JP2011175109A - Variable wavelength optical filter and variable wavelength laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a variable wavelength optical filter which has a wide-band and variable wavelength characteristics, and a variable wavelength laser which has a wide band and the laser beam wavelength of which is variable. <P>SOLUTION: The variable wavelength laser includes a multi-mode interference-type waveguide portion including a core portion; a first optical input/output portion provided to one end in the length direction of the multi-mode interference type waveguide portion; and a second optical input/output portion provided to the other end of the multi-mode interference type waveguide portion. The multi-mode interference type waveguide portion includes an optical filter portion which has loss wavelength characteristics for light input from the first optical input/output portion and light output from the second optical input/output portion; a side-clad portion which is provided to the side of a core portion of the multi-mode interference type waveguide portion and has a refractive index or equivalent refractive index lower than that of the core portion; and a refractive index adjustment mechanism which makes the refractive index or the equivalent refractive index of the clad portion vary. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a wavelength tunable optical filter and a wavelength tunable laser using the same.

  As a method of performing wavelength tuning in a wide wavelength range in a wavelength tunable laser, a grating element in which the wavelength dependence of reflectance periodically peaks, and a wavelength in which loss or reflectance wavelength dependence has a relatively wide full width at half maximum, There is a method of combining with a wavelength tunable optical filter capable of changing the peak wavelength. For example, in Non-Patent Document 1, a wavelength tunable laser is configured by combining a sampled grating (SG) and a grating assist directional coupler optical filter as a wavelength tunable optical filter.

  On the other hand, a wavelength tunable laser that combines a multimode interference type (Multi Mode Interference, MMI) waveguide (see Non-Patent Document 2), which is a wavelength tunable filter having a wide loss peak with a full width at half maximum, and a sampled grating is disclosed. (See Patent Document 1 and Non-Patent Document 3).

US Pat. No. 6,687,267

M. Oberg, et al., IEEE Photonics Technology Letters, vol.5, No.7, pp.735-738, 1993 Pierre A. Besse, et al., Journal of Lightwave Technology, vol.12, No.6, pp.1004-1009, 1994 H. G. Bukkems, et al., IEEE Journal of Quantum Electronics, vol.43, No.7, pp.614-621, 2007

  For example, in a large capacity WDM (Wavelength Division-Multiplexing) communication, a wide wavelength range is used. Therefore, in order to realize a wavelength tunable laser with a wider wavelength tunable range, wavelength characteristics such as reflectance and loss can be changed. There is a need for a tunable optical filter having a wider range.

  The present invention has been made in view of the above, and an object thereof is to provide a wavelength tunable optical filter whose wavelength characteristics are variable in a wide band, and a wavelength tunable laser in which the wavelength of laser light is variable in a wide band. .

  In order to solve the above-described problems and achieve the object, a wavelength tunable optical filter according to the present invention includes a multimode interference waveguide section having a core section, and a length direction of the multimode interference waveguide section. A first optical input / output unit provided at one end; and a second optical input / output unit provided at the other end of the multimode interference waveguide unit; An optical filter unit having a loss wavelength characteristic with respect to light input from the first optical input / output unit and output from the second optical input / output unit, and provided at a side of the core unit of the multimode interference waveguide unit And a side cladding part having a refractive index or equivalent refractive index lower than that of the core part, and a refractive index adjusting mechanism for changing the refractive index or equivalent refractive index of the side cladding part.

  In addition, the wavelength tunable optical filter according to the present invention includes a multimode interference waveguide section having a core section, and a first optical input / output section provided at one end in the length direction of the multimode interference waveguide section. And a reflection means provided at the other end of the multimode interference waveguide section. The multimode interference waveguide section is input from the first light input / output section and reflected by the reflection means. An optical filter unit having a reflection wavelength characteristic with respect to light output from the first light input / output unit, and a refractive index higher than the core unit provided on a side of the core unit of the multimode interference waveguide unit; A side clad part having a small equivalent refractive index, and a refractive index adjusting mechanism for changing a refractive index or an equivalent refractive index of the side clad part are provided.

  In the wavelength tunable optical filter according to the present invention as set forth in the invention described above, the optical filter unit is provided at the one end of the multimode interference waveguide unit, and is input from the first optical input / output unit and is reflected. A second light input / output unit that outputs a part of the light reflected by the means is further provided.

  In the tunable optical filter according to the present invention as set forth in the invention described above, the multi-mode interference waveguide section has a ridge structure.

  In the tunable optical filter according to the present invention as set forth in the invention described above, the multi-mode interference waveguide section has a buried structure.

  In the tunable optical filter according to the present invention as set forth in the invention described above, the multi-mode interference waveguide section has a high mesa structure.

  In the tunable optical filter according to the present invention as set forth in the invention described above, the first optical input / output unit has a high mesa structure.

  The tunable optical filter according to the present invention is the above-described tunable optical filter, wherein the upper cladding layer and the lower cladding, which are formed above and below the side cladding portion in the stacking direction and have a lower refractive index than the side cladding portion, are provided. It is characterized by comprising a layer.

  A wavelength tunable laser according to the present invention includes the wavelength tunable optical filter according to any one of the above inventions.

  Further, the wavelength tunable laser according to the present invention is characterized in that, in the above-mentioned invention, an optical resonator comprising a periodic reflecting means in which the wavelength dependence of reflectance periodically peaks.

  In the wavelength tunable laser according to the present invention as set forth in the invention described above, the periodic reflecting means is a DBR mirror.

  According to the present invention, by changing the refractive index or equivalent refractive index of the side cladding portion provided on the side portion of the core portion of the multimode interference waveguide, the light confinement state in the core portion is changed, There is an effect that it is possible to realize a wavelength tunable optical filter whose wavelength characteristic is variable in a wide band and a wavelength tunable laser in which the wavelength of the laser beam is variable in a wide band.

FIG. 1 is a schematic plan view of a wavelength tunable optical filter according to the first embodiment. FIG. 2 is a diagram showing a cross section taken along line AA of the wavelength tunable optical filter shown in FIG. 1 and an equivalent refractive index distribution shape. FIG. 3 is a diagram illustrating a calculation result of the change in the wavelength characteristic of the loss when the equivalent refractive index of the side cladding portion is changed in the tunable optical filter shown in FIG. FIG. 4 is a diagram illustrating a schematic cross section of the wavelength tunable optical filter according to the second embodiment and a distribution shape of an equivalent refractive index. FIG. 5 is a diagram illustrating a schematic cross section of the wavelength tunable optical filter according to the third embodiment and a distribution shape of an equivalent refractive index. FIG. 6 is a schematic plan view of a wavelength tunable optical filter according to the fourth embodiment. FIG. 7 is a cross-sectional view of the main part of the wavelength tunable optical filter shown in FIG. FIG. 8 is a diagram showing a change in loss characteristics when the equivalent refractive index of the side cladding portion is changed in the tunable optical filter shown in FIG. FIG. 9 is a schematic plan view of a wavelength tunable optical filter according to the fifth embodiment. FIG. 10 is a diagram illustrating a cross section taken along the line C-C and an equivalent refractive index or a refractive index distribution shape of the tunable optical filter illustrated in FIG. 9. FIG. 11 is a schematic plan view of a wavelength tunable optical filter according to the sixth embodiment. FIG. 12 is a schematic plan view of the wavelength tunable laser according to the seventh embodiment. 13 is a cross-sectional view of the main part of the DD line of the wavelength tunable laser shown in FIG. 14 is a cross-sectional view of the main part of the wavelength variable laser shown in FIG. 15 is a cross-sectional view of a principal part of the wavelength tunable laser shown in FIG. FIG. 16 is a diagram schematically illustrating an example of the wavelength characteristic of the reflectance of the sampled grating waveguide portion. FIG. 17 is a diagram for explaining the oscillation wavelength of the wavelength tunable laser shown in FIG. FIG. 18 is a diagram for explaining the relationship between the oscillation wavelength and the longitudinal mode of the wavelength tunable laser shown in FIG. FIG. 19 is a diagram for explaining an example of a manufacturing method of the wavelength tunable laser shown in FIG.

  Hereinafter, embodiments of a wavelength tunable optical filter and a wavelength tunable laser according to the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the embodiments. Moreover, in each drawing, the same code | symbol is attached | subjected suitably to the same or corresponding component. Furthermore, it should be noted that the drawings are schematic, and the relationship between the thickness and width of each layer, the ratio of each layer, and the like may differ from the actual ones. Even between the drawings, there are cases in which portions having different dimensional relationships and ratios are included.

(Embodiment 1)
First, the wavelength tunable optical filter according to the first embodiment of the present invention will be described. FIG. 1 is a schematic plan view of a wavelength tunable optical filter according to the first embodiment. FIG. 2 is a diagram showing a cross section taken along the line AA of the variable wavelength optical filter shown in FIG. 1 and the distribution shape of the equivalent refractive index. Hereinafter, the wavelength tunable optical filter 10 will be described with reference to FIGS.

  The wavelength tunable optical filter 10 includes an optical input / output unit 12b, 12c, and 12d provided in one end or the other end of the MMI waveguide unit 12a and the length direction of the MMI waveguide unit 12a formed in the semiconductor multilayer structure 11. , 12e, an electrode 13 provided on the side portion 15 of the MMI waveguide portion 12a on the semiconductor multilayer structure 11, and an electrode 14 provided on the back surface of the semiconductor multilayer structure 11. ing. The MMI waveguide portion 12a has a width W1 and a length L1.

  The semiconductor multilayer structure 11 has a structure in which a lower cladding layer 11a, a core layer 11b, and an upper cladding layer 11c are sequentially stacked. When light having a wavelength of 1.55 μm is input, the lower cladding layer 11a and the upper cladding layer 11c are made of, for example, n-InP and p-InP, respectively. The core layer 11b includes a core portion 11ba in the MMI waveguide portion 12a and a side cladding portion 11bb in the side portion 15. The core layer 11b is made of InGaAsP adjusted to have a composition of 1.4Q, for example, so that the refractive index is higher than that of InP. Note that 1.4Q means that the band gap wavelength is 1.4 μm.

  A part of the upper cladding layer 11c protrudes to form a ridge portion 11ca. A region where the ridge portion 11ca is formed is an MMI waveguide portion 12a having a ridge structure. Further, due to the presence of the ridge portion 11ca, the equivalent refractive index of the core layer 11b with respect to the stacking direction of the semiconductor stacked structure 11 has a distributed shape like a shape P11 in the width direction. As a result, light of a predetermined wavelength (for example, 1.55 μm band) input to the core layer 11b is confined in the core portion 11ba and guided by the difference in equivalent refractive index between the core portion 11ba and the side cladding portion 11bb. To do.

  The optical input / output units 12b, 12c, 12d, and 12e also have a ridge waveguide structure, but are configured to guide light of a predetermined wavelength in a single mode.

  Next, the characteristics of the wavelength tunable optical filter 10 will be described. First, the characteristics of the optical filter unit 12 will be described. When the single-mode light IL1 having a predetermined wavelength is input from the light input / output unit 12b serving as the first light input / output unit, the MMI waveguide unit 12a is confined in the core unit 11ba by the side clad unit 11bb, and is MMI. Waveguide portion 12a is guided in multimode, and is output as single mode light OL1 from light input / output portion 12d as the second input / output portion.

This MMI waveguide section 12a has a loss wavelength characteristic that has a minimum value with respect to the light input from the optical input / output section 12b and output from the optical input / output section 12d due to the multimode interference effect. Yes. In order to realize this loss wavelength characteristic, the MMI waveguide unit 12a can be connected to the optical input / output unit 12b by using a design parameter such as an equivalent refractive index of the waveguide by a method described in Non-Patent Document 1, for example. , 12c, 12d, and 12e are used as input / output ports so that the length L1 and width W1 of the MMI waveguide portion 12a and the positions of the optical input / output portions 12b, 12c, 12d, and 12e are set. You only have to set it. At this time, for example, if the propagation constants of the 0th-order mode and the 1st-order mode of the MMI waveguide 12a at the wavelength at which the loss is a minimum value (minimum loss wavelength) are β ₀ and β ₁ , respectively, the value of the length L1 is 3π / {2 (β ₀ −β ₁ )}.

  Here, when a voltage is applied between the electrode 13 and the electrode 14 as the refractive index adjusting mechanism and a current is injected into the side portion 15, the equivalent refractive index of the side cladding portion 11bb is shown in FIG. It changes like a shape P12. When the equivalent refractive index of the side cladding portion 11bb changes, the light confinement state in the core portion 11ba of the MMI waveguide portion 12a changes, and the minimum loss wavelength also changes. Therefore, since the wavelength characteristic of loss can be changed by the above current injection, the wavelength tunable optical filter 10 functions as a wavelength tunable optical filter.

  FIG. 3 is a diagram illustrating a calculation result of a change in the wavelength characteristic of the loss when the equivalent refractive index of the side cladding portion 11bb is changed in the tunable optical filter 10 illustrated in FIG. In FIG. 3, “reference” indicates the wavelength characteristic of loss in a state where no current flows through the side portion 15. In FIG. 3, the tunable optical filter 10 is designed so that the minimum loss wavelength is about 1.56 μm in the “reference” state. Further, the difference in the equivalent refractive index between the core portion 11ba and the side cladding portion 11bb is designed to be 0.06. The value of the length L1 is set to 505 μm, and the value of the width W1 is set to 10 μm.

  Next, a current is supplied to the side portion 15 so that the equivalent refractive index of the side clad portion 11bb is 0.08%, 0.38%, and 0.58% with respect to the equivalent refractive index of “reference”. Just lower. In FIG. 3, “−0.08%”, “−0.38%”, and “−0.58%” indicate a state in which the equivalent refractive index is decreased at a rate of the numerical value. As shown in FIG. 3, the minimum loss wavelength shifts to the short wavelength side as the equivalent refractive index is lowered. In FIG. 3, the arrow Ar1 indicates the variable range of the minimum loss wavelength between the “reference” state and the “−0.58%” state, but the variable range indicated by the arrow Ar1 is as extremely as about 50 nm. Broadband. That is, the wavelength tunable optical filter 10 according to the first embodiment has a variable wavelength characteristic of loss in a very wide band.

  Note that the variable range of the minimum loss wavelength depends on the difference in equivalent refractive index between the core portion 11ba and the side cladding portion 11bb of the MMI waveguide portion 12a. That is, the smaller the difference in equivalent refractive index is, the larger the light oozes out from the core portion 11ba to the side cladding portion 11bb. Therefore, the minimum loss wavelength when the equivalent refractive index of the side cladding portion 11bb is changed by a predetermined amount. The variable range becomes even wider. In particular, in the tunable optical filter 10 according to the first embodiment, since the MMI waveguide portion 12a has a ridge structure, the difference in equivalent refractive index between the core portion 11ba and the side cladding portion 11bb is 0.06. It can be easily realized, and the variable range of the minimum loss wavelength can be made extremely wide as about 50 nm.

  As described above, the wavelength variable optical filter 10 according to the first embodiment has a variable wavelength characteristic of loss in a very wide band.

(Embodiment 2)
Next, a wavelength tunable optical filter according to Embodiment 2 of the present invention will be described. Since the structure of the wavelength tunable optical filter according to the second embodiment viewed from above is the same as the structure of the wavelength tunable optical filter 10 shown in FIG. 1, the description thereof will be omitted, and the sectional structure thereof will be described below. FIG. 4 is a diagram illustrating a schematic cross section of the wavelength tunable optical filter according to the second embodiment and a distribution shape of an equivalent refractive index.

  The wavelength tunable optical filter 20 includes an MMI waveguide portion 22a and four optical input / output portions provided at one end or the other end of the MMI waveguide portion 22a in the length direction, which are formed in the semiconductor multilayer structure 21. An optical filter unit, an electrode 23 provided on the side portion 25 of the MMI waveguide portion 22 a on the semiconductor multilayer structure 21, and an electrode 24 provided on the back surface of the semiconductor multilayer structure 21 are provided.

  The semiconductor multilayer structure 21 has a structure in which a lower cladding layer 21a, a core layer 21b, and an upper cladding layer 21c are sequentially stacked. The lower cladding layer 21a and the upper cladding layer 21c are made of, for example, n-InP and p-InP, respectively.

  The core layer 21b includes a core portion 21ba in the MMI waveguide portion 22a and a side cladding portion 21bb in the side portion 25, and the core portion 21ba is embedded in the side cladding portion 21bb. The core portion 21ba is made of InGaAsP adjusted to have a composition of 1.4Q, for example, so that the refractive index is higher than that of InP. Further, the side cladding portion 21bb is made of InGaAsP whose composition is adjusted to be 1.23Q, for example, so that the refractive index is higher than that of InP and lower than that of the core portion 21ba.

  Since the composition of the core portion 21ba and the side clad portion 21bb is adjusted as described above, the equivalent refractive index of the core layer 21b with respect to the stacking direction of the semiconductor stacked structure 21 has a distributed shape like the shape P2 in the width direction. have. Light of a predetermined wavelength input to the core layer 21b is confined in the core portion 21ba and guided by the difference in equivalent refractive index between the core portion 21ba and the side cladding portion 21bb.

  The four optical input / output units have a ridge waveguide structure similar to the optical input / output units 12a to 12e of the wavelength tunable optical filter 10, and are configured to guide light of a predetermined wavelength in a single mode. ing.

  The loss wavelength characteristic of the wavelength tunable optical filter 20 has a loss wavelength characteristic that has a minimum value like the wavelength tunable optical filter 10. When a voltage is applied between the two electrodes 23 and 24 serving as a refractive index adjusting mechanism and a current is injected into the side portion 25, the equivalent refractive index of the side cladding portion 21bb is lowered due to the plasma effect. As a result, the minimum loss wavelength of the wavelength tunable optical filter 20 also changes, so that it functions as a wavelength tunable optical filter.

  In this wavelength tunable optical filter 20, if the core portion 21ba is made of 1.4Q InGaAsP and the side cladding portion 21bb is made of 1.23Q InGaAsP, the difference in the equivalent refractive index is, for example, 0 at a wavelength of 1.56 μm. .06 or so. In this case, if the equivalent refractive index of the side cladding portion 21bb is changed by a ratio of -0.58%, the variable range of the minimum loss wavelength of the wavelength tunable optical filter 20 can be made extremely wide as about 50 nm.

  As described above, the wavelength variable optical filter 20 according to the second embodiment has a variable wavelength characteristic of loss in a very wide band.

(Embodiment 3)
Next, a wavelength tunable optical filter according to Embodiment 3 of the present invention will be described. Since the structure of the wavelength tunable optical filter according to the third embodiment viewed from above is the same as the structure of the wavelength tunable optical filter 10 shown in FIG. 1, the description thereof will be omitted, and the sectional structure thereof will be described below. FIG. 5 is a diagram showing a schematic cross section of the wavelength tunable optical filter according to the third embodiment and a distribution shape of the equivalent refractive index.

  The tunable optical filter 30 includes an MMI waveguide portion 32a formed in the semiconductor multilayer structure 31 and four optical input / output portions provided at one end or the other end in the length direction of the MMI waveguide portion 32a. An optical filter section, an electrode 33 provided on the side portion 35 of the MMI waveguide section 32 a on the semiconductor multilayer structure 31, and an electrode 34 provided on the back surface of the semiconductor multilayer structure 31 are provided.

  The semiconductor multilayer structure 31 has a structure in which a lower cladding layer 31a, a core layer 31b, and an upper cladding layer 31c are sequentially stacked. The lower cladding layer 31a and the upper cladding layer 31c are made of, for example, n-InP and p-InP, respectively.

  The core layer 31b includes a core portion 31ba in the MMI waveguide portion 32a and a side cladding portion 31bb in the side portion 35. The core part 31ba is formed thicker than the side clad part 31bb. The core layer 31b is made of, for example, InGaAsP adjusted to have a composition of 1.4Q.

  As described above, the core layer 31b is formed such that the core portion 31ba is thicker than the side clad portion 31bb. Therefore, the equivalent refractive index of the core layer 31b with respect to the stacking direction of the semiconductor stacked structure 31 is the width direction. Has a distribution shape such as shape P3. As a result, light of a predetermined wavelength input to the core layer 31b is confined and guided in the core portion 31ba due to the difference in equivalent refractive index between the core portion 31ba and the side cladding portion 31bb.

  The four optical input / output units have a ridge waveguide structure similar to the optical input / output units 12a to 12e of the wavelength tunable optical filter 10, and are configured to guide light of a predetermined wavelength in a single mode. ing.

  The loss wavelength characteristic of the wavelength tunable optical filter 30 also has a loss wavelength characteristic that has a minimum value like the wavelength variable optical filters 10 and 20. When a voltage is applied between the two electrodes 33 and 34 serving as a refractive index adjusting mechanism and a current is injected into the side portion 35, the equivalent refractive index of the side cladding portion 31bb decreases due to the plasma effect. As a result, the minimum loss wavelength of the wavelength tunable optical filter 30 also changes, so that it functions as a wavelength tunable optical filter.

  In this wavelength tunable optical filter 30, the core part 31ba and the side cladding part 31bb are made of 1.4Q InGaAsP, the layer thickness of the core part 31ba is 0.6 μm, and the layer thickness of the side cladding part 31bb is 0.31 μm. Then, the equivalent refractive index difference is, for example, about 0.06 at a wavelength of 1.56 μm, for example. In this case, if the equivalent refractive index of the side cladding portion 31bb is changed by a ratio of -0.58%, the variable range of the minimum loss wavelength of the wavelength tunable optical filter 30 can be made extremely wide as about 50 nm.

  As described above, the wavelength variable optical filter 30 according to the third embodiment has a variable wavelength characteristic of loss in a very wide band.

(Embodiment 4)
Next, a wavelength tunable optical filter according to Embodiment 4 of the present invention will be described. FIG. 6 is a schematic plan view of the wavelength tunable optical filter according to the fourth embodiment.

  The wavelength tunable optical filter 40 includes an MMI waveguide portion 42a formed in the semiconductor multilayer structure 41 and light input / output portions 42b, 42c, and 42d provided at one end or the other end in the length direction of the MMI waveguide portion 42a. , 42e, an electrode 43 provided on the side of the MMI waveguide portion 42a on the semiconductor multilayer structure 41, and an electrode provided on the back surface of the semiconductor multilayer structure 41. .

  The MMI waveguide portion 42a, its side portions, and each electrode are the same as the MMI waveguide portion 12a, the side portion 15 and the electrodes 13, 14 of the wavelength tunable optical filter 10 shown in FIGS. To do.

  FIG. 7 is a cross-sectional view of the main part of the wavelength tunable optical filter shown in FIG. As shown in FIG. 7, in the cross section taken along the line BB, the semiconductor multilayer structure 41 has a structure in which a core layer 41b and an upper clad layer 41c are sequentially laminated on a protruding portion 41aa of the lower clad layer 41a. An optical input / output unit 42b is configured. That is, the optical input / output unit 42b is a single mode optical waveguide having a so-called high mesa structure. The lower clad layer 41a, the upper clad layer 41c, and the core layer 41b are made of, for example, n-InP and InGaAsP and p-InP having a composition of 1.4Q, respectively.

  The other light input / output units 42c, 42d, and 42e also have the same high mesa structure as the light input / output unit 42b.

  Since the wavelength tunable optical filter 40 includes the MMI waveguide portion 42a having the same ridge structure as that of the MMI waveguide portion 12a, the loss wavelength characteristic is changed by changing the equivalent refractive index of the side portion using the electrode 43. Can be changed. The bandwidth of the wavelength characteristic of the loss of the wavelength tunable optical filter 40 is narrower than that of the wavelength tunable optical filter 10, and the wavelength selection characteristic is high.

This will be specifically described below. The band of the loss characteristic of the MMI waveguide is expressed by the following formula (1) (see Non-Patent Document 2).
2 | δλ | = [Z (α) × π × d ₀ ² × n] / L (1)
However, in Equation (1), 2 | δλ | is a bandwidth whose loss is from a minimum value to a value larger by α, Z (α) is a function of α, n is an equivalent refractive index of the core portion of the MMI waveguide, and d ₀ Indicates the Gaussian beam diameter of light when light is input to the MMI waveguide, and L indicates the length of the MMI waveguide.

In the tunable optical filter 40, since the light input and output portion 42b is the first light output unit is high mesa structure, a Gaussian beam diameter d ₀ of the light input to the MMI waveguide portion 42a, than the ridge structure Can also be narrowed. As a result, according to the equation (1), the bandwidth 2 | δλ | of the wavelength tunable optical filter 40 is narrowed.

  FIG. 8 is a diagram showing changes in loss characteristics when the equivalent refractive index of the side cladding portion is changed in the wavelength tunable optical filter 40 shown in FIG. In FIG. 8, “reference” indicates the wavelength characteristic of loss in a state where no current flows through the side portion. Comparing the “reference” in FIG. 8 with the “reference” in FIG. 3, the optical input / output section is made a high mesa structure and the Gaussian beam diameter of the input light is narrowed despite the identical structure of the MMI waveguide. It is confirmed that the bandwidth is narrower in the case of 8 “reference”.

  In FIG. 8, “−0.5%” indicates that the current flowing through the side portion is equivalent to 0.5% of the equivalent refractive index of the “reference” equivalent refractive index of the side cladding portion. The lowered state is shown. In FIG. 8, the arrow Ar2 indicates the variable range of the minimum loss wavelength between the “reference” state and the “−0.5%” state, but the variable range indicated by the arrow Ar2 is extremely about 40 nm. Broadband.

  As described above, the wavelength tunable optical filter 40 according to the fourth embodiment has a variable wavelength characteristic of loss in a very wide band, and has a higher wavelength selectivity.

(Embodiment 5)
In the above embodiment, all the side portions of the MMI waveguide portion have a planar waveguide structure, but hereinafter, Embodiment 5 in which the side portion does not have a waveguide structure will be described.

  FIG. 9 is a schematic plan view of a wavelength tunable optical filter according to the fifth embodiment. FIG. 10 is a diagram illustrating a cross section taken along the line C-C and an equivalent refractive index or a refractive index distribution shape of the wavelength tunable optical filter illustrated in FIG. 9.

  Hereinafter, the wavelength tunable optical filter 50 will be described with reference to FIGS. The wavelength tunable optical filter 50 includes an MMI waveguide portion 52a and light input / output portions 52b, 52c, 52d provided at one end or the other end in the length direction of the MMI waveguide portion 52a formed in the semiconductor multilayer structure 51. , 52e, the side cladding member 56 provided on the side portion 55 of the MMI waveguide portion 52a on the semiconductor multilayer structure 51, the side cladding member 56, and the back surface of the semiconductor multilayer structure 51. And a refractive index adjusting mechanism 53. In FIG. 9, the description of the refractive index adjustment mechanism 53 on the side clad member 56 is omitted for the sake of explanation.

  The semiconductor multilayer structure 51 has a structure in which a lower clad layer 51a, a core layer 51b serving as a core portion, and an upper clad layer 51c are sequentially laminated in a cross section taken along the line CC, and is an MMI waveguide having a high mesa structure. Part 52a is configured. The lower cladding layer 51a and the upper cladding layer 51c are made of, for example, n-InP and p-InP, respectively. The core layer 51b is made of InGaAsP adjusted to have a composition of 1.4Q, for example, so that the refractive index is higher than that of InP.

  The side clad member 56 is made of a material having a refractive index lower than the equivalent refractive index of the core layer 51 b with respect to the stacking direction of the semiconductor stacked structure 51. As a result, the equivalent refractive index of the core layer 51b and the refractive index of the side cladding member 56 with respect to the stacking direction of the semiconductor stacked structure 51 have a distributed shape such as the shape P4 in the width direction. As a result, light of a predetermined wavelength input to the core layer 51 b is confined in the core layer 51 b and guided by the difference between the equivalent refractive index of the core layer 51 b and the refractive index of the side cladding member 56.

  The light input / output units 52b, 52c, 52d, and 52e also have a high mesa structure, and are configured to guide light of a predetermined wavelength in a single mode.

  The loss wavelength characteristic of the wavelength tunable optical filter 50 has a loss wavelength characteristic that has a minimum value like the wavelength tunable optical filter 10. Then, by changing the refractive index of the side cladding member 56 by the refractive index adjusting mechanism 53, the minimum loss wavelength of the wavelength tunable optical filter 50 also changes, so that it functions as a wavelength tunable optical filter.

  The side clad member 56 and the refractive index adjusting mechanism 53 are appropriately selected according to the principle used for changing the refractive index of the side clad member 56 and are not particularly limited. For example, when the refractive index of the side clad member 56 is changed using the electro-optic effect, the side clad member 56 is made of a semiconductor, ferroelectric, glass, resin, or the like having an electro-optic effect. The refractive index adjusting mechanism 53 is an electrode. When the refractive index of the side clad member 56 is changed using the thermo-optic effect, the side clad member 56 is made of a resin having a thermo-optic effect, and the refractive index adjusting mechanism 53 is a heater or Peltier. An element for heating or cooling the element or the like is used.

(Embodiment 6)
In the above embodiment, the MMI waveguide section is a transmissive element. However, Embodiment 6 in which the MMI waveguide section is a reflective element will be described below. FIG. 11 is a schematic plan view of a wavelength tunable optical filter according to the sixth embodiment.

  The wavelength tunable optical filter 60 is provided at the other end of the MMI waveguide portion 62a, the optical input / output portions 62b and 62c provided at one end of the MMI waveguide portion 62a, formed in the semiconductor multilayer structure 61. An optical filter portion 62 having a high light reflection film 67 as a reflecting means, an electrode 63 provided on the side of the MMI waveguide portion 62 a on the semiconductor multilayer structure 61, and an electrode provided on the back surface of the semiconductor multilayer structure 61 And. The MMI waveguide portion 62a has a width W2 and a length L2. The reflectance of the high light reflection film 67 is 90% or more, for example.

  The light input / output units 62b and 62c and the electrode 63 have the same structure as the light input / output units 12b and 12c and the electrode 13 of the wavelength tunable optical filter 10 shown in FIGS. On the other hand, the MMI waveguide portion 62a has the same cross-sectional structure as the MMI waveguide portion 12a, and the width W2 is also the same as the width W1, but the length L2 is set to ½ of the length L1. ing. That is, the wavelength tunable optical filter 60 has a structure in which the wavelength tunable optical filter 10 is cut along the center line in the length direction, and the high light reflection film 67 is provided on the cut surface.

  As a result of this wavelength tunable optical filter 60 having the above structure, the single-mode light IL2 having a predetermined wavelength input from the light input / output unit 62b serving as the first light input / output unit causes the MMI waveguide unit 62a to be in multimode. The multimode similar to that of the MMI waveguide portion 12a is guided in the path that is guided and reflected by the high light reflection film 67, and again guided in the multimode through the MMI waveguide portion 62a and output as the light OL2 from the light input / output portion 62b. Interference effect will be received. As a result, due to the multimode interference effect, this MMI waveguide section 62a has a reflection wavelength characteristic that has a maximum value with respect to the light input from the light input / output section 62b and output from the light input / output section 62b. Will have. Since the MMI waveguide section 62a functions as a 2 × 2 coupler similarly to the MMI waveguide section 12a, the light OL3 having the same characteristics as the light OL2 is also output from the light input / output section 62c.

In order to realize such a reflection wavelength characteristic, an MMI waveguide that functions as a 2 × 2 coupler by using a design parameter such as an equivalent refractive index of the waveguide by the method described in Non-Patent Document 1, for example. The length and width of the part and the positions of the four light input / output parts may be set, and a highly reflective film may be provided at the position of the center line in the length direction of the MMI waveguide part. At this time, for example, if the propagation constants of the 0th order mode and the 1st order mode of the MMI waveguide portion 62a at the wavelength at which the reflectance is maximized (maximum reflection wavelength) are β ₀ and β ₁ respectively, the value of the length L2 is 3π / {4 (β ₀ −β ₁ )}.

  In the wavelength tunable optical filter 60, when the refractive index of the side portion of the MMI waveguide portion 62a is changed using the electrode 63, the maximum reflection wavelength of the MMI waveguide portion 62a also changes. Therefore, the wavelength characteristic of the reflectance can be changed in a very wide band by the above current injection, so that the tunable optical filter 60 functions as a tunable optical filter.

  As described above, the wavelength tunable optical filter 60 according to the sixth embodiment has a wavelength characteristic of reflectance that is very wide and tunable, and at the same time, is approximately ½ shorter than the wavelength tunable optical filter 10. A small wavelength tunable optical filter is obtained.

(Embodiment 7)
Next, a tunable laser using a reflective tunable optical filter as in the sixth embodiment will be described as a seventh embodiment of the present invention. Hereinafter, the structure of the wavelength tunable laser according to the seventh embodiment will be described first, the operation thereof will be described, and finally an example of the manufacturing method will be described.

(Construction)
FIG. 12 is a schematic plan view of the wavelength tunable laser 100 according to the seventh embodiment. As shown in FIG. 12, the wavelength tunable laser 100 includes a sampled grating waveguide unit 101 that is a DBR (Distributed Bragg Reflector) mirror as a periodic reflecting means constituting an optical resonator, and an optical amplification waveguide unit 102. A phase adjusting waveguide section 103, an MMI waveguide section 104, a curved waveguide section 105, an optical amplification waveguide section 106, and a curved waveguide section 107 to which laser light LL is to be output. The optical waveguides are arranged so as to be optically connected in this order and to be folded back with the MMI waveguide portion 104 as a base point. Further, the upper surface of the wavelength tunable laser 100 is covered with a protective film 111 made of, for example, SiNx and having a thickness of about several tens to several hundreds of nm, except for a contact portion between a p-side electrode and a contact layer, which will be described later.

  Next, a laminated structure of the wavelength tunable laser 100 will be described with reference to FIGS. First, FIG. 13 is a cross-sectional view of the main part of the DD line of the wavelength tunable laser 100 shown in FIG. As shown in FIG. 13, the sampled grating waveguide portion 101 has a lower cladding made of n-InP that also serves as a buffer layer on a substrate 122 made of n-InP having an n-side electrode 121 formed on the back surface. A layer 123, a core layer 124 made of InGaAsP, an upper clad layer 125 made of p-InP, and a grating layer 126 made of InGaAsP, in which a grating region 126a in which a short-period grating is formed is arranged at a predetermined period, , P-InP upper cladding layers 127 and 128 and InGaAs contact layers 129 are sequentially stacked. A p-side electrode 112 is formed on the contact layer 129. Here, the composition of the core layer 124 is adjusted to 1.4Q, and the composition of the grating layer 126 is adjusted to 1.5Q.

  The optical amplification waveguide unit 102 includes a substrate 122 and a lower cladding layer 123 that are common to the sampled grating waveguide unit 101, a SCH (Separate confinement heterostructures) layer 130a made of InGaAsP, and a multiple quantum well (MQW) made of InGaAsP. ), An active layer 130 having a structure, an SCH layer 130b, upper cladding layers 131 and 128 made of p-InP, and a contact layer 129 are sequentially stacked. A p-side electrode 113 is formed on the contact layer 129.

  The phase adjusting waveguide section 103 and the MMI waveguide section 104 include a common substrate 122 and a lower cladding layer 123, a core layer 124, upper cladding layers 132 and 128 made of p-InP, and a contact layer 129. It has a stacked structure. A p-side electrode 114 is formed on the contact layer 129. A p-side electrode 115 is formed on the side of the MMI waveguide portion 104.

  Further, separation grooves 118 to 120 for electrically separating these are provided between the sampled grating waveguide portion 101, the optical amplification waveguide portion 102, the phase adjustment waveguide portion 103, and the MMI waveguide portion 104. Is formed. The separation groove 120 has a part of a side wall of the groove formed in a cutout shape. The protective film 111 also covers the surfaces in the separation grooves 118 to 120.

  The optical amplification waveguide unit 106 has a stacked structure similar to that of the optical amplification waveguide unit 102, and a p-side electrode 116 is formed on the contact layer 129. Further, the bent waveguide portions 105 and 107 have the same laminated structure as that of the phase adjusting waveguide portion 103 except that the bent waveguide portions 105 and 107 do not have the p-side electrode.

  As described above, the wavelength tunable laser 100 uses an InGaAsP-based semiconductor material in order to output laser light having a wavelength of 1.55 μm. Further, the sampled grating waveguide section 101, the optical amplification waveguide section 102, the phase adjustment waveguide section 103, the MMI waveguide section 104, the curved waveguide section 105, the optical amplification waveguide section 106, and the curved waveguide section 107 are provided. , And monolithically integrated on the same substrate 122.

  Further, as shown in FIG. 12, an antireflection film 108 is formed on the end face of the laminated structure on the side of the curved waveguide portion 107 of the wavelength tunable laser 100. On the other hand, as shown in FIGS. 12 and 13, a high light reflection film 109 having a reflectivity of, for example, 90% or more is formed as a reflecting means on the end face of the laminated structure on the MMI waveguide section 104 side.

  Next, FIG. 14 is a cross-sectional view of an essential part of the wavelength variable laser 100 shown in FIG. As shown in FIG. 14, the sampled grating waveguide portion 101 and the optical amplification waveguide portion 106 have a buried mesa structure. Specifically, in the sampled grating waveguide portion 101, a part from the lower cladding layer 123 to the upper cladding layer 127 has a mesa structure. This mesa structure has a structure in which a buried layer 133 made of p-InP and a current blocking layer 134 made of n-InP are buried, and further, an upper clad layer 128, a contact layer 129, and a p side The electrodes 112 are sequentially stacked. On the other hand, in the optical amplification waveguide section 106, a part from the lower cladding layer 123 to the upper cladding layer 131 has a mesa structure. The mesa structure is buried with a buried layer 133 and a current blocking layer 134, and an upper clad layer 128, a contact layer 129, and a p-side electrode 116 are sequentially laminated thereon. These current blocking layers 134 have a function of efficiently injecting current. In addition, a separation groove 117 is formed between the sampled grating waveguide portion 101 and the optical amplification waveguide portion 106 to electrically separate them. The protective film 111 also covers the surface in the separation groove 117. The optical amplification waveguide section 102, the phase adjustment waveguide section 103, and the curved waveguide sections 105 and 107 also have the same embedded mesa structure. The mesa width of these embedded mesa optical waveguides is set so as to guide light of a predetermined wavelength in a single mode, and is 2 μm, for example, when the predetermined wavelength is in the 1.55 μm band.

  On the other hand, FIG. 15 is a cross-sectional view of the main part of the wavelength variable laser 100 shown in FIG. As shown in FIG. 15, the protruding portion of the upper cladding layer 132, the upper cladding layer 128, and the contact layer 129 form a ridge portion R, and constitute an MMI waveguide portion 104 having a ridge structure. A contact layer 135 is formed on the upper cladding layer 132 at the side of the ridge portion R. A p-side electrode 115 is formed on the contact layer 135. The surface of the ridge portion R is covered with a protective film 111. The MMI waveguide section 104 is provided with two optical input / output sections having a buried mesa structure.

  The MMI waveguide section 104, the high light reflection film 109, the p-side electrode 115, and the n-side electrode 121 constitute a reflection-type wavelength tunable optical filter 110 similar to that in the sixth embodiment.

  Also, as shown in FIG. 12, the protective film 111 is formed so as to cover the surface of the region other than the buried mesa structure and each p-side electrode.

(Operation)
Next, the operation of the wavelength tunable laser 100 according to the seventh embodiment will be described. Hereinafter, the sampled grating waveguide section 101 will be described first, and then the operation of the wavelength tunable laser 100 will be described.

  The sampled grating waveguide section 101 has a characteristic that the wavelength dependency of the reflectance periodically peaks. The wavelength and period of the peak of this reflectance adjust the refractive index of each layer from the laminated lower cladding layer 123 to the upper cladding layer 127, the period of the short period grating in the grating region 126a, and the period of arrangement of the grating region 126a. Can be set.

  FIG. 16 is a diagram schematically illustrating an example of the wavelength characteristic of the reflectance of the sampled grating waveguide unit 101. In FIG. 16, regarding the characteristics of the grating layer 126, the composition is 1.5Q, the thickness is 50 nm, the period of the short-period grating in the grating region 126a is 241 nm, and the arrangement period of the grating region 126a is 37.4 μm. It is set. The refractive index of the grating layer 126 is 3.49. The length of the sampled grating waveguide portion 101 is 600 μm. As shown in FIG. 16, this sampled grating waveguide section 101 has a wavelength dependency in which the reflectivity has a peak in the vicinity of a wavelength of 1.55 μm and a peak at a period of about 10 nm over a band of about 60 nm. It will have. Further, the full width at half maximum of each peak is about 1 nm.

  In this sampled grating waveguide section 101, when a voltage is applied between the n-side electrode 121 and the p-side electrode 112 and current is injected into the grating layer 126, the equivalent refractive index of the grating layer 126 changes due to the plasma effect. . Thereby, the peak wavelength of the reflectance can be changed. For example, in the case shown in FIG. 16, if the peak wavelength of reflectance is changed by 10 nm by current injection, the peak of reflectance can be arranged without gaps over the entire range of 1.515 to 1.585 μm.

  Next, the operation of the wavelength tunable laser 100 will be described with reference to FIGS. FIG. 17 is a diagram for explaining the oscillation wavelength of the wavelength tunable laser 100 shown in FIG. First, it is assumed that the wavelength tunable optical filter 110 has a wavelength dependency of reflectance such as a reflection spectrum 70 shown in FIG. In this case, light is reflected by the sampled grating waveguide section 101 and the wavelength tunable optical filter 110 at the wavelength of the reflection peak 71 having the largest overlap with the reflection spectrum 70 among the reflectance peaks of the sampled grating waveguide section 101. A resonator is formed.

  Here, when a voltage is applied between the n-side electrode 121 and the p-side electrode 113 and current is injected into the active layer 130 of the optical amplification waveguide section 102, the optical amplification waveguide section 102 emits light. Then, the peak wavelength substantially corresponding to the reflection peak 71 due to the optical amplification function by the stimulated emission of the optical amplification waveguide section 102 and the action of the optical resonator formed by the sampled grating waveguide section 101 and the wavelength tunable optical filter 110. The laser beam having the intensity spectrum 72 of oscillates. Next, the bent waveguide section 105 guides the oscillated laser light. On the other hand, for the optical amplification waveguide section 106, a voltage is applied between the n-side electrode 121 and the p-side electrode 116, and a current is injected into the active layer 130 of the optical amplification waveguide section 106, whereby a semiconductor optical amplifier is obtained. Function as. The optical amplification waveguide unit 106 receives the laser light guided by the curved waveguide unit 105, optically amplifies the laser light, and outputs the amplified laser beam to the curved waveguide unit 107. The bent waveguide section 107 guides the optically amplified laser light and outputs it as laser light LL. Since the bent waveguide portion 107 is an optical waveguide having an angle with respect to the end surface on which the antireflection film 108 is formed, a part of the laser light LL is returned as reflected light by the end surface reflection in the wavelength tunable laser 100. Therefore, the intensity of the laser beam LL is further stabilized. Note that it is preferable to employ a window structure on the output side of the optical amplification waveguide section 106 because return light is further suppressed.

  Next, it is assumed that the wavelength dependence of the reflectivity of the wavelength tunable optical filter 110 is changed as shown by a reflection spectrum 73 shown in FIG. 17 by current injection. In this case, light is reflected by the sampled grating waveguide unit 101 and the tunable optical filter 110 at the wavelength of the reflection peak 74 having the largest overlap with the reflection spectrum 73 among the reflectance peaks of the sampled grating waveguide unit 101. A resonator is formed. As a result, when current is injected into the active layer 130 of the optical amplification waveguide section 102, laser light having an intensity spectrum 75 having a peak wavelength substantially corresponding to the reflection peak 74 oscillates. Thereafter, the bent waveguide section 105 guides the oscillated laser light, and the optical amplification waveguide section 106 receives the laser light guided by the bent waveguide section 105 and optically amplifies it, and the bent waveguide section. Reference numeral 107 guides the optically amplified laser light and outputs it as laser light LL.

  As described above, the wavelength tunable laser 100 outputs laser light LL having a wavelength corresponding to one of the reflectance peaks of the sampled grating waveguide unit 101 by current injection into the wavelength tunable optical filter 110. Can do. Furthermore, as described above, the sampled grating waveguide portion 101 can change the peak wavelength of the reflectance without gaps by current injection. Therefore, the wavelength tunable laser 100 is a laser that is tunable over a wide band.

  By the way, as described above, for example, the intensity spectrum 72 of the oscillating laser beam has a peak wavelength substantially corresponding to the reflection peak 71, but strictly speaking, the peak wavelength of the intensity spectrum 72 is that of the reflection peak 71. The wavelength corresponds to the longitudinal mode of the optical resonator existing in the vicinity. This will be specifically described below.

  FIG. 18 is a diagram for explaining the relationship between the oscillation wavelength of the wavelength tunable laser 100 and the longitudinal mode. In FIG. 18, reference numeral 76 indicates a superimposed spectrum of the reflection spectrum 70 of the wavelength tunable optical filter 110 and the reflection peak 71 of the sampled grating waveguide section 101. On the other hand, reference numeral 77 indicates a longitudinal mode of the optical resonator. These longitudinal modes 77 are arranged at a predetermined wavelength interval 78 on the wavelength axis. Strictly speaking, the laser oscillation wavelength is a wavelength in the vicinity of the peak of the superposition spectrum 76 and is any wavelength of the longitudinal mode 77.

  In this wavelength tunable laser 100, the phase adjustment waveguide unit 103 can adjust the wavelength of the longitudinal mode of the optical resonator to adjust the laser oscillation wavelength. That is, when a voltage is applied between the n-side electrode 121 and the p-side electrode 114 and current is injected into the core layer 124 in the phase adjustment waveguide section 103, the equivalent refractive index of the core layer 124 changes due to the plasma effect. . Thereby, the optical length of the optical resonator can be changed to adjust the wavelength of the longitudinal mode, and the final laser oscillation wavelength can be adjusted.

  As described above, the wavelength tunable laser 100 according to the seventh embodiment uses a wavelength tunable optical filter 110 that has a variable peak reflectance wavelength and a short length in a wide band. The wavelength of the laser is variable, the laser is small, and the wavelength of the laser light is stable.

(Production method)
Next, a method for manufacturing the wavelength tunable laser 100 according to the first embodiment will be described. FIG. 19 is a diagram for explaining an example of a manufacturing method of the wavelength tunable laser shown in FIG.

  First, using a MOCVD crystal growth apparatus, a part 123a of the lower cladding layer 123, an SCH layer 130a, an active layer 130, an SCH layer 130b, and a part 131a of the upper cladding layer 131 are sequentially grown on the substrate 122. Next, a mask made of a SiNx film for protecting the region A1 in which the optical amplification waveguide unit 102 and the optical amplification waveguide unit 106 are formed is formed, and using this mask, the SCH layer 130a in the region other than the region A1 is formed. Then, the active layer 130, the SCH layer 130b, and a part 131a of the upper cladding layer 131 are removed by etching. Next, in a region other than the region A1, the remaining portion 123b for forming the lower cladding layer 123, the core layer 124, the upper cladding layer 125, and the grating layer 126 are formed on a portion 123a of the lower cladding layer 123 that has been removed by etching. A grating forming layer 136 made of InGaAsP for forming is formed by butt joint growth.

  Next, after forming a SiNx film on the entire surface, patterning of the grating region 126a is performed on a region where the sampled grating waveguide portion 101 is to be formed. In addition, patterning is also applied to regions where the phase adjusting waveguide portion 103, the MMI waveguide portion 104, the bent waveguide portion 105, and the bent waveguide portion 107 are to be formed. Etching is performed using this SiNx film as a mask to form the grating region 126a in the grating forming layer 135 in the region where the sampled grating waveguide portion 101 is formed, and all of the grating forming layer 136 in other regions is removed. Next, after removing the mask of the SiNx film, a p-InP layer is again grown on the entire surface. Thereby, the upper cladding layers 127, 131, and 132 are formed.

  Next, a mask of an SiNx film is formed, and an optical waveguide mesa structure other than the MMI waveguide section 104 is formed by dry etching. Thereafter, the buried mesa structure is formed by embedding the buried layer 133 and the current blocking layer 134. Next, after removing the mask of the SiNx film, an upper cladding layer 128 and a contact layer 129 are formed on the entire surface.

  Next, a mask of the SiNx film is formed, and a ridge structure of the optical waveguide of the MMI waveguide section 104 is formed by dry etching. Next, a patterned mask is formed in the region of the buried mesa structure, and the isolation grooves 117 to 120 are formed by dry etching.

  Next, a mask of an SiNx film having an opening in a region where the contact layer 135 is to be formed is formed, and the contact layer 135 is regrown. Subsequently, after removing the mask of the SiNx film, a protective film 111 is formed on the entire surface, a patterned mask is formed on the protective film 111, a predetermined region of the protective film 111 is removed, an opening is provided, and a lift-off method is performed. For example, p-side electrodes 112 to 116 having an AuZn / Au structure are formed. Next, after polishing the back surface of the substrate 122, an n-side electrode 121 having an AuGeNi / Au structure, for example, is formed on the entire back surface. After that, the end surfaces where the antireflection film 108 and the high light reflection film 109 are to be formed are formed by cleavage, and the antireflection film 108 and the high light reflection film 109 made of, for example, a dielectric multilayer film are formed. .

  In the wavelength tunable laser 100, the cleaved surface formed by cleaving without forming the highly reflective film 109 may be used as a cleaved mirror as a reflecting means, or the etched surface formed by etching as an etched mirror. May be used.

  In the seventh embodiment, the sampled grating waveguide is used as the DBR mirror constituting the optical resonator. However, there is no particular limitation as long as the wavelength dependency of the reflectance shows a peak periodically. For example, another DBR mirror such as a super-periodic structure grating may be used. For example, the sampled grating may be formed in the core layer, the lower clad layer, or the like.

  In the seventh embodiment, the wavelength tunable laser is configured using the reflection type wavelength tunable optical filter. However, if the wavelength tunable optical filter according to the present invention is used, the configuration of the wavelength tunable laser is described. Is not particularly limited. For example, a tunable laser having a structure in which the respective elements are linearly arranged as disclosed in Patent Document 1 may be configured using the transmission-type tunable optical filter according to the first to fifth embodiments.

  Further, in the above-described embodiment, as the structure of each optical waveguide, for example, a buried mesa structure is employed for the sampled grating waveguide section 101, and a ridge structure is employed for the MMI waveguide section 104. However, the structure of any optical waveguide including the optical input / output unit is not limited to that employed in the above-described embodiment. For example, an embedded structure, a high mesa structure, or a ridge structure is used depending on required characteristics. Etc. can be arbitrarily adopted.

  In each of the above embodiments, an InGaAsP-based semiconductor material is used in order to use the wavelength tunable optical filter in the wavelength 1.55 μm band, but the semiconductor material to be used is appropriately selected according to the wavelength band to be used. You can choose.

  In the above-described embodiment, both the light input to and output from the wavelength tunable optical filter are single mode. However, the wavelength tunable optical filter may be designed so that at least one of the input and output light is multimode light. . In the above embodiment, the tunable optical filter functions as a 2 × 2 coupler. However, an optical input / output unit is appropriately provided to function as an N × M coupler (N and M are integers of 1 or more). You may make it do. As described above, the mode of light to be input / output and the number of branches can be set by adjusting the width, length, equivalent refractive index, and the like of the MMI waveguide, the position of the light input / output unit, and the like.

  Further, the present invention is not limited by the above embodiment. What comprised each component of each said embodiment combining suitably is also contained in this invention. In each embodiment, not only the refractive index or equivalent refractive index of the side cladding part is changed, but also the equivalent refractive index of the core part of the MMI waveguide part is changed to change the wavelength of the optical filter part. The characteristics may be controlled. In order to change the equivalent refractive index of the core portion of the MMI waveguide portion, for example, an MMI waveguide portion provided with a current injection structure to the core portion may be used.

10-60, 110 Wavelength tunable optical filter 11-61 Semiconductor laminated structure 11a-51a Lower cladding layer 11b-51b Core layer 11ba-31ba Core part 11bb-31bb Side cladding part 11c-51c Upper cladding layer 11ca Ridge part 12, 42 ~ 62 Optical filter part 12a ~ 62a, 104 MMI waveguide part 12b ~ 12e, 42b ~ 42e, 52b ~ 52e, 62b, 62c Optical input / output part 13 ~ 43, 14 ~ 34, 63 Electrode 15 ~ 55 Side part 41aa Projection Part 53 Refractive index adjusting mechanism 56 Side clad member 67, 109 High light reflection film 70, 73 Reflection spectrum 71, 74 Reflection peak 72, 75 Intensity spectrum 76 Superposition spectrum 77 Longitudinal mode 78 Wavelength interval 100 Wavelength variable laser 101 Sampled grating Waveguide Path part 102 Optical amplification waveguide part 103 Phase adjustment waveguide part 105, 107 Curved waveguide part 106 Optical amplification waveguide part 108 Antireflection film 111 Protective film 112-116 P-side electrode 117-120 Separation groove 121 N-side electrode 122 Substrate 123 Lower cladding layer 123a Part of lower cladding layer 123b Remaining part forming lower cladding layer 124 Core layer 125 Upper cladding layer 126 Grating layer 126a Grating region 127, 128, 131, 132 Upper cladding layer 129, 135 Contact layer 130 Active Layer 130a, 130b SCH layer 131a part of upper cladding layer 133 buried layer 134 current blocking layer 136 grating forming layer A1 region Ar1, Ar2 arrow IL1, IL2, OL1-OL3 light L1, L2 length LL Light P11, P12, P2 to P4 shape R ridge W1, W2 width

Claims (11)

A multimode interference waveguide section having a core section; a first optical input / output section provided at one end of the multimode interference waveguide section in the length direction; and another multimode interference waveguide section. A second optical input / output unit provided at one end, and the multimode interference type waveguide unit is a loss wavelength with respect to light input from the first optical input / output unit and output from the second optical input / output unit. An optical filter portion having characteristics;
Provided on the side of the core portion of the multi-mode interference waveguide portion, a side cladding portion having a lower refractive index or equivalent refractive index than the core portion;
A refractive index adjusting mechanism for changing a refractive index or an equivalent refractive index of the side cladding portion;
A wavelength tunable optical filter comprising: A multimode interference waveguide section having a core section; a first optical input / output section provided at one end of the multimode interference waveguide section in the length direction; and another multimode interference waveguide section. Reflection means provided at one end, and the multi-mode interference waveguide section receives light from the first light input / output section, reflected by the reflection means, and output from the first light input / output section. And an optical filter portion having reflection wavelength characteristics,
A side clad part having a refractive index or an equivalent refractive index smaller than that of the core part, provided on a side part of the core part of the multimode interference waveguide part;
A refractive index adjusting mechanism for changing a refractive index or an equivalent refractive index of the side cladding portion;
A wavelength tunable optical filter comprising:   The optical filter unit is provided at the one end of the multi-mode interference waveguide unit, and a second optical input / output unit that outputs a part of the light input from the first optical input / output unit and reflected by the reflecting means The tunable optical filter according to claim 2, further comprising:   The wavelength tunable optical filter according to claim 1, wherein the multimode interference waveguide section has a ridge structure.   The wavelength tunable optical filter according to any one of claims 1 to 3, wherein the multimode interference type waveguide section has a buried structure.   The wavelength tunable optical filter according to claim 1, wherein the multimode interference waveguide section has a high mesa structure.   The wavelength tunable optical filter according to claim 1, wherein the first light input / output unit has a high mesa structure.   The upper clad layer and the lower clad layer, which are formed above and below the side clad portion in the stacking direction and have a lower refractive index than the side clad portion, are provided. Wavelength tunable optical filter described in 1.   A tunable laser comprising the tunable optical filter according to claim 1.   The wavelength tunable laser according to claim 9, further comprising an optical resonator configured by periodic reflection means in which the wavelength dependency of reflectance periodically peaks.   The tunable laser according to claim 10, wherein the periodic reflection unit is a DBR mirror.

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