JP2013093414A - Variable wavelength semiconductor laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a variable wavelength semiconductor laser which allows for prevention of characteristic deterioration by reducing unintended phase shift due to fluctuation of refractive index variation near a mask during selective growth.SOLUTION: The variable wavelength semiconductor laser has two laser parts A1, A2 fabricated by etching a part of an active waveguide layer 12 formed on a semiconductor substrate 11, and growing a non-active waveguide layer 13 having a composition or a layer structure different from that of the active waveguide layer 12 selectively. A diffraction grating 15 is formed over the full length of the active waveguide layer 12 and non-active waveguide layer 13. For the equivalent phase shift of -ΔΩ resulting from refractive index variation between the active waveguide layer 12 and non-active waveguide layer 13 during selective growth, a correction phase shift of ΔΩ is inserted into a position of the diffraction grating 15 corresponding to the bonding face of the active waveguide layer 12 and non-active waveguide layer 13.

Description

  The present invention relates to a wavelength tunable semiconductor laser used as a light source for optical fiber communication and a light source for optical measurement, and in particular, characteristics of a light source for optical wavelength (frequency) multiplexing system in optical communication and a light source for optical measurement that covers a wide wavelength band. Regarding improvement.

  In the wavelength division multiplexing communication system in optical fiber communication, a plurality of laser beams having different frequencies (wavelengths) are transmitted using a single optical fiber at intervals determined by the standard. Each frequency is called a channel, and a wavelength tunable laser capable of switching the oscillation frequency at high speed is required for high-speed channel switching.

In a wavelength tunable semiconductor laser for communication, a laser that oscillates at a single wavelength called a single mode laser is used. In order to obtain a single mode, for example, a diffraction grating having periodic irregularities in a waveguide is used. It is used. The semiconductor optical waveguide in which the diffraction grating is formed serves as a distributed reflector (DBR) that selectively reflects at the Bragg wavelength λ _B obtained from the diffraction grating period Λ and the equivalent refractive index n of the optical waveguide. Here, the relational expression between the Bragg wavelength λ _B , the diffraction grating period Λ, and the equivalent refractive index n of the optical waveguide is
λ _B = 2nΛ (1)
It becomes. A tunable semiconductor laser produced by giving a gain to a distributed reflector is called a distributed feedback (DFB) laser.

From formula (1), it can be seen that the Bragg wavelength λ _B can be changed by changing the equivalent refractive index n of the distributed reflector. In other words, a wavelength tunable semiconductor laser capable of changing the oscillation wavelength by changing the equivalent refractive index n can be configured by configuring a resonator using a distributed reflector that can selectively change the wavelength of reflection. Is possible. As a wavelength tunable semiconductor laser using a diffraction grating, it has a plurality of reflection peaks by a distributed reflection type laser (DBR laser) using a uniform diffraction grating distributed reflector, or by periodically providing a diffraction grating. SG (Sampled Grating) -DBR laser, SSG (Super Structure Grating) -DBR laser, etc. using a distributed reflector are known.

As a wavelength tunable semiconductor laser capable of continuously changing the wavelength, there is a distributed active DFB laser (TDA-DFB laser). FIG. 8 is a sectional view showing the basic structure of a conventional distributed active DFB laser. This distribution activity DFB laser, as shown in FIG. 8, on the lower cladding layer 1, an active waveguide layer 2 of a length L _a, inactive length L _t of different composition from the active waveguide layer 2 Waveguide layers (wavelength control layers) 3 are alternately connected periodically. A diffraction grating 5 in which periodic irregularities are formed between the active waveguide layer 2 and the inactive waveguide layer 3 and between the upper cladding layer 4 and the equivalent refractive index of the waveguide is periodically modulated is formed. Has been. Electrodes 7 and 8 are formed on the upper cladding 4 so as to correspond to the active waveguide layer 2 and the inactive waveguide layer 3, respectively. While the common electrode 9 is formed at the lower part of the substrate, the electrodes 7 and 8 formed at the upper part of the substrate are separated by the region of the active waveguide layer 2 and the region of the inactive waveguide layer 3. The electrodes 7 of the active waveguide layer 2 and the electrodes 8 of the inactive waveguide layer 3 are short-circuited on the element.

As described above, the distributed active DFB laser has a structure in which the active waveguide layer 2 and the inactive waveguide layer 3 are alternately and periodically connected in the light propagation direction. Although light is emitted and gain is generated by injecting the current I _a into the active waveguide layer 2, a diffraction grating 5 is formed in each of the waveguide layers 2 and 3, and only a wavelength corresponding to the period of the diffraction grating 5 is formed. Selective reflection causes laser oscillation. On the other hand, by injection of current I _t to the inactive waveguide layer 3 the refractive index by the plasma effect in accordance with the carrier density changes, the optical period of the diffraction grating 5 of the non-active waveguide layer 3 changes. The equivalent refractive index of the inactive waveguide layer 3 changes, and the resonant longitudinal mode wavelength is shifted to the short wavelength side by the ratio of the length of the wavelength control region to the length of one period. If the active waveguide layer 2 of the optical active region length along the propagation direction L _a of the wavelength control region length in the direction of propagation of light in the non-active waveguide layer 3 and L _t, 1 cycle of repeating structural is the length L L _t + L _a, and the ratio [Delta] [lambda] _r / lambda _r of change in the resonance longitudinal mode wavelength lambda _r,
Δλ _r / λ _r = (L _t / (L _t + L _a )) · (Δn / n) (2)
(For example, see Non-Patent Document 1).

On the other hand, each wavelength of the plurality of reflection peaks is also shifted to the short wavelength side as a result of a change in equivalent refractive index due to current injection. The reflection peak wavelength is proportional to the average equivalent refractive index change of the repeating structural one cycle, the ratio [Delta] [lambda] _s / lambda _s of the change in the reflection peak wavelength lambda _s is
Δλ _s / λ _s = (L _t / (L _t + L _a )) · (Δn / n) (3)
It becomes. From equations (2) and (3), the reflection peak wavelength λ _s and the resonance longitudinal mode wavelength λ _r are shifted by the same amount. Therefore, in this laser, the wavelength continuously changes while maintaining the first oscillation mode. However, in the case of the structure shown in FIG. 8, since the diffraction grating 5 is continuously formed at the same period, the wavelength that originally satisfies the oscillation phase condition deviates from the reflection peak wavelength, and the single mode property is poor. In order to improve the single mode characteristic, it is necessary to insert a phase shift (λ / 4) for satisfying the phase condition of the resonator near the center of the resonator.

  In the distributed active DFB laser disclosed in Patent Document 1, active waveguide layers and inactive waveguide layers are alternately and periodically connected on the lower clad, and the upper clad is formed thereon. In this structure, electrodes corresponding to the active waveguide layer and the inactive waveguide layer are formed on the upper clad, and a common electrode is formed below the lower clad. In this distributed active DFB laser, the diffraction grating is formed only in a part, but the wavelength continuously changes in the same manner as the distributed active DFB laser shown in FIG. However, since the diffraction grating is periodically formed in the resonator (referred to as a sample diffraction grating), a plurality of reflection peaks can be formed, and thus it is necessary to improve the single mode property.

  Therefore, Patent Document 1 discloses a structure of a distributed active DFB laser in which two lasers having different repetition periods are integrated in series on the same substrate, and a diffraction grating is formed in each active waveguide layer. ing. The interval between the multiple reflection peaks depends on the sample period of the diffraction grating. By changing this period on the left and right sides of the resonator, the reflection peaks other than the reflection main peak do not overlap on the left and right sides of the resonator. Has improved.

  Furthermore, in Patent Document 2, a structure in which two lasers having different repetition periods of an active waveguide layer and an inactive waveguide layer are connected in cascade and a diffraction grating is formed over the entire resonator, or the active waveguide layer and the inactive waveguide layer are inactive. A structure in which a plurality of laser parts having different repetition periods of the waveguide layer are connected in cascade, and a plurality of laser parts having different repetition periods of the active waveguide layer and the inactive waveguide layer in order to suppress spatial hole burning. A structure in which a phase shift is inserted between the connected laser units is disclosed.

Japanese Patent No. 3237733 JP 2008-103466 A

Ishii et al, "A Tunable Distributed Amplification DFB Laser Diode (TDA-DFB-LD)," IEEE Photonics Technology Letters, vol. 10, no. 1, Jan 1998.

  The basic operating principle of the distributed active DFB laser and the operating principle of the conventional example are described in detail in Non-Patent Document 1, Patent Document 1, and Patent Document 2. However, all are described based on an ideal structure, and no problem in manufacturing is pointed out.

  In the distributed active DFB laser, an active waveguide layer and an inactive waveguide layer are repeatedly formed. Ideally, the active waveguide layer and the inactive waveguide layer desirably have a uniform refractive index along the resonance direction. The repeating structure of the active waveguide layer and the inactive waveguide layer is, for example, by periodically removing the substrate on which the active waveguide layer is formed, and selectively removing the inactive waveguide layer only in the removal region from the organic metal. It is formed by regrowth by a vapor deposition method or the like. The selective growth is performed using SiO2 or the like as a mask. Ideally, it is desirable that the layer thickness and composition be uniform, but in reality, the growth amount and composition are slightly different between the vicinity of the mask and the portion away from the mask. Therefore, the refractive index of each waveguide layer does not become uniform along the resonator direction due to fluctuations in the layer thickness and crystal composition. Fluctuations can be reduced by appropriately setting the selective growth method and conditions, but it is very difficult to eliminate them completely. In addition, such slight refractive index fluctuation between the active waveguide layer and the inactive waveguide layer affects the phase relationship of the diffraction grating of each waveguide layer. There is a risk of causing mode hops.

  Accordingly, the present invention provides a wavelength tunable semiconductor laser capable of reducing an unintended phase shift caused by a fluctuation in refractive index near the mask during selective growth and preventing characteristic deterioration. With the goal.

  A wavelength tunable semiconductor laser according to a first invention for solving the above-described problems is obtained by etching a part of a first waveguide layer formed on a semiconductor substrate, the composition of the first waveguide layer being Or a tunable semiconductor laser having at least two or more waveguide structures produced by selectively growing a second waveguide layer having a different layer structure inside the resonator, wherein the first waveguide layer or the The second waveguide layer has gain, a diffraction grating is formed over the entire length of the first waveguide layer and the second waveguide layer, and the first waveguide layer generated during the selective growth and the A junction surface between the first waveguide layer and the second waveguide layer with respect to an equivalent phase shift of −ΔΩ due to a refractive index variation between the second waveguide layer and the second waveguide layer. The correction position of the phase shift amount ΔΩ at the position of the diffraction grating corresponding to Characterized in that the insertion of the shift.

  A wavelength tunable semiconductor laser according to a second invention for solving the above-mentioned problem is the wavelength tunable semiconductor laser according to the first invention, wherein the first waveguide layer has a gain and a diffraction grating is formed. The second waveguide layer has no gain while a diffraction grating is formed, and the first waveguide layer and the second waveguide layer are alternately arranged at a fixed ratio and at least one or more. In addition to the correction phase shift, the first waveguide layer and the second waveguide layer are formed in the diffraction grating corresponding to the gap between the laser portions. A phase shift is set so as to satisfy the phase condition of the resonator when the refractive index is uniform along the resonance direction.

  A wavelength tunable semiconductor laser according to a third aspect of the present invention for solving the above problem is the wavelength tunable semiconductor laser according to the first aspect of the present invention, wherein the first waveguide layer has a gain and a diffraction grating is formed. A distributed feedback laser is formed, the second waveguide layer has no gain, while a diffraction grating is formed to form a distributed Bragg reflector, and the first waveguide layer and the second waveguide layer In addition to the correction phase shift, the refractive index of the first waveguide layer and the second waveguide layer is uniform along the resonance direction in the diffraction grating corresponding to the joint surface with A phase shift set to satisfy the phase condition of the resonator is inserted.

  A wavelength tunable semiconductor laser according to a fourth invention for solving the above-described problems is obtained by etching a part of a first waveguide layer formed on a semiconductor substrate, the composition of the first waveguide layer being Or a wavelength tunable semiconductor laser having at least two or more waveguide structures produced by selectively growing a second waveguide layer having a different layer structure inside the resonator, wherein the first waveguide layer or the The second waveguide layer has gain, a diffraction grating is formed on the first waveguide layer or the second waveguide layer, and the waveguide layer on which the diffraction grating is formed is inside the resonator. At least two, and an equivalent phase shift of −ΔΩ due to a refractive index variation between the first waveguide layer and the second waveguide layer generated during the selective growth is corrected. So that the diffraction grating is formed. The diffraction grating is formed so that the phase of the diffraction gratings adjacent to each other is shifted by n × ΔΩ with respect to the number n of the joint surfaces existing between the waveguide layers.

  According to the wavelength tunable semiconductor laser of the present invention, it is possible to eliminate an unintended phase shift caused by a change in refractive index near the mask during selective growth, and to prevent characteristic deterioration.

It is explanatory drawing which shows the cross-section of the wavelength tunable semiconductor laser which concerns on 1st embodiment of this invention. 1 is a top view of a wavelength tunable semiconductor laser according to a first embodiment of the present invention. It is explanatory drawing which shows the manufacturing process of the waveguide of the wavelength tunable semiconductor laser which concerns on 1st embodiment of this invention. It is explanatory drawing which shows the example of the connection part of a waveguide. It is explanatory drawing which shows the cross-section of the wavelength tunable semiconductor laser which concerns on 2nd embodiment of this invention. It is explanatory drawing which shows the cross-section of the wavelength tunable semiconductor laser which concerns on 3rd embodiment of this invention. It is a schematic diagram explaining the phase relationship of the wavelength tunable semiconductor laser which concerns on 3rd embodiment of this invention. It is explanatory drawing which shows the basic structure of a general distributed active DFB laser.

  The details of the wavelength tunable semiconductor laser according to the present invention will be described below with reference to the drawings.

(First embodiment)
A first embodiment of a wavelength tunable semiconductor laser according to the present invention will be described below with reference to FIGS.

FIG. 1 is a schematic diagram showing a cross section along the waveguide direction of the wavelength tunable semiconductor laser according to the first embodiment of the present invention.
As shown in FIG. 1, in the wavelength tunable semiconductor laser according to the present embodiments, n-type on the InP lower cladding layer 11, GaInAsP active waveguide layer 12 _a1 of the first length in the laser unit A ₁ L _a1 The active waveguide layer 12 _a1 is alternately and periodically connected to a GaInAsP inactive waveguide layer (wavelength control layer) 13 _t1 having a length L _{t1 having} a different composition. Further, in the second laser portion A ₂ , the GaInAsP active waveguide layer 12 _a2 having a length L _{a2, and} a GaInAsP inactive waveguide layer 13 _t2 having a length L _{t2 having} a different composition from the active waveguide layer 12 _a2 , Are alternately connected periodically. Hereinafter, the GaInAsP active waveguide layers 12 _a1 and 12 _a2 are collectively referred to as GaInAsP active waveguide layers 12 and the GaInAsP inactive waveguide layers 13 _t1 and 13 _t2 are collectively referred to as GaInAsP inactive waveguide layers 13. .

  Periodic irregularities were formed between the GaInAsP active waveguide layer 12 and the GaInAsP inactive waveguide layer 13 and the p-type InP upper cladding layer 14 to periodically modulate the equivalent refractive index of the waveguide. A diffraction grating 15 is formed. A highly doped p-type InGaAs contact layer 16 is provided on the InP upper clad 14 for ohmic contact, and electrodes 17, 17 corresponding to the active waveguide layer 12 and the inactive waveguide layer 13, respectively, are provided thereon. 18 is formed. The electrode 19 formed in the lower part of the substrate is common, but the electrode formed in the upper part of the substrate is separated into the region of the active waveguide layer 12 and the region of the inactive waveguide layer 13. Specifically, the contact layer 16 and the electrodes 17 and 18 are separated by the region of the active waveguide layer 12 and the region of the inactive waveguide layer 13, and further, as shown in FIG. The electrodes 17 and the electrodes 18 of the inactive waveguide layer 13 are short-circuited on the element.

Further, in the wavelength tunable semiconductor laser according to the present embodiment, the repetition period of the active waveguide layer 12 and the inactive waveguide layer 13 is set to L ₁ , in the first laser part A ₁ and the second laser part A ₂ . in place of the L ₂ has a structure in which are connected in series. Further, a phase shift (hereinafter referred to as a correction phase shift) of a phase shift amount ΔΩ is put in the diffraction grating near the boundary (junction surface) between all the active waveguide layers 12 and the inactive waveguide layer 13, and the first laser unit In addition to the correction phase shift, a phase shift for satisfying the phase condition of the resonator having the phase shift amount λ / 4 is inserted between A ₁ and the second laser unit A ₂ . Hereinafter, the phase shift inserted between the first laser part A ₁ and the second laser part A _{2 is called} the inter-laser part phase shift 21, and the phase shift inserted in the other part is called the waveguide interlayer phase shift 20. To do. It is assumed that the phase shift amount ΔΩ is determined so as to cancel out the phase shift due to the refractive index fluctuation caused by the fluctuation of the layer thickness and composition during crystal growth that occurs in the vicinity of the connection surface of each waveguide layer.

  Here, when GaInAsP having a band gap wavelength of 1.55 μm is used for the active waveguide layer 12, the non-active waveguide 13 uses a shorter band gap wavelength, for example, GaInAsP having a band gap wavelength of 1.4 μm. Thus, the carrier density does not become constant because it does not contribute to the laser oscillation gain. Thereby, the refractive index can be changed greatly by current injection.

  Further, the active waveguide layer 12 and the inactive waveguide layer 13 do not have to be a bulk material. For example, a quantum well structure, a multilayer quantum well structure in which quantum wells are stacked with a barrier layer interposed therebetween, a quantum dot, It may have a low-dimensional quantum well structure such as a quantum wire. Further, in order to enhance light confinement and carrier confinement in the active layer, a separate confinement heterostructure in which a layer having an intermediate refractive index is introduced between the active layer and the cladding layer may be introduced.

  Since it is important that the refractive index of the diffraction grating 15 fluctuates periodically, the position where the diffraction grating 15 is formed is between the active waveguide layer 12 or the inactive waveguide layer 13 and the upper cladding 14. For example, it may be formed between the waveguide layers 12 and 13 and the lower cladding 11 or at a position away from each layer.

  The semiconductor used in this element is not limited to the combination of InP and GaInAsP, and other semiconductors such as GaAs, GaInNAs, and AlGaInAs may be used. The band gap between the active waveguide layer 12 and the inactive waveguide layer 13 may be used. The combination of wavelengths is not limited to the above.

  In the tunable semiconductor laser according to the present embodiment, although not shown, Fe, which is a semi-insulating material, is doped on both sides of the waveguide so that the current Ia is efficiently injected into the active waveguide layer 12. A buried heterostructure (BH) in which InP is buried and regrown is formed. Instead of Fe-doped InP, p-type and n-type semiconductors may be alternately stacked to form a current blocking layer. Moreover, it is good also as an InP layer made into high resistance by doping Ru instead of Fe.

The waveguide structure employs a buried heterostructure in this embodiment, but the principle of the present invention can also be used in a general ridge structure or high mesa structure.
In the first laser part A ₁ and the second laser part A ₂ , the repetition period L of the active waveguide layer 12 and the inactive waveguide layer 13 is different from L ₁ and L ₂ , respectively, but the active waveguide layer 12 And the ratio of the inactive waveguide layer 13 (L _a1 / L _t1 and L _a2 / L _t2 ) are the same. In this embodiment, this ratio is 1/2. Further, an inter-laser phase shift 21 is inserted between the first laser part A ₁ and the second laser part A ₂ to change the phase of the diffraction grating 15 by ¼ wavelength + ΔΩ. As a result, the phases of the reflected wave at the first laser part A ₁ and the reflected wave at the second laser part A ₂ are matched so as to satisfy the oscillation condition.

Further, as described above, the electrodes 17 and 18 provided on the active waveguide layer 12 and the wavelength control non-active waveguide layer 13 are separated from each other. As shown in FIG. The electrodes 17 on 12 and the electrodes 18 on the inactive waveguide layer 13 are connected on the element. By short-circuiting the electrodes 17 and 18 in each region on the element in this way, the current I _a or wavelength control can be performed in each region simply by bonding a metal bonding wire one by one. it can be injected current I _t.

  Next, a method for manufacturing a wavelength tunable semiconductor laser according to this embodiment will be briefly described. First, the active waveguide layer 12 and the inactive waveguide layer 13 are formed on the n-type InP lower clad layer 11 by using a metal organic vapor phase epitaxial growth (MOCVD) method and a selective growth method based thereon. Specifically, this is performed as shown in FIG. First, an etching mask 22 such as SiO 2 is formed on the substrate on which the active waveguide layer 12 is formed (FIG. 3A). Subsequently, the active waveguide layer 12 is processed into an island shape using the etching mask 22 (FIG. 3B). Then, the active waveguide layer 12 and the inactive waveguide layer 13 are connected by re-growing the inactive waveguide layer 13 while leaving the etching mask 22 as it is (FIG. 3C).

  By the way, in FIG. 3, the layer thicknesses of the active waveguide layer 12 and the non-active waveguide layer 13 are drawn uniformly for a schematic diagram. However, in actuality, in the case of selective growth using the etching mask 22, the material on the etching mask 22 moves to a place without the etching mask 22 and grows only at a place without the etching mask 22. The growth rate changes only in the vicinity of the mask 22. In addition, because the growth rate differs depending on the crystal plane, the composition and thickness in the vicinity of the etching mask 22 are not uniform due to the shape of the etched surface.

  To explain this further, FIG. 4 (a) shows an ideal waveguide connection surface, but in practice the thickness in the vicinity of the connection surface is not uniform as shown in FIG. 4 (b). The refractive index fluctuates near the waveguide connection surface. Here, FIG. 4B is an example, and fluctuations in the layer thickness and composition vary depending on the etching shape and growth conditions. Moreover, the place which becomes thick and the place which becomes thin also arise. Usually, the core layer of the waveguide is made of a material having a higher refractive index than that of the clad.

  In the example of FIG. 4B, the inactive waveguide layer 13 is thick near the connection surface. Although there is a portion that is slightly thinner, since it is thicker on average, the equivalent refractive index near the connection surface is higher than in the ideal case of FIG. Therefore, even if the diffraction grating 15 is uniformly formed from the active waveguide layer 12 to the inactive waveguide layer 13, the optical length in the vicinity of the connection surface is long, so that the phase shift is equivalently −ΔΩ. It is equivalent to containing.

The amount of change in the average equivalent refractive index in the vicinity of the connection surface compared to the ideal case is Δn, and the region in which the refractive index is changed compared to the ideal case is ΔL. The phase shift amount -ΔΩ of the phase shift is
−ΔΩ = Δn × ΔL
Can be considered. However, the sign of the phase shift may be reversed depending on the definition of space. More specifically, ΔL is further refined and considered as δL, and the product of the refractive index change δn due to the fluctuation of the layer thickness may be integrated in the region of ΔL.

Thereafter, the pattern of the diffraction grating 15 is transferred to the applied resist using an electron beam exposure method, and etching is performed using the transfer pattern as a mask to form the diffraction grating 15. At this time, as shown in FIG. 1, the diffraction grating 15 includes a waveguide interlayer phase shift 20 of ΔΩ between the waveguide layers 12 and 13, and the first laser part A ₁ and the second laser part A ₂ . A phase shift 21 between the laser portions of 1/4 + ΔΩ is inserted between them.

  The phase shift amount ΔΩ is determined so as to correct the refractive index change due to the fluctuation of the layer thickness and composition generated during the selective growth described above. This is because, for example, it is possible to confirm fluctuations such as layer thickness by observing with a scanning electron microscope (SEM), etc. Therefore, after performing test growth and observing the cross section with SEM, Are calculated as an equivalent phase shift of the phase shift amount −ΔΩ, and when the laser is actually manufactured, the phase shift amount ΔΩ of the diffraction grating is determined in order to cancel this. As a result, an equivalent phase shift of −ΔΩ caused by layer thickness fluctuation or the like can be canceled by a correction phase shift of the phase shift amount ΔΩ inserted into the diffraction grating 15.

  Where the layer thickness fluctuates, although depending on the thickness of the mask and the growth conditions, the variation within 10 micrometers from the mask is generally significant, so the thickness between the vicinity of the mask and 10 micrometers can be observed by SEM. That's fine. If the active waveguide layer is etched by dry etching and then wet etching or the like is used to allow side etching to enter the lower portion of the etching mask, the portion may be considered including that portion.

  In addition, if a micro-PL apparatus capable of measuring photoluminescence (PL) in a minute region is used, composition fluctuations can also be known. However, when a general growth method is used, composition fluctuations also occur, but the fluctuation of the layer thickness has a larger influence on the refractive index. Therefore, simply, it is possible to perform only the SEM observation of the cross section and calculate the refractive index change from the layer thickness change.

  In the present embodiment, the diffraction grating 15 has the same period for both the active waveguide layer 12 and the inactive waveguide layer 13.

  After the p-type InP upper cladding layer 14 and the p-type InGaAs contact layer 16 are grown, in order to control the transverse mode, the waveguide is processed into a stripe shape having a width of 1.2 μm and FeP doped on both sides thereof. Growing block layer. After the electrodes 17 and 18 are formed, the p-type InGaAs contact layer 16 between the electrodes 17 and 18 is removed in order to electrically isolate the active layer drive electrode 17 and the wavelength control electrode 18. Further, when the waveguide layers 12 and 13 are separated, a separation groove may be formed.

  The semiconductor growth method is not limited to the metal organic vapor phase epitaxial growth method, and a molecular beam epitaxial growth method or other means may be used. The method of forming the diffraction grating 15 is not limited to the electron beam exposure method, and a two-bundle interference exposure method or other means may be used.

In this embodiment, the number of repetitions of the active waveguide layer 12 and the inactive waveguide layer 13 in the first laser part A ₁ and the second laser part A ₂ is set to 6, respectively. Since the first laser part A ₁ and the second laser part A ₂ use the diffraction grating 15 having the same coupling coefficient, the second laser having a long repetition period of the active waveguide layer 12 and the inactive waveguide layer 13 is used. reflectivity for better parts a ₂ becomes larger the coupling coefficient and the product of length increases. Therefore, when the number of repetitions is the same, the output is naturally asymmetric, and the output from the first laser unit A ₁ having a low reflectivity is larger than the output from the second laser unit A ₂ having a high reflectivity. Therefore, the output can be efficiently extracted from the first laser part A ₁ side. Note that the number of repetitions of the active waveguide layer 12 and the inactive waveguide layer 13 is not limited to 6, and the number of repetitions does not have to be the same in the first laser part A ₁ and the second laser part A _2. Therefore, the repetition period and the number of repetitions may be designed according to the required reflectance.

In this embodiment, the repetition period of the first laser part A ₁ is 57 μm, and the repetition period of the second laser part A ₂ is 72 μm. The coupling coefficient of the diffraction grating 15 is 25 cm ⁻¹ .

In the present embodiment, in addition to the phase shift of the phase shift amount ΔΩ for correcting the refractive index variation during selective growth between the diffraction gratings 15 of the first laser part A ₁ and the second laser part A _2. The phase shift 21 between the laser parts having a quarter wavelength is included. The quarter wavelength shift is usually in the ideal state, i.e., even when the refractive indices of the active waveguide layer 12 and the inactive waveguide layer 13 are uniform along the resonance direction, Inserted to satisfy the phase condition. As a result, the most phase matching condition is obtained. However, it is not always necessary to use a ¼ wavelength shift. For example, the phase condition may be shifted by using a 波長 wavelength shift or a 3/8 wavelength shift. It is possible to suppress the concentration of light in the phase shift region. Concentration of light in the phase shift region causes a decrease in carrier density and increases a refractive index. This is called hole burning and may cause problems such as mode instability at high output. That is, if the wavelength shift is shifted from the quarter wavelength shift, hole burning can be suppressed and the mode can be stabilized.

In addition, even in an ideal state, the position of the phase shift (1/4 wavelength shift in this embodiment) that is inserted into the diffraction grating 15 to satisfy the phase condition of the resonator is not necessarily the same as that of the first laser part A ₁ . There is no need to be between the second laser parts A ₂ , and the phase condition can be satisfied if there is a phase shift within a range of about 1/3 of the total resonator length at the center of the resonator. If the phase shift for satisfying the phase condition of the resonator is not in the vicinity of the waveguide junction plane, this phase shift and the phase of ΔΩ to compensate for the equivalent phase shift caused by the fluctuation of the junction plane Shifts may be inserted separately.

As described above, in the present embodiment, the ratio between the active waveguide layer 12 and the inactive waveguide layer 13 of the first laser part A ₁ and the second laser part A ₂ is 1: 2. Since the average equivalent refractive index change can be increased by increasing the proportion of the inactive waveguide 13, the amount of wavelength change can be increased. However, when the ratio of the inactive waveguide layer 13 is increased, the ratio of the active waveguide layer 12 is inevitably decreased, and it may be difficult to obtain a gain necessary for laser oscillation. Therefore, it is necessary to adjust the ratio according to the design such as the number of active layers and the loss of the waveguide. However, the principle of the present invention is that the first laser part A ₁ and the second laser part A _{2 are used.} Since the ratio of the active waveguide layer 12 and the inactive waveguide layer 13 is the same, the ratio can be changed as required.

  As described above, in the wavelength tunable semiconductor laser according to the present embodiment, a normal wavelength tunable semiconductor laser is manufactured only in that the active waveguide layer 12 and the inactive waveguide layer 13 are alternately and periodically arranged. It can be easily manufactured using a method. Inserting the ΔΩ waveguide interlayer phase shift 20 and the λ / 4 + ΔΩ inter-laser phase shift 21 into the diffraction grating 15 can be realized only by changing the electron beam drawing pattern. The diffraction grating 15 can be realized by periodically repeating line and space by electron beam drawing. The waveguide interlayer phase shifts 20 and 21 are the positions of lines or spaces where the waveguide interlayer phase shifts 20 and 21 are to be inserted. Can be realized by shifting a desired amount.

  Further, the waveguide interlayer phase shifts 20 and 21 do not necessarily have to be directly above the connection surface between the active waveguide 12 and the inactive waveguide 13. Actually, since the layer thickness and composition fluctuation in the range of about 10 μm from the vicinity of the connection surface are remarkable, the effect becomes remarkable if the waveguide interlayer phase shifts 20 and 21 are provided in the range.

  In the present embodiment, the diffraction grating 15 having the same period is used in all regions, but the essence of the present invention is to insert an equivalent phase shift due to a refractive index variation deviated from an ideal state by selective growth into the diffraction grating 15. Since the correction is performed by the waveguide interlayer phase shifts 20 and 21, the principle of the present invention can be applied even if the period and coupling coefficient of the diffraction grating 15 in each region are different.

  Further, since the layer structure of a normal pn diode type can be easily integrated with a semiconductor amplifier, a modulator, etc., it can be used as a light source element that is an element of a high-performance multifunctional element.

(Second embodiment)
Hereinafter, a second embodiment of the wavelength tunable semiconductor laser according to the present invention will be described with reference to FIG.
In the present invention, when different waveguides are butt-joined by selective growth (butt joint), the layer thickness and composition of the regrowth layer in the vicinity of the selective growth mask are disturbed, and the equivalent is caused by the change in refractive index. It is characterized by compensating for such a phase shift by adding a phase shift to the diffraction grating formed on the waveguide. That is, the present invention can be applied to an element having a junction surface of different types of waveguides by selective growth and having a diffraction grating formed on different types of waveguides. Therefore, in this embodiment, unlike the first embodiment described for the distributed active DFB laser, a distributed Bragg reflector (DBR) having a diffraction grating, and a distributed feedback (DFB) laser having a gain and a diffraction grating. As an example, a structure in which is connected will be described.

  FIG. 5 is a diagram for explaining the wavelength tunable semiconductor laser according to the present embodiment. As shown in FIG. 5, in the wavelength tunable semiconductor laser according to the present embodiment, an inactive waveguide layer 32 and an active waveguide layer 33 are connected on the lower cladding layer 31. A diffraction grating 35 is formed in which periodic irregularities are formed on the inactive waveguide layer 32 and the active waveguide layer 33 and between the upper cladding layer 34 and the equivalent refractive index of the waveguide is periodically modulated. Has been. Electrodes 36 and 37 are formed on the upper cladding layer 34. A common electrode 38 is formed in the lower part of the substrate. In the upper part, the electrodes 36 and 37 are separated by the region of the inactive waveguide layer 32 and the region of the active waveguide layer 33, and the inactive waveguide is further separated. The electrodes 36 of the layer 32 and the electrodes 37 of the active waveguide layer 33 are short-circuited on the element.

That is, the wavelength tunable semiconductor laser according to the present embodiment is connected to a distributed Bragg reflector (DBR) B ₁ having a diffraction grating 35a and a distributed feedback (DFB) laser B ₂ having a gain and diffraction grating 35b. It has a structure. This is also called a distributed reflection (DR) laser. Thus, distributed Bragg reflectors B _1, respectively diffraction grating 35a to distributed feedback laser B _2, 35b are formed, the diffraction grating of the distributed Bragg reflector B ₁ of the diffraction grating 35a with distributed feedback laser B ₂ It determines the stability degree by the phase relationship with 35b, determines the phase relationship by placing the phase shift 39 between the distributed Bragg reflector B ₁ and the distributed feedback laser B _2.

  Here, in the wavelength tunable semiconductor laser according to this embodiment, the phase shift 39 inserted into the diffraction grating 35 near the boundary between the inactive waveguide layer 32 and the active waveguide layer 33 is changed to a λ / 4 shift. The phase shift is obtained by adding a correction phase shift of ΔΩ. The phase shift amount ΔΩ is determined so as to cancel the equivalent phase shift due to the refractive index fluctuation caused by the fluctuation of the layer thickness or composition during crystal growth occurring in the vicinity of the connection surface of each waveguide layer.

As described in the first embodiment, the connection structure of the distributed Bragg reflector B ₁ and the distributed feedback laser B ₂ includes, for example, the active waveguide layer 33 of the distributed feedback laser B ₂ on the lower cladding layer 31. some of the grown substrate was etched SiO2 as a mask, selective growth of the non-active waveguide layer 32 remains distributed Bragg reflector B ₁ leaving the SiO2 mask. Thereafter, the diffraction grating 35 is formed and the upper clad layer 34 is grown.

However, as in the first embodiment, however, the refractive index may deviate from the design due to fluctuations in layer thickness and composition during regrowth, and an equivalent phase shift may be inserted. Therefore, in order to avoid this, it is necessary in the case of an ideal connection structure in which there is no equivalent phase shift caused by disturbance such as layer thickness between the distributed Bragg reflector B ₁ and the distributed feedback laser B _2. A phase shift 39 obtained by adding a correction phase shift of ΔΩ that cancels an equivalent phase shift caused by a disturbance such as a layer thickness to the phase shift for satisfying the phase condition of a simple resonator is inserted.

Here, in the present embodiment, the phase shift for satisfying the phase condition of the resonator necessary for an ideal connection structure is set to ¼ wavelength, and the distributed Bragg reflector B ₁ and the distributed feedback laser B ₂ are used. In between, a phase shift 39 of λ / 4 + ΔΩ is introduced. As a result, a stable operation close to an ideal design can be obtained.

  In the present embodiment, the present invention can be applied regardless of the material and the laser structure, as in the first embodiment. Further, the phase shift and the correction phase shift for satisfying the phase condition of the resonator can be determined independently, and the phase shift for satisfying the phase condition of the resonator is not limited to ¼ wavelength.

(Third embodiment)
Hereinafter, a wavelength tunable semiconductor laser according to a third embodiment of the present invention will be described with reference to FIGS. FIG. 6 is a diagram for explaining the third embodiment.

In the first embodiment, the distributed active DFB laser in which the diffraction grating 15 is formed over the entire resonator has been described as an example. In the present embodiment, the location where the diffraction grating is formed is periodically and partially limited. A distributed active DFB laser having a sample diffraction grating will be described as an example.
Hereinafter, the same members as those shown in FIGS. 1 to 4 and described in the first embodiment are denoted by the same reference numerals, and redundant description will be omitted, and different points will be mainly described.

  As shown in FIG. 6, in the present embodiment, the active waveguide layer 23 has a diffraction grating 25, and the inactive waveguide layer 24 has no diffraction grating. Since the non-active waveguide layer 24 has no diffraction grating, the phase relationship between the active waveguide layer 23 and the non-active waveguide layer 24 cannot be determined as in the first embodiment. The phase relationship of the diffraction grating 25 between the waveguide layers 23 is important.

That is, for example, in the present embodiment, the light propagated through the active waveguide layer 23 _a11 followed by non-active waveguide layer 24 _t1 becomes the entering the next active waveguide layer 23 _a12. Therefore, the phase relationship of the diffraction grating 25 between the active waveguide layer 23 _a11 and the active waveguide layer 23 _a12 is determined by the optical length of the inactive waveguide layer 24 _t1 . This may be considered that the inactive waveguide layer 24 is a phase shift of the diffraction grating 25 formed in the active waveguide layer 23.

  FIG. 7 is a diagram for explaining that the inactive waveguide layer 24 becomes a phase shift of the diffraction grating 25 formed in the active waveguide layer 23. For the sake of simplicity, the description here assumes that the average refractive indexes of the active waveguide layer 23 and the inactive waveguide layer 24 are equal.

The active waveguide layer 23 has a diffraction grating 25, and the inactive waveguide layer 24 has no diffraction grating. In FIG. 7, the diffraction grating when the diffraction grating 25 _{a11 of} the left active waveguide layer 23 _a11 is assumed to continue to the non-active waveguide layer 24 _t1 is written with a dotted line. In such a wavelength tunable semiconductor laser, it is understood that the phase of the dotted line and the diffraction grating 25 _a12 of the active waveguide layer 23 _a12 on the right side do not match. In other words, by adopting a configuration in which the phases of the diffraction gratings 25 of the adjacent active waveguide layers 23 are not continuous with each other, the same effect as that in which a phase shift is present between the diffraction gratings 25 of the adjacent active waveguide layers 23 is obtained. An effect can be obtained.

  In FIG. 7, the average refractive index of the active waveguide layer 23 and the inactive waveguide layer 24 is assumed to be the same. However, when the refractive indexes are different, the refractive index and the length are not the actual length. What is necessary is just to calculate based on the optical length that can be expressed by the product.

  The wavelength tunable semiconductor laser according to the present embodiment operates stably if the phase relationship between the active waveguide layers 23 adjacent to each other with the inactive waveguide layer 24 interposed therebetween is brought close to the ideal state. Therefore, in order to compensate for an equivalent phase shift due to a change in refractive index caused by a layer thickness or composition disturbance when the inactive waveguide layer 24 is selectively grown with respect to the active waveguide layer 23, What is necessary is just to make it the optical length of the active waveguide layer 24 be a phase shift corresponding to two connection surfaces, that is, 2ΔΩ. Thereby, the phase relationship between the active waveguide layers 23 can be brought close to an ideal state, and stable operation can be obtained.

  In the present embodiment, a diffraction grating 25 is formed in the active waveguide layer 23 and there is no diffraction grating in the inactive waveguide layer 24, but conversely, a diffraction grating is formed in the inactive waveguide layer 24 to activate the active waveguide. The present invention can be applied even when the waveguide layer 23 has no diffraction grating. In that case, if the active waveguide 26 and the inactive waveguide 27 are replaced, it can be easily analogized.

  In this embodiment, the active waveguide layer 23 and the inactive waveguide layer 24 are alternately and periodically arranged. For example, the active waveguide layer in which the diffraction grating is formed and the first inactive waveguide layer are formed. When three types of waveguides of the waveguide layer and the second inactive waveguide layer are alternately and repeatedly connected, the phase relationship of the diffraction gratings between the active waveguide layers where the diffraction gratings are formed is expressed as follows. The product n × ΔΩ of the number n of the joint surfaces existing in the region and the equivalent phase shift ΔΩ around one joint surface may be used. Needless to say, when there is a necessary phase shift θ in an ideal state, θ + n × ΔΩ.

  INDUSTRIAL APPLICABILITY The present invention is suitable for application to a wavelength tunable semiconductor laser having a configuration in which different types of waveguides are butt-joined by selective growth and a structure in which a diffraction grating is formed on at least one type of waveguide. It is.

11, 31 n-type InP lower cladding layer 12, 32 GaInAsP active waveguide layer 13, 33 GaInAsP inactive waveguide layer (wavelength control layer)
14, 34 p-type InP upper cladding layer 15, 35 diffraction grating 16 p-type InGaAs contact layer 17, 37 active layer electrode 18, 36 wavelength control electrode 19, 38 electrode 20 phase shift between waveguide layers 21 phase shift between laser parts 22 etching Mask 32 GaInAsP inactive waveguide layer 33 GaInAsP active waveguide layer 39 Phase shift

Claims (4)

A part of the first waveguide layer formed on the semiconductor substrate is etched, and a second waveguide layer having a composition or layer structure different from that of the first waveguide layer is selectively grown. A tunable semiconductor laser having at least two waveguide structures inside a resonator,
The first waveguide layer or the second waveguide layer has a gain;
A diffraction grating is formed over the entire length of the first waveguide layer and the second waveguide layer,
With respect to an equivalent phase shift of −ΔΩ due to a refractive index variation between the first waveguide layer and the second waveguide layer that occurs during the selective growth, the first guide A wavelength tunable semiconductor laser, wherein a correction phase shift of a phase shift amount ΔΩ is inserted at a position of the diffraction grating corresponding to a joint surface between a waveguide layer and the second waveguide layer. The first waveguide layer has a gain and a diffraction grating is formed;
The second waveguide layer has no gain while a diffraction grating is formed,
The first waveguide layer and the second waveguide layer are alternately arranged at a constant ratio and have a plurality of laser parts having at least one repetition period,
When the refractive index of the first waveguide layer and the second waveguide layer is uniform along the resonance direction, in addition to the correction phase shift, in the diffraction grating corresponding to between the laser parts, 2. The wavelength tunable semiconductor laser according to claim 1, wherein a phase shift set so as to satisfy a phase condition of the resonator is inserted. The first waveguide layer has a gain and a diffraction grating is formed to constitute a distributed feedback laser;
The second waveguide layer has no gain while a diffraction grating is formed to form a distributed Bragg reflector,
In addition to the correction phase shift, the first waveguide layer and the second waveguide layer are added to the diffraction grating corresponding to the bonding surface between the first waveguide layer and the second waveguide layer. 2. The wavelength tunable semiconductor laser according to claim 1, wherein a phase shift set so as to satisfy the phase condition of the resonator is inserted when the refractive index of the laser is uniform along the resonance direction. A part of the first waveguide layer formed on the semiconductor substrate is etched, and a second waveguide layer having a composition or layer structure different from that of the first waveguide layer is selectively grown. A tunable semiconductor laser having at least two or more waveguide structures inside a resonator,
The first waveguide layer or the second waveguide layer has a gain;
A diffraction grating is formed on the first waveguide layer or the second waveguide layer,
There are at least two waveguide layers in which the diffraction grating is formed inside the resonator,
The diffraction grating is configured to correct an equivalent phase shift of -ΔΩ due to a refractive index variation between the first waveguide layer and the second waveguide layer that occurs during the selective growth. The tunable semiconductor is characterized in that the diffraction grating is formed so that the phase of the diffraction grating adjacent to each other is n × ΔΩ with respect to the number n of the joint surfaces existing between the waveguide layers formed with laser.

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