JP6897498B2 - Reflective wavelength filter - Google Patents

Description

The present invention relates to an optical device, and more particularly to a reflective wavelength filter capable of reflecting light of an arbitrary 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 combined on the transmitting side to form a single fiber. On the receiving side, it is possible to transmit a large amount of information with one optical fiber by demultiplexing the received light for each wavelength and receiving the light for each wavelength.

In constructing a WDM communication system, a wavelength filter capable of selectively extracting a specific wavelength or combining different wavelengths into one optical path is an important optical device in constructing a WDM system.

In particular, a waveguide type wavelength filter manufactured by using a multilayer thin film substrate made of different materials in multiple layers is space-saving and expensive in terms of its small size and functionality when monolithic with other functional elements. It is advantageous for functionalization. Often, it is required to adjust the spectral characteristics 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 type filter. In Non-Patent Document 1, the transmission spectrum of an AWG made of Si is changed by utilizing the temperature dependence of the refractive index of Si. This uses the fact that when focusing on the pair of input / output ports of the AWG, its transmission peak wavelength is proportional to the refractive index of the waveguide forming the array diffraction grating.

In addition to the AWG, there are many reports of adjusting the filter with a ring resonator. In Non-Patent Document 2, a tunable light source (TLS) is used as a tunable filter 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 also shifts in proportion to the refractive index of the material, similar to the AWG.

Further, as a typical filter that can be applied to other waveguide types, a distributed Bragg reflector (DBR) that reflects light of a specific wavelength can be mentioned. Similar to the AWG and the ring resonator, the DBR also shifts its characteristic spectrum in proportion to the refractive index of the waveguide material. Non-Patent Document 3 describes that the reflection spectrum of the DBR is changed by changing the refractive index of the minute DBR portion by local heating by a microheater. In this method, TLS is formed by sandwiching the optical gain medium between 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-Resonator Tunable Lasers" , 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 similarly 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.M. 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.

As a common problem of the filters described above, there is a problem that the filter characteristics are proportional to the refractive index of the waveguide. In other words, the tunable amount is determined by the control range of the refractive index of the waveguide. There are many methods such as temperature, current, electric field, and stress to change the refractive index of the waveguide, but in any case, it can be said that it is at most 1% or less, which is not sufficient as a tunable amount. ..

For example, if the peak wavelength of one filter is set to 1550 nm used in the optical communication wavelength band, the wavelength tunable amount is 10 nm or less, although it depends on the refractive index of the material. It can be seen that the wavelength band of optical communication has a width of 40 nm even if it is limited to the C band band where the optical loss of the optical fiber is the smallest, and the above tunable amount of wavelength is insufficient. A common cause of all the above problems is that the spectrum of the filter has a one-to-one correspondence with the refractive index that constitutes the filter.

By the way, in recent years, there has been a report that the filter characteristics are realized not by the refractive index of the waveguide but by the phase of the light propagating in the waveguide. The reflection type transversal filter (RTF) using the multi-mode interference waveguide (MMI) of Non-Patent Document 4 is a light incident on an MMI having an N × N port configuration. Is N-branched, reciprocates through N reflective delay lines of different lengths, and is incident on the MMI again, reflecting the phase relationship of the light from the N delay lines, and a specific input. It has the function of returning light to the port. That is, the light is focused on a specific port only when the phases of the light from each delayed waveguide have a specific relative relationship.

What is important in a 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 of the propagating optical waveguide and the change in the refractive index of the material, and is inversely proportional to the wavelength (proportional to the frequency). Therefore, it can be seen that even if the amount of change in the refractive index is small, the phase change can be increased by increasing the length of the waveguide that changes the refractive index.

FIG. 1 is a diagram showing a configuration of a phase control type RTF as in Non-Patent Document 4. 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 provided with reflection portions at the ends. The filter characteristics of RTF are defined by the difference in the length of each delay line, and there is a degree of freedom as to how long the shortest delay line (called a reference delay line) should be. That is, by increasing the length of the reference delay line and providing an electrode for changing the refractive index of the waveguide here, it is possible to increase the change in the phase reciprocating in the delay line. In this case, deterioration of the filter characteristics due to carelessly lengthening the delay line actually exists (Non-Patent Document 4), but if the extension length is limited, the degree of deterioration can be sufficiently reduced. it can.

The principle of controlling the filter characteristics with the phase as described above can also be realized in Non-Patent Document 5 in which the RTF by MMI is not a reflection type. The advantage of the reflection type as in Non-Patent Document 4 is that the light reciprocates in the delay line, so the phase changes when a refractive index change is given to a certain waveguide as compared with the case where the same principle is realized in the transmission type. Has the advantage of doubling.

Strictly speaking, controlling the filter characteristics based on the phase rather than the refractive index of the waveguide is not physically impossible even in the AWG, but it is actually very difficult or its effect is small. This is due to the large number of delay lines and the continuity of the phase of the light emitted from each delay line, which is a major difference between the AWG and the transversal filter.

As a result of diligent studies on the above-mentioned conventional problems, the present inventors diligently studied a transversal filter using a multimode interference waveguide, particularly a filter based on the phase of light propagating through the waveguide using a reflective transversal filter. We have found that the wavelength variable amount can be made larger than before by controlling the characteristics, and have reached the present invention.

The present invention has been made in view of the above-mentioned 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 wavelength tunable amount. It is in.

In order to solve the above problems, the invention described in one embodiment includes a multi-mode interference waveguide having an M (M is an integer of 1 or more) × K (K is an integer of 2 or more) port configuration and the above-mentioned multiple. A reflection type wavelength filter having K delay lines having different lengths connected to each of the K ports of the mode interference waveguide, and the first K delay lines are provided for each of the K delay lines. At least one of the first electrode, the second electrode, and the third electrode is provided, and at least one of the first electrodes is provided on the K delay lines. At least one of the electrodes of the above and at least one of the third electrode is provided, and ri (i = 1, 2, ,,,, K), dL, L0, Le1, Le2, Le3 is zero or more. real, when B is to be the largest value or more in the ri, the K delay line is L0 + ri × dL length, the length of the first electrode is ri × Le1, wherein the length of the second electrode is (B-ri) × Le2, the length of the third electrode is a reflective-type wavelength tunable filter, which is a Le3.

It is a schematic diagram for demonstrating the advantage of a phase control type filter element. It is a cross-sectional structure of the waveguide of the reflection type tunable filter used in Example 1. It is a numerical calculation result of the phase change when a voltage is applied to the waveguide structure of FIG. It is a figure which shows the schematic structure including the electrode structure of the reflection type tunable filter used in Example 1. It is a figure which shows the spectral characteristic and the modulation characteristic of the reflection type tunable filter of FIG. It is a figure which shows the spectral characteristic when r1 and r2 are non-integers in the reflection type tunable filter of FIG. 4, and the modulation characteristic thereof. It is a figure which shows the reflection type tunable filter which has the characteristic of FIG. It is a schematic diagram of the multi-wavelength light source or the multi-wavelength receiver used in Example 3. It is a schematic diagram of the tunable light source used in Example 4. It is a schematic diagram of the tunable light source used in Example 5. It is the reflection spectrum and the product thereof in the two reflection type tunable filters in Example 5. It is a figure which shows the reflection spectrum and the product of the two reflection type tunable filters before and after shifting the spectrum of one reflection type tunable filter by about 4 nm in the tunable light source of FIG. It is a schematic diagram of the wavelength variable light source integrated type Mach-Zehnder modulator used in Example 6.

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

In Example 1, an example using a reflection-type tunable filter (RTF) using a multimode interference waveguide (MMI) will be described.

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

A laminated substrate is formed by depositing different types of semiconductors on the substrate S in the form of a multilayer film. InP is used as the substrate S, and MQW made of InAlGaAs and InAlAs having a total layer thickness of 0.5 μm and InP as a clad layer CL of 2.0 μm are laminated on the core layer CO.

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 by a thin-film deposition step.

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

In this embodiment, if the clad CL side is a p-doped semiconductor and the substrate S is an n-doped semiconductor, if a negative voltage is applied to the clad CL side with reference to the potential of the substrate S, the voltage value yields. Below the voltage, the current flowing through this structure is small.

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

As will be described later, the present invention is in that even if the change in the refractive index is small, the amount of change in the filter characteristics can be increased by designing the electrode length. Therefore, there is a great benefit to physics such as QCSE, which has high power efficiency but has a fixed maximum value of the amount of change in the refractive index.

FIG. 3 shows the phase change when a voltage is applied to the waveguide structure when the length of the waveguide structure shown in FIG. 2 (b) in the waveguide direction is 100 μm, based on the method of Non-Patent Document 6. It is a numerical calculation. The relationship between the applied voltage and the phase change is shown for each input wavelength (unit: micrometer). For example, when a voltage of 10 V is applied to the waveguide for light of 1.55 μm, a phase change of about 0.3π occurs. In this embodiment, the reflection type wavelength filter shown in FIG. 4 is configured by using the above waveguide structure, and its characteristics are observed.

FIG. 4A is a diagram showing an example of a reflection type wavelength filter configured by using the waveguide structure of FIG. 2B. As shown in FIG. 4A, the reflection type wavelength filter of this embodiment includes a multimode interference waveguide 30 in which 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 each of the five input ports of 30, and five delay lines 20 connected to each of the five output ports of the multimode interference waveguide 30. For the sake of explanation, the five delay lines 20 are given identification numbers i = ₁ , ₂ , ₃ , ₄ , _{5 in} order from the top (201, 202, 203, 204, 2005). In Figure 4, the shortest delay line (see delay line) 20 ₃ a third (i = 3) to its length and L0. Other delay line 20 _5, 20 _4, 20 _2, 20 _1, and i = 5, i = 4, i = 2, i = the delay line in the first order is gradually longer, each the length Li Then, Li = L0 + ri × dL can be set. Here, L0 and dL are real numbers of zero or more. A mirror 22 is provided at the other end of the five delay lines 20 that are not connected to the output port of the multimode interference waveguide 30.

Each delay line 20 is provided with a fixed phase shifter 21 that does not dynamically move the phase. The reflection type tunable filter in this embodiment and other examples is configured based on the so-called reflection type transversal filter as shown in FIG. 1, and the portion of the fixed phase shifter 21 is a reflection type. The tunable filter functions as a transversal filter. That is, the phase shift amount by the fixed phase shifter 21 is adjusted so as to match the transfer function of the MMI 30, and the reflection type wavelength filter is appropriately selected so as to exhibit the filter characteristics. When ri is an integer from 1 to 4, the circuit of FIG. 4 is a reflection type 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 a certain arbitraryness as described in Non-Patent Document 5.

When the circuit of FIG. 4 is a discrete Fourier transform element, when light is incident on a specific port, the light is branched into five by the MMI 30 and reciprocates on each delay line, and the light is output to each port again. The wavelength (frequency) of the output light has a certain frequency interval Δf / 5. That is, when 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 refractive index of the waveguide, and factor 2 is derived from the fact that the filter of FIG. 4 is a reflective type. Δf is generally called the free spectral range (FSR). That is, the FSR of each port can be controlled by the design of dL. Here, dL = 31.3 μm, r1 = 4, r2 = 3, r3 = 0, r4 = 2, r5 = 1.

Further, each delay line is provided with a maximum of three electrically separated electrodes Pi, Ni, and Ji. For 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 in ri). , The length of the electrode Ji can be set to Le3, and B = 4 is adopted in this example. Then, Le1 = Le2 = 120 μm and Le3 = 600 μm. Le1, Le2, and Le3 are real numbers of zero or more.

The three electrodes Pi, Ni, and Ji need to be electrically separated in each delay line 20, but for electrodes of the same group (Pi to each other, Ni to each other, Ji to each other) among different delay lines 20. May be short-circuited as shown in FIG. 4 (b). With the configuration shown in FIG. 4B, when an electric signal is applied to the short-circuited terminals, a phase change occurs in proportion to the lengths of the electrodes Pi, Ni, and Ji provided on the respective delay lines 20. In order to cause the spectrum shift described later, there is an advantage that the control becomes simple.

FIG. 5A shows a numerical calculation result of the intensity of light returned to each port on the input side of the MMI 30 when light is incident on the port 3 on the input side of the MMI 30. When light is incident on the input side port 3 of the MMI 30, the spectrum at the same port 3 is the reflection spectrum when viewed from the 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 having a different wavelength every 2 nm appears in each port, and when focusing on one port, a peak wavelength appears every 10 nm. That is, the FSR when focusing on one port is 10 nm.

In FIG. 5B, for the sake of readability of the graph, paying attention only to the port 2, when no voltage is applied to the electrodes, when a voltage of 5V is applied to all of the electrodes Pi, 5V is applied to all of the electrodes Ni. The spectrum in each case when the voltage of is applied 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 attention was paid only to port 2 in FIG. 5B, this wavelength shift occurs in all ports other than port 2. This can be understood as follows.

In the transversal filter by MMI30, the phase θi of the light reciprocating on the reference delay line of the identification number i is expressed by 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 the identifier i (Li = L0 + ri × dL), and φi is the voltage applied to the electrodes Pi, Ni, and Ji. The phase shift due to the applied external signal is shown.

The relative phase of each delay line is given by the following equation (2), where _{θ i = ref} is the phase of the light that reciprocates the reference delay line.

Wavelength-frequency dispersion relation f = c / λ (f: light frequency, c: speed of light) and refractive index are expressed as n = n'λ + ng (n': refractive index dispersion, ng: group refractive index) Then, the following equation (3) is obtained.

Here, assuming that the spectral characteristics shift from f to f + Δf when the signal is not applied to the variable phase shifters (electrodes Pi, Ni, Ji) of each delay line and when the signal is applied.

Is established.

For example, when no signal is applied to the reference delay line (φ _{i = ref} = 0),

Therefore, if a phase shift amount proportional to the ri of each delay line is given in addition to the reference delay line, the spectrum will be shifted. That is, as described above regarding the length of the phase adjusting electrode Pi, if the length of the phase adjusting electrode Pi is set by the product of ri and the length of the constant Le1, when the same voltage is applied to all Pis, , The spectrum shifts to the long wavelength side. However, paying attention to the above equation (5), if the physics of the refractive index change for the phase shift is the same, the sign of the phase change φi is basically the same. For example, when φi> 0, Δf <0, and the shift can be made only to the low frequency side (in the case of wavelength, the long wavelength side).

Therefore, assuming that the phase shift amount φ _{i = ref of} the reference delay line is a positive finite value, from Eq. (4)

Therefore, it can be seen that Δf> 0 if _{φi <φ i = ref.}

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

In order for φi ≧ 0 for all the delay lines of the identification number i, paying attention to Δf ≧ 0, the following equation (8) is obtained.

That is, as described above regarding the length of the phase adjusting electrode Ni, the length of the phase adjusting electrode Ni is the product of B-ri (however, B is greater than or equal to the largest value in ri) and the length of the constant Le2. If set, the spectrum shifts to the short wavelength side when the same voltage is applied to all Ni.

FIG. 5C also focuses on the spectrum of the port 2 and shows the spectra when no voltage is applied to the electrodes and when a voltage of 9 V is applied to all the electrode 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 peaks of the existing filter. Here, the electrodes Ji are provided only on the electrodes i = 2 and i = 5, and their lengths are all L3 = 600 μm as described above.

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

On the other hand, the phase relationship between the spectrum peaks, that is, when the frequency is shifted from f0 by the FSR of the spectrum, is

If the phases of equation (9) and equation (10) are equal, the following equation (11) 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, no signal is applied to the delay lines of the identification numbers i = 3, 4, and 1, but the voltage may be applied only to the delay lines of the identification numbers i = 2, 5 so that the respective phase changes are π. Specifically, when the group of electrodes Ji is provided with electrodes having the same length and the same voltage is applied to them, the action shown in FIG. 5C can be obtained. More generally, which delay line the electrode Ji is provided on depends on the number of delay lines and the like, but in any case, it is important to provide electrodes having the same length on some of the delay lines.

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

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

At the end of this embodiment, when the electrode length is lengthened, the length of the reference delay line L0 is made finite, and the wavelength shift efficiency is improved when the electrode Pi on the delay line is lengthened. Compare with the case of AWG.

Since the electrode Pi on the delay line needs to be proportional to ri from the above explanation, when the electrode length obtained by extending the reference delay is Δl, the electrode on the longest delay line can be lengthened by Δl first. it can. The identification number of the delay line at this time is i = m. At that time, if the electrode (length Li) on a certain delay line becomes the length Li', the rate of change is expressed by the following equation (12) from the above-mentioned necessary condition that the ratio between the electrodes is constant.

Therefore, the longer the electrode of the longest delay line, in other words, the longer the longest delay line length, the smaller the benefit of improving the modulation efficiency. An AWG with a K-port configuration generally requires 3 to 4 times the delay line of K, that is, an AWG with a maximum delay line length of 4 times that of a transversal filter is also described in FIG. The benefit of modulation efficiency due to the expansion of the reference delay line as described above is small.

After all, for the above reason, the refractive index of the entire element (entire delay line) is controlled as in Non-Patent Document 1 rather than the expansion of the refractive index change (that is, phase control) by extending the length of the odor modulation region in the AWG. Therefore, it can be said that the control of the filter characteristics is more advantageous. Furthermore, one advantage of the reflective tunable filter of the present embodiment is that the benefit of increasing the electrode length by expanding the reference delay line is large as long as the element size allows.

FIG. 7 is a diagram showing a configuration of a reflection type tunable 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 Each of the electrode groups was short-circuited, but in this embodiment, one or more of the electrode groups of the electrode N or the electrode group of the electrode P provided on the delay line and one K-1. A reflective wavelength variable filter having 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 is used. Other configurations of the reflective tunable filter are similar.

In preparation for understanding the advantages of the configuration of the reflective tunable filter used in this embodiment, it is necessary to intentionally remove the factor ri that determines the length of the delay line from the regularity so that the filter characteristics have an envelope. Think.

In the first embodiment, there is no special restriction on how to select ri except that it is a real number of zero or more, and an integer value from 0 to 4 is used. However, if a special selection method is used for this, the filter characteristics can be made more sophisticated. There is. For example, among the ri selected in Example 1, r1 = 5.4 and r2 = 4.05 for r1 and r2. 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. 6A. Due to the delay line length factor of 0.35, an envelope appears in the filter characteristics, as can be seen in FIG. 6 (a). The period of this envelope is approximately 28 nm, which corresponds to 1 / 0.35 times the FSR (10 nm) of one port given by dL in FIG. 4 (a). Such filter characteristics are important as a wavelength selection mechanism for a multi-wavelength light source or a tunable wavelength light source, which will be described, for example, in Examples 3 and subsequent examples. In this way, by specially designing the length of a part of the delay line, its spectral characteristics can be changed.

Here, in the reflective tunable filter of the present embodiment, as shown in FIG. 7, the electrodes of i = 1 and 2 and the electrodes of i = 4 and 5 are short-circuited to form the electrode group of the terminal P12 and the electrode group of P45, respectively. Divide into. Here, the spectrum when a voltage of 5 V is applied only to the terminal P45 is shown in 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.

Further, FIG. 6C 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 keeping the position of the envelope.

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

FIG. 8 is a diagram showing a configuration of a multi-wavelength light source or a multi-wavelength receiver adopted in this embodiment. In this embodiment, a light source 40 is provided on the input side of the input / output waveguide 10 of the reflection type tunable filter, and a light source 50 is provided on the output side of the multi-wavelength light source or the reflection type tunable filter to form a multi-wavelength receiver. I am using the one that I did. 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 optical signal incident from outside the element changes, the transmittance between the input / output port and the light source or receiver is maximized by using the multi-wavelength light source or multi-wavelength receiver. The electrodes Pi, Ni, and Ji are adjusted in such a way or intentionally to block a specific wavelength. For example, by controlling Pi and Ni, the maximum transmittance for a given wavelength of light can be maximized.

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

Further, in many of these applications, it is necessary to continuously apply an electric signal to the electrodes for a long period of time to keep the filter characteristics of the device constant.

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

FIG. 9 is a diagram showing a configuration of a tunable 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 reflection type tunable filter of the first or second embodiment is adopted. ing. Some or all of the M ports on the input side of the reflective tunable filter (FIG. 9A shows all cases) show a mirror-terminated 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 delay line No. 4) is not mirror-terminated. Therefore, each semiconductor optical amplifier 60 receives the wavelength selectivity of the reflection type tunable filter, a specific wavelength oscillates, and light is output from the fourth delay line. Since the wavelength of the reflected light is different for each SOA, each semiconductor optical amplifier 60 oscillates at 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 tunable filter of Example 1 or 2, if the semiconductor optical amplifier 60 that is excited by injecting a current is selected, the wavelength from the element is correspondingly selected. Means that you can choose. A signal may be applied to each electrode for fine adjustment of the wavelength.

Further, if the reflection type tunable filter of Example 1 or 2 is adjusted so that the spectral characteristics shown in FIG. 6 have an envelope, the wavelength selectivity is improved, which is higher than that in 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. It can be easily obtained by cleavage of the semiconductor constituting the low reflection mirror semiconductor optical amplifier 60. In this case, each SOA oscillates due to the reflection characteristics of the low reflection mirror and the reflection type wavelength variable filter of Example 1 or 2, and the oscillated light is obtained as transmitted light from the low reflection mirror of each semiconductor optical amplifier 60. ..

In Example 1, if the ri of the reflective wavelength variable filter of Example 1 or 2 is assigned an integer from 0 to 4 as in Example 1, the reflective wavelength variable filter becomes a discrete Fourier transform element. As described above, the peak interval of the light reflectance felt by each semiconductor optical amplifier 60 is determined by the wavelength interval reflecting dL (2 nm in the case of Example 1). That is, when all the semiconductor optical amplifiers 60 are excited and each is oscillated, it is considered that the semiconductor optical amplifiers oscillate at wavelengths having a specific wavelength interval. Therefore, it can be used as a light source for a system that requires light of different wavelengths at one time with a constant wavelength interval, such as super channel transmission whose practicality has been discussed in recent years.

9 (c) shows an electrode provided on the TLS MMI 30 having the configuration of FIG. 9 (a). In general, TLS needs to adjust the wavelength dependence (longitudinal mode) of the gain reflecting the length of the entire resonator in addition to the characteristics of the wavelength selection filter. By providing an electrode for longitudinal mode control on the MMI as shown in FIG. 9C, the longitudinal mode of the resonator felt by each of the semiconductor optical amplifiers 60 can be controlled regardless of which semiconductor optical amplifier 60 is excited. Therefore, the TLS electrode. The structure can be simplified. This configuration can also be applied to other TLSs such as Example 5.

In this embodiment, as a reflection type tunable filter, if the QCSE effect using MQW on the InP substrate is used as in Example 1, it is possible to monolithically integrate with the semiconductor optical amplifier 60 which can also be manufactured on the InP substrate. Therefore, it can be expected that the TLS will be miniaturized and the mounting cost will be reduced to make it more economical.

FIG. 10 is a diagram showing a configuration of a tunable light source adopted in this embodiment. In this embodiment, a wavelength tunable light source configured by providing the reflection type tunable filters of Examples 1 or 2 on both sides of the semiconductor optical amplifier 60 is adopted. In the fifth embodiment, the mirrors of the semiconductor optical amplifier 60 are not particularly limited, but by replacing them with the reflective tunable filter of the first or second embodiment as shown in FIG. 10, a highly functional tunable light source can be obtained. realizable.

Here, the two reflective tunable filters are frontMIR (first reflective tunable filter) 30a and rearMIL (second reflective tunable filter) 30b. In FIG. 10, the electrodes Pi, Ni, and Ji of the reflective tunable filters 30a and 30b are omitted. Focusing on one SOA, the light guided through the SOA reciprocates in the SOA with a reflectance corresponding to the product of the reflection spectra of frontMIR30a and rearMIR30b, and the peak wavelength of the product oscillates. As in Figure 10, it 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 spectrum of the product of frontMIR30a and rearMIR30b, and for the sake of easy viewing of the graph, only the reflection spectrum felt by the SOA of one of frontMIR30a and rearMIR30b is shown. The spectrum so far has been a dB display spectrum, but here, a linear scale display is used. In FIG. 11, the spectrum of each port is indicated by "#port number" or "series port number".

Here, the front MIR 30a and the rear MIR 30b 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 reflectance at the other wavelengths is low. Therefore, it is considered that the corresponding SOA oscillates in the vicinity of 1550 nm.

Here, for example, consider applying a signal to the electrodes P, N, and J of the delay line 20 of the frontMIR30a to shift the spectrum to the 0.4 nm long wave side (FIG. 12). Then, it can be seen that the product of the reflections of the two reflective tunable filters completely jumps next to the conventional peak (the wavelength at which the front peak and the rear peak match shifts to the next peak). This is a principle called the vernier effect, and it is known as a method of forming a resonator for a specific wavelength in a wide wavelength range by combining reflectors having multiple peaks, but conventionally, a DBR or a ring resonator is used. It has been realized.

The present invention can utilize this effect even in a small change in the refractive index such as QCSE, and can oscillate light in a wider wavelength range by realizing a vernier effect in each SOA by utilizing the multiportability of MMI. Can be done.

FIG. 13 is a diagram showing a configuration of a tunable light source adopted in this embodiment. In this embodiment, a Mach-Zehnder modulator (Mach-Zehnder) connected via any of the delay lines connected to the K ports of the multimode interference waveguide of the frontMIR30a wavelength tunable light source used in the fifth embodiment. A Mach-Zehnder modulator with a variable wavelength light source, which is configured by providing a modulator (MZM) 70, is adopted. The configuration of this embodiment is that low power consumption physics such as QCSE of the semiconductor MQW structure can be used as described above.

On the other hand, in recent years, there have been many reports on the Mach-Zehnder modulator 70 using MQW QCSE on InP (for example, Non-Patent Document 6). That is, for example, the TLS and MZM shown in Example 5 can be manufactured even if their semiconductors are exactly the same except for the SOA part.

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

10 Input / output waveguide 20 Delay line 30 Multimode interference waveguide 40 Light source 50 Receiver 60 Semiconductor optical amplifier 70 Mach-Zehnder modulator

Claims (8)

A multimode interference waveguide having an M (M is an integer of 1 or more) × K (K is an integer of 2 or more) port configuration and a mutual length connected to each of the K ports of the multimode interference waveguide. A reflective wavelength filter with 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, and the K delay lines are provided with the above. At least one of the first electrodes, at least one of the second electrodes, and at least one of the third electrodes are provided.
It is assumed that ri (i = 1,2, ,,,, K), dL, L0, Le1, Le2, Le3 are real numbers of zero or more, and B is the largest value or more in ri, and the K delay lines are described above. Is when the length is L0 + ri × dL
The length of the first electrode is ri × Le1, the length of the second electrode is (B-ri) × Le2, and wherein the length of said third electrode is Le3 Reflective tunable filter. All the electrodes of the first electrode group composed of the plurality of first electrodes are short-circuited, all the electrodes of the second electrode group composed of the plurality of second electrodes are short-circuited, and the plurality of the first electrodes are short-circuited. The reflective wavelength variable filter according to claim 1, wherein all the electrodes of the third electrode group including the three electrodes are short-circuited. One or more of the electrode groups of either the first electrode group composed of the plurality of the first electrodes provided on the delay line or the second electrode group composed of the plurality of the second electrodes and K-1. The reflective type according to claim 1, wherein the number of the first electrodes or the second electrodes is short-circuited, and the remaining first electrodes or the second electrodes are also short-circuited separately. Variable wavelength filter. To two or more and M-1 ports of the reflection type wavelength variable filter according to any one of claims 1 to 3 and the M ports of the multimode interference waveguide of the reflection type wavelength variable filter. It is a multi-wavelength light source provided with connected light sources having different wavelengths, and light of different wavelengths is combined and output from the port to which the light source is not connected among the M ports. A featured multi-wavelength light source. The reflection type tunable filter according to any one of claims 1 to 3, and two or more and M-1 ports out of the M ports of the multimode interference waveguide of the reflection type tunable filter. It is a multi-wavelength receiver provided with a receiver connected to each, and the light incident from the port to which the receiver is not connected among the M ports is demultiplexed for each wavelength component. A multi-wavelength receiver characterized in that it is incident on the corresponding receiver. One end is connected to at least one or more ports of the reflective wavelength variable filter according to any one of claims 1 to 3 and the M ports of the multimode interference waveguide of the reflective wavelength variable filter. A wavelength-variable light source including a semiconductor optical amplifier and a reflection mirror that terminates the other ends of the semiconductor optical amplifier. The multimode of the reflection type wavelength variable filter of one of the two reflection type wavelength variable filters, which is a wavelength variable light source provided with two reflection type wavelength variable filters according to any one of claims 1 to 3. It is further provided with a semiconductor optical amplifier having one end connected to the M ports of the interference waveguide and the other end connected to the M ports of the multimode interference waveguide of the other reflective wavelength variable filter. A characteristic wavelength variable light source. A Mach-Zehnder modulator connected via any 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 reflective tunable filter. A Mach-Zehnder modulator with an integrated wavelength tunable light source.

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