JP2019082633A - Reflection type wavelength filter - Google Patents

Abstract

To provide a waveguide type wavelength filter in which a wavelength variable amount is wider than a variable wavelength region limited by a refractive index and is sufficient.SOLUTION: Provided is a reflection type wavelength variable filter comprising a multimode interference waveguide of M×K port configuration (M is an integer greater than or equal to 1, K is an integer greater than or equal to 2), and K delay lines connected to each of the K ports of the multimode interference waveguide and mutually differing in length. Each of the K delay lines is provided with at least one of a first electrode, a second electrode and a third electrode, and assuming that ri (i=1, 2, etc., to K), dL, L0, Le1, Le2, Le3 are real numbers greater than or equal to zero and B is the largest value among ri's or greater, when the length of the K delay lines is L0+ri×dL, the length of the first electrode is ri×Le1, the length of the second electrode is (B-ri)×Le2, and the length of the third electrode is Le3.SELECTED DRAWING: Figure 4

Description

  The present invention relates to an optical device, and more particularly to a reflective wavelength filter capable of reflecting light of any wavelength in a wavelength range within a certain range.

  One factor of the large capacity of optical communication is largely due to wavelength-division multiplexing (WDM) technology, that is, laser light of different wavelengths is used as a carrier and this is multiplexed on the transmission side to form a single fiber. It is possible to transmit a large amount of information through one optical fiber by demultiplexing the reception light for each wavelength and receiving the light for each wavelength on the reception side.

  In constructing a WDM communication system, wavelength filters capable of selectively extracting specific wavelengths or combining different wavelengths into one optical path are important optical devices in constructing a WDM system.

  In particular, a waveguide type wavelength filter manufactured using a multilayer thin film substrate in which different materials are formed in multiple layers can save space and increase the space of the system from the viewpoint of its compactness and functionality when monolithically made with other functional elements. It is advantageous for functionalization. Often it is required to adjust the spectral properties of this waveguide-type filter with an external signal.

  Non-Patent Document 1 describes a method of adjusting the transmission spectrum of an arrayed waveguide grating (AWG) which is a typical example of a waveguide filter. In Non-Patent Document 1, the transmission spectrum of AWG made of Si is changed using the temperature dependence of the refractive index of Si. This is based on the fact that the transmission peak wavelength is proportional to the refractive index of the waveguide forming the array diffraction grating when focusing on the pair of input / output ports of the AWG.

  Besides the AWG, there are many reports on adjusting a filter with a ring resonator. In Non-Patent Document 2, a wavelength tunable filter for a tunable light source (TLS) is obtained by injecting a current into a ring resonator made of a compound semiconductor to change the refractive index. The characteristic spectrum of the ring resonator shifts in proportion to the refractive index of the material, as in the AWG.

  In addition, as a typical filter that can cope with other waveguide types, a distributed Bragg reflector (DBR) that reflects light of a specific wavelength can be mentioned. Similar to AWGs and ring resonators, the characteristic spectrum of DBR shifts in proportion to the refractive index of its waveguide material. Non-Patent Document 3 describes changing the reflection spectrum of DBR by changing its refractive index in a minute DBR portion by local heating with a microheater. In this method, TLS is formed by sandwiching the optical gain medium with two DBRs.

Jie Hyun Lee, Heuk Park, Sae-Kyoung Kang, Jong Hyun Lee and Jyung Chan Lee, "Silicon AWG as a tunable filter in tunable ONU for TWDM-PON application", in Proc. of COIN 2014, FA1-5. 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-Driven Surfaces , Vol. E94-C, pp. 1439-1446, 2011. Hiroyuki Ishii, Fumiyoshi Kano, Yuichi Tohmori, Yasuhiro Kondo, Toshiaki Tamamura, Yuzo Yoshikuni, "Narrow spectral linewidth under wavelength tuning in thermally tunable super-structure-grating (SSG) DBR lasers", IEEE JSTQE, vol. 1, pp. 401-407 (1995). Yuta Ueda, Takeshi Fujisawa, and Masaki Kohtoku, "Eight-Channel Reflection-type Transversal Filter for Compact and Easy-to-Fabricate InP-Based Optical Multiplexer", J. Org. lightw. Technol. , Vol. 34, pp. 2684-2691 (2016). Yuta Ueda, Takeshi Fujisawa, Kiyoto Takahata, Masaki Kohtoku, and Hiroyuki Ishii, "InP-based compact transversal filter for monolithically integrated light source array", Optics Express, vol. 22, no. 7, pp. 7844-7851 (2014). Y. Ueda, Y. Ogiso N. Kikuchi, "InP PIC technologies for high-performance Mach-Zehnder modulator", in Proc. of SPIE Photonics WEST 2017 OPTO 10129-4.

  A common problem with the filters described above is that the filter characteristics are proportional to the refractive index of the waveguide. In other words, the variable amount of wavelength is determined by the control range of the refractive index of the waveguide. There are many methods to change the refractive index of the waveguide, such as temperature, current, electric field, stress, but in any case, although it is at most 1% or less, it was not sufficient as a wavelength variable. .

  For example, if the peak wavelength of one filter is set to 1550 nm used in the optical communication wavelength band, the variable wavelength amount is 10 nm or less, although it depends on the refractive index of the material. The wavelength band of the optical communication is, for example, 40 nm wide even if it is limited to the C band where the optical loss of the optical fiber is the smallest, and it is understood that the above-mentioned variable wavelength amount is insufficient. The reason common to all the above problems is that the spectrum of the filter is in one-to-one correspondence with the refractive index constituting the filter.

  In recent years, it has been reported that the filter characteristic is realized not by the refractive index of the waveguide but by the phase of light propagating through the waveguide. A reflection-type transversal filter (RTF) using multi-mode interference (MMI) of Non-Patent Document 4 is a light incident on an MMI in an N × N port configuration. Is branched into N, reciprocates N reflective delay lines of different lengths, and enters the MMI again, reflecting the phase relationship of light from the N delay lines, and It has the function of returning light to the port. That is, light is collected at a specific port only when the phases of light from the delay waveguides are in a specific relative relationship.

  The important point in the multimode interference waveguide as in Non-patent Document 4 is that the filter characteristics can be controlled not by the refractive index but by the phase. The phase change of light is proportional to the length change of the propagating optical waveguide and the refractive index change of the material, and inversely proportional to the wavelength (proportional to the frequency). Therefore, it is understood that the phase change can be increased by increasing the length of the waveguide for changing the refractive index, even if the amount of change in refractive index is small.

  FIG. 1 is a diagram showing the configuration of a phase control RTF as in Non-Patent Document 4. As shown in FIG. The RTF has a configuration in which a multimode interference waveguide 30 is provided between the input / output waveguide 10 and the delay lines 20 having different lengths each provided with a reflective portion at the end. The filter characteristic of RTF is defined by the difference in length of each delay line, and there is a degree of freedom as to the length of the shortest delay line (referred to as a reference delay line). That is, by providing an electrode for changing the refractive index of the waveguide by increasing the length of the reference delay line, it is possible to increase the change in the phase reciprocating the delay line. In this case, deterioration of the filter characteristics due to the careless lengthening of the delay line also practically exists (Non-Patent Document 4), but if the length of expansion is limited, the degree of deterioration may be sufficiently reduced. it can.

  The principle of controlling the filter characteristics with the phase as described above can also be realized by Non-Patent Document 5 in which RTF by MMI is not reflective. The advantage of the reflective type as in Non-Patent Document 4 is that light travels back and forth in the delay line, so phase change occurs when a refractive index change is given to a certain waveguide as compared with the transmissive type that realizes the same principle. There is an advantage that is doubled.

  The control of the filter characteristics not by the refractive index but by the phase of the waveguide is strictly not physically impossible even with the AWG, but it is extremely difficult in practice, or its effect is small. The main difference between the AWG and the transversal filter is the large number of delay lines and the continuity of the phase of light emitted from each delay line.

  The inventors of the present invention have intensively studied the above-mentioned problems, and have found that the transversal filter using a multimode interference waveguide, in particular, the filter according to the phase of light propagating through the waveguide using a reflective transversal filter that is a reflection type. By controlling the characteristics, it has been found that the variable amount of wavelength can be made larger than that of the prior art, resulting in the present invention.

  The present invention has been made in view of the above-described conventional problems, and an object of the present invention is to provide a waveguide type wavelength filter which is wider than the variable wavelength range limited by the refractive index and has a sufficient amount of variable wavelength. It is in.

  In order to solve the above-mentioned problems, the invention described in one embodiment comprises: a multimode interference waveguide having M (M is an integer of 1 or more) × K (K is an integer of 2 or more) port configuration; A reflective wavelength filter comprising K delay lines of different lengths connected to each of K ports of the mode interference waveguide, wherein each of the K delay lines is And at least one of the second electrode and the third electrode is provided, and ri (i = 1, 2,..., K), dL, L0, Le1, Le2, Le3. Is a real number greater than or equal to zero, B is greater than or equal to the largest value among ri, and the length of the K delay lines is L0 + ri × dL, the length of the first electrode is ri × Le1 , The length of the second electrode is (B−ri) × Le 2 and the length of the third electrode is Le 3 A reflection-type wavelength tunable filter, wherein the door.

It is a schematic diagram for demonstrating the advantage of the phase control type filter element. 7 is a cross-sectional structure of a waveguide of the reflective wavelength tunable filter used in the first embodiment. It is a numerical calculation result of the phase change at the time of applying a voltage to the waveguide structure of FIG. FIG. 2 is a view showing a schematic configuration including an electrode structure of a reflective wavelength tunable filter used in Example 1. It is a figure which shows the spectrum characteristic of the reflection type wavelength tunable filter of FIG. 4, and its modulation characteristic. It is a figure which shows the spectrum characteristic at the time of making r1 and r2 into a non-integer in the reflection type wavelength tunable filter of FIG. 4, and its modulation characteristic. It is a figure which shows the reflection type wavelength-variable filter which has the characteristic of FIG. FIG. 7 is a schematic view of a multi-wavelength light source or a multi-wavelength receiver used in Example 3. FIG. 10 is a schematic view of a wavelength tunable light source used in Example 4. FIG. 16 is a schematic view of a wavelength tunable light source used in Example 5. It is a reflection spectrum and its product in two reflection type wavelength tunable filters in Example 5. FIG. It is a figure which shows the reflection spectrum in two reflection type wavelength tunable filters before and behind which shifted the spectrum of one reflection type wavelength tunable filter about 4 nm in the wavelength tunable light source of FIG. 10, and its product. FIG. 18 is a schematic view of a wavelength tunable light source integrated Mach-Zehnder modulator used in Example 6.

  Hereinafter, embodiments of the present invention will be described in detail.

  In the first embodiment, an example in which a reflection-type transversal filter (RTF) using a multi-mode interference (MMI) is used will be described.

  As a material of the waveguide, an InP-based compound semiconductor (FIG. 2A) having a multi-quantum well (MQW) structure with a photoluminescence wavelength of 1375 nm is used. Here, although the above-described materials are taken as an example, other materials such as Si and glass may be used.

  Different types of semiconductors are deposited in the form of a multilayer film on the substrate S to form a laminated substrate. InP is used as the substrate S, and MQW made of InAlGaAs and InAlAs with a total layer thickness of 0.5 μm is stacked thereon as the core layer CO, and InP is stacked as the cladding layer CL of 2.0 μm.

  The laminated substrate of FIG. 2A is processed into a waveguide shape by dry etching as shown in FIG. 2B, and electrodes Pi, Ni and Ji are provided on the waveguide path by a vapor deposition process.

  The quantum-confined Stark effect (QCSE), in which the refractive index changes when an electric field is applied to the MQW, utilizes the refractive index change due to voltage control. Refractive index change due to voltage control has an advantage that the target material is made insulating, and if the leak current is sufficiently small, the power consumption becomes as small as possible.

  In the present embodiment, if the cladding CL side is a p-doped semiconductor and the substrate S is an n-doped semiconductor, the voltage value is broken if a negative voltage is applied to the cladding CL side based on the potential of the substrate S. Below the voltage the current through this structure is small.

  On the other hand, as a demerit of refractive index change due to voltage control, the refractive index change is about 1/10 compared to current injection or local heating, and if the voltage is made too large to obtain a large refractive index change, There is a problem that the load on the circuit is increased, or the current flows out beyond the breakdown voltage of the semiconductor material in the first place.

  As described later, according to the present invention, even if the change in refractive index is small, the amount of change in filter characteristics can be increased by designing the electrode length. Therefore, although the power efficiency is high, such as QCSE, the benefit is great for the physics in which the maximum value of the refractive index change is determined.

  FIG. 3 shows the phase change when a voltage is applied to the waveguide when the length of the waveguide direction shown in FIG. 2 (b) is 100 μm, based on the method of Non-Patent Document 6 Calculated numerically. Each shows the relationship between the applied voltage and the phase change for each input wavelength (unit: micrometer). For example, for a light of 1.55 μm, applying a voltage of 10 V to the waveguide causes a phase change of approximately 0.3π. In the present embodiment, the reflection type wavelength filter shown in FIG. 4 is configured using the above waveguide structure, and the characteristics thereof are observed.

FIG. 4 (a) is a view showing an example of a reflection type wavelength filter configured using the waveguide structure of FIG. 2 (b). As shown in FIG. 4A, the reflection type wavelength filter of the present embodiment has a multimode interference waveguide 30 with M (number of input ports) = K (number of output ports) = 5, and a multimode interference waveguide. The configuration includes five input / output waveguides 10 connected to five input ports of 30 and five delay lines 20 connected to five output ports of multimode interference waveguide 30, respectively. For the purpose of explanation, identification numbers are given to the five delay lines 20 in the order of i = ₁ , ₂ , ₃ , ₄ and ₅ from the top (201, 202, 203, 204, 205). In Figure 4, the shortest delay line (see delay line) 20 ₃ a third (i = 3) to its length and L0. The delay lines of the other delay lines 20 ₅ , 20 ₄ , 20 ₂ , and 20 ₁ become longer in the order of i = 5, i = 4, i = 2, i = 1, and the lengths of the delay lines become Li Then, it can be set as Li = L0 + ri × dL. Here, L0 and dL are real numbers greater than or equal to zero. A mirror 22 is provided at the other end of the five delay lines 20 not connected to the output port of the multimode interference waveguide 30.

  Each delay line 20 is provided with a fixed phase shifter 21 which does not move the phase dynamically. The reflection type variable wavelength filter in the present embodiment and the other embodiments is configured based on a so-called reflection type transversal filter as shown in FIG. 1, and the portion of this fixed phase shifter 21 is a reflection type. The wavelength tunable filter functions as a transversal filter. That is, the amount of phase shift by this fixed phase shifter 21 is adjusted to match the transfer function of the MMI 30, and the reflective wavelength filter is appropriately selected so as to exhibit filter characteristics. When ri is an integer of 1 to 4, the circuit of FIG. 4 is a reflective discrete Fourier transform element. The order of the lengths of the delay lines 20 and the fixed phase shift amount of the phase shifter 21 corresponding thereto can be set based on certain arbitraryness as described in Non-Patent Document 5.

  In the case where the circuit of FIG. 4 is a discrete Fourier transform element, when light enters a specific port, the light is branched into five by the MMI 30 and reciprocates each delay line, and light is output again to the port. The wavelength (frequency) of the light to be output is a certain frequency interval Δf / 5. That is, focusing on a specific port, light is periodically reflected at each frequency interval Δf.

  Here, there is a relationship of Δf = c / ng / 2 / dL. c is the speed of light, ng is the group index of the waveguide, and factor 2 is derived from the fact that the filter of FIG. 4 is reflective. Δf is generally referred to as free spectral range (FSR). That is, the design of dL can control the FSR of each port. Here, assuming that dL = 31.3 μm, r1 = 4, r2 = 3, r3 = 0, r4 = 2, and r5 = 1.

  In addition, up to three electrically separated electrodes Pi, Ni, Ji are provided on each delay line. In these electrodes, the length of the electrode Pi is ri × Le1 and the length of the electrode Ni is (B−ri) × Le2 (where B ≧ max (ri): B is greater than or equal to the largest value among ri) The length of the electrode Ji can be set to Le3, and B = 4 is adopted in this example. Then, it is assumed that Le3 = 600 μm, assuming that Le1 = Le2 = 120 μm. Le1, Le2, Le3 are real numbers greater than or equal to zero.

  The three electrodes Pi, Ni, and Ji need to be electrically separated in each delay line 20, but the electrodes of the same group (Pi to each other, Ni to each other, Ji to each other) between different delay lines 20 are required. May be short circuited as shown in FIG. 4 (b). When an electrical signal is applied to the short-circuited terminal by adopting the configuration of FIG. 4B, a phase change proportional to the length of the electrodes Pi, Ni, and Ji provided on the respective delay lines 20 occurs. In order to cause the spectrum shift described later, there is an advantage that the control is simplified.

  FIG. 5A shows the result of numerical calculation of the intensity of light returned to each port on the input side of MMI 30 when light is incident on port 3 on the input side of MMI 30. The spectrum at the same port 3 when light is incident on the port 3 on the input side of the MMI 30 is a reflection spectrum as viewed from this port 3. In FIG. 5A, the spectrum of each port is indicated by # port number.

  According to FIG. 5A, it can be seen that light of different wavelengths appears at each port every 2 nm, and when focusing on one port, peak wavelengths appear every 10 nm. That is, the FSR when focusing on one port is 10 nm.

  In FIG. 5 (b), in the case where the voltage is not applied to the electrodes focusing on only port 2 and the voltage of 5 V is applied to all the electrodes Pi, 5 V is applied to all the electrodes Ni. The spectrum of each case at the time of applying the voltage of is shown.

  According to FIG. 5B, it can be seen that the spectrum shifts to the long wave side when applied to the electrode Pi, and the spectrum shifts to the short wave side when applied to the electrode Ni. Although only port 2 is focused on in FIG. 5B, this wavelength shift occurs at all ports other than port 2. This can be understood as follows.

  In the transversal filter according to the MMI 30, the phase θi of light reciprocated in the reference delay line of the identification number i is expressed as in the following equation (1).

  In equation (1), λ is the wavelength of light, n is the refractive index at wavelength λ, Li is the length of the delay line of identifier i (Li = L0 + ri × dL), and φi is the voltage to electrodes Pi, Ni, Ji The phase shift by the applied external signal is shown.

_Assuming that the phase of light reciprocated in the reference delay line is θi _{= ref} , the relative phase of each delay line is given by the following equation (2).

  Dispersion relationship between wavelength and frequency f = c / λ (f: frequency of light, c: speed of light), and refractive index as n = n'λ + ng (n ': dispersion of refractive index, ng: group refractive index) Then, the following equation (3) is obtained.

  Here, assuming that the spectral characteristics shift from f to f + Δf when no signal is applied to the variable phase shifters (electrodes Pi, Ni, Ji) of the delay lines and when the signals are applied,

Is true.

For example, when a signal is not applied to the reference delay line (φ _{i = ref} = 0),

  If the phase shift amount proportional to ri of each delay line is given other than the reference delay line, the spectrum will be shifted. That is, as described above regarding the length of the phase adjustment electrode Pi, if the length of the phase adjustment electrode Pi is set by the product of ri and the length of the constant Le1, the same voltage is applied to all the Pi. , The spectrum shifts to the long wavelength side. However, if attention is paid to the above equation (5), the sign of the phase change φi is basically one as long as the physics of the refractive index change for phase shift is the same. For example, if .phi.i> 0, .DELTA.f <0, and it is possible to shift only to the low frequency side (long wavelength side if it is wavelength).

Then, assuming that the phase shift amount φ _{i = ref of} the reference delay line is a positive finite value, according to equation (4)

Therefore, it can be understood that if φi <φi _{= ref,} then Δf> 0.

Here, φ _{i = ref} = ABΔf (B> 0) and the phase shift amount of the reference delay line is represented by the product of the arbitrary localization number B and the phase shift amount of Δf. At that time φ i

  If it is noted that .DELTA.f.gtoreq.0 in order to satisfy .phi.i.gtoreq.0 for all delay lines with identification numbers i, the following equation (8) is obtained.

  That is, as described in relation to the length of the phase adjustment electrode Ni, the length of the phase adjustment electrode Ni is the product of B-ri (where B is the largest value of ri or more) and the length of the constant Le2. If set, when the same voltage is applied to all Ni, the spectrum shifts to the short wavelength side.

  FIG. 5C also shows only the spectrum of port 2 and shows spectra in the case where no voltage is applied to the electrodes and in the case where a voltage of 9 V is applied to all the electrodes Ji. According to FIG. 5C, it can be seen that when 9 V is applied to all the electrodes Ji, a peak appears in the middle of the peak of the existing filter. Here, the electrodes Ji are provided only on the electrodes of i = 2 and i = 5, and the lengths thereof are all L3 = 600 μm as described above.

  This principle will be described using equation (3). Now, assuming that the spectrum has a peak at the frequency f = f0, the equation (3) can be modified as shown in the following equation (9).

  On the other hand, the middle of the spectral peak, that is, the phase relationship when the frequency is shifted from f0 by FSR of the spectrum is

  If the phases of Equation (9) and Equation (10) are equal, Equation (11) below can be derived.

Since ri is an integer from 0 to 4, if no voltage is applied to the reference delay line (φ _{i = ref} = 0), no signal is applied to the delay lines of ri = 0, 2 and 4. That is, a signal is not applied to the delay lines of identification numbers i = 3, 4 and 1, but a voltage may be applied such that the phase change of each of the delay lines of identification numbers i = 2 and 5 is π. Specifically, when the groups of the electrodes Ji are provided with electrodes of the same length and the same voltage is applied to them, the action shown in FIG. 5C is obtained. More generally, it depends on the number of delay lines and the like as to which delay line the electrode Ji is provided, but in any case, it is important to provide an electrode with the same length in some of the delay lines.

  Incidentally, the method of changing the filter characteristics by controlling the operation of the equation (11), that is, controlling only the phase of a specific delay line, is impossible with the AWG in which the phase continuity for each delay line is important.

  In the first embodiment described above, the wavelength adjustment is premised on QCSE. However, in the phase adjustment using not only QCSE but any refractive index change, the longer the electrode length, the higher the efficiency.

  Finally, in the case of increasing the electrode length, the length of the reference delay line L0 is made finite, and the improvement of the wavelength shift efficiency when the electrode Pi on the delay line is increased. Compare with AWG.

  Since the electrode Pi on the delay line needs to be proportional to ri according to the above description, when the electrode length obtained by the extension of the reference delay is Δl, the electrode of the longest delay line may first be elongated by Δl it can. The identification number of the delay line at this time is i = m. At that time, assuming that the electrode (length Li) on a certain delay line becomes the length Li ′, the rate of the change becomes the following equation (12) from the above-mentioned requirement that the ratio between the electrodes is constant.

  Therefore, the longer the delay line electrode, in other words, the longest delay line length, the smaller the benefit of improving the modulation efficiency. AWGs with K port configurations generally require a delay line of 3 to 4 times K, ie an AWG with 4 times the length of the largest delay line compared to the transversal filter is also described in Figure 1 The benefit of modulation efficiency from the expansion of the reference delay line as described above is small.

  After all, the refractive index of the entire element (the entire delay line) is controlled as in Non-Patent Document 1 rather than the expansion of the refractive index change (ie, phase control) by the extension of the modulation region length in the AWG for the above reasons. It can be said that the control of the filter characteristic by that is advantageous. Furthermore, in the reflection type wavelength tunable filter of the present embodiment, one advantage is that the benefit of the electrode length enlargement by the reference delay line enlargement is large as far as the element size allows.

FIG. 7 is a view showing the configuration of a reflection type variable wavelength filter adopted in this embodiment. Example 1 (the N from 20 ₂ 20 _5, P 20 _1, 20 _2, 20 _4, 20 ₅₎ electrodes N or electrode P is the delay line 20 which is provided all about the electrode N electrode group or electrode P Although the electrode groups are short-circuited in this embodiment, in this embodiment, at least one and K-1 of the electrode groups of the electrode group of the electrodes N provided on the delay line or the electrode group of the electrodes P are provided. A reflective wavelength tunable filter is used which has a configuration in which the following electrodes N or electrodes P are short-circuited, and the remaining electrodes N or electrodes P are also short-circuited separately. The other configurations of the reflective wavelength tunable filter are similar.

  In preparation for understanding the advantages of the configuration of the reflective wavelength tunable filter used in the present embodiment, the filter characteristics are made to have an envelope by intentionally removing from the regularity the factor ri that determines the length of the delay line. Think.

  In the first embodiment, the method of selecting ri is not particularly limited except that it is a real number of 0 or more, and is an integer value of 0 to 4. However, when the special selection is made, the filter characteristic can be made more sophisticated. There is. For example, of ri selected in the first embodiment, r1 and r2 are set to r1 = 5.4 and r2 = 4.05, respectively. When this is intentionally decomposed, r1 = 4 × (1 + 0.35) and r2 = 3 × (1 + 0.35).

  The filter spectrum when light is incident on the port 3 at this time is shown in FIG. Due to the factor 0.35 of the delay line length described above, an envelope appears in the filter characteristics, as can be seen from FIG. 6 (a). The period of this envelope is about 28 nm, which corresponds to 1 / 0.35 times the FSR (10 nm) of one port given by dL in FIG. 4A. Such filter characteristics are important as, for example, a wavelength selection mechanism for a multi-wavelength light source or a wavelength variable light source described in the third and subsequent embodiments. Thus, the spectral characteristics can be changed by specially designing the lengths of some delay lines.

  Here, in the reflection type wavelength tunable filter of the present embodiment, as shown in FIG. 7, the electrodes of i = 1, 2 and i = 4, 5 are short-circuited respectively, and the electrode group of terminal P12 and the electrode group of P45 Divided into The spectrum at the time of applying the voltage of 5V only to the terminal P45 here is FIG.6 (b). According to FIG. 6 (b), it can be seen that the envelope is shifted as compared with FIG. 6 (a). In FIG. 6, the spectrum of each port is indicated by # port number.

  FIG. 6 (c) is a spectrum when a voltage of 5 V is applied to both P12 and P45. In this case, it can be seen that only the spectrum of each port shifts while the position of the envelope remains unchanged.

  In this way, a part of the delay line is changed with another design rule of the delay line (here, ri is an integer and a non-integer), and the changed and short electrodes are separately shorted, and the electric signal is individually It can be seen that by applying, the envelope of the spectral shape can be changed.

FIG. 8 is a view showing the configuration of a multi-wavelength light source or a multi-wavelength light receiver adopted in this embodiment. In this embodiment, the light source 40 is provided on the input side of the input / output waveguide 10 of the reflection type wavelength tunable filter, and the light receiver 50 is provided on the output side of the multiwavelength light source or the reflection type wavelength tunable filter. Using the In Figure 8, only the input and output waveguides 10 _3, which is arbitrarily selected light source 40 may not be provided the light receiver 50.

  When the wavelength of light from the light source or the light signal incident from the outside of the device changes, the transmissivity between the input / output port and the light source or light receiver is maximized using the multi-wavelength light source or the multi-wavelength light receiver Adjust the electrodes Pi, Ni and Ji to block specific wavelengths in a similar or intentional manner. For example, controlling Pi and Ni can maximize the maximum transmittance for a given wavelength of light.

  Further, as in the second embodiment, an envelope is provided in the spectrum, and the cutoff wavelength can be controlled by the electrode Ji when blocking other than the target wavelength.

  Furthermore, in many of these applications it is often necessary to keep the filter characteristics of the device constant by continuing to apply an electrical signal to the electrodes over time.

  The configuration adopted in this embodiment has low power consumption but has high utility value in physics (for example, QCSE) having a small change in refractive index because the length of the electrode is arbitrary (the above-mentioned degree of freedom of L0). . That is, it is advantageous in terms of power consumption in an application in which an electrical signal is continuously applied for a long time as in the present embodiment.

  FIG. 9 is a diagram showing the configuration of the wavelength variable light source adopted in this embodiment. In this embodiment, a tunable laser source (TLS) configured by providing a semiconductor optical amplifier (SOA) 60 on the input side of the reflective wavelength tunable filter of the embodiment 1 or 2 is adopted. ing. A part (or all of the cases shown in FIG. 9A) of the M ports on the input side of the reflection type tunable filter (a case of all is shown in FIG. 9A) shows a tunable tunable light source.

  The other end of each semiconductor optical amplifier 60 is basically terminated by a mirror 22. However, in FIG. 9A, one of the delay lines (here, the fourth delay line) is not mirror-terminated. Accordingly, each semiconductor optical amplifier 60 receives the wavelength selectivity of the reflective wavelength tunable filter, oscillates a specific wavelength, and light is output from the fourth delay line. Since the wavelength of light reflected is different for each SOA, each semiconductor optical amplifier 60 oscillates for a different wavelength of light.

  This is because, when the semiconductor optical amplifier 60 is connected to a plurality of ports of the reflection type wavelength tunable filter of the first or second embodiment, if the semiconductor optical amplifier 60 which injects and excites current is selected, the wavelength from the element is correspondingly selected. It means that you can choose. A signal may be applied to each electrode for fine adjustment of the wavelength.

  Further, if the reflection type wavelength tunable filter of the embodiment 1 or 2 is adjusted so that the envelope has the spectrum characteristic shown in FIG. 6, the wavelength selectivity is improved, which is higher than the case where the envelope is not used. The oscillation wavelength can be selected with accuracy.

  Further, FIG. 9B shows a configuration in which the reflectance of the termination mirror 22 of the semiconductor optical amplifier 60 is lowered instead of terminating all the delay lines 20. Such a low reflection mirror semiconductor optical amplifier 60 can be easily obtained by cleavage of the semiconductor constituting the same. In this case, each SOA oscillates in response to the reflection characteristics of the low reflection mirror and the reflection type wavelength tunable filter of Embodiment 1 or 2, and the oscillated light is obtained as the transmitted light from the low reflection mirror of each semiconductor optical amplifier 60. .

  If ri of the reflection type wavelength tunable filter of the embodiment 1 or 2 is respectively assigned integers from 0 to 4 as in the embodiment 1, the reflection type wavelength tunable filter becomes a discrete Fourier transform element in the embodiment 1 As described above, the peak interval of the reflectance of light sensed by each semiconductor optical amplifier 60 is determined by the wavelength interval reflecting dL (2 nm in the case of the first embodiment). That is, when all the semiconductor optical amplifiers 60 are excited and oscillated, it is considered that oscillation occurs at a wavelength of a specific wavelength interval. Therefore, it can be used as a light source of a system in which light of different wavelengths whose wavelength spacing is adjusted to be constant, such as super channel transmission whose utility has been recently discussed, is required.

  In FIG. 9 (c), an electrode is provided on the TLS MMI 30 of the configuration of FIG. 9 (a). In general, it is necessary to adjust the wavelength dependency (longitudinal mode) of the gain reflecting the length of the entire resonator, in addition to the filter characteristics of wavelength selection in TLS. By providing an electrode for longitudinal mode control on the MMI as shown in FIG. 9C, it is possible to control the longitudinal mode of the resonator which each senses regardless of which semiconductor optical amplifier 60 is excited. The structure can be simplified. This configuration is also applicable to other TLS such as the fifth embodiment.

  This embodiment can be monolithically integrated with the semiconductor optical amplifier 60 which can be similarly fabricated on an InP substrate by utilizing the QCSE effect using the MQW on the InP substrate as the reflection type wavelength tunable filter as in the first embodiment. Therefore, economics can be expected by downsizing of TLS and implementation cost reduction.

  FIG. 10 is a diagram showing the configuration of the wavelength variable light source adopted in this embodiment. In this embodiment, a variable-wavelength light source configured by providing the reflection type variable-wavelength filters according to the first or second embodiment on both sides of the semiconductor optical amplifier 60 is employed. In the fifth embodiment, the mirrors of the semiconductor optical amplifier 60 are not particularly limited. However, as shown in FIG. 10, the high-performance wavelength tunable light source can be obtained by replacing the mirrors with the reflective wavelength tunable filters of the first or second embodiment. realizable.

Here, two reflection type wavelength tunable filters are referred to as frontMIR (first reflection type wavelength tunable filter) 30 a and rearMIR (second reflection type wavelength tunable filter) 30 b. In FIG. 10, the electrodes Pi, Ni, and Ji of the reflective wavelength tunable filters 30a and 30b are omitted. Focusing on one SOA, light guided in the SOA travels back and forth in the SOA with a reflectance corresponding to the product of the reflection spectra of the frontMIR 30a and the rear MIR 30b, and the peak wavelength of the product oscillates. As in Figure 10, is outputted light oscillated from the port in the same manner as in Example 4 because there is no end mirror into a single delay line 20 _3, FrontMIR30a.

  FIG. 11 shows the reflection spectra of the frontMIR 30a and the rearMIR 30b and their products, and only the reflection spectra felt by one of the SOAs of the frontMIR 30a and the rearMIR 30b are shown for the sake of graph clarity. In addition, although the spectrum until now was a spectrum of dB indication, it is displaying of a linear scale here. In FIG. 11, the spectrum of each port is indicated by "# port number" or "series port number".

  Here, the front MIR 30 a and the rear MIR 30 b design the delay line 20 so that the FSR of each port is 4 nm and 4.4 nm. Reflecting this FSR shift, the product of the two filters has a peak at a wavelength of 1550 nm and the reflectivity of the other wavelengths is low. Therefore, it can be considered that the corresponding SOA oscillates near 1550 nm.

  Here, for example, a signal is applied to the electrodes P, N, and J of the delay line 20 of the front MIR 30 a to shift the spectrum to the 0.4 nm long wave side (FIG. 12). Then, it can be seen that the product of reflection of the two reflection type wavelength tunable filters completely jumps next to the conventional peak (the wavelength at which the front peak and the rear peak coincide with each other shifts to the next peak). This is a principle called the vernier effect, which is known as a method of forming a resonator for a specific wavelength in a wide wavelength range by combining reflectors having multipeaks, but conventionally it has been performed by means of a DBR or a ring resonator. It has been realized.

  The present invention can utilize this effect even in small refractive index changes such as QCSE, and make the light oscillate in a wider wavelength range by realizing the vernier effect in each SOA by making use of the multiport property of MMI. Can.

  FIG. 13 is a diagram showing the configuration of the wavelength variable light source adopted in this embodiment. In this embodiment, a Mach-Zehnder modulator (Mach-Zehnder modulator) connected via any of delay lines connected to the K ports of the multimode interference waveguide of the front MIR 30a of the wavelength tunable light source used in the fifth embodiment. A variable wavelength light source integrated type Mach-Zehnder modulator configured by providing a modulator (MZM) 70 is adopted. As described above, the configuration of the present embodiment is that low power consumption physics such as QCSE of semiconductor MQW structure can be used.

  On the other hand, in recent years, there have been many reports on a Mach-Zehnder modulator 70 using the QCSE of MQW on InP (for example, Non-Patent Document 6). That is, for example, the TLS and MZM shown in the fifth embodiment can be manufactured even if the semiconductors are completely the same except for the SOA part.

  Therefore, by integrating TLS and semiconductor MZM according to the present invention monolithically on the substrate S (semiconductor InP substrate) as shown in FIG. 13, the economy of the optical coupling loss between the light source and the modulator is reduced and the mounting cost is reduced. TLS-type integrated MZM can be realized.

10 input / output waveguide 20 delay line 30 multimode interference waveguide 40 light source 50 light receiver 60 semiconductor optical amplifier 70 Mach-Zehnder modulator

Claims (8)

M (M is an integer of 1 or more) × K (K is an integer of 2 or more) A multimode interference waveguide having a port configuration and lengths of mutually connected K ports of the multimode interference waveguide A reflective wavelength filter comprising K different delay lines,
Each of the K delay lines is provided with at least one of a first electrode, a second electrode, and a third electrode,
Let ri (i = 1,2 ,,,, K), dL, L0, Le1, Le2, Le3 be a real number greater than or equal to zero, B be greater than or equal to the largest value of ri, and the K delay lines Is when the length is L0 + ri × dL
Reflective type characterized in that the length of the first electrode is ri × Le 1, the length of the second electrode is (B−ri) × Le 2, and the length of the third electrode is Le 3 Wavelength tunable filter.   All of the electrodes of the first electrode group consisting of the plurality of first electrodes are short-circuited, and all of the electrodes of the second electrode group consisting of the plurality of second electrodes are short-circuited, The reflective wavelength tunable filter according to claim 1, wherein all the electrodes of the third electrode group consisting of three electrodes are short-circuited.   At least one of the first electrode group including the plurality of first electrodes provided in the delay line or the second electrode group including the plurality of second electrodes and K-1 or more 2. The reflection type according to claim 1, wherein the number of first electrodes or less or the second electrodes is short-circuited, and the remaining first electrodes or second electrodes are also short-circuited separately. Wavelength tunable filter.   The reflective wavelength tunable filter according to any one of claims 1 to 3, and two or more of the M ports and M-1 ports of the multimode interference waveguide of the reflective wavelength tunable filter. A multi-wavelength light source provided with light sources different in wavelength from one another, wherein light having different wavelengths is combined and output from the port to which the light source is not connected among the M ports Multi-wavelength light source characterized by   The reflective wavelength tunable filter according to any one of claims 1 to 3, and two or more of the M ports and M-1 ports of the multimode interference waveguide of the reflective wavelength tunable filter. A light receiving device connected to each of the plurality of light receiving devices, wherein light incident from a port to which the light receiving device is not connected among the M ports is split into wavelength components thereof The multi-wavelength light receiver characterized in that it is incident on the corresponding light receiver.   One end is connected to at least one port of the reflective wavelength tunable filter according to any one of claims 1 to 3 and M ports of the multimode interference waveguide of the reflective wavelength tunable filter What is claimed is: 1. A wavelength variable light source comprising: a semiconductor optical amplifier; and reflection mirrors respectively terminating the other end of the semiconductor optical amplifier.   A wavelength tunable light source comprising two reflection type wavelength tunable filters according to any one of claims 1 to 3, wherein the multimode of the reflection type wavelength tunable filter of one of the two reflection type wavelength tunable filters And a semiconductor optical amplifier connected at one end to the M ports of the interference waveguide and at the other end to the M ports of the multimode interference waveguide of the other reflective wavelength tunable filter. Characteristic variable wavelength light source.   A Mach-Zehnder modulator connected via any one of the wavelength tunable light source according to claim 7 and a delay line connected to the K ports of the multimode interference waveguide of the other reflection type wavelength tunable filter. What is claimed is: 1. A wavelength tunable light source integrated Mach-Zehnder modulator comprising:

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