JP2013015728A - Semiconductor tunable filter and semiconductor tunable laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor tunable laser that has a wider tunable bandwidth than ever before and that simplifies accurate oscillation wavelength control.SOLUTION: A filter area 420 of a semiconductor tunable laser 400 includes a first MZI 421 and a second MZI 422 that are arranged in parallel and are connected to two output ports of a 2×2 optical coupler 423, respectively. Each MZI is a symmetric MZI that has two isomorphic arm waveguides between two 2×2 optical couplers. Each of the arm waveguides comprises a Fabry-Perot etalon that includes: a first mirror that is connected to the output port of one 2×2 optical coupler; and a second mirror that is connected to the first mirror through an optical waveguide with predetermined length (first length L1 for the first MZI 421 and second length L2 for the second MZI 422) and is connected to an input port of the other 2×2 optical coupler. The output ports of the other 2×2 optical coupler are provided with high reflective mirrors 424 and 425 with a reflectivity of 90% or more, respectively.

Description

  The present invention relates to a semiconductor wavelength tunable filter and a semiconductor wavelength tunable laser.

  In recent years, due to an increase in traffic on the Internet, transmission capacity is increased by using wavelength multiplexing for transmission between nodes. In such wavelength division multiplexing transmission, a wavelength tunable laser is an indispensable optical component.

  FIG. 1 shows a monolithic integrated wavelength tunable laser using a conventionally proposed double ring resonator (see Non-Patent Document 1). The ring resonator is a transmission type optical filter having a feature that the transmission intensity increases at a constant frequency interval (FSR). In this laser, two ring resonators having different FSRs are used to reduce the minimum of two FSRs. It is possible to obtain a wavelength tunable operation in the common multiple frequency range (Vernier effect).

  In order to obtain a large wavelength variable band that covers a communication wavelength band, for example, the C band (1530 to 1570 nm), it is necessary to increase the FSR, that is, to decrease the resonator length of the ring resonator, A high-mesa optical waveguide is used in which a semiconductor is etched vertically to the cladding layer and a steep bend radius can be realized. FIG. 2 shows a cross-sectional view of the high mesa optical waveguide. A multimode interference (MMI) optical coupler having an optical coupling efficiency of 50% is used for the optical coupler portion of the ring resonator. In order to change the oscillation wavelength of the laser, current is independently injected into each of the two ring resonators, and the resonance peak wavelength is adjusted by changing the refractive index of the optical waveguide. Refractive index modulation can be performed at high speed in about nanoseconds by current injection. Further, by providing an optical waveguide for phase adjustment (phase adjustment region) in the laser resonator, the longitudinal mode interval can be finely adjusted and adjusted to a desired oscillation wavelength accurately. This phase adjustment is also performed by current injection.

IEEE Photonics Technology Letters, vol. 19, 2007, pp. 1322-1324 IEEE 21st International Semiconductor Laser Conference (ISLC 2008), 2008, pp. 153-154

  However, in the structure of FIG. 1, it is difficult to further expand the wavelength variable band. In the current optical communication system using the wavelength division multiplexing transmission system, not only the aforementioned C band but also the wavelength band such as the L band (1570 to 1610 nm) and the S band (1460 to 1530 nm) as the communication capacity increases. It is used. Accordingly, it is required to have a wider wavelength tunable band that can cover a plurality of wavelength bands with a single wavelength tunable laser. However, in the configuration of FIG. 1, the wavelength selection performance determined by the finesse of the ring resonator From this tradeoff, there is a limit that the submode suppression ratio is at most 30 dB and the wavelength variable band is up to 50 nm.

  As shown in FIG. 3, two ring resonators and an asymmetric Mach-Zehnder interferometer (hereinafter referred to as “MZI”) are connected in series, as shown in FIG. 3, in order to reduce the number of passes through the ring resonator and to use no end face reflection. A one-chip integrated semiconductor wavelength tunable laser arranged in a loop mirror by a Sagnac interferometer has been proposed (see Non-Patent Document 2). In this configuration, the light from the gain region is equally branched by the 1 × 2 optical coupler, circulates in the loop clockwise and counterclockwise, and enters the 1 × 2 optical coupler again. Since the right-handed light and the left-handed light re-enter the 1 × 2 optical coupler with the same phase, they are fed back to the gain region with an optical coupling efficiency of almost 100%. At this time, the method of expanding the wavelength tunable band by the vernier effect by making the FSRs of the two ring resonators different is the same as the configuration of FIG. Become. Although the loss due to the ring resonator can be reduced by reducing the number of passes, the wavelength selection performance deteriorates. Therefore, in order to compensate for the deterioration of the filter performance, an asymmetric MZI is inserted into the loop as a third filter, and an attempt is made to achieve both wavelength variable band expansion and wavelength selection performance. However, there is a problem in adding the third filter for wavelength control. That is, the wavelength control mechanism of the wavelength tunable laser becomes complicated, and for example, a long time measurement is required to obtain the wavelength tunable characteristics.

  In the ring resonator or the MZI filter, the design value and the actual measurement value of the FSR and the transmission peak wavelength do not always coincide with each other, and the actual value has a variation determined by a manufacturing error. Therefore, after manufacturing the wavelength tunable laser, the relationship between the laser oscillation wavelength and the current injected into the wavelength control filter (applied power when controlled by heat, applied voltage when controlled by voltage) must be actually investigated. The laser oscillation wavelength cannot be accurately controlled. For example, it is assumed that an injection current of 10 mA is required to move the resonance peak of the ring resonator to the next resonance peak. When the wavelength tunable band is expanded by the vernier effect with the configuration of FIG. 1, all wavelengths within the wavelength tunable band can be selected by injecting a current of 10 mA at the maximum into the two ring resonators. When acquiring the above-described relationship between the oscillation wavelength and the current in steps of 0.1 mA, numerical data of 100 × 100 = 10000 points is required. When a measurement system (for example, configured with a current source and a wavelength meter) that takes 1 second per point is used, the time required to acquire the characteristics of one tunable laser is about 2.8 hours. When the same number of current steps is measured with the configuration of FIG. 3 to which a wavelength control filter based on asymmetric MZI is added, approximately 280 hours (100 × 100 × 100 = 1000000 points) are required. The above example depends on the required oscillation wavelength accuracy, filter shape, measurement speed, and the like, but when a ring resonator having steep filter characteristics is included, a sufficient number of measurement steps is required. Therefore, the configuration of FIG. 3 in which a wavelength control filter is newly added complicates the wavelength control mechanism, which causes a significant reduction in mass productivity.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a semiconductor wavelength tunable laser that has a wider wavelength tunable band than in the past and is simple in accurate oscillation wavelength control. is there. Another object of the present invention is to provide a semiconductor wavelength tunable filter that enables the semiconductor wavelength tunable laser.

  In order to achieve such an object, according to the first aspect of the present invention, two arm waveguides are connected between two 2 × 2 optical couplers, and each arm waveguide includes the two 2 × 2 optical couplers. A first mirror connected to the output port of one 2 × 2 optical coupler of the two optical couplers, and the first mirror is connected to the two 2 × 2 light beams by an optical waveguide having a predetermined length. A Fabry-Perot etalon having a second mirror connected to an input port of the other 2 × 2 optical coupler of the couplers, and a high reflection mirror provided at an output port of the other 2 × 2 optical coupler A tunable semiconductor wavelength filter.

  According to a second aspect of the present invention, in the first aspect, the two arm waveguides have the predetermined length of the optical waveguide connecting the first mirror and the second mirror. In contrast, the length of the optical waveguide connecting the one 2 × 2 optical coupler and the first mirror and the length of the optical waveguide connecting the second mirror and the other 2 × 2 optical coupler are equal. It is characterized by.

  Further, according to a third aspect of the present invention, the first and second semiconductor wavelength tunable filters are connected in parallel to a 2 × 2 optical cover, and each semiconductor wavelength tunable filter is the semiconductor wavelength tunable filter according to the first aspect. The optical fiber connecting the first mirror and the second mirror in the first semiconductor wavelength tunable filter has the same structure of the Fabry-Perot etalon provided in each of the two arm waveguides. The predetermined length of the waveguide is a first length, and the predetermined length of the optical waveguide connecting the first mirror and the second mirror in the second semiconductor wavelength tunable filter. Is a second length different from the first length.

  According to a fourth aspect of the present invention, the first and second semiconductor wavelength tunable filters are connected in series to a 2 × 2 optical cover, and each semiconductor wavelength tunable filter is the semiconductor wavelength tunable filter according to the first aspect. The two arm waveguides have the same structure, and the output port of the other 2 × 2 optical coupler of the first semiconductor wavelength tunable filter is not the high reflection mirror. The optical waveguide connected to the input port of the one 2 × 2 optical coupler of the semiconductor wavelength tunable filter, and connecting the first mirror and the second mirror in the first semiconductor wavelength tunable filter The predetermined length is a first length, and in the second semiconductor wavelength tunable filter, the predetermined length of the optical waveguide connecting the first mirror and the second mirror is Said first length Characterized in that it is a second different lengths.

  A fifth aspect of the present invention provides a semiconductor wavelength comprising a gain region, a filter region having a wavelength selection function for light from the gain region, and a phase adjustment region between the gain region and the filter region In the tunable laser, the filter region is configured by the semiconductor wavelength tunable filter according to any one of the first to fourth aspects.

  According to the semiconductor wavelength tunable filter and the semiconductor wavelength tunable laser including the filter according to the present invention, two arm waveguides are connected between two 2 × 2 optical couplers, and each arm waveguide has two A first mirror connected to the output port of one of the 2 × 2 optical couplers of the 2 × 2 optical coupler, and the first mirror is connected to the two mirrors by an optical waveguide having a predetermined length. A Fabry-Perot etalon having a second mirror connected to the input port of the other 2 × 2 optical coupler of the two optical couplers, and a high reflection mirror provided at the output port of the other 2 × 2 optical coupler As a result, it is possible to provide a semiconductor wavelength tunable laser that has a wider wavelength tunable band than in the past and that is simple in accurate oscillation wavelength control.

It is a figure which shows the monolithic integrated type wavelength tunable laser using the double ring resonator proposed conventionally. It is a figure which shows sectional drawing of a high mesa optical waveguide. It is a figure which shows the prior art example of the semiconductor wavelength tunable laser which reduced the frequency | count of passage to a ring resonator, and did not use an end surface reflection. It is a figure which shows the semiconductor wavelength tunable laser which concerns on Embodiment 1 of this invention. (A) is a transmission spectrum of a Fabry-Perot etalon, and (b) is a characteristic diagram of its phase. FIG. 3 is a detailed diagram of each MZI of the semiconductor wavelength tunable laser 400 according to the first embodiment. 6 is a diagram showing a reflection spectrum of each MZI and a reflection spectrum from the entire filter region 420 of the semiconductor wavelength tunable laser 400 according to Embodiment 1. FIG. It is a figure which shows the electron microscope (SEM) image of the gap mirror which comprises an etalon. It is a figure which shows the relationship between the reflectance of the gap mirror calculated | required by calculation, and the gap space | interval d of the transmittance | permeability. It is a figure which shows the SEM image of the high reflection mirror produced in the output port of a 2 * 2 optical coupler. It is a figure which shows the semiconductor wavelength variable filter with which the semiconductor wavelength variable laser which concerns on Embodiment 2 is provided. It is a figure which shows the result of having calculated | required the relationship between reflectivity difference (DELTA) R and M about Embodiment 1 of parallel connection, and Embodiment 2 of series connection. It is a figure which shows the semiconductor wavelength variable laser which concerns on Embodiment 3 of this invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
(Embodiment 1)
FIG. 4 shows a semiconductor wavelength tunable laser according to Embodiment 1 of the present invention. As illustrated, the semiconductor tunable laser 400 includes a gain region 410, a filter region 420 having a wavelength selection function for light from the gain region 410, and a phase adjustment region 430 between the gain region 410 and the filter region 420. Is provided.

  In the filter region 420, the first MZI 421 and the second MZI 422 are arranged in parallel, and are connected to the two output ports of the 2-input 2-output type (2 × 2) optical coupler 423, respectively. Each MZI is a symmetric MZI having two arm waveguides of the same structure between two 2 × 2 optical couplers.

  Each arm waveguide includes a first mirror connected to the output port of one 2 × 2 optical coupler, a first mirror and a predetermined length (first length L1, second length in the first MZI 421). The MZI 422 includes a Fabry-Perot etalon having a second mirror connected to the input port of the other 2 × 2 optical coupler and connected by an optical waveguide having a second length L2). For example, the reflectance of each mirror can be set to 35%. High reflection mirrors 424 and 425 having a reflectance of 90% or more are provided at the output port of the other 2 × 2 optical coupler.

  The Fabry-Perot etalon is known as a transmission filter or a reflection filter. FIG. 5A shows a transmission spectrum of a Fabry-Perot etalon, and FIG. 5B shows a characteristic diagram of its phase. Here, the transmission peak interval FSR of the Fabry-Perot etalon is

It can be expressed. c is the speed of light, n _eff is the effective refractive index of the optical waveguide, and L is the length of the Fabry-Perot etalon. The transmission spectrum of an etalon has a plurality of transmission peaks like a ring resonator. Furthermore, it can be seen from FIG. 5B that the phase difference is always π between adjacent transmission peak wavelengths.

  In the first embodiment, the Fabry-Perot etalon having the same structure is disposed in the two arm waveguides of the symmetric MZI in order to spatially separate transmitted light and reflected light. FIG. 6 shows a detailed view of each MZI of the semiconductor wavelength tunable laser 400 according to this embodiment. Of the two 2 × 2 optical couplers, the light input to the MZI is equally divided by one 2 × 2 optical coupler on the gain region 420 side, and enters each Fabry-Perot etalon. The reflected light from the etalon again enters the 2 × 2 optical coupler on the gain region 420 side. Since the two Fabry-Perot etalons are identical in structure and the distance between the 2 × 2 optical coupler and the Fabry-Perot etalon is equal (because of symmetric MZI), no phase difference is imparted to the light propagating through the arm waveguide. . That is, all the reflected light from the Fabry-Perot etalon is output to the output port 1. On the other hand, the transmitted light of the Fabry-Perot etalon is similarly output to the output port 2 because no phase difference is given. In this embodiment, only the transmitted light of the Fabry-Perot etalon is used, and the reflected light is discarded from the radiation waveguide connected to the 2 × 2 optical coupler on the gain region 420 side. The transmitted light is reflected by the high reflection mirror formed at the output port 2 of the MZI and re-enters the MZI.

  In the present embodiment, the first MZI 421 and the second MZI 422 include Fabry-Perot etalons having different FSRs. By connecting them in parallel to the 2 × 2 optical coupler 423, the wavelength tunable band due to the Vernier effect is expanded and the phase difference between the transmission peaks of the two sets of Fabry-Perot etalon is used to further increase the wavelength tunable band. It is possible to increase. Furthermore, by forming the current injection electrode on the waveguide of the phase adjustment region 430, it is possible to continuously adjust the cavity mode of the laser resonator, and to accurately control the oscillation wavelength. A Fabry-Perot etalon generally has a bulk form in which a dielectric multilayer film is formed on a glass substrate or the like. In this embodiment, each component is planarized on a InP substrate by using a semiconductor process technology such as photolithography. It is realized by arranging in the. By this method, the device can be reduced in size as compared with the case where a bulk Fabry-Perot etalon and a gain medium are spatially arranged.

  FIG. 7 shows the reflection spectrum of each MZI and the reflection spectrum from the entire filter region 420 of the semiconductor wavelength tunable laser 400 according to this embodiment. From FIG. 7, the maximum filter reflectivity is obtained at the wavelength where the reflection peaks of the first MZI 421 and the second MZI 422 coincide, while the reflection peak of both MZIs coincides with the short wavelength side or the long wavelength side next. It can be seen that the reflectance is minimized at the wavelength. This is because the reflected light from each MZI is coupled to the 2 × 2 optical coupler 423 in the same phase (phase difference 0) or opposite phase (phase difference π) at the wavelength where the reflection peaks coincide. In the embodiment, this is made possible by setting the FSR of two sets of Fabry-Perot etalon so as to satisfy the relationship of the following equation.

Here, FSR1 <FSR2.

Here, a manufacturing method of the semiconductor wavelength tunable laser 400 according to the first embodiment will be described. The laser active layer of the device includes an n-InP layer on an n-InP substrate, an active layer of InGaAsP / InP multiple quantum well structure (MQW) (photoluminescence peak wavelength 1.53 μm), and SCH (Separate- It was set as the structure confined by the (confinement heterostructure) layer. Next, a SiO ₂ film is formed by sputtering, removed by etching except for a portion to be a gain region, and further the active layer is removed using the patterned SiO ₂ film as a mask. Next, an InGaAsP optical waveguide layer having a 1.4Q composition and a film thickness of 0.3 μm is butt-joint grown by selective growth, and then the SiO ₂ layer is removed to grow a p-InP layer and a p ⁺ -InGaAs layer on the entire substrate. did. In the first embodiment, a gain region, a phase adjustment region, and a ridge waveguide in which a Fabry-Perot etalon portion is etched to a position immediately above the core layer are used, and a filter region including the other optical coupler portion is manufactured with a high mesa optical waveguide structure. did. In the first embodiment, a 3 dB multimode interference (MMI) optical coupler that can be manufactured easily and with low loss is used as a 2 × 2 optical coupler. The ridge waveguide and high mesa waveguide were produced by photolithography and dry etching. After forming the waveguide, benzocyclobutene (BCB), which is an organic material that can be embedded in a local region and has excellent planarization, is applied by spin coating, and an O ₂ / C ₂ F ₆ mixed gas is used. Etchback was performed until the substrate surface was exposed by RIE, and the substrate surface was planarized. Thereafter, a current injection electrode is formed on the gain region, the phase adjustment region, and the p ⁺ -InGaAs layer of the etalon ridge waveguide, and a substrate back electrode is further formed.

  The mirror that constitutes the Fabry-Perot etalon in this embodiment is realized by a so-called gap mirror that forms a reflective gap by forming a spatial gap (gap) perpendicular to the light propagation direction in the MZI arm waveguide. can do. FIG. 8 shows an electron microscope (SEM) image of the produced gap mirror. The gap mirror is disposed between the etalon ridge optical waveguide and the high-mesa optical waveguide on the optical coupler side. The gap mirror is manufactured at the same time as the manufacturing process of the high mesa optical waveguide used in the optical coupler, that is, the pattern formation by photolithography and the InP processing by dry etching, and can be realized without any additional process.

  FIG. 9 shows the relationship between the reflectance of the gap mirror and the gap distance d of transmittance obtained by calculation. Here, the gap is filled with BCB (refractive index of 1.54), and the radiation loss due to diffraction is set to 1 dB / μm from the calculation by the three-dimensional BPM method (Beam propagation method). From FIG. 9, it can be seen that the reflectivity of the gap mirror increases or decreases due to interference with respect to d. In this embodiment, d was set to 0.75 μm from the pattern resolution of photolithography used for the production, and the reflectance was set to 35%. In general, as for the transmission characteristics of a Fabry-Perot etalon filter, when the reflectance of a mirror is increased, the transmission spectrum is sharpened and the wavelength selection performance is improved. Therefore, the BCB filled in the gap is locally removed, and the refractive index difference from the semiconductor is increased (increasing the mirror reflectivity) by changing to a material having a refractive index lower than that of air or BCB. It is also possible to improve the wavelength selection performance of this embodiment. The reflectance of the mirror used in the Fabry-Perot etalon of this embodiment is preferably about 10% to 60%.

  In the present embodiment, as a mirror used for the Fabry-Perot etalon, a gap mirror using interference reflection by forming a spatial gap (gap) in the waveguide can be used. However, in order to reflect the light transmitted through the MZI and feed it back to the gain region, it is necessary to integrate a high reflection mirror having a higher reflectivity, ideally 100% reflectivity, on the output side of the MZI. As an example, a mirror having high reflectivity was realized by depositing Au on the end face of the optical waveguide. FIG. 10 shows an SEM image of the produced high reflection mirror. A metal reflector using Au or Ag has characteristics that a current flows, a high reflectivity due to plasma oscillation of free electrons in the metal is obtained, and formation is easy (by a vacuum deposition method). In the embodiment of FIG. 10, an end surface perpendicular to the light propagation direction is formed by dry etching when a high mesa optical waveguide is manufactured (an etching region as shown in FIG. 12 is created in advance on the mask pattern), and Au is formed on the end surface. Reflectance of 93% or more was obtained by oblique deposition (film thickness: 300 nm). The reflectance of the high reflection mirror is preferably 90% or more. Further, even if a dielectric multilayer film is formed on the cleaved end face by sputtering or the like, a highly reflective mirror having the same reflectance can be produced.

  In the present embodiment, the first MZI 421 and the second MZI 422 are connected to the 2 × 2 optical coupler 423 in parallel, but the length of the optical waveguide to be connected may be different between the two MZIs. Between the second MZI and the 2 × 2 optical coupler

By providing an optical waveguide of length L _Phase, an effect of expanding the wavelength tunable range using the phase difference of transmitted light can be expected even if the length is unequal. Here, L1 and L2 are the lengths of the Fabry-Perot etalon in the first MZI 421 and the second MZI 422, respectively.

(Embodiment 2)
FIG. 11 shows a semiconductor wavelength tunable filter provided in the semiconductor wavelength tunable laser according to the second embodiment. In the configuration of Non-Patent Document 1, which is a conventional technique, two ring resonators having different FSRs are connected in series, and the wavelength variable band is expanded by the vernier effect. Also in the semiconductor wavelength tunable laser 400 according to the first embodiment, MZIs can be connected in series as shown in FIG.

  If the FSRs of the first MZI 421 and the second MZI 422 are FSR1 and FSR2, respectively, the wavelength variable band Δλ ′ of the filter region 420 is caused by the vernier effect as shown in Non-Patent Document 1.

It can be expressed. For comparison, FIG. 7 shows Δλ ′ and Δλ. Compared with the equation (2), in the first embodiment, the wavelength variable band can be doubled as compared with the case where filter elements having a plurality of peaks are connected in series. In the first embodiment, when FSR1 is 400 GHz, FSR2 is 444 GHz, M is 10 and N is 9, the wavelength variable band Δλ is 8000 GHz (64 nm). However, the wavelength is variable when only the vernier effect shown in FIG. 11 is used. The band Δλ ′ is 4000 GHz (32 nm), and the wavelength variable performance twice that of the second embodiment can be obtained by the configuration of the first embodiment. However, the semiconductor wavelength tunable filter according to the second embodiment can provide a wide wavelength tunable band as compared with a wavelength tunable filter using a conventional ring resonator. Usually, in order to obtain a large wavelength tunable band due to the vernier effect, it is necessary to increase M (or N) or increase the FSR from Equation (7). In order to increase the FSR, the length L of the Fabry-Perot etalon is reduced from the equation (1), or in the case of a ring resonator, the length of the resonator (in the case of a ring resonator having a constant curvature, the length is 2πR). R should be a small bend radius). However, there is a lower limit to the bending radius of the bending waveguide constituting the ring resonator, and in the case of the semiconductor high mesa optical waveguide used in this embodiment, the optical loss increases rapidly when the bending radius is made smaller than about 10 μm. Therefore, the FSR of the ring resonator has an upper limit of about 700 GHz considering the length of the optical coupler constituting the ring resonator. On the other hand, in the case of a Fabry-Perot etalon, since it is composed of a simple linear waveguide, there is no restriction due to the bending radius. Therefore, a 10 THz FSR exceeding the FSR of the ring resonator can also be realized. It should be noted that accurate oscillation wavelength control of the semiconductor tunable laser can be easily realized by forming the refractive index control electrodes on the Fabry-Perot etalon and the waveguide in the phase adjustment region, respectively.

  As described above, in order to obtain a large wavelength tunable band by the vernier effect, it is necessary to increase M (or N) or increase the FSR. In order to increase M, it is necessary to decrease the difference between the two FSRs. In that case, the overlap of the transmission spectra of the two sets of Fabry-Perot etalon becomes large. That is, in the reflection spectrum from the filter region, the reflectance difference ΔR between the main peak and the adjacent peak decreases. ΔR is an important parameter for determining the wavelength selection performance in the filter region, that is, the sub-mode suppression ratio (SMSR) of the wavelength tunable laser, and has a trade-off relationship with the wavelength tunable region.

  FIG. 12 shows the result of calculation of the relationship between the reflectance difference ΔR and M in order to compare the first embodiment in parallel connection and the second embodiment in series connection. In the calculation, the loss of the optical waveguide was determined as 5 dB / cm, the loss of the coupler as 0.5 dB, and the FSR1 as 400 GHz. As is clear from FIG. 12, ΔR is improved in Embodiment 1 compared to Embodiment 2 for the same M. This is because light is output also to the radiation waveguide side according to the difference between the phase and amplitude to the 2 × 2 optical coupler 423, and light other than the transmission peak wavelength has a reduced coupling efficiency to the gain region 410. . Thereby, the reflectance difference ΔR between the main peak and the adjacent peak is improved as compared with the second embodiment in series connection, and the sub-mode suppression ratio is improved. That is, in the first embodiment, the wavelength tunable band is doubled and a high SMSR wavelength tunable laser can be realized. In the first embodiment, ΔR is 1.2 dB and the SMSR can be 40 dB or more, and a wavelength variable laser having a wide wavelength variable band and a high SMSR can be manufactured as compared with Non-Patent Document 1, which is a conventional technique.

(Embodiment 3)
FIG. 13 shows a semiconductor wavelength tunable laser according to Embodiment 3 of the present invention. The semiconductor wavelength tunable laser 1300 according to the third embodiment includes a gain region 410, a filter region 1320, and a phase adjustment region 430. The filter region 1320 includes two arms connected between two 2 × 2 optical couplers. An MZI 1321 in which Fabry-Perot etalons having different FSRs are arranged in a waveguide is provided. The two arm waveguides have different lengths of the optical waveguides connecting the mirrors constituting the Fabry-Perot etalon, so that the MZI 1321 is an asymmetric MZI. However, the length of the optical waveguide connecting the 2 × 2 optical coupler and the mirror is equal in the two arm waveguides.

  With this configuration, as in the first embodiment, by using the phase of the transmitted light of the Fabry-Perot etalon, the wavelength tunable band is increased twice as compared with the second embodiment using the simple vernier effect. Is possible. Furthermore, the number of Fabry-Perot etalon used is halved, and the device area can be reduced. Furthermore, the power required for wavelength tuning is also halved. Further, the number of optical couplers in the laser resonator is five in the first embodiment and two in the third embodiment, so that optical loss due to the optical coupler can be reduced.

  In this embodiment, two Fabry-Perot etalon are connected in parallel to two 2 × 2 optical couplers, but the length of the optical waveguide to be connected is equal between the two arm waveguides. However, for example, between a Fabry-Perot etalon of length L2 and one 2 × 2 optical coupler

By providing an optical waveguide of length L _Phase, an effect of expanding the wavelength tunable range using the phase difference of transmitted light can be expected even if the length is unequal. Here, L1 and L2 are the lengths of the Fabry-Perot etalon, respectively, and K is a positive integer.

  The semiconductor wavelength tunable laser 1300 according to the present embodiment can be manufactured using a ridge waveguide structure in the Fabry-Perot etalon part as in the first and second embodiments, but other than the high mesa waveguide and the core layer, An embedded waveguide or a rib waveguide in which both sides are embedded with a semiconductor is also conceivable.

Finally, in this embodiment, an InP-based compound semiconductor can be used, but the same applies if a gain medium is also integrated in a GaAs-based or quartz-based, or a silicon thin wire waveguide composed of Si, SiO ₂ , polyimide, or the like. Note that this can be achieved. In this embodiment, a change in refractive index due to current injection can be used, but a wavelength-variable operation can also be obtained using a change in refractive index due to voltage, heat, or pressure.

400 Semiconductor wavelength tunable laser 410 Gain region 420 Filter region (equivalent to “semiconductor wavelength tunable filter”)
421 First MZI
422 Second MZI
423 2 × 2 optical coupler 424, 425 High reflection mirror 430 Phase adjustment region 1300 Semiconductor wavelength tunable laser 1320 Filter region (corresponding to “semiconductor wavelength tunable filter”)
1321 MZI
1322 High Reflective Mirror

Claims (5)

Two arm waveguides are connected between two 2 × 2 optical couplers,
Each arm waveguide is
A first mirror connected to the output port of one 2 × 2 optical coupler of the two 2 × 2 optical couplers;
A fabric having a first mirror and a second mirror connected by an optical waveguide having a predetermined length and connected to an input port of the other 2 × 2 optical coupler of the two 2 × 2 optical couplers. With a Perot etalon,
A semiconductor wavelength tunable filter, wherein a high reflection mirror is provided at an output port of the other 2 × 2 optical coupler. The two arm waveguides differ in the predetermined length of the optical waveguide connecting the first mirror and the second mirror,
The lengths of the optical waveguide connecting the one 2 × 2 optical coupler and the first mirror, and the optical waveguide connecting the second mirror and the other 2 × 2 optical coupler are equal. The semiconductor wavelength tunable filter according to claim 1. First and second semiconductor tunable filters are connected in parallel to a 2 × 2 optical turntable,
Each semiconductor wavelength tunable filter is the semiconductor wavelength tunable filter according to claim 1, wherein the structures of the Fabry-Perot etalon provided in each of the two arm waveguides are the same,
In the first semiconductor wavelength tunable filter, the predetermined length of the optical waveguide connecting the first mirror and the second mirror is a first length;
In the second semiconductor wavelength tunable filter, the predetermined length of the optical waveguide connecting the first mirror and the second mirror is a second length different from the first length. A tunable semiconductor wavelength filter. First and second semiconductor tunable filters are connected in series to a 2 × 2 optical turntable,
Each semiconductor wavelength tunable filter is the semiconductor wavelength tunable filter according to claim 1, wherein the structures of the two arm waveguides are the same,
The output port of the other 2 × 2 optical coupler of the first semiconductor tunable filter is connected to the input port of the one 2 × 2 optical coupler of the second semiconductor tunable filter instead of a high-reflection mirror. Connected,
In the first semiconductor wavelength tunable filter, the predetermined length of the optical waveguide connecting the first mirror and the second mirror is a first length;
In the second semiconductor wavelength tunable filter, the predetermined length of the optical waveguide connecting the first mirror and the second mirror is a second length different from the first length. A tunable semiconductor wavelength filter. A gain region;
A filter region having a wavelength selection function for light from the gain region;
A semiconductor wavelength tunable laser comprising a phase adjustment region between the gain region and the filter region,
The said filter area | region is comprised with the semiconductor wavelength tunable filter in any one of Claim 1 to 4, The semiconductor wavelength tunable laser characterized by the above-mentioned.