral 224 denotes an output waveguide, 224a denotes a wavelength variable control electrode on the output waveguide side, and 225-1 to 225-N denote optical couplers of the output side waveguide.

The layer structure of the epitaxial substrate used in the sixth embodiment is the same as the structure shown in FIG. The wavelength variable operation can be obtained by changing the refractive indexes of L ₁ and L ₂ of the ladder interference type wavelength variable filter shown in the sixth embodiment, as in the case of the embodiment 2. Further, the band variable operation can be obtained by controlling k of the MZI optical coupler as described in the fifth embodiment. However, in order to keep the flat wavelength spectrum constant during wavelength variable operation due to the excess optical path length difference of the arrayed waveguide, it is necessary to optimize the electrode length between the ladders that give the excess optical path length difference.

That is, if the above-described electrode length is not optimized, the wavelength spectrum shape gradually collapses in proportion to the amount of change in refractive index during wavelength variable operation, and at the same time, the flatness of the wavelength spectrum is lost. This problem is solved by adjusting the electrode length in proportion to the excess optical path length change (p × ΔS: p is an integer, where p ≠ 0). In the structure of the filter element shown in FIG. 22, the excess optical path length change is obtained by controlling k of the MZI optical coupler from an arbitrary value to 1. In this case, if the electrode lengths L ₁ and L ₂ on the input / output waveguides connecting the respective arrayed waveguides are set to be constant, an excess optical path length change (p × ΔS) is given to the light wave propagating through the ladder interferometer. In this case, the electrode lengths are p × L ₁ and p × L ₂ . Accordingly, it is possible to keep the flat wavelength spectrum shape constant during wavelength variable operation.

FIG. 23 is a wavelength variable and band variable characteristic diagram of the ladder interference type wavelength tunable filter shown in FIG. 22 and shows the band variable and wavelength variable operations of the ladder interference type wavelength filter. In this case, when current is injected into the L ₁ electrode, λ ₀ is shifted to the short wavelength side, and when current is injected into the L ₂ electrode, λ ₀ is shifted to the long wavelength side. However, in the case of a ladder interferometer in which the optical path length difference decreases as light incident from the input waveguide propagates, when current is injected into the L ₁ electrode, λ ₀ shifts to the long wavelength side, and current flows into the L ₂ electrode. Λ ₀ shifts to the short wavelength side. In this case, the diffraction order is 120, and λ ₀ is set to 1550 nm. As shown in FIG. 23, it can be seen that the wavelength can be varied in the frequency range of 1.25 THz by injecting current into the electrodes L ₁ and L ₂ and changing the effective refractive index of the optical waveguide by 0.18%.

  By controlling k of the MZI optical coupler, the transmission band of the flat frequency spectrum corresponding to the number of arrayed waveguides can be controlled independently of the wavelength variable operation, and the flat frequency spectrum is kept constant during the wavelength variable operation. (Shape factor = 0.54).

  A ladder interference type wavelength filter having a flat frequency spectrum can easily increase the band variable range and the wavelength variable range to several THz and several tens of THz by changing only the diffraction order. In the case of a ladder interference type wavelength filter that is exactly the same as the waveguide parameter of the filter element of the present invention and in which only the diffraction order is set to 7, the band variable range and the wavelength variable range are each 0.22% in refractive index change. It can be increased to 1 and 250 THz.

It is a block diagram for demonstrating the ladder interference type | mold wavelength filter which concerns on this invention. It is a figure which shows the frequency spectrum characteristic with respect to the number of array waveguides (N) when the optical coupling factor (k) of the optical coupler of the ladder interference type | mold wavelength filter shown in FIG. 1 is set to 0.85. FIG. 3 is a structural diagram of an MZI including two 3 dBMMI couplers. It is a figure which shows the coupling coefficient with respect to the refractive index change amount in the refractive index change area | region of the optical coupler of MZI shown in FIG. It is a block diagram for demonstrating Embodiment 1 of the ladder interference type | mold wavelength filter of this invention. FIG. 6 is a band variable characteristic diagram of the ladder interference type wavelength filter shown in FIG. 5. FIG. 4 is a configuration diagram of a ladder interference type tunable filter that is apodized with the MZI optical coupler shown in FIG. 3. FIG. 8 is a band variable characteristic diagram of the ladder interference type wavelength filter shown in FIG. 7. FIG. 6 is a frequency band characteristic diagram of the number of arrayed waveguides of the ladder interference type wavelength filter according to the second embodiment. It is a block diagram for demonstrating Embodiment 2 of the ladder interference type | mold wavelength filter which concerns on this invention. It is a band variable characteristic figure of the ladder interference type | mold wavelength filter shown in FIG. It is a block diagram for demonstrating Embodiment 3 of the ladder interference type | mold wavelength filter which concerns on this invention. It is a figure which shows the layer structure of the epitaxial substrate of the ladder interference type | mold wavelength variable filter shown in FIG. FIG. 13 is a wavelength variable characteristic diagram of the ladder interference type wavelength filter shown in FIG. 12. FIG. 4 is a characteristic diagram of the optical coupler at the bar port of the ladder interference type wavelength filter shown in FIG. 1, where (a) shows a case where the coupling coefficient k of the optical coupler is 0.85 and (b) shows that k is 0.72. The transmission characteristics are shown. FIG. 4 is a characteristic diagram at the bar port of the optical coupler of the ladder interference type wavelength filter shown in FIG. 1, (a) shows transmission characteristics when k is 0.5 and (b) shows k when 0.15. Show. It is a block diagram for demonstrating Embodiment 4 of the ladder interference type | mold wavelength filter which concerns on this invention. It is a flatness wavelength spectrum characteristic view of the ladder interference type wavelength filter shown in FIG. It is a block diagram for demonstrating Embodiment 5 of the ladder interference type | mold wavelength variable filter which concerns on this invention. FIG. 20 is a configuration diagram of a ladder interference type wavelength filter that solves the problem of the ladder interference type wavelength tunable filter shown in FIG. 19. It is a flatness wavelength spectrum band variable characteristic figure of the ladder interference type | mold wavelength variable filter shown in FIG. It is a block diagram for demonstrating Embodiment 6 of the ladder interference type | mold wavelength variable filter which concerns on this invention. FIG. 23 is a wavelength variable and band variable characteristic diagram of the ladder interference type wavelength variable filter shown in FIG. 22. It is explanatory drawing which shows notionally the structure of the conventional band variable wavelength filter, and is a figure which shows an AWG type | mold band variable wavelength filter. It is explanatory drawing which shows notionally the structure of the conventional band variable wavelength filter, and is a figure which shows a ring resonance type band variable wavelength filter. It is explanatory drawing which shows notionally the structure of the conventional band variable wavelength filter, and is a figure which shows a transversal type | mold band variable wavelength filter.

Explanation of symbols

11, 51, 71, 101, 121, 171, 191, 221 Input waveguides 12-1 to 12-N, 52-1 to 52-N, 72-1 to 72-N, 102-1 to 102-N, 122-1 to 122 -N, 172-1 to 172 -N, 192-1 to 192 -N, 222-1 to 222 -N Input side waveguide optical couplers 13-1 to 13 -N, 53-1 -53-N, 73-1 to 73-N, 103-1 to 103-N, 123-1 to 123-N, 173-1 to 173-N, 193-1 to 193-N, 223-1 to 223 -N array waveguide 14, 54, 74, 104, 124, 174, 194, 224 Output waveguide 15-1 to 15-N, 55-1 to 55-N, 75-1 to 75-N, 105-1 -105-N, 125-1 to 125-N, 175-1 to 175-N, 195-1 to 195-N, 2 5-1 to 225-N Output side waveguide optical coupler 31 Optical waveguide 31a Refractive index change region 32 3 dB coupler 121a, 221a Wavelength variable control electrode 124a, 224a on input waveguide side Wavelength variable on output waveguide side Control electrode 131 InP substrate 132 n-type InP layer 133 GaInAsP layer (λg = 1.4 μm)
134 p ^-- InP layer 135 p ⁺ -InP layer 136 p ⁺ -InGaAs layer

Claims (6)

  A pair of input waveguides and output waveguides, optical couplers arranged at regular intervals in the input waveguides and output waveguides, and N pieces connecting the input / output waveguides via the optical couplers (N: a natural number of 2 or more) array waveguides, and the length from the incident end to the exit end is increased by the same length difference in order from the path to the array waveguide connected to the incident end side or A wavelength filter comprising a ladder interference structure that decreases, and a filter band can be varied by using a coupling rate variable coupler as the optical coupler.   2. The wavelength filter according to claim 1, wherein the diffraction order of the ladder interferometer is changed by periodically changing the optical coupling rate using the coupling rate variable coupler.   By changing the refractive index of the input / output waveguide by heat, voltage application or current injection, the selected wavelength can be varied, and the selected wavelength variable operation and the band variable operation can be performed independently. The wavelength filter according to claim 1 or 2, characterized in that:   Only the optical path length difference generated by the ladder interferometer in the N-th stage array waveguide and the (N-1) -th stage array waveguide is set to be larger or smaller than the optical path length difference up to the (N-1) -th stage. The wavelength filter according to claim 1, wherein the filter frequency spectrum characteristic has flatness.   5. The wavelength filter according to claim 4, wherein a transmission band of a selected wavelength can be varied while maintaining a flat filter spectral characteristic by using the coupling rate variable coupler. 6. The wavelength filter according to claim 4, wherein the selected wavelength is variable by changing the refractive index of the input / output waveguide by heat, voltage application, or current injection, and the refractive index changing region length is guided to the array. By adjusting in proportion to the optical path length difference between the waveguides, the flatness spectrum wavelength characteristics can be kept constant during wavelength variable operation, and the selected wavelength variable operation and band variable operation can be performed independently. Tunable wavelength filter.

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