JP2011142191A - Semiconductor wavelength tunable laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a low-loss semiconductor wavelength tunable laser that has a wide wavelength variable region and can be manufactured easily. <P>SOLUTION: The wavelength tunable laser includes a gain region, a filter region having a wavelength selection function on light from the gain region, a phase adjusting region between the gain region and filter region, and an output edge. The filter region is a Sagnac interferometer functioning as a loop mirror, and has a structure in which ring resonators 1, 2 structured of a 2×2 optical coupler and optical waveguide are arranged in a loop. The ring resonators 1, 2 are directly connected by the optical waveguide and are arranged in series. The ring resonators 1, 2 expand a wavelength variable region by having different FSRs from each other. A high mesa optical waveguide is used in connecting the 2×2 optical coupler and the ring resonators 1, 2, and in connecting the ring resonators 1, 2. In addition, a low-loss MMI optical coupler, which is manufactured easily, is used for an optical coupling part of the ring resonators 1, 2 and the 2×2 optical coupler. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a semiconductor wavelength tunable laser that is an important optical component for supporting wavelength division multiplexing large capacity communication.

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

  Under such circumstances, as shown in Non-Patent Document 1, a monolithic integrated type tunable laser using a double ring resonator has been proposed. FIG. 1 shows a structural diagram. The ring resonator is a transmission type optical filter having a feature that transmission intensity increases at a constant frequency interval (FSR). In this laser, the minimum of two FSRs is obtained by using a ring resonator having two different FSRs. It is possible to obtain a wavelength variable operation in the frequency range of the common multiple (Vernier effect). In order to obtain a large wavelength tunable range that covers the 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, and to realize a steep bend radius. A high mesa optical waveguide is used. FIG. 2 shows a cross-sectional structure diagram of the high mesa optical waveguide. In this structure, the semiconductor is etched vertically to the core layer and the lower cladding layer. 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, the resonance peak wavelength is adjusted by changing the refractive index of the optical waveguide by independently injecting currents into the two ring resonators. Current injection enables refractive index modulation at high speed in about nanoseconds. Further, by providing an optical waveguide for phase adjustment (phase adjustment region) in the laser, the longitudinal mode interval can be finely adjusted so that it can be accurately adjusted to a desired oscillation wavelength. This phase adjustment is also performed by current injection.

  However, the structure shown in FIG. 1 has the following problems. The first problem is that it is difficult to increase the output. In the configuration of FIG. 1, light matching the resonance peak wavelength of two ring resonators from light from the gain region effectively passes through the filter region, is reflected by the end face, passes through the filter region again, and is coupled to the gain region. To do. The ring resonator has a loss of several dB even for light having a resonance peak wavelength, and the loss increases every time it passes through the ring resonator. Further, the reflectivity from the end face was only about 30% with the cleaved end face, and there was a mirror loss of about 5.2 dB. Although it is possible to reduce the mirror loss by forming a highly reflective film on the end face, the reflectivity from the end face on which the highly reflective film is formed is not actually 100%, but a loss of several to tens of percent. was there. Therefore, the feedback light to the gain region actually receives a loss of about 10 dB due to the passage of the ring resonator twice and the mirror loss at the end face, leading to a decrease in laser light output. The second problem is that it is difficult to further expand the wavelength variable range. In the current optical communication system using the wavelength division multiplex transmission system, the wavelength band is not limited to the C band as described above, and the L band (1570-1610 nm), the S band (1460-1530 nm), etc. as the communication capacity increases. Wavelength bands are also used. Therefore, it is required to have a wider wavelength tunable band that can cover a plurality of wavelength bands with one wavelength tunable laser. However, in the configuration of Non-Patent Document 1, the wavelength variable range has a limit of 50 nm at the maximum because of the trade-off with the wavelength selection performance determined by the finesse of the ring resonator.

  As a configuration in which the number of times of passage to the ring resonator is reduced and end face reflection is not used, the configuration shown in FIG. This is a one-chip integrated semiconductor wavelength tunable laser in which a Sagnac interferometer in which two ring resonators and an asymmetric Mach-Zehnder interferometer are connected in a loop shape is arranged in a filter region (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 range by the vernier effect by making the FSRs of the two ring resonators different is the same as the configuration of FIG. 1, but the number of passes through each ring resonator is one time by the loop configuration. 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, an asymmetric Mach-Zehnder interferometer is inserted as a third filter into the loop of the Sagnac interferometer. With such a configuration, an attempt is made to achieve both expansion of the wavelength variable range and wavelength selection performance.

Japanese Patent Application No. 2006-239142

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

  However, adding the third filter for wavelength control has a serious problem in production. That is, the wavelength control mechanism of the wavelength tunable laser is complicated, and for example, a great amount of measurement time is required to obtain the wavelength tunable characteristics. In a ring resonator or a Mach-Zehnder interferometer, design values and actual measurement values of FSR and transmission peak wavelength do not always match, and actual values have variations determined by manufacturing errors. Therefore, after manufacturing the tunable laser, unless the relationship between the laser oscillation wavelength and the current injected into the filter region (applied power when controlled by heat, applied voltage when controlled by voltage) is not actually investigated, the laser The oscillation wavelength cannot be controlled accurately. 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 range is expanded by the vernier effect with the configuration of FIG. 1, all wavelengths within the wavelength tunable range can be selected by injecting currents 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 using a measurement system (for example, composed of a current source and a wavelength meter) that takes 1 second per point, the time required for acquiring the characteristics of one tunable laser is about 2.8 hours. Further, if the same number of current steps is measured in the configuration of FIG. 3 to which a wavelength control filter is added, about 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 low loss and has a wide wavelength tunable range and is easy to produce.

  In order to achieve such an object, according to a first aspect of the present invention, there is provided a wavelength tunable laser including a filter region having a wavelength selection function for light from a gain region, wherein the filter region has a coupling efficiency with respect to a cross port. First and second ring resonators having optical couplers of less than 50% and having different frequency intervals (FSRs), and light from the gain region to the first and second ring resonators A Sagnac interferometer that functions as a loop mirror.

  According to a second aspect of the present invention, in the first aspect, the first ring resonator and the second ring resonator are directly connected by an optical waveguide and arranged in series. And

  According to a third aspect of the present invention, in the first aspect, the first ring resonator is a second light that further equally divides one light that is equally branched by the first optical coupler. A third Sagnac interferometer is connected to the output port of the coupler, and the second ring resonator further splits the other light equally split by the first optical coupler. The second Sagnac interferometer is configured to be connected to the output port of the optical coupler, and the first and second Sagnac interferometers are arranged in parallel.

  According to a fourth aspect of the present invention, in any one of the first to third aspects, the first and second ring resonators include an optical coupler having a coupling efficiency with respect to a cross port of 50% or more. The ring resonator is characterized in that the coupling efficiency is effectively less than 50% by making the radiation waveguide that discards light other than the resonance peak wavelength inside the circumference of the ring resonator.

  According to a fifth aspect of the present invention, in any one of the first to fourth aspects, each of the optical couplers is a multimode interference coupler.

  According to a sixth aspect of the present invention, in the fifth aspect, the multimode interference coupler has a coupling efficiency of 15% and is three times as long as a multimode interference coupler having a coupling efficiency of 85%. It is characterized by that.

  According to a seventh aspect of the present invention, in the fifth aspect, the multimode interference coupler has a coupling efficiency of 28% and is three times as long as a multimode interference coupler having a coupling efficiency of 72%. It is characterized by that.

  According to an eighth aspect of the present invention, in any of the first to fourth aspects, each of the optical couplers is a directional coupler.

  According to a ninth aspect of the present invention, in the eighth aspect, the directional coupler is formed of a ridge type optical waveguide.

  According to a tenth aspect of the present invention, in any one of the first to ninth aspects, a phase adjustment region for adjusting a phase of guided light is provided between the gain region and the filter region. And the oscillation wavelength can be finely adjusted by the phase adjustment region.

  According to the present invention, the finesse of the ring resonator is improved, and a high reflectance to the gain region can be obtained without forming a reflective film. That is, it is possible to easily produce a wavelength selective filter that has simple wavelength control, a wide wavelength variable band, and low loss characteristics. Therefore, it is possible to provide a high-performance and low-cost semiconductor wavelength tunable laser that has not been realized so far.

It is a block diagram of the conventional semiconductor wavelength tunable laser using a ring resonator. It is sectional drawing of a high mesa optical waveguide. It is a block diagram of a conventional loop type semiconductor wavelength tunable laser. It is a block diagram of the semiconductor wavelength tunable laser concerning Embodiment 1 of this invention. It is a characteristic view which shows the transmission spectrum of the ring resonator of Embodiment 1, and the reflection spectrum from a filter area | region. FIG. 4 is a structural diagram ((a) is a top view and (b) is a cross-sectional view) of an MMI coupler having a coupling efficiency of 15% according to the first embodiment. FIG. 5A is a structural diagram of an MMI coupler having a coupling efficiency of 15% according to the first embodiment, and FIG. 5B is a diagram illustrating a propagation state (simulation). FIG. 3 is a structural diagram ((a) is a top view and (b) is a cross-sectional view) of an MMI coupler having a coupling efficiency of 28% according to the first embodiment. FIG. 6 is a characteristic diagram showing a relationship between a reflectance difference ΔR and a multiplication factor M according to the first embodiment and the related art. It is a characteristic view which shows the relationship between the loss of the filter area | region and multiplication factor M which concern on Embodiment 1 and a prior art. It is a block diagram which shows the semiconductor wavelength variable laser which concerns on Embodiment 2 of this invention. It is a characteristic view which shows the relationship between the loss of the filter area | region which concerns on Embodiment 1 and 2, and the intensity | strength coupling efficiency of a coupler. (A) is a ring resonator of a prior art, (b) is a figure which shows the structure of the ring resonator of Embodiment 3. FIG. FIG. 7 is a structural diagram (a is a plan view and FIG. B is a cross-sectional view) of a ring resonator of a semiconductor wavelength tunable laser according to a fourth embodiment of the present invention. (A) is a figure which shows the mode (simulation) of propagation of the directional coupler using the ridge type | mold optical waveguide of Embodiment 4, (b) is a characteristic view which shows the relationship between coupling efficiency and the length of a coupler. .

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

(Embodiment 1)
FIG. 4 shows the semiconductor variable laser according to the first embodiment. As illustrated, the wavelength tunable laser includes a gain region, a filter region having a wavelength selection function for light from the gain region, a phase adjustment region between the gain region and the filter region, and an output end face. The filter region is a Sagnac interferometer that functions as a loop mirror, and has a configuration in which ring resonators 1 and 2 including a two-input two-output (2 × 2) optical coupler and an optical waveguide are arranged in a loop. . The ring resonators 1 and 2 are directly connected by an optical waveguide and arranged in series. The 2 × 2 optical coupler may be a 1-input 2-output type (1 × 2) optical coupler.

  The light from the gain region is equally split by a 2 × 2 optical coupler (corresponding to the “first optical coupler”), circulates in the loop clockwise and counterclockwise, and enters the 2 × 2 optical coupler again. . Since the right-handed light and the left-handed light re-enter the 2 × 2 optical coupler with the same phase, they are fed back to the gain region with an optical coupling efficiency of 100%.

  The ring resonators 1 and 2 in the loop are arranged in series and have different FSRs to expand the wavelength variable range. Here, the FSR of the ring resonator is

It can be expressed. Here, c is the speed of light, n _eff is the effective refractive index of the optical waveguide, and L is the resonator length of the ring resonator. Assuming that the FSRs of the ring resonator 1 and the ring resonator 2 are FSR1 and FSR2, respectively, the wavelength variable region Δλ is as shown in Non-Patent Document 1,

It can be expressed. Here, the multiplication factor M is

It can be expressed. From Equations (1) to (3), in order to obtain a large wavelength variable range, it is necessary to increase the multiplication factor M or increase the FSR. In the example of this embodiment, FSR1 is 400 GHz, FSR2 is 417 GHz, M is about 25, and the wavelength variable range Δλ is 10,000 GHz (80 nm). This is a wavelength tunable performance that is 1.5 times or more compared to the prior art described in Non-Patent Document 1 and twice or more that of Non-Patent Document 2.

  FIG. 5 shows a transmission spectrum of the ring resonator used in the present embodiment and a reflection spectrum from the filter region. In order to increase M, it is necessary to reduce the difference between the two FSRs, as can be seen from the above equation (3). In that case, the overlap of the transmission spectra of the two ring resonators 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 of the filter region, that is, the side mode suppression ratio of the wavelength tunable laser. The sharper the resonance of the ring resonator, the smaller the overlap of the transmission spectrum and the better ΔR. In the example of this embodiment, ΔR is 1.5 dB, and a side mode suppression ratio of 40 dB or more is obtained.

  With reference to FIGS. 6-8, the detail of the filter area | region with which the semiconductor wavelength tunable laser of this embodiment is provided is demonstrated. In this embodiment, a high-mesa optical waveguide capable of a steep bend radius is used for the connection between the 2 × 2 optical coupler and the ring resonators 1 and 2 and the connection between the ring resonators 1 and 2. Thereby, even if the bending radius is reduced to about 10 μm, the loss due to bending can be ignored. Further, a multimode interference (MMI) optical coupler that can be easily manufactured with low loss is used for the optical coupling portion of the ring resonators 1 and 2 and the 2 × 2 optical coupler. In order to reduce the resonator length L, the length of the optical coupling portion (optical coupling length) must also be reduced. However, a known directional coupler composed of a high mesa optical waveguide has a manufacturing problem. Since the high mesa optical waveguide has a large relative refractive index difference with air, the light seepage in the lateral direction is small. Therefore, when a directional coupler composed of a high mesa optical waveguide is used in the optical coupling portion, the distance between the two high mesa optical waveguides must be 0.1 microns or less in order to shorten the optical coupling length. However, it is very difficult in processing to form a deep groove (generally 3 to 4 microns) having a width of about 0.1 microns by etching or the like.

While the MMI optical coupler has the above advantages, it has a drawback that only fixed optical coupling efficiency can be obtained. In particular, in the case of a 2 × 2 MMI optical coupler used for a ring resonator, only three types of optical coupling efficiencies of light incident on an input port to a cross port are considered to be 50%, 72%, and 85%. The lengths L _MMI of the three types of MMI optical couplers are respectively expressed by the following equations.

Here, n _eq is the equivalent refractive index, W _wg is the width of the input / output waveguide, W _gap is the interval between the input / output waveguides, and λ is the wavelength used. The ring resonator is characterized in that the finesse is improved as the optical coupling efficiency to the cross port is reduced in the optical coupling portion. That is, the light goes around the ring resonator more and the resonance becomes sharper, and the wavelength selection performance is improved. In the prior art, as shown in Non-Patent Document 1 or 2, there is no choice but to use a 50% MMI coupler (3 dB-MMI coupler) which is the lowest coupling efficiency among the three types. However, in the present embodiment, the light from the gain region passes through the filter region only once due to the loop configuration, and the number of times of passage is half that of Non-Patent Document 1, and in an MMI coupler with a coupling efficiency of 50%, The wavelength selection performance must be reduced. Therefore, in the present embodiment, a low coupling efficiency of less than 50% is realized by appropriately designing the structure of the MMI coupler, and the wavelength selection performance in the filter region is improved.

FIG. 6 shows a structural diagram of the MMI coupler according to the present embodiment. In the example of this embodiment, W _wg is 1.2 μm, W _gap is 0.5 μm, W _MMI is 3.4 μm, and 3L _MMI-85% is 72 μm. It arrange | positioned at the both ends of the coupling | bond part. The MMI coupler is characterized in that the position and intensity of image formation in the incident light field are determined by the relative positional relationship between the incident waveguide and the W _MMI . In a 2 × 2 MMI coupler having a coupling efficiency of 85%, light input to the input port is branched into two by the coupling efficiency and imaged on the output ports (cross port and bar port). When the length of the MMI coupler is doubled to 2L _MMI-85% , the input optical field is equally split and operates as a coupler with a coupling efficiency of 50%. When the length of the MMI coupler is further increased to 3L _MMI-85% , which is three times, this time, the coupling efficiency is 15%, and the image is split into two images at the output port. That is, by adding the length 2L _MMI-85% , the input waveguide is equivalent to the input from the symmetrical position, and the branching ratio is inverted. As a result, a 2 × 2 MMI coupler having a low coupling efficiency of 15% was realized. FIG. 7 shows a simulation result obtained by the beam propagation method.

  In this embodiment, an MMI coupler with a coupling efficiency of 85% is used. However, even if the length of the MMI coupler with a coupling efficiency of 72% is tripled, the branching ratio is similarly reversed. FIG. 8 shows another example of the MMI coupler according to this embodiment. By using this structure, a 2 × 2 MMI coupler with a coupling efficiency of 28% can be realized.

  Thus, by appropriately designing the structure of the MMI coupler, a low coupling efficiency of less than 50% can be realized and the wavelength selection performance of the filter region can be improved. However, the length of the 2 × 2 MMI coupler is 3 As a result, the structural parameters of the ring resonator are limited. A ring resonator having a resonator length L is composed of two 2 × 2 MMI couplers and a bent waveguide (see FIG. 4). When the curvature of the bending waveguide is constant, the bending radius R can be expressed by the following equation.

When a high mesa optical waveguide is used for the bent waveguide, as described above, R can be reduced to about 10 μm with a small loss. Further, from the equations (4) to (6), it can be seen that the length of the MMI optical coupler can be reduced by reducing the structural parameters W _wg and W _gap of the MMI optical coupler. Considering the above, it is understood that W _wg , W _gap , and FSR should satisfy the following conditions in this embodiment.
(1) The input / output waveguide width W _wg is a condition in which light propagates in a single mode, and the condition in which R is 10 μm or more in Expression (7). The range is about 0.5 to 3 μm. In the above embodiment, 1.2 μm.
(2) The input / output waveguide interval Wgap is a condition that R is 10 μm or more in the equation (7). The minimum value is determined by the precision of the narrow gap, and the maximum value is about 2 μm. In the above embodiment, 0.5 μm.
(3) The frequency interval FSR is a condition in which R is 10 μm or more in the equation (7). The range from conditions (1) and (2) is about 50 to 600 GHz. In the above-described embodiment, FSR1 is 400 GHz and FSR2 is 417 GHz.

  FIG. 9 shows the result of calculating the relationship between ΔR that determines the wavelength selection performance of the filter region and the multiplication factor M in order to compare the configuration of Non-Patent Document 1 as the prior art with the configuration of this embodiment. . In the calculation, the loss of the optical waveguide is 5 dB / cm, the loss of the optical coupler is 0.5 dB, and the FSR1 is 400 GHz. Increasing M increases the overlap of the transmission spectra of the two ring resonators, thus reducing ΔR. However, since the 2 × 2 MMI coupler having a coupling efficiency as low as 15% is used in the example of the present embodiment, M can be increased to 25 regardless of the loop configuration. This corresponds to an improvement in the wavelength variable band that is approximately twice that of the prior art. FIG. 10 similarly shows the relationship between the loss generated in the filter region and the multiplication factor M. It can be seen that by using a 2 × 2 MMI coupler with a coupling efficiency of 28%, it is possible to reduce the loss by about 6.6 dB while slightly increasing the wavelength tuning performance. As a result, the oscillation threshold of the wavelength tunable laser is greatly reduced without forming a high reflection film, and the laser light output can be increased to +13 dBm or more.

Finally, a manufacturing method of the semiconductor wavelength tunable laser according to this 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 thickness of 0.3 μm is grown by selective growth, and then the SiO ₂ layer is removed to form a p-InP layer and a p ⁺ -InGaAs layer on the entire substrate. grown. Next, a ridge type optical waveguide etched up to just above the core layer was produced by photolithography and wet etching to form a gain region and a phase adjustment region. The filter region including the ring resonator portion was formed by batch production with a high mesa optical waveguide structure by photolithography and dry etching. Finally, the current injection electrode and a ridge type optical waveguide in the gain region and the phase control region p ⁺ -InGaAs layer, further high mesa optical waveguide p ⁺ -InGaAs layer constituting the input and output waveguides of the ring resonators, And it is completed on the back surface of the substrate.

(Embodiment 2)
A semiconductor tunable laser according to the second embodiment is shown in FIG. In the present embodiment, the filter region is a Sagnac interferometer that functions as a loop mirror, and the ring resonators 1 and 2 including a 2 × 2 optical coupler and an optical waveguide are arranged in the loop. As in the first embodiment, the ring resonators 1 and 2 are different from each other in that they form separate Sagnac interferometers. Sagnac interferometers are connected in parallel to the output ports of the 2 × 2 optical coupler. As a result, the phase of the light propagating in the laser can be effectively used, and the laser light output can be increased, the wavelength variable range, or the side mode suppression ratio can be further improved.

  The light from the gain region is first equally split by a 2 × 2 optical coupler (corresponding to a “first optical coupler”). The equally split light is further split equally by a 1 × 2 optical coupler (corresponding to “second optical coupler” and “third optical coupler”), and circulates in the loop clockwise and counterclockwise. To do. Right-handed light and left-handed light pass through the ring resonator connected to the output port of the 1 × 2 optical coupler and then re-enter the 1 × 2 optical coupler in the same phase, so that the optical coupling efficiency is 100%. Is fed back in the same way as in the first embodiment. In addition, the ring resonators arranged in the loop have different FSRs and expand the wavelength variable range.

Both the fed back lights reenter the 2 × 2 optical coupler. The length of path 2 (path passing through ring resonator 2) is such that the phase difference between path 1 (path passing through ring resonator 1) and path 2 is always 0 at the resonance peak wavelength. A phase adjustment unit of L _Phase is provided. The length L _Phase is determined by the difference between the two path lengths.

  It can be expressed. L1 and L2 are the path lengths of the paths 1 and 2, respectively. By making the path lengths of the path 1 and the path 2 equal, the light of the wavelength whose peak coincides among the infinite resonance peaks of the two ring resonators always re-enters the 2 × 2 optical coupler in the same phase. , 100% feedback to the gain region. More importantly, the light of other wavelengths has a difference in phase and amplitude, and is branched and output by the 2 × 2 optical coupler. That is, light is also output to the radiation waveguide side of the 2 × 2 optical coupler in accordance with the difference between the phase and amplitude, and the light other than the resonance peak wavelength has reduced coupling efficiency to the gain region. By this effect, the reflectance difference ΔR between the main peak and the adjacent peak is improved as compared with the first embodiment, and the wavelength variable region and the side mode suppression ratio are improved. At this time, the phase relationship between the paths is maintained even when current is injected into the ring resonator and the peak wavelength is changed. Furthermore, changing the ring resonator from a series connection configuration to a parallel connection configuration is nothing but the filter transmittance from the product to the sum. For example, when the peak transmittance of one ring resonator is 0.5, the reflectance from the filter region is 0.5, which is a branched sum from 0.25, which is the product. That is, the reflectance from the filter region is improved and the laser light output is increased.

  FIG. 12 shows the result of calculating the relationship between the loss of the filter region and the coupling efficiency of the coupler in order to compare the configurations of the first and second embodiments. In the calculation, the loss of the optical waveguide is 5 dB / cm, the loss of the optical coupler is 0.5 dB, FSR1 is 400 GHz, and M is 20. When the coupling efficiency of the optical coupler is reduced, the loss in the filter region increases in both the first and second embodiments. However, the increase in loss is slower in the second embodiment, and when a 2 × 2 MMI coupler with a coupling efficiency of 28% is used, the loss is reduced by about 3 dB while increasing the wavelength tunable performance as compared with the first embodiment. It was possible to make it. As a result, the oscillation threshold of the wavelength tunable laser is greatly reduced without forming a high reflection film, and the laser light output can be further increased. The structure and manufacturing method of the optical coupler and the optical waveguide in the ring resonator of this embodiment are exactly the same as those of the first embodiment. In addition, a current adjusting electrode for phase adjustment may be formed on the waveguide of path 1 or path 2 in order to compensate for the phase difference between paths caused by manufacturing errors. Further, the 2 × 2 optical coupler that equally branches light from the gain region may be a 1 × 2 optical coupler.

(Embodiment 3)
FIG. 13B is a structural diagram of the ring resonator of the semiconductor wavelength tunable laser according to the third embodiment. In the prior art, when a ring resonator is configured using 2 × 2 MMI couplers, the MMI couplers are arranged in parallel as shown in FIG. 13A, and the input / output waveguides inside the two MMI couplers are bent. They are connected by a waveguide. The other input / output waveguides of the MMI coupler are used for input / output to the ring resonator, or are used as radiation waveguides that discard light other than the resonance peak wavelength. In the case of the first or second embodiment, it is possible to increase the bandwidth or increase the output of the wavelength tunable laser only when the length of the MMI coupler is tripled. However, as shown in FIG. 13B, the input / output side to the ring resonator is connected as usual, and the input / output waveguide on the other side is connected between the outer waveguides and the inner waveguide is connected. If the waveguide is a radiation waveguide, this is equivalent to inversion of the branching ratio of the coupler. That is, it is possible to improve the performance of the laser while maintaining the MMI coupler having a large coupling efficiency of 72% and 85% without triple the length. With this configuration, 3L _{MMI in} the second term on the right side of Expression (7) becomes L _MMI , and the above-described constraint conditions are relaxed. That is, a wavelength tunable laser having the same R and a larger FSR (specifically, up to about 900 GHz) is possible. The manufacturing method is the same as that of the first embodiment.

(Embodiment 4)
FIGS. 14A and 14B are structural diagrams of ring resonators of the semiconductor wavelength tunable laser according to the fourth embodiment. As described above, the high-mesa optical waveguide has strong optical confinement in the lateral direction, and it is technically difficult to manufacture a sufficiently short directional coupler as in the first to third embodiments. Therefore, in the first to third embodiments, the optical coupler of the ring resonator is a 2 × 2 MMI coupler. However, in this embodiment, a sufficiently short directional coupler can be easily formed by appropriately mode-converting the high-mesa optical waveguide from the high-mesa optical waveguide to the ridge-type waveguide before and after the optical coupler, and configuring the directional coupler with the ridge-type optical waveguide. It was prepared. The ridge-type optical waveguide is an optical waveguide etched up to just above the core layer, and has a structure used for the gain region and the phase adjustment region. The ring resonator of this embodiment has an affinity with the manufacturing process described in the first embodiment. Is extremely high. In the example of this embodiment, the ridge-type optical waveguide width W _wg is 1.2 μm, and the ridge-type optical waveguide interval W _gap is 0.5 μm that can be manufactured by the above-described manufacturing process.

FIG. 15 shows the state of light propagation of a directional coupler using a ridge-type optical waveguide (simulation by a beam propagation method), and the relationship between the coupling efficiency and the length of the directional coupler. FIG. 15 shows that a coupling efficiency of 15% can be realized when the length of the directional coupler is 20 μm. This is due to the fact that the ridge-type optical waveguide has a larger amount of light in the lateral direction than the high-mesa optical waveguide. Thereby, it becomes possible to realize arbitrary coupling efficiency by adjusting the length of the directional coupler and the waveguide interval W _gap, and the degree of freedom of design is further increased. The amount of light oozing depends mainly on the waveguide width, and (1) the waveguide width W so that the coupling efficiency is 1% or more and less than 50% as the range of the structural parameter of the directional coupler of this embodiment. _wg is 0.5 to 3.0 μm,
(2) The waveguide interval W _gap is 0.1 to 2.0 μm,
(3) The length of the directional coupler is 5 to 200 μm.
Can be considered.

  In this embodiment, it is important to produce a connection from the high mesa optical waveguide to the ridge optical waveguide with low loss. Ridge type optical waveguide has a large lateral seepage (light field distribution is wide horizontally), and the waveguide width of the high-mesa optical waveguide is increased by about 1.4 μm and 0.2 μm so that the coupling is 98% or more. Efficiency was achieved. In addition, it is possible to connect with a lower loss by introducing a tapered shape conversion structure into the connection between the high mesa optical waveguide and the ridge waveguide as shown in Patent Document 1. Furthermore, in this embodiment, a ridge-type optical waveguide structure is used for the directional coupler portion. However, other waveguides that are light confinement than the high-mesa optical waveguide, for example, embedded in which both sides of the core layer are embedded with a semiconductor. A type waveguide or a rib type waveguide is also conceivable.

Finally, in this embodiment, an InP-based compound semiconductor is used, but the same can be realized by integrating a gain medium even in a GaAs-based or quartz-based, or a silicon fine wire waveguide composed of Si, SiO ₂ , polyimide, or the like. Is noted. In this embodiment, the refractive index change due to current injection is used. However, the wavelength variable operation can be obtained even when the refractive index change due to voltage, heat or pressure is used.

Claims (10)

In a wavelength tunable laser including a filter region having a wavelength selection function for light from a gain region,
The filter region is
First and second ring resonators having optical couplers with a coupling efficiency of less than 50% with respect to the crossport and having mutually different frequency spacings (FSRs), and light from the gain region, A tunable laser comprising a first optical coupler that branches equally toward two ring resonators and that functions as a loop mirror.   2. The semiconductor wavelength tunable laser according to claim 1, wherein the first ring resonator and the second ring resonator are directly connected by an optical waveguide and arranged in series. The first ring resonator is connected to an output port of a second optical coupler that further branches one of the lights equally split by the first optical coupler to form a first Sagnac interferometer. And
The second ring resonator is connected to an output port of a third optical coupler that further branches the other light equally split by the first optical coupler to constitute a second Sagnac interferometer. And
2. The semiconductor wavelength tunable laser according to claim 1, wherein the first and second Sagnac interferometers are arranged in parallel. The first and second ring resonators are:
An optical coupler with a coupling efficiency of 50% or more for the cross port is provided.
4. The ring resonator according to claim 1, wherein a coupling efficiency is less than 50% by making a radiation waveguide for discarding light other than the resonance peak wavelength inside the ring resonator. The semiconductor wavelength tunable laser according to any one of the above.   5. The semiconductor wavelength tunable laser according to claim 1, wherein each of the optical couplers is a multimode interference coupler.   6. The semiconductor wavelength tunable laser according to claim 5, wherein the multimode interference coupler has a coupling efficiency of 15% and a length three times that of the multimode interference coupler having a coupling efficiency of 85%.   6. The semiconductor wavelength tunable laser according to claim 5, wherein the multimode interference coupler has a coupling efficiency of 28% and is three times as long as a multimode interference coupler having a coupling efficiency of 72%.   5. The semiconductor wavelength tunable laser according to claim 1, wherein each of the optical couplers is a directional coupler.   9. The semiconductor wavelength tunable laser according to claim 8, wherein the directional coupler is composed of a ridge type optical waveguide. A phase adjustment region for adjusting the phase of the guided light is provided between the gain region and the filter region,
10. The semiconductor wavelength tunable laser according to claim 1, wherein an oscillation wavelength can be finely adjusted by the phase adjustment region.

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