JP4648647B2 - Tunable semiconductor laser - Google Patents

Description

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

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

  In such a situation, as shown in Non-Patent Document 1 or Non-Patent Document 2, Grating Assisted Co-directional Coupler (GACC). A tunable laser (GACC-SG-LD) using a directional optical coupler has been proposed.

  As shown in FIG. 16, the GACC type tunable laser (GACC-SG-LD) includes a gain region 1610, a GACC region 1620, a phase adjustment region 1630, and a super-periodic structure distributed reflector region (Sampled Grating: SG or Super -Structure Grating (SSG) 1640. Bias currents Ia, Ig, Ip, and Ir are input to each region. 16A is a longitudinal sectional view along the light propagation direction of the GACC-SG-LD, FIG. 16B is a transverse sectional view of the gain region 1610, and FIG. 16C is the GACC region 1620 and the phase adjustment region. FIG. 16D is a cross-sectional view of 1630, and FIG.

  The GACC region 1620 has a function of coupling only a specific wavelength of light input from one waveguide 1625 to the other waveguide 1623, and further changing the refractive index of the waveguide 1623. It functions as a wavelength tunable filter having a large wavelength tunable range operable in the entire wavelength tunable region. The SG region 1640 is a reflective grating (diffraction grating) having a feature that the reflection intensity increases at a constant frequency interval (free spectral range: FSR). Therefore, the oscillation wavelength of the GACC-SG-LD is controlled by coarse adjustment in the GACC region 1620 having a large wavelength variable range and fine adjustment in the SG region 1640 and the phase adjustment region 1630.

Pierre-Jean Rigole, et. Al. “114-nm Wavelength Tuning Range of a Vertical Grating Assisted Codirectional Coupler Laser with a Super Structure Grating Distributed Bragg Reflector” IEEE Photonics Technology Letters, Vol. 7, No. 7, JULY (1995) , pp. 697-699 M. Oberg, et. Al. "74nm Wavelength Turning Range of an InGaAsP / InP Vertical Grating Assisted Codirectional Coupler Laser with Rear Sampled Grating Reflector" IEEE Photonics Technology Letters, Vol.5, No.7, JULY (1993), pp. 735-738

  However, in the conventional GACC-SG-LD as shown in FIG. 16, it is necessary to form several layers having different semiconductor compositions and structures in each region in the plane. The regrowth process by a growth method (Metal Organic Chemical Vapor Deposition: MOCVD) or the like must be repeated five times or more. Also, the formation of the SG or SSG requires an electron beam lithography process that requires a lot of process time. As a result, the conventional GACC-SG-LD and its module are inevitably in a situation where the manufacturing cost is inevitably high.

  Furthermore, in the case of realizing a wavelength tunable laser having a wavelength tunable width of 100 nm, for example, an arbitrary wavelength of 100 GHz interval can be output using the GACC-SG-LD, the SG region 1640 is as shown in FIG. In addition, a structure in which a large number of 400 μm long unit units 1644 having a grating region 1642 having a period of about 240 nm and a length of 5 μm are connected in series in the light guiding direction is required, and therefore, the total element length must be increased. I don't get it. As a result, the longitudinal mode interval becomes narrow and the laser oscillation becomes unstable. Therefore, it is necessary to control the refractive index of the phase adjustment region 1630 in addition to the GACC region 1620 and the SG region 1640. For this refractive index control, specifically, in FIG. 16, the three currents Ig, Ir, and Ip are controlled, and the oscillation bias current Ia is added to the gain region, so the oscillation wavelength control is complicated. In addition, there is a point that wavelength variation or the like due to deterioration due to long-term use occurs.

  The present invention has been made in view of such a state of the art, and an object of the present invention is to provide a wavelength tunable laser capable of stably outputting arbitrary wavelengths at a constant frequency interval by a simple control method and a simple manufacturing method. Is to realize.

In order to achieve the above object, a wavelength tunable semiconductor laser according to the present invention includes a gain region of an optical waveguide having a gain medium and two or more waveguides having different refractive indexes connected to the output side of the gain region. A wavelength filter of a directional optical coupler type having a structure in which a grating (diffraction grating) is formed in one or more of the waveguides, and the two or more of the wavelength filters. By changing the refractive index of at least one waveguide among the waveguides, the oscillation wavelength of the wavelength filter can be varied at a constant frequency interval, and connected to the output side of the wavelength filter to be constant. ; and a wavelength locking filter to change in transmittance at the frequency intervals on a common semiconductor substrate, wherein the wavelength locking filter is small among the two or more waveguides of said wavelength filter A waveguide structure equal to at least one waveguide, and the wavelength filter includes a structure in which a grating is formed on a sidewall of one or more of the two or more waveguides; The core layer of any one of the two or more waveguides is used as the gain medium in the gain region, and the refractive index of one of the remaining waveguides is changed by the refractive index control means. Thus, the oscillation wavelength of the wavelength filter is made variable at a constant frequency interval .
That is, in the present invention, in the GACC type wavelength filter, by using the core layer of one optical waveguide as a gain medium, a separate gain medium is not formed in the laser resonator, that is, no regrowth process is required. In addition, it is possible to realize a wavelength tunable semiconductor laser. Further, in the present invention, in order to omit the regrowth process for manufacturing the GACC region, the grating is not formed in the plane as in the conventional GACC-SG-LD, but is formed on the side wall of the optical waveguide. As a result, it can be formed in a lump during the waveguide structure fabrication process.

  Here, as the wavelength locking filter, a ring resonator or a Mach-Zehnder interferometer having a characteristic that transmission intensity increases at a constant frequency interval can be used. That is, in the present invention, a ring resonator or a Mach-Zehnder interferometer having a characteristic that the transmission intensity increases with a constant FSR is used instead of using SG or SSG as in the conventional GACC-SG-LD. Used as a wavelength lock filter. A ring resonator or a Mach-Zehnder interferometer can be easily manufactured by a more commonly used photolithography and dry etching process. Therefore, an electron beam drawing process, which is indispensable for SG manufacturing, and a regrowth process by MOCVD or the like. Can be omitted. Furthermore, the ring resonator or the Mach-Zehnder interferometer can realize an element length sufficiently smaller than that of SG or SSG because of its structure. Therefore, a wavelength tunable laser capable of stably outputting an arbitrary wavelength at a relatively small frequency interval of 100 GHz. Can be realized.

The wavelength filter may have a structure in which the two or more waveguides are stacked in a vertical direction .

The wavelength filter may have a structure in which the two or more waveguides are arranged in parallel in an in-plane direction .

  Further, a phase adjustment region is provided, and the oscillation wavelength of the wavelength filter can be finely adjusted through the phase adjustment region. That is, in the present invention, by providing a phase adjusting waveguide in the laser resonator, the longitudinal mode interval is finely adjusted, and the wavelength variable operation can be accurately performed at a desired frequency interval.

  Further, it may further include a refractive index control means for changing a refractive index of the phase adjustment region. Further, it may further include a refractive index control means for changing a refractive index of the oscillation wavelength locking filter. That is, according to the present invention, the wavelength can be controlled at a wavelength between the discrete oscillation wavelengths of the wavelength tunable laser by changing the resonance wavelength by changing the refractive index of the wavelength lock filter and / or the phase adjustment region. Operation can be realized.

  In addition, by using the cleavage surface of the semiconductor element of the wavelength tunable semiconductor laser as a reflection mirror, or by forming a high reflection film on the cleavage surface and using the high reflection film as a reflection mirror, a feedback function is achieved. It can be characterized as having been added. That is, in the present invention, a laser is oscillated by providing a feedback function using reflection of the end face of the laser.

  The set of the wavelength filter, the wavelength locking filter, and the gain region may be a ring resonator including an optical coupler. That is, in the present invention, the entire laser is made to have a ring structure so that it can oscillate without using reflection at the end face.

  In addition, by removing a portion of the waveguide of the constituent elements other than the wavelength filter and the gain region by etching, it has a structure in which the influence on the laser oscillation of the non-selected light in the wavelength filter is removed. It can be. In other words, the GACC type filter has a function of coupling only a specific wavelength to the other waveguide among the light input from one waveguide, but the light of the wavelength not selected remains in the incident waveguide. Propagates and adversely affects laser oscillation. Therefore, in the present invention, the waveguide can be removed by etching and non-selective light can be emitted. Further, the angle of the end face removed by the etching is set such that non-selective light is totally transmitted to the air side at the interface between the waveguide exposed on the end face and air. Can do.

  In addition, by using the gain region as an absorption region, it is possible to have a structure in which the influence of the non-selected light on the laser oscillation in the wavelength filter is removed. That is, in the present invention, a gain region in which no current is injected, that is, an absorption region is intentionally arranged in the laser resonator, and the non-selective light of the GACC filter can be absorbed.

  Further, at least one of an optical modulator, a light receiving element, or a semiconductor optical amplifier is integrated at the output end of the wavelength tunable semiconductor laser. That is, according to the present invention, it is possible to monolithically integrate an optical modulator, a light receiving element, or a booster semiconductor optical amplifier in the output waveguide of the optical coupler, and various functions can be realized.

  With the above configuration, according to the present invention, a wavelength tunable semiconductor laser that is easy to control can be realized at low cost. At the same time, according to the present invention, a large wavelength variable range can be realized. In the present invention, the wavelength locking filter that determines the frequency interval is not affected by heat due to the GACC wavelength filter and the current change in the gain region, so that the wavelength is stable even when the oscillation wavelength is switched sharply. There are features.

The best mode for carrying out the present invention will be described below in detail with reference to the drawings.
[First Embodiment]
FIG. 1 shows a schematic configuration of a wavelength tunable semiconductor laser according to the first embodiment of the present invention. As shown in FIG. 1, a wavelength tunable semiconductor laser 100 according to this embodiment includes a gain region 110 as a gain medium, a GACC type wavelength filter 120, and a ring resonator 130 as a wavelength lock filter as main components. It is configured and optically connected in series in this order. Reference numeral 140 denotes a refractive index control electrode, 141 denotes a wiring wire (lead wire) that connects the refractive index control electrode 140 and the GACC type wavelength filter 120, 150 denotes an output side waveguide, and 160 denotes a ring resonator 130. And an optical coupler that couples the waveguide 150. In FIG. 1, the ring resonator 130 is used as the wavelength locking filter, but a Mach-Zehnder interferometer may be applied instead of the ring resonator 130 as in a second embodiment described later.

  Further, the element of the wavelength tunable semiconductor laser 100 constitutes a laser resonator using the end face 102 formed by cleavage (cleavage) as a reflection mirror. Furthermore, in order to eliminate connection loss caused by the difference in spot size that may exist between the GACC region waveguide and the ring resonator waveguide, the GACC region 120 is also a waveguide that is exactly the same as the ring resonator 130 as described later. The structure is a high-mesa waveguide. For example, when a ring resonator having an FSR with a frequency of about 100 GHz used in the present invention is realized with a semiconductor, the bending radius of the ring resonator 130 must be reduced to about 100 μm. Therefore, when the ring resonator 130 is used as a wavelength lock filter in the present invention, the waveguide structure is a high-mesa structure with a high optical confinement etched to the bottom of the core layer, and the bending loss can be greatly reduced. Yes.

  FIG. 2 shows a cross-sectional structure of the wavelength tunable semiconductor laser device of this embodiment. The gain region 110 and the GACC type wavelength filter 120 are configured by stacking two waveguides 123 and 125 in the direction perpendicular to the substrate 121. The active layer 115 in the gain region 110 and the upper waveguide core layer 125 of the GACC type wavelength filter 120 are connected. In other words, the light amplified by the gain region 110 is incident on the upper waveguide core layer 125 of the GACC type wavelength filter 120.

  As shown in FIG. 3, the component part of the GACC-type wavelength filter 120 in the present embodiment includes two waveguides (a lower waveguide core layer and an upper waveguide core layer) 123 and 125 stacked in the vertical direction, and A grating (diffraction grating) 170 having the same shape and period Λ is formed on both side walls of the high-mesa waveguide. In other words, the concave and convex cuts of the grating 170 are formed not in the vertical direction of the filter but in the side wall direction thereof.

  The GACC-type wavelength filter 120 is a directional coupler having a grating with a period Λ that satisfies the following formula (1). Of the light input from one waveguide 123 or 125, the light having the wavelength λ is converted to the other. It has a function of coupling to the waveguide 125 or 123.

Here, n _eq1 and n _eq2 are the equivalent refractive indexes of the two waveguides 123 and 125 in the GACC type wavelength filter 120. This period Λ can be realized on the order of several tens of μm, and for the production of GACC, high-resolution lithography such as electron beam drawing is not required, and it can be sufficiently produced by more general photolithography. is there.

In this embodiment, the waveguides 123 and 125 are manufactured with a width of 1.5 μm, an amplitude Dg of the grating 170 of 50 nm, and a waveguide interval S of 1.0 μm. These numerical values determine the transmission characteristics of the filter. It is an important parameter. In realizing the GACC type wavelength filter 120, it is preferable that the following conditions are satisfied in the present embodiment.
1) The height and width of the core layers of the waveguides 123 and 125 are 0.3 μm to 2.0 μm under the condition that light propagates through both waveguides in a single mode.
2) The amplitude Dg of the grating 170 determines the transmission characteristics, and is 0.01 μm to 1.0 μm.
3) The waveguide interval S determines transmission characteristics, and is 0.5 μm to 2.0 μm.

  In the present embodiment, of the light incident on the upper waveguide 125, the light having the wavelength λ is coupled to the lower waveguide 123 and propagates to enter the ring resonator 130. Light of other wavelengths propagates through the upper waveguide 125 as it is. At this time, the GACC region of the GACC type wavelength filter 120 is also exactly the same as that of the ring resonator 130 in order to eliminate the connection loss caused by the difference in spot size that may exist between the lower waveguide 123 and the waveguide of the ring resonator 130. A high-mesa waveguide having a waveguide structure is used.

Further, in this embodiment, in order to omit the regrowth process for manufacturing the GACC region, the grating 170 is not formed on the surface as in the prior art, but on the side wall of the high mesa optical waveguide as described above. The GACC type wavelength filter 120 and the ring resonator 130 were formed at the same time during the manufacturing process of the high mesa structure. More specifically, in this embodiment, as shown in FIG. 2, an n-InP lower clad layer 122, an n-added 1.1Q composition InGaAsP core layer (lower optical waveguide core layer) are formed on a common n-InP substrate 121. ) 123, the n-InP intermediate layer 124, and the gain region 110 are composed of the InGaAsP / InP MQW active layer 115, followed by the GACC type wavelength filter 120 and the ring resonator 130 with the 1.4Q composition. An InGaAsP core layer (upper optical waveguide core layer) 125, a p-InP upper cladding layer 126, and a P ⁺ -InGaAs layer 127 were sequentially grown by metal organic chemical vapor deposition (MOCVD). Next, a high mesa optical waveguide structure including a gain region 110, a GACC type wavelength filter 120 having a grating 170 on the side wall as shown in FIG. 3, and a ring resonator 130 is continuously formed by photolithography and dry etching. Formed together.

  Further, the GACC type wavelength filter 120 is provided with a refractive index control electrode 140, and a current input via the refractive index control electrode 140 passes through the wiring wire 141 to the upper waveguide 125 of the GACC type wavelength filter 120. It is designed to be entered. In this configuration, since the GACC type wavelength filter 120 is a wavelength filter having a large wavelength variable range that can operate in all of the wavelength variable regions, the current value input via the refractive index control electrode 140 is changed, This is because the transmission peak wavelength of the light passing through the GACC type wavelength filter 120 is changed by changing the refractive index of the upper waveguide 125 of the GACC type wavelength filter 120.

  Next, the transmission characteristics of the GACC wavelength filter 120 and the ring resonator 130 obtained by calculation are shown in FIG. The ring resonator 130 is set to FSR 100 GHz. The solid line in the figure shows the transmission characteristics when both filters are connected. The maximum transmittance can be obtained at a wavelength of 1550 nm, and the transmittance on both sides is as small as 2 dB. Since such a filter characteristic is provided in the laser resonator 130, laser oscillation can be obtained with only one wavelength in the present embodiment shown in FIG. Therefore, when the GACC wavelength filter 120 is tuned (wavelength adjustment) through the refractive index control electrode 140 provided in the GACC wavelength filter 120, laser oscillation is performed while repeating mode hopping at 100 GHz intervals in the GACC wavelength variable region. Wavelength tuning of light becomes possible.

  In this embodiment, since the ring resonator 130 is used as a wavelength lock filter, the GACC type wavelength filter 120 has a high mesa structure because of ease of manufacture and reduction of optical coupling loss. However, a Mach-Zehnder interferometer is used as the wavelength lock filter. In the case of using (not shown), the waveguide structure does not need to have a high mesa structure, and therefore the GACC type wavelength filter 120 has a structure other than the high mesa structure, for example, a raised portion at the top of the filter as shown in FIG. Alternatively, a ridge type structure in which the grating 170 is formed only on the both side walls may be manufactured. In addition, even when the ring resonator 130 is used as a wavelength lock filter, the GACC type wavelength filter 120 can be manufactured as a ridge type structure.

[Second Embodiment]
FIG. 6 shows a schematic configuration of a wavelength tunable semiconductor laser according to the second embodiment of the present invention. As shown in FIG. 6, the wavelength tunable semiconductor laser 100 according to the second embodiment includes a gain region 110 as a gain medium, a GACC wavelength filter 120, and a Mach-Zehnder interferometer 180 as a wavelength lock filter. And are optically connected in series in this order. Further, the element of the wavelength tunable semiconductor laser 100 constitutes a laser resonator using the end face 102 formed by cleavage (cleavage) as a reflection mirror.

  FIG. 7 conceptually shows the structure of the GACC type wavelength filter 120 of the second embodiment. In the second embodiment, in the component part of the GACC type wavelength filter 120, a 1.4Q optical waveguide layer 703 is laminated on the InP substrate 701, and two waveguides 704 and 705 of the GACC type wavelength filter 120 are further formed thereon. Are arranged in parallel in the in-plane direction of the substrate, and each has a ridge structure. A grating (diffraction grating) 710 having a period Λ determined from the equivalent refractive index of the two waveguides is formed symmetrically on both side surfaces of the ridge (bump) portion of one waveguide 704. Both side surfaces of the ridge portion of the other waveguide 705 are flat surfaces on which no grating is formed. Further, in order to bring about a difference in refractive index between the two waveguides 704 and 705, the width and height of the ridge portions of the two waveguides 704 and 705 are different from each other, and the waveguide propagates through the waveguide 704. The mode profile is different.

  The light amplified by the gain region 110 enters one waveguide of the GACC type wavelength filter 120, that is, the right waveguide 704 in FIG. Further, light in the vicinity of the wavelength λ is coupled to the left waveguide 705 by the grating 710 having the period Λ and propagates to the constituent region of the Mach-Zehnder interferometer 180.

  Further, the left waveguide 705 of the GACC type wavelength filter 120 in FIG. 7 is provided with a refractive index control electrode 140, and the current input via the refractive index control electrode 140 is left of the GACC type wavelength filter 120. The signal is input to the waveguide 705. As described above, the GACC type wavelength filter is configured by changing the refractive index of the left waveguide 705 of the GACC type wavelength filter 120 by changing the current value input through the refractive index control electrode 140. This is to change the transmission peak wavelength of light passing through 120. As a result, a wavelength tunable laser having the same function as that of the first embodiment can be realized.

  In the GACC type wavelength filter 120 of the second embodiment, the grating 710 is formed on the side wall of the ridge portion of one waveguide 704, but the same can be realized even if the ridge structure is a high mesa structure. It can also be realized by forming the grating on the upper surface of the waveguide, and can also be realized by forming it on both waveguides simultaneously. Furthermore, a grating may be formed between the two waveguides.

  In the second embodiment, in the GACC type wavelength filter 120, the optical waveguides of both the waveguides 704 and 705 have the same composition. However, different compositions such as 1.. It is also possible to fabricate both waveguides 704 and 705 with different compositions such as 4Q and 1.1Q to realize a GACC type wavelength filter.

[Third Embodiment]
FIG. 8 shows a schematic configuration of a wavelength tunable semiconductor laser according to the third embodiment of the present invention. Here, reference numeral 800 denotes a ground electrode described later, and reference numeral 801 denotes a wiring wire that connects the ground electrode 800 and the GACC type wavelength filter 120. Further, the element of the wavelength tunable semiconductor laser 100 constitutes a laser resonator using the end face 102 formed by cleavage (cleavage) as a reflection mirror.

  The GACC type wavelength filter 120 constituting the wavelength tunable semiconductor laser 100 of the third embodiment has a structure having two waveguides in an upper part and a lower part as will be described later with reference to FIG. By using the core layer of the optical waveguide as a gain medium, it is possible to realize a tunable semiconductor laser without forming a separate gain medium in the laser resonator, that is, without requiring any regrowth process. .

FIG. 9 shows a cross-sectional view of the elements of the constituent parts of the actually manufactured GACC type wavelength filter 120. As shown in FIG. 9, on a p-InP substrate 121, a p-InP buffer layer 122, an i-1.3Q composition InGaAsP layer (lower optical waveguide core layer) 123, an n-InP layer (ground layer) 124, i An InGaAsP / InP multiple quantum well structure (MQW) active layer (also used as an upper optical waveguide core layer) 925, a p-InP layer 126, and a P ⁺ -InGaAs layer 127 were grown in advance by MOCVD. Next, a high mesa optical waveguide structure (see FIG. 3) including a GACC type wavelength filter and a ring resonator portion was formed in a lump by photolithography and dry etching. Thereafter, a ground electrode 800 connected to the n-InP layer 124 located in the middle between the upper and lower optical waveguides 925 and 123 is formed from the side wall of the high mesa structure via the wiring wire 801, and then current is injected into the upper gain medium 925. The electrode 140 for the refractive index, and finally the electrode 910 for changing the refractive index for the lower waveguide 123 are formed on the back surface of the element to complete.

  With this configuration, the region of the GACC wavelength filter 120 has a structure in which two p · i · n double heterojunctions are formed in opposite directions of p · i · n · i · p. The upper gain region waveguide 925 and the lower refractive index control waveguide 123 can be independently controlled in current. In this case, the current value input through the refractive index control electrode 910 of the lower waveguide 123 is changed, and the transmission peak wavelength of the light passing through the GACC wavelength filter 120 is changed. Wavelength tuning will be performed.

  With this structure, it becomes possible to realize a wavelength tunable semiconductor laser in which the regrowth process by the MOCVD method and the electron beam drawing process, which have been costly, are omitted.

  In the third embodiment, a GACC type wavelength filter having a high mesa structure in which waveguides are stacked in the direction perpendicular to the substrate is used. In the ridge structure as shown in FIG. 5, the core of one waveguide is used. A similar effect can be expected by using a layer as a gain medium. Similarly, in the GACC type wavelength filter arranged in parallel in the in-plane direction of the substrate in the second embodiment, the regrowth process is omitted by using the core layer of one waveguide as a gain medium. A semiconductor laser can be realized.

[Fourth Embodiment]
FIG. 10 shows a schematic configuration of a wavelength tunable semiconductor laser according to the fourth embodiment of the present invention. In the wavelength tunable semiconductor laser 100 of the fourth embodiment, a phase adjustment region 1010 is provided in a laser resonator formed between the end face of the element and the wavelength locking ring resonator 130. A refractive index adjusting electrode (refractive index controlling electrode) 1020 is provided on the waveguide. By supplying a control current from the refractive index adjusting electrode 1020 to the phase adjusting region 1010 through the wiring wire 1030, the deviation between the absolute value of the longitudinal mode and the longitudinal mode interval of the laser resonator can be corrected. The element of the wavelength tunable semiconductor laser 100 constitutes a laser resonator with the end face 102 formed by cleavage (cleavage) as a reflection mirror.

  In the case of the first embodiment of FIG. 1, since the element length of the wavelength tunable semiconductor laser is about 2 mm as described above, the longitudinal mode interval is about 20 GHz. Therefore, if the absolute value of the longitudinal mode or the longitudinal mode interval deviates from the FSR of the ring resonator: 100 GHz, there is a problem that the oscillation wavelength is not exactly 100 GHz apart, and the longitudinal mode interval is determined from the FSR of the wavelength locking ring resonator. The oscillation wavelength can be controlled only with the frequency accuracy of the value obtained by adding half the value. For this reason, in the fourth embodiment, by providing the phase adjustment region 1010 as described above, it is possible to correct such a deviation between the absolute value of the longitudinal mode and the longitudinal mode interval of the laser resonator. .

[Fifth Embodiment]
FIG. 11 shows a schematic configuration of a wavelength tunable semiconductor laser according to the fifth embodiment of the present invention. In the fifth embodiment, the gain region 110, the GACC type wavelength filter 120, the wavelength locking ring resonator 130, and the phase adjustment region 1110 are optically connected in series in this order, and the wavelength tunable semiconductor laser 100 is connected. Is configured. In the present embodiment, a refractive index control electrode 1130 that causes a change in refractive index in the ring resonator 130 and the phase adjustment region 1110 is formed on the waveguide. By supplying a control current from the refractive index control electrode 1130 to the ring resonator 130 or the phase adjustment region 1110 through the wiring wire 1140, a refractive index change is caused in the ring resonator 130 and the phase adjustment region 1110, and the ring The resonance frequency of the resonator 130 and the longitudinal mode interval of the laser resonator can be finely adjusted.

  In the present embodiment, the output end face 1150 of the wavelength tunable semiconductor laser is provided with a high reflection film (HR) coat 1160, which enables oscillation with a smaller amount of current injection into the gain region 110. The threshold can be set, and the power consumption of the wavelength tunable laser can be reduced.

[Sixth Embodiment]
FIG. 12 shows a schematic configuration of a wavelength tunable semiconductor laser according to the sixth embodiment of the present invention. In the first to fifth embodiments of the present invention described so far, the element constitutes a resonator using the end face 102 formed by cleavage (cleavage) as a reflection mirror, but the wavelength tunable of the sixth embodiment. In the semiconductor laser 100, as shown in FIG. 12, the gain region 110, the GACC type wavelength filter 120, the wavelength locking ring resonator 130, and the phase adjustment region 1110 are connected in a ring shape via the optical coupler 160. Thus, the overall structure is a ring resonator, and the wavelength tunable ring laser emits output light to the output waveguide 1202 via another optical coupler 1201. The output waveguide 1202 is arranged in a state in which both end faces of the element are connected, and a pair of anti-reflection (AR) coats 1160 are applied to both end faces.

  In the sixth embodiment, the longitudinal mode interval of the ring laser is not dependent on cleavage unlike the first to fifth embodiments of the present invention described so far, so the longitudinal mode interval is set accurately and with good reproducibility. It is characterized by what it can do.

  FIG. 13 shows a concept of a waveguide between the GACC type wavelength filter 120 and the ring resonator 130 of the sixth embodiment. The GACC type wavelength filter 120 has a function of coupling only a specific wavelength selected from the light input from the upper waveguide (upper waveguide core layer) 125 to the lower waveguide (lower waveguide core layer) 123. However, light having a wavelength that has not been selected propagates through the upper waveguide 125 as it is, and has an adverse effect that laser oscillation occurs at an unintended wavelength. Therefore, in the sixth embodiment, all of the upper waveguide 125 in the constituent part of the ring resonator 130 is removed by etching at an end face angle at the terminal position of the constituent part of the GACC type wavelength filter 120 by etching. The non-selection light is configured not to propagate in the laser resonator 130. In addition, as described above, the selection light coupled to the lower waveguide 123 is changed from the two-layer waveguides 125 and 123 to the single-layer waveguide 123 at the connection point between the GACC type wavelength filter 120 and the ring resonator 130. Re-coupling to the upper waveguide 125 at the ring resonator 130 is prevented.

  The etching described above is performed not only in the ring resonator 130 but also in the entire waveguide shown by a thin line in FIG. 12 including the waveguide including the ring resonator 130 and the phase adjustment region 1110, the optical coupler 1201, and the output waveguide 1202. It is given to the waveguide. The end face angle is set to an angle at which the non-selective light is totally transmitted to the air side at the boundary surface between the upper waveguide 125 and air. Specifically, the equivalent refractive index of the upper waveguide 125 and the air It is calculated | required from the refractive index difference with the refractive index. Further, the end face having the angle is formed between the GACC type wavelength filter 120 and the ring resonator 130 and between the gain region 110 and the optical coupler 1201 when the upper waveguide 125 is removed by etching. Is done.

[Seventh Embodiment]
FIG. 14 shows a schematic configuration of a wavelength tunable semiconductor laser according to the seventh embodiment of the present invention. In the seventh embodiment, in addition to the configuration similar to that of FIG. 12, the light absorption regions 1401 and 1402 are arranged between the GACC type wavelength filter 120 and the ring resonator 130 and between the gain region 110 and the light in the laser resonator. They are respectively formed between the coupler 1201 and the coupler 1201. These light absorption regions 1401 and 1402 make it possible to eliminate non-selective light propagating through the upper waveguide 125 in the GACC type wavelength filter 120.

  The light absorption regions 1401 and 1402 can be formed by providing a gain medium region, that is, a waveguide region having the same structure as the gain region 110 in the upper waveguide (upper waveguide core layer) 125 in the two-layer waveguide structure. Yes (see FIG. 9). This utilizes the feature that the gain region 110 becomes an optical waveguide having a propagation loss of about several hundred dB / cm unless current injection is performed.

  Further, in order to prevent the selective light coupled to the lower waveguide (lower waveguide core layer) 123 from being recoupled to the upper waveguide 125, the waveguide including the ring resonator 130 and the phase adjustment region 1110, and the optical coupling The upper waveguide 125 is removed by etching in all the waveguides illustrated by the thin lines in FIG. 14 including the device 1201 and the output waveguide 1202. In addition, in this embodiment, since it has the light absorption area | regions 1401 and 1402, the end surface angle demonstrated using FIG. 12, FIG. 13 by 6th Embodiment is not necessarily required.

[Eighth Embodiment]
FIG. 15 shows a schematic configuration of a wavelength tunable semiconductor laser according to the eighth embodiment of the present invention. In the eighth embodiment, similarly to the sixth and seventh embodiments described with reference to FIGS. 12 and 14, the gain region 110, the GACC type wavelength filter 120, the wavelength locking ring resonator 130, and the phase adjustment are performed. The wavelength variable ring laser is connected to the region 1110 via the optical coupler 160 to form a ring resonator as a whole, and the output light is output to the output waveguide 1202 via another optical coupler 1201. It has become.

  As shown in FIG. 15, a light receiving element 1501 for monitoring the light intensity is provided at one end of the output waveguide 1202. By monitoring the current of the light receiving element 1501 with a display device or the like (not shown), it is possible to perform a constant output operation. Further, a Mach-Zehnder optical modulator 1502 and a booster semiconductor optical amplifier (optical amplifier) 1503 for amplifying the optical output deteriorated by the optical modulator 1502 are provided on the other end side of the output waveguide 1202. ing. Further, an antireflection (AR) coating 1160 is applied to at least one end face of the element.

  The Mach-Zehnder type optical modulator 1502 can be manufactured with the same epi configuration as that of the lower waveguide 123 of the GACC type wavelength filter 120, which is advantageous for monolithic integration. Of course, the same monolithic integration effect can be obtained even when an electroabsorption optical modulator (not shown) using the quantum confined Stark effect using the MQW structure is used instead of the Mach-Zehnder optical modulator 1502. be able to. As described above, according to the present embodiment, it is possible to manufacture a highly functional wavelength tunable laser in which an optical modulator is also integrated.

[Other embodiments]
In the above, the preferred embodiment of the present invention has been described by way of example. However, the embodiment of the present invention is not limited to the above-described example, and the constituent members thereof are within the scope of the claims. Various modifications such as replacement, change, addition, increase / decrease in number, change in shape design, etc. are all included in the embodiments of the present invention.

For example, in the configuration of the above-described embodiments of the present invention, but using a compound semiconductor of an InP-based, even when a silicon wire waveguide configured such in GaAs-based or Si and SiO ₂ or polyimide, a gain medium If 110 is hybrid-connected, the same can be realized.

  In the configuration of each of the embodiments of the present invention described above, 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. For example, in the GACC type wavelength filter, of two or more waveguides, the core layer of one waveguide is made of a material having a positive refractive index temperature coefficient, and the core layer of the other waveguide is made a negative refractive index temperature. By using a material having a coefficient, the direction of the refractive index change due to the thermo-optic effect can be made different between the waveguides, and the wavelength variable amount of the wavelength filter can be increased.

  Further, in the configuration of each embodiment of the present invention described above, a configuration is shown in which a grating is formed on the side wall of the GACC type wavelength filter. However, it is formed by modulating the amplitude or period of the grating on the side wall. It may be. In this case, the grating on the side wall may be formed so as to have a maximum amplitude at the central portion of the wavelength filter and be modulated so that the amplitude decreases as the distance from the center increases. In addition, as a function of the modulation, a Gaussian function that maximizes the central part, a square of the Sinc function, a triangle function, a Hamming window function, a Hanning window function, a Sinc function with a variable of −π to π, and a variable of 0 to π Any one of the sine function and its square may be used. Alternatively, the grating period may be modulated at a position corresponding to the negative value of the Sinc function.

  Further, the number of waveguides of the GACC type wavelength filter in the present invention is not limited to two, and the present invention can be similarly applied to a GACC type wavelength filter having a plurality of arbitrary three or more waveguides.

  The wavelength tunable semiconductor laser of the present invention can be provided as a light source for wavelength-multiplexed large-capacity optical communication that is easy to manufacture and simple in wavelength tunable control compared to conventional wavelength tunable light sources. Expected to contribute.

1 is a schematic top view showing a schematic configuration of a wavelength tunable semiconductor laser according to a first embodiment of the present invention. It is sectional drawing which shows the cross-section of the wavelength tunable semiconductor laser element in the 1st Embodiment of this invention. 1 is a conceptual perspective view showing a structure of a high mesa structure GACC type wavelength filter of a wavelength tunable semiconductor laser according to a first embodiment of the present invention. It is a characteristic view which shows the transmission spectrum characteristic of the part of the GACC type | mold wavelength filter and ring resonator calculated | required by calculation. 1 is a conceptual perspective view showing a structure of a ridge structure GACC type wavelength filter of a wavelength tunable semiconductor laser according to a first embodiment of the present invention. It is a typical top view which shows schematic structure of the wavelength tunable semiconductor laser in the 2nd Embodiment of this invention. It is a conceptual perspective view which shows the structure of the ridge structure GACC type | mold wavelength filter of the wavelength tunable semiconductor laser in the 2nd Embodiment of this invention. It is a typical top view which shows schematic structure of the wavelength tunable semiconductor laser in the 3rd Embodiment of this invention. It is sectional drawing which shows the cross-section of the wavelength tunable semiconductor laser element in the 3rd Embodiment of this invention. It is a typical top view which shows schematic structure of the wavelength tunable semiconductor laser in the 4th Embodiment of this invention. It is a typical top view which shows schematic structure of the wavelength tunable semiconductor laser in the 5th Embodiment of this invention. It is a typical top view which shows schematic structure of the wavelength tunable semiconductor laser in the 6th Embodiment of this invention. It is a conceptual perspective view which shows the waveguide structure between the GACC type | mold wavelength filter and ring resonator of the wavelength tunable semiconductor laser in the 6th Embodiment of this invention. It is a typical top view which shows schematic structure of the wavelength tunable semiconductor laser in the 7th Embodiment of this invention. It is a typical top view which shows schematic structure of the wavelength tunable semiconductor laser in the 8th Embodiment of this invention. The structure of the conventional GACC-SG-LD is shown, (A) is a longitudinal sectional view along the light propagation direction of the GACC-SG-LD, (B) is a transverse sectional view of the gain region 1610, and (C) is the GACC. A cross-sectional view of the region 1620 and the phase adjustment region 1630, (D) is a cross-sectional view of the superperiodic structure distributed reflector region 1640. It is an expanded sectional view which shows the structure of the grating area | region in the superperiodic structure distribution reflector area | region of the conventional GACC-SG-LD.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 Wavelength tunable semiconductor laser 101 Waveguide 102 Element end surface 110 Gain region 115 Active layer 120 GACC type wavelength filter 121 Substrate 123 Lower optical waveguide core layer (lower optical waveguide)
125 Upper optical waveguide core layer (upper optical waveguide)
130 Ring resonator (wavelength lock filter)
140 Refractive index control electrode (electrode for current injection into the upper gain region)
141 Wiring wire 150 Output waveguide 160 Optical coupler 170 Grating 180 Mach-Zehnder interferometer (wavelength lock filter)
701 Substrate 703 Optical waveguide layer 704 Right optical waveguide (ridge portion)
705 Left optical waveguide (ridge)
710 Grating 800 Ground electrode 801 Wiring wire 900 Electrode for current injection into lower gain region 925 Active layer 1010 Phase adjustment region 1020 Refractive index control electrode 1030 Wiring wire 1110 Phase adjustment region 1130 Refractive index control electrode 1140 Wiring wire 1150 Output end face 1160 HR coat 1201 optical coupler 1202 output waveguide 1401 and 1402 light absorption region 1501 light receiving element 1502 Mach-Zehnder optical modulator 1503 semiconductor optical amplifier

Claims (13)

A gain region of an optical waveguide having a gain medium;
A structure in which two or more waveguides having different refractive indexes are connected to the output side of the gain region, and a grating (diffraction grating) is formed in one or more of the waveguides A directional optical coupler type wavelength filter having:
A refractive index control means for changing an oscillation wavelength of the wavelength filter at a constant frequency interval by changing a refractive index of at least one of the two or more waveguides of the wavelength filter;
A wavelength-locking filter that is connected to the output side of the wavelength filter and whose transmittance changes at a constant frequency interval;
On a common semiconductor substrate ,
The wavelength locking filter has a waveguide structure equal to at least one of the two or more waveguides of the wavelength filter,
The wavelength filter has a structure in which a grating is formed on a side wall of one or more waveguides of the two or more waveguides, and any one of the two or more waveguides. By changing the refractive index of one of the remaining waveguides by the refractive index control means, the oscillation wavelength of the wavelength filter can be varied at constant frequency intervals. A tunable semiconductor laser characterized in that   2. The wavelength tunable semiconductor laser according to claim 1, wherein a ring resonator or a Mach-Zehnder interferometer having a characteristic that transmission intensity increases at a constant frequency interval is used as the wavelength locking filter. The wavelength tunable semiconductor laser according to claim 1, wherein the wavelength filter has a structure in which the two or more waveguides are stacked in a vertical direction. The wavelength tunable semiconductor laser according to claim 1, wherein the wavelength filter has a structure in which the two or more waveguides are arranged in parallel in an in-plane direction. A phase adjusting region, the wavelength tunable semiconductor laser according to any one of claims 1 to 4, characterized in that to enable fine adjustment of the oscillation wavelength of the wavelength filter via the phase adjusting region. 6. The wavelength tunable semiconductor laser according to claim 5 , further comprising refractive index control means for changing a refractive index of the phase adjustment region. Wavelength-tunable semiconductor laser according to any one of claims 1 to 6, characterized in that it further comprises a refractive index control means for changing the refractive index of the oscillation wavelength locking filter. A feedback function was provided by using the cleavage surface of the semiconductor element of the wavelength tunable semiconductor laser as a reflection mirror, or by forming a high reflection film on the cleavage surface and using the high reflection film as a reflection mirror. wavelength-tunable semiconductor laser according to any one of claims 1 to 7, characterized in that. The wavelength tunable according to any one of claims 1 to 7 , wherein the set of the wavelength filter, the wavelength locking filter, and the gain region forms a ring resonator including an optical coupler. Semiconductor laser. It has a structure in which the influence on the laser oscillation of the non-selective light in the wavelength filter is removed by removing a part of the waveguide of the component other than the wavelength filter and the gain region by etching. wavelength-tunable semiconductor laser according to any one of claims 1 to 9. 10. the angle of the end face is removed by the etching, the unselected light at the interface between the waveguide and the air exposed to the end surface, characterized in that it is set to an angle such that total transmission on the air side The wavelength tunable semiconductor laser described in 1. By using said gain region as the absorption region, the wavelength tunable semiconductor according to claim 1 to 11, characterized in that it has an effect on the laser oscillation of the non-selected light in the wavelength filter is removed structure laser. The wavelength tunable semiconductor laser of the optical modulator to an output end, or the light receiving element or a wavelength tunable semiconductor laser according to any one of claims 1 to 12, characterized in that it has integrated at least one of the semiconductor optical amplifier, .

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