JP2015184593A - optical wavelength filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical wavelength filter in which a flat top region of a reflection wavelength filter using an MMI element is enlarged.SOLUTION: When N is an integer of 2 or more, the optical wavelength filter has: a multi-mode interference waveguide C7 having NxN ports in which N input-output ports IO7-1 to 3 are connected to N delay waveguides D7-1 to 3 each having different lengths; and reflectors M7-1, 2 provided at tip parts of the respective delay waveguides D7-1 to 3. At least one delay waveguide D7-2 of the N delay waveguides D7-1 to 3 has a resonator structure (Fabry-Perot etalon E7).

Description

  The present invention relates to an optical wavelength filter. More specifically, the present invention relates to a wavelength demultiplexing function for emitting light to different waveguides for each wavelength, or to emit light of different wavelengths incident on different waveguides from the same optical waveguide. The present invention relates to an optical wavelength filter having a wavelength multiplexing function.

  The increase in capacity of optical communication systems is largely due to wavelength division multiplexing (WDM) technology. Laser beams of different wavelengths are prepared as carrier waves, a plurality of laser beams are combined on the transmitting side and incident on one optical fiber, and the wavelength division multiplexed light is demultiplexed for each wavelength on the receiving side. A single optical fiber can transmit as much information as the number of wavelengths. An element that realizes the function of multiplexing and demultiplexing light is an optical wavelength filter.

  As optical devices used as optical wavelength filters, array waveguide diffraction grating (AWG) elements and multimode interference (MMI) waveguide elements are known. Among these, the MMI element having a plurality of input / output ports has a feature that the port from which light is output differs depending on the relative relationship of the phase of light input to each input port. For example, Non-Patent Document 1 discloses an optical wavelength filter using an MMI element.

  FIG. 1 shows a reflective wavelength filter using a conventional MMI element. Wavelength multiplexed light (wavelengths 1 to N) incident from an appropriate input / output port (IOk) is branched into N by an NxN port MMI coupler (C1) and branched into N ports. A delay waveguide (D) having a different length is connected to each of the N ports, and a reflecting mirror (M) is provided at the tip of the delay waveguide. The branched light is reflected by the reflecting mirror and then enters the MMI coupler (C1) again.

  The light reciprocating through the delay waveguide (D) has a different phase change for each wavelength because the length of the delay waveguide (D) is different. Due to the phase relationship of light incident on the MMI coupler (C1) from the N ports, light is output from a specific input / output port for each different wavelength. As shown in FIG. 1, wavelength multiplexed light (wavelengths 1 to N) is output from N input / output ports (IO1 to ION) for each different wavelength, and functions as an optical wavelength filter.

  A reflective wavelength filter using an MMI element is very compact and has a simple structure, and thus is useful as a light wavelength filter that is small and easy to manufacture.

Y. Ueda, T. Fujisawa, K. Takahata, M. Kohtoku, H. Takahashi, and H. Ishii, “InP-based Compact Reflection-Type Transversal Filter”, in Proc. Of IPRM2013, TuD4-3, 2013. M. Bachmann, P. A. Besse, and H. Melchior, “General self-imaging properties in N x N multimode interference couplers including phase relations”, Appl. Opt. Vol. 33, p. 3905, 1994. Toru SEGAWA, Shinji MATSUO, Takaaki KAKITSUKA, Yasuo SHIBATA, Tomonari SATO, Yoshihiro KAWAGUCHI, Yasuhiro KONDO, and Ryo TAKAHASHI, “Monolithically Integrated Wavelength−Routing Switch Using Tunable Wavelength Converters with Double−Ring−Resonator Tunable Lasers Electronics”, IEICE TRANSACTIONS on Electronics , Vol.E94-C, pp.1439-1446, 2011. Takeshi Fujisawa, Sigeru Kanazawa, Kiyoto Takahata, Wataru Kobayashi, Takashi Tadokoro, Hiroyuki Ishii, and Fumiyoshi Kano, “Large−Output−Power, Ultralow−Driving−Voltage (0.5 Vpp) Operation of 1.3−μm, 4 × 25G, EADFB Laser Array for Driverless 100GbE Transmitter ”, in Proc. Of ECOC2011, Mo.1.LeSaleve, 2012.

  The optical wavelength filter including the reflective wavelength filter using the MMI element desirably has a so-called “flat top” transmission intensity in the vicinity of the wavelength of the input light that gives the maximum transmission intensity. That is, it is important that the fluctuation of the light transmission intensity is small in the vicinity of the wavelength of the input light that gives the maximum transmission intensity. Having a given “flat-top” region allows manufacturing errors when making optical wavelength filters, or ensures stable transmission even when the wavelength of light incident on the optical wavelength filter fluctuates. An optical wavelength filter showing the intensity can be obtained. Expansion of the “flat top” region of optical wavelength filters is desired.

  An object of the present invention is to provide an optical wavelength filter in which a flat-up region of a reflective wavelength filter using an MMI element is enlarged.

  In order to achieve the above object, according to the present invention, in the first embodiment, when N is an integer of 2 or more, N input / output ports and N delay waveguides each having a different length are used. In an optical wavelength filter including an NxN-port multi-mode interference waveguide connected to each other and a reflecting mirror provided at a tip portion of each delay waveguide, at least one of the N delay waveguides The delay waveguide has a resonator structure.

  In the second embodiment, when N is an integer equal to or larger than 2, N × N-port multimode interference waveguides in which N input / output ports and N delay waveguides each having a different length are connected to each other, A plurality of optical wavelength filters including a plurality of optical wavelength filters each including a reflecting mirror provided at a distal end portion of each delay waveguide, wherein at least one of the plurality of optical wavelength filters is used. The filter has a resonator structure in at least one of the N delay waveguides.

  As described above, according to the present invention, since at least one delay waveguide has a resonator structure, the value of the phase difference of the light traveling back and forth through the delay waveguide is maintained in a wider wavelength range. It becomes possible to enlarge the flat-up area.

It is a figure which shows the reflection type wavelength filter using the conventional MMI element. It is a figure which shows the optical wavelength filter of the 1x2 port structure using the MMI element of the conventional 3x3 port. It is a figure which shows the cross section of the reflection type wavelength filter using an MMI element. It is a figure which shows the calculation result of the light transmission characteristic of the reflection type wavelength filter using an MMI element. It is a figure which shows the structure of a Fabry-Perot etalon resonator. It is a figure which shows the calculation result of the reflective characteristic of a Fabry-Perot etalon resonator. It is a figure which shows the structure of a ring waveguide resonator. 1 is a diagram illustrating an optical wavelength filter having a 1 × 2 port configuration using a 3 × 3 port MMI element according to Example 1. FIG. It is a figure which shows the calculation result of the light transmission characteristic of the optical wavelength filter concerning Example 1. FIG. 6 is a diagram illustrating a configuration of an optical wavelength filter according to Example 2. FIG. It is a figure which shows the light transmission characteristic of the output side filter of the optical wavelength filter concerning Example 2. FIG. It is a figure which shows the calculation result of the light transmission characteristic of the optical wavelength filter for a comparison. It is a figure which shows the calculation result of the light transmission characteristic of the optical wavelength filter concerning Example 2. FIG. It is a figure which shows the structure of the multiwavelength light source to which the optical wavelength filter of this embodiment is applied. It is a figure which shows the structure of the optical receiver which applied the optical wavelength filter of this embodiment.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(Example of calculation of light transmission characteristics)
First, light transmission characteristics will be described using a reflection type wavelength filter using a conventional MMI element.

  FIG. 2 shows a conventional 1 × 2 port optical wavelength filter using a 3 × 3 port MMI element. The optical wavelength filter includes an MMI coupler (C2) in which three input / output ports (IO2-1 to IO2-3) and three delay waveguides (D2-1 to D2-3) are connected. Reflecting mirrors (M2-1 to M2-3) are provided at the distal ends of the delay waveguides (D2-1 to D2-3).

  The lengths of the delay waveguides D2-1 to D2-3 have a relationship of D2-1> D2-3> D2-2 in the long order. Each length is an integral multiple of approximately half (ΔL / 2) of the unit delay length ΔL determined from the wavelength interval of the peak of the light transmission characteristics (see Non-Patent Document 1). Strictly speaking, in order to make the MMI coupler C2 function as a coupler of an optical wavelength filter, the length of each delay waveguide is finely adjusted from an integral multiple of ΔL / 2 to a length shorter than the wavelength of light. The reason why it is an integral multiple of “ΔL / 2” is because light is reciprocated through each delay waveguide because it is a reflective wavelength filter.

  FIG. 3 shows a cross section of a reflective wavelength filter using an MMI element. On the InP substrate S3, a high mesa waveguide is formed in which an InP lower clad layer CL3-1, a core layer C3 based on InGaAsP, and an InP upper clad layer CL3-2 are sequentially laminated. Further, a clad CL3-3 is laminated around the high mesa waveguide to form a buried waveguide. The core layer C3 is an InGaAsP-based quaternary mixed crystal, but any material can be used as long as it can be grown on an InP substrate and has a higher refractive index than the cladding layer. The core layer C3 has a composition that emits light having a photoluminescence peak wavelength in the vicinity of 1.15 μm.

  Next, a manufacturing method will be described. First, the InP lower cladding layer CL3-1, the core layer C3, and the upper cladding layer CL3-2 are epitaxially grown on the InP substrate S3. Next, a waveguide pattern constituting the input / output port, the delay waveguide, and the MMI coupler is formed by photolithography, and etched to the InP substrate S3 by dry etching to form a high mesa waveguide. Then, a reflecting mirror is formed by vapor-depositing Au on the tip portion of the delay waveguide using a vapor deposition device.

  Although the reflecting mirror is a metal mirror here, a highly reflecting film using a dielectric multilayer film, a reflecting mirror using reflection between simple different materials such as a semiconductor and air, or the like may be used. The material of the optical waveguide itself is also an InP-based semiconductor material, but other materials may be used as long as they can form a waveguide such as a dielectric such as glass or silicon.

  FIG. 4 shows the calculation result of the light transmission characteristics of the reflective wavelength filter using the MMI element. 4A shows the respective light intensities of light emitted from the input / output ports IO2-1 to IO2-3 when wavelength multiplexed light (wavelengths 1 to 3) is incident on the input / output port IO2-2. Show. In the numerical calculation, it was assumed that the MMI coupler C2 has no wavelength dependence and has ideal characteristics. That is, when light is incident on the MMI coupler C2 from any of the input / output ports, the light is equally branched into three regardless of the wavelength of the incident light. The material of the optical waveguide was an InP-based semiconductor material, and a metal mirror made of Au was used as a reflecting mirror. The phase relationship in the output of the three-branched light was assumed as described in Non-Patent Document 2.

  Calculation was performed for light incident at a wavelength of 1.285 μm to 1.315 μm. In this calculation, a wavelength of 1.3 μm band is selected, but the effect is the same for light having a wavelength of 1.55 μm band where the propagation loss is minimized in the optical fiber. For example, it can be seen that the light emitted from the input / output port IO2-1 has the strongest light intensity around 1.29 μm. In addition, it can be seen that the incident light is emitted from a specific input / output port according to its wavelength, and it can be understood that it operates as an optical wavelength filter.

  Since the input / output port IO2-2 also serves as a waveguide through which light enters, it is necessary to connect a circulator to the input / output port IO2-2 in order to extract light from the input / output port IO2-2. At this time, the optical wavelength filter is a 1 × 3 port optical wavelength filter. When light is not extracted from the input / output port IO2-2, the optical wavelength filter is a 1 × 2 port.

  FIG. 4B shows the phase change when light reciprocates between the delay waveguide D2-1 and the delay waveguide D2-3 with reference to the phase change in the shortest delay waveguide D2-2. When the light intensity of light near 1.29 μm emitted from the input / output port IO2-1 is maximized, the phase of the light reciprocating through the delay waveguide D2-3 reciprocates through the shortest delay waveguide D2-2. There is a phase difference of about 0.33π compared to the phase of light. There is a phase difference of about 0.67π when compared with the phase of the light reciprocated through the delay line D2-1.

  Therefore, in order to obtain an optical wavelength filter in which the flat-top region is enlarged, the phase difference of each light traveling back and forth in the delay waveguide with respect to the input wavelength giving the peak of the light transmission characteristic is in a wider wavelength region. Thus, it is sufficient that the values of 0.33π and 0.67π can be maintained.

(Resonator structure)
Therefore, in order to realize a reflective wavelength filter using MMI having a flat-top transmission characteristic, a resonator structure is incorporated in the delay waveguide.

  FIG. 5 shows the structure of a Fabry-Perot etalon resonator. The Fabry-Perot (FP) etalon resonator is an optical waveguide made of an InP-based semiconductor material having an incident side boundary surface with an electric field reflectance r1 and a reflection side boundary surface with an electric field reflectance r2. Here, r1 = 0.6 and r2 = 0.9, and the distance 3ΔL between both the boundary surfaces.

  FIG. 6 shows the calculation results of the reflection characteristics of the Fabry-Perot etalon resonator. As shown in FIG. 6A, it can be seen that the light incident on the FP etalon resonator is largely reflected at a specific wavelength due to the resonance phenomenon between the two boundary surfaces. Further, as shown in FIG. 6B, it can be seen that in the wavelength band where the light is largely reflected, the phase change of the reflected light with respect to the incident light is small compared to the wavelength band where the light reflection is small.

  That is, if the FP etalon resonator is used as a reflecting mirror of a delay waveguide of a reflective wavelength filter using an MMI element, the phase change of light in the delay line can be reduced only in a desired wavelength band. Compared with the case of using only a reflecting mirror, an optical wavelength filter having flat-top transmission characteristics in a wide wavelength range can be obtained.

  The FP etalon resonator is selected as the resonator structure for confining the light. For example, another resonator may be used as long as the phase change of the reflected light due to the input wavelength change is caused by the resonance phenomenon by confining the light. The structure has the same effect. For example, as shown in FIG. 7, a ring resonator type resonator structure in which a ring waveguide is brought close to a linear waveguide terminated by a mirror may be used. By adjusting the radius of the ring waveguide and the distance between the linear waveguide and the ring waveguide, desired reflection characteristics can be obtained.

(Optical wavelength filter)
In this embodiment, an optical wavelength filter having a 1 × 2 port configuration using a 3 × 3 port MMI element will be described as an example of a reflective wavelength filter using an MMI device.

  FIG. 8 shows an optical wavelength filter having a 1 × 2 port configuration using a 3 × 3 port MMI element according to the first embodiment. The optical wavelength filter includes a 3 × 3 port MMI coupler (C7) in which three input / output ports (IO7-1 to IO7-3) and three delay waveguides (D7-1 to D7-3) are connected. An FP etalon E7 having a distance 3ΔL between both boundary surfaces is connected to the tip of the shortest delay waveguide D7-2. Reflecting mirrors (M7-1, M7-3) are provided at the tip portions of the other delay waveguides (D7-1, D7-3).

  Of the boundary surfaces of the FP etalon E7, the boundary surface (input / output side) on the MMI coupler C7 side has a structure in which a gap of several hundred nm is formed in the semiconductor. For example, according to Non-Patent Document 3, if the gap width is appropriately selected, an electric field reflectance of about 60% (power reflectance of about 30%) can be obtained. Further, metal (Au) is formed on the other boundary surface (reflection side) of the FP etalon E7. Therefore, in calculating the light transmission characteristics of the optical wavelength filter according to Example 1 described below, the electric field reflectance on the incident / exit side of the FP etalon E7 is 0.60, and the electric field reflectance on the reflection side is 0.95. It was. Note that the loss at the reflection-side interface is ignored in this calculation.

  According to the present embodiment, if an NxN-port MMI element is used, a 1x (N-1) -port optical wavelength filter can be configured. Further, if one of the input / output ports is a port that serves both as an input and an output, a 1 × N port optical wavelength filter can be configured. When N is an integer equal to or greater than 2, N × N port multimode interference waveguides connected to N N input / output ports and N delay waveguides of different lengths, and respective delays In an optical wavelength filter having a reflecting mirror provided at a tip portion of a waveguide, it is only necessary that at least one delay waveguide out of N delay waveguides has a resonator structure.

(Example of calculation of light transmission characteristics)
FIG. 9 shows the calculation results of the light transmission characteristics of the optical wavelength filter according to the first example. FIG. 9A shows light emitted from the input / output ports IO7-1 to IO7-3 when light having a wavelength of 1.260 μm to 1.360 μm is incident on the input / output port IO7-2 of the optical wavelength filter. Each light intensity is shown. As apparent from the comparison with FIG. 4A, it can be seen that a flat-top characteristic with a small change in transmittance is obtained near the wavelength where the light transmittance reaches a peak, and the region is enlarged. .

  FIG. 9B shows the phase change when light reciprocates between the delay waveguide D7-1 and the delay waveguide D7-3, with reference to the phase change in the shortest delay waveguide D7-2. As shown in FIG. 6, in the FP etalon E7, the phase change of the reflected light is gentle in the vicinity of the wavelength where the reflectance reaches a peak. Therefore, when the phase change of the delay waveguides D7-1 and D7-3 is seen with reference to the phase change of the delay waveguide D7-2 having the FP etalon E7, the delay waveguide D7-1 in a specific wavelength band is seen. , D7-3 shows a gradual phase change. The phase of the light reciprocating through the delay waveguide D7-3 has a phase difference of about 0.33π compared to the phase of the light reciprocating through the delay waveguide D7-2, and the light reciprocated through the delay line D2-1. There is a phase difference of about 0.67π compared to the phase of. Compared with the conventional example shown in FIG. 4, it can be seen that the value of this phase difference is maintained in a wider wavelength region, and the flat-up region is enlarged.

  FIG. 10 shows the configuration of the optical wavelength filter according to the second embodiment. By connecting a plurality of optical wavelength filters in multiple stages, a multi-stage optical wavelength filter having more channels can be produced. The optical wavelength filter according to the second embodiment includes a 1 × 2 port incident side filter F9-1 and a 1 × 4 port output side filter F9-2 and F9-3 connected in multiple stages to expand the number of ports of the optical wavelength filter. Yes. It is effective to use the top flat type optical wavelength filter of this embodiment as one of such multistage connection type optical wavelength filters.

  FIG. 11 shows the light transmission characteristics of the emission side filter of the optical wavelength filter according to the second embodiment. FIG. 11A shows the light transmission characteristics of the output side filter F9-2, and FIG. 11B shows the light transmission characteristics of the output side filter F9-3.

  FIG. 12 shows a calculation result of the light transmission characteristics of the optical wavelength filter for comparison. The light transmission characteristics of the filters connected in multiple stages, that is, from the input port I9-1 of the incident side filter F9-1 to the output ports O9-2-1 to O9-2-4 of the output side filter F9-2, The light transmission characteristics up to the output ports O9-3-1 to O9-3-4 of the emission filter F9-3 are shown. A reflection type wavelength filter using the conventional MMI having the light transmission characteristics shown in FIG. 4 is applied to the incident side filter F9-1. Reflecting the light transmission characteristics shown in FIG. 4, in FIG. 12, the outputs of the output ports O9-2-1 and O9-2-4, and O9-3-1 and O9-3-4 are lowered. Yes.

  FIG. 13 shows the calculation result of the light transmission characteristics of the optical wavelength filter according to the second embodiment. The light wavelength filter of the present embodiment having the light transmission characteristics shown in FIG. 9 is applied to the incident side filter F9-1. Reflecting the light transmission characteristics shown in FIG. 4, the light transmission characteristics of the entire multistage connection type optical wavelength filter are the same at all output ports without impairing the light transmission characteristics of the output side filter shown in FIG. It can be seen that the light is emitted with the light intensity of.

  As described in Non-Patent Document 4, light from a plurality of laser elements manufactured on the same substrate is combined and output using a multiplexer that is also manufactured on the same substrate. Monolithic multi-wavelength light sources are known. Such a multi-wavelength light source is compact and can multiplex light from a plurality of laser elements having different wavelengths with low loss. If a conventional optical wavelength filter is used as the multiplexer used here, the light output intensity from the multi-wavelength light source will fluctuate when the laser oscillation wavelength fluctuates due to manufacturing errors, chip temperature, etc. . When the optical wavelength filter of the present embodiment is used, a multi-wavelength light source in which the flat-up region is enlarged and the output light intensity is stable can be obtained.

  FIG. 14 shows a configuration of a multi-wavelength light source to which the optical wavelength filter of this embodiment is applied. The optical wavelength filter includes an MMI coupler (C13) in which three input / output ports (IO13-1 to IO13-3) and three delay waveguides (D13-1 to D13-3) are connected. An FP etalon E13 having a distance of 3ΔL between both boundary surfaces is connected to the tip of the shortest delay waveguide D13-2. Reflecting mirrors (M13-1, M13-3) are provided at the tip of the other delay waveguides (D13-1, D13-3).

  If light sources 13-1 and 13-2 having different wavelengths are connected to the two input / output ports IO13-1 and IO13-3, wavelength multiplexed light can be obtained from the input / output port IO13-2. According to the optical wavelength filter of the present embodiment, even if the wavelength of light from the laser fluctuates, the fluctuation of the output is small, the laser element caused by the manufacturing error of the light sources 13-1 and 13-2 and the change of the external environment Thus, it is possible to obtain a monolithic multi-wavelength light source having a stable light intensity even when the oscillation wavelength fluctuates.

  FIG. 15 shows the configuration of a light receiving device to which the optical wavelength filter of this embodiment is applied. If the photodetectors (PD14-1 and 14-2) are connected instead of the light sources 13-1 and 13-2 of the multiwavelength light source shown in FIG. 14, light reception using the optical wavelength filter as a duplexer circuit is also possible. A device can be configured. Even if each wavelength of the WDM signal incident on the input / output port IO13-2 fluctuates, a stable light intensity can be detected by the photodetector.

IO I / O Port C MMI Coupler D Delay Waveguide M Reflector E FP Etalon F Filter I Input Port O Output Port PD Photodetector

Claims (8)

When N is an integer of 2 or more, a multimode interference waveguide of NxN ports in which N input / output ports and N delay waveguides having different lengths are connected to each other, In an optical wavelength filter including a reflecting mirror provided at a tip portion,
An optical wavelength filter having a resonator structure in at least one of the N delay waveguides.   The optical wavelength filter according to claim 1, wherein the resonator structure is a Fabry-Perot etalon resonator.   The optical wavelength filter according to claim 1, wherein the resonator structure is a ring resonator. When N is an integer of 2 or more, a multimode interference waveguide of NxN ports in which N input / output ports and N delay waveguides having different lengths are connected to each other, In a multi-stage connection type optical wavelength filter comprising a plurality of optical wavelength filters including a reflecting mirror provided at the tip portion, connected in multiple stages,
At least one optical wavelength filter among the plurality of optical wavelength filters has a resonator structure in at least one delay waveguide among the N delay waveguides.   5. The multistage optical wavelength filter according to claim 4, wherein the resonator structure is a Fabry-Perot etalon resonator.   The multistage connection type optical wavelength filter according to claim 4, wherein the resonator structure is a ring resonator.   4. A monolithic multi-wavelength light source, wherein a laser element is connected to at least one input / output port of the optical wavelength filter according to claim 1, 2 or 3.   4. A light receiving device, wherein a photodetector is connected to at least one input / output port of the optical wavelength filter according to claim 1, 2 or 3.