JP5702262B2 - Tunable semiconductor laser - Google Patents

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.

For example, in the case of a distributed active DFB laser in which two lasers having different repetition periods L ₁ and L ₂ (hereinafter referred to as a first laser unit and a second laser unit) are integrated in series on the same substrate, a diffraction grating A quarter wavelength phase shift is inserted between the first laser part and the second laser part so as to satisfy the phase condition of the resonator.

  The wavelength change is caused by passing a current through the inactive waveguide layer and injecting carriers. The refractive index of the semiconductor decreases as carriers are injected, and the oscillation wavelength is shortened.

  Also, in the case of such a resonator structure, when ignoring the optical loss of the inactive waveguide layer, the oscillation is most likely to occur, that is, the threshold gain is low because the active waveguide layer and the inactive waveguide layer are low. This is when the refractive index of the layer matches. At this time, if the periods of the diffraction gratings of the active waveguide and the inactive waveguide are the same, the Bragg wavelengths coincide with each other, so that the reflectance becomes the highest.

FIG. 3 shows the difference in refractive index between the active waveguide layer and the inactive waveguide layer (the refractive index of the inactive waveguide layer−) for the distributed active DFB laser having the first laser portion and the second laser portion described above. the refractive index) of the active waveguide layer as a parameter, the normalized threshold gain difference divided by the threshold gain g _th1 of the threshold gain difference Delta] g _th first resonance mode and a second resonance mode first resonant mode Δg _th / g _th1 (Values indicated by black circles and solid lines in the figure; corresponding to the left axis) and threshold gain g _th1 of the first resonance mode (values indicated by white circles and broken _{lines in the} figure; corresponding to the right axis). The normalized threshold gain difference Δg _th / g _th1 is an important parameter in considering single mode characteristics, and the larger this number, the better the single mode property. That is, the intensity ratio (SMSR) between the first mode and the second mode is the largest. Therefore, in the case of the structure of the distributed active DFB laser having the first laser portion and the second laser portion described above, the threshold gain g _th is the lowest and the single mode property is the best when there is no difference in refractive index. become.

In the case of a distributed active DFB laser, it is considered that the refractive index of the active waveguide layer does not change and the refractive index changes by injecting current into the inactive waveguide layer. For example, assuming that the refractive index change of the inactive waveguide layer is about 0.03, if the refractive index difference is used in the range of 0.015 to −0.015 centering on the refractive index difference 0, the average will be averaged. The lowest threshold gain g _th is the lowest, and the normalized threshold gain g _th is the largest. That is, the refractive index difference is set to 0.015 in a state where carriers are not injected into the inactive waveguide layer.

However, in actual cases, as the carrier density increases, absorption loss such as free electron absorption increases, so that the threshold gain g _th increases. Therefore, in an actual device, by assigning the refractive index change range uniformly around the refractive index difference 0, when the refractive index change becomes large and the refractive index difference exceeds 0 and becomes a negative value, the threshold gain is originally set. in addition to g _th is increased, the threshold gain g _th is exerted properties due to the loss increase may be deteriorated.

  Therefore, an object of the present invention is to provide a wavelength tunable semiconductor laser capable of continuing oscillation while suppressing a large characteristic deterioration.

A wavelength tunable semiconductor laser according to a first aspect of the present invention for solving the above-described problems is obtained by alternately and periodically forming an active waveguide layer having gain and an inactive waveguide layer for controlling the wavelength on a semiconductor substrate. A tunable semiconductor laser having a structure formed repeatedly in which a diffraction grating is formed over the entire length of the active waveguide layer and the inactive waveguide layer, and the active waveguide layer and the inactive waveguide layer wherein prior to the refractive index of having at least one phase shift Omega _C is set to satisfy the phase condition so co oscillator if it is uniform along the resonance direction in the resonator, a current is injected The initial refractive index difference ΔNs between the inactive waveguide layer and the active waveguide layer is made to correspond to the shortest wavelength of the wavelength tunable region in which the wavelength variable operation is performed by injecting current into the refractive index of the inactive waveguide layer. When the value is changed Serial as ΔNt the refractive index variation of the non-active waveguide layer, and sets so as to satisfy the following equation (4).

A wavelength tunable semiconductor laser according to a second invention for solving the above-described problem is the wavelength tunable semiconductor laser according to the first invention, wherein the active waveguide layer and the inactive waveguide layer have at least two or more. It has a different repetition period.

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 or second aspect of the invention, wherein the phase shift Ω _C satisfies the following expression (5): It is characterized by that.

  According to the wavelength tunable semiconductor laser of the present invention, by gradually matching the resonance conditions when changing the wavelength, an increase in loss caused when a refractive index change is caused by current injection into the inactive waveguide layer is overcome. It is possible to cancel the oscillation, and it is possible to continue oscillation while suppressing a large 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 a figure explaining the characteristic of a first embodiment of the present invention. It is a figure explaining the effect of 1st embodiment of this invention. It is a figure explaining a phase shift. 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 active waveguide layer 12 region and the inactive waveguide layer 13 region. 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 . It has a structure in which L _{2 is} changed and connected in series. Further, a quarter wavelength phase shift 20 is inserted between the first laser part A ₁ and the second laser part A ₂ .

  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 The combination of wavelengths is not limited to the above.

  In the present invention, the difference in refractive index between the active waveguide layer and the inactive waveguide layer is important, and this will be described in detail later.

In the wavelength tunable semiconductor laser according to the present embodiment, although not shown in the drawing, Fe that 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 periods of the active waveguide layer 12 and the inactive waveguide layer 13 are 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. In addition, a phase shift 20 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. 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. In this way, by short-circuiting the electrodes 17 and 18 in each region on the element, the current Ia or the wavelength control current can be applied to each region only by bonding a metal bonding wire one by one. It can be injected.

  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, an etching mask 22 such as SiO2 is formed on the substrate on which the active waveguide layer 12 is formed, the active waveguide layer 12 is processed into an island shape using the etching mask 22, and the etching mask 22 is left as it is. Thus, the active waveguide layer 12 and the inactive waveguide layer 13 are connected by re-growing the inactive waveguide layer 13.

Thereafter, the diffraction grating pattern 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. The diffraction grating is provided with a phase shift of λ / 4 between the first laser part A ₁ and the second laser part A ₂ .

  Here, 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 .

As described above, in the present embodiment, a quarter wavelength phase shift 20 is inserted between the diffraction gratings 15 of the first laser part A ₁ and the second laser part A ₂ . 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.

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 of the number of active layers and the like, and the loss of the waveguide. The principle of the distributed active DFB laser is that the first laser part A ₁ and the second laser part for the proportion between the active waveguide layer 12 by a ₂ inactive waveguide layer 13 is to the same, the ratio can be changed on demand.

  Here, in the wavelength tunable semiconductor laser according to the present embodiment, the refractive index difference between the active waveguide layer 12 and the inactive waveguide layer 13 is important. Although the refractive index of the inactive waveguide layer 13 depends on the layer structure and wavelength, it changes by about 0.025 to 0.03 by current injection. The inactive waveguide layer 13 of this embodiment has a structure in which a 300 nm thick GaInAsP layer having a band gap wavelength of 1.4 μm is sandwiched between InP clad layers as a core layer. In this case, a refractive index change of about 0.027 can be caused. Therefore, the refractive index difference between the active waveguide layer 12 and the inactive waveguide layer 13 is set to 0.022 before current injection into the inactive waveguide layer 13.

That is, the wavelength change occurs in the range of the refractive index difference between the active waveguide layer 12 and the inactive waveguide layer 13 of −0.005 to 0.022. In this way, even if the loss increases due to current injection into the inactive waveguide layer 13 and the threshold gain g _th increases, the active waveguide layer 12 and the inactive waveguide layer 13 are used as resonators. refractive index difference is eliminated with, since the Bragg wavelength of the two match now high reflectance can be obtained, to work the effect of lowering the threshold gain g _th improved characteristics of the resonator, a large threshold gain g _th It is possible to prevent an increase in

  That is, more than half of the amount of change in the refractive index of the inactive waveguide layer 13 is the difference in refractive index between the active waveguide layer 12 and the inactive waveguide layer 13 on the horizontal axis in FIG. If the refractive index difference between the initial active waveguide layer 12 and the inactive waveguide layer 13 is set so as to move in a positive region of the refractive index of the layer 13 minus the refractive index of the active waveguide layer 12, the above effect is obtained. Can be obtained.

More effectively, when the refractive index of the active waveguide layer 12 does not change when the refractive index of the inactive waveguide layer 13 is changed to the maximum, the active waveguide layer 12 and the inactive waveguide are not changed. It is desirable that the refractive index difference with the layer 13 is 0 to −0.01. In practice, this is because the carrier density of the active layer is increased by the amount of increase in the loss due to current injection into the inactive waveguide layer 13 and the increase in the threshold gain g _th . This is because the refractive index slightly decreases.

  The amount by which the refractive index decreases depends on other parameters such as the waveguide loss and the coupling coefficient of the diffraction grating, and is difficult to generalize. However, the important point of the present invention is that the initial refractive index difference is set so that the characteristics as a resonator are improved as a result of changing the refractive index.

Accordingly, the refractive index difference between the active waveguide layer 12 and the inactive waveguide layer 13 is changed from 0 to −0.01 when the refractive index of the inactive waveguide layer 13 is changed to the maximum in FIG. In fact, considering the effect that the refractive index of the active waveguide layer 12 is lowered, the difference in refractive index is in the vicinity of 0, and the threshold gain g _th is the lowest for the resonator.

If this range is set, even if the refractive index change of the active waveguide layer 12 occurs 0.01 or more, the final refractive index difference between the active waveguide layer 12 and the inactive waveguide layer 13 is Since the threshold gain g _th is monotonously decreasing from the initial state when considered from the initial state, it can be said that the threshold gain g _th works in a direction to cancel the increase of the threshold gain g _th due to the increase in loss, and the effect of the present invention is obtained. It can be said that.

On the contrary, if almost no change in the refractive index of the active waveguide layer 12 occurs, the refractive index difference between the final active waveguide layer 12 and the inactive waveguide layer 13 is -0.01. Threshold gain g _th when the threshold gain g _th and the refractive index difference -0.01 when the refractive index difference 0 3 is only a difference of about 5%. Therefore, it can be said that the increase in the threshold gain g _th is slight, which is an allowable range. For this reason, the refractive index difference between the active waveguide layer 12 and the inactive waveguide layer 13 when the refractive index change of the inactive waveguide layer 13 is maximized is set to 0 to −0.01. It can be said that setting is good.

In the following, the above-described points will be described as mathematical formulas.
Let ΔNt be the amount of change in the refractive index of the inactive waveguide layer 13. Difference in refractive index between the active waveguide layer 12 and the inactive waveguide layer 13 at an initial stage (before current injection into the inactive waveguide layer 13) (refractive index of the inactive waveguide layer 13−refractive index of the active waveguide layer 12) ) Is ΔNs. When it is assumed that there is no change in the refractive index of the active waveguide layer 12, the difference in refractive index from the active waveguide layer 12 when the maximum change in refractive index of the inactive waveguide layer 13 is obtained is ΔNe. These relationships are expressed by the following formula (6).
ΔNs−ΔNt = ΔNe (6)

  Here, as described above, the refractive index difference ΔNe is set as in the following equation (7).

  From the equations (6) and (7), the initial refractive index difference ΔNs is expressed by the following equation (8).

  When the setting conditions of this embodiment are applied, the amount of change in the refractive index of the inactive waveguide layer 13 is ΔNt = 0.027, and the initial refraction between the active waveguide layer 12 and the inactive waveguide layer 13 is as follows. Since the rate difference is ΔNs = 0.022, the initial refractive index difference ΔNs is in the range of 0.017 to 0.027, so it can be seen that the formula (8) is satisfied.

  The initial refractive index difference ΔNs can be controlled by changing the layer structure. For example, although the active waveguide layer 13 has a multilayer quantum well structure, the equivalent refractive index increases as the number of quantum wells increases. The equivalent refractive index can also be increased by making the barrier composition a composition having a longer bandgap wavelength. In addition, the equivalent refractive index can be designed by changing the thickness and composition of the layer forming the waveguide core.

By the way, in the distributed active DFB laser having the repeated structure of the active waveguide layer 12 and the non-active waveguide layer 13, when the refractive index difference between the active waveguide layer 12 and the non-active waveguide layer 13 increases, In addition, secondary reflection peaks occur at intervals inversely proportional to the repetition period. In the structure in which two laser parts A ₁ and A ₂ having different repetition periods shown in FIG. 1 are connected in series, the interval between the main reflection peak and the sub-reflection peak is the first laser part A ₁ and the second laser part. Because of the difference in A ₂ , resonance does not occur at the secondary reflection peak, and oscillation continues at the main reflection peak even if the refractive index difference increases.

  In the present invention, the initial refractive index difference ΔNs needs to be relatively large. However, in the case of a distributed active DFB laser having only a single repetition period, the initial refractive index difference ΔNs is large, so that it has a single mode property. Can get worse. Therefore, in order to apply the present invention, a distributed active DFB laser having two or more repetition periods of the active waveguide layer 12 and the inactive waveguide layer 13 is desirable.

(Second embodiment)
Hereinafter, a second embodiment of the wavelength tunable semiconductor laser according to the present invention will be described with reference to FIGS.

  In this embodiment, the phase shift 20 described in the first embodiment is changed to a phase shift smaller than λ / 4 instead of λ / 4. Specifically, in this embodiment, the phase shift is λ / 5. Other structures are the same as those of the first embodiment, and hereinafter, the same members are denoted by the same reference numerals, and redundant description is omitted, and different points will be mainly described. The initial refractive index difference ΔNs between the active waveguide layer 12 and the inactive waveguide layer 13 was set to 0.02.

4 shows a normalized threshold gain difference Δg _th / g _th1 (left axis) and threshold gain g when three types of phase shifts (λ / 4, λ / 6, λ / 8) are applied as the phase shift 20. _th (right axis). It can be seen that a large difference appears in the normalized threshold gain difference Δg _th / g _th1 by changing the amount of phase shift. On the other hand, there is no significant difference in the threshold gain g _th in the first mode. Since the phase shift of this embodiment is λ / 5, it can be easily analogized that the characteristic is between λ / 4 and λ / 6 in FIG.

  Here, the phase shift has a relationship defined in FIG. FIG. 5 is a schematic diagram of the refractive index fluctuation of the diffraction grating when the horizontal axis is the position and the vertical axis is the refractive index. FIG. 5A shows a case where the phase shift 20 is zero, and FIG. When the phase shift 20 is λ / 8, FIG. 5C shows the case where the phase shift 20 is λ / 4.

In the present invention, as described in the first embodiment, the initial refractive index difference ΔNs is set in consideration of the maximum refractive index change, and in particular, the refractive index is changed so that the refractive index difference becomes a substantially positive region. . As can be seen from FIG. 4, when the phase shift 20 is λ / 4, the normalized threshold gain difference Δg _th / g _th1 is the largest at the refractive index difference 0, and it is smaller as the refractive index difference is larger. Centering on a refractive index difference of 0, it has a symmetrical shape with positive and negative refractive index differences. On the other hand, by making the phase shift 20 smaller than λ / 4, the peak position of the normalized threshold gain difference Δg _th / g _th1 can be made a region where the refractive index difference is positive.

In this embodiment, since the initial refractive index difference moves from about 0.02 to about −0.005, the phase shift 20 is set so that an average large normalized threshold gain difference Δg _th / g _th1 is obtained in this range. λ / 5. Here, the smaller the phase shift 20, the smaller the normalized threshold gain difference Δg _th / g _th1 at the peak position. However, when considered as a wavelength tunable laser, it is better to obtain a submode suppression ratio (SMSR) averaged over a range in which the wavelength is changed.

  As described above, if the wavelength is changed around a region where the refractive index difference is positive, an average single mode characteristic can be obtained over the entire wavelength change range by reducing the phase shift amount. .

However, if the phase shift 20 is too small, the single mode characteristics will be deteriorated. As shown in FIG. 4, when the phase shift 20 is λ / 8, the peak is in the vicinity of the refractive index difference of 0.02 to 0.025. When the refractive index difference is 0.03, both the phase shift of λ / 6 and the phase shift of λ / 8 abruptly decrease the normalized threshold gain difference Δg _th / g _th1 . Therefore, if the phase shift 20 is smaller than λ / 8, the effect of obtaining an average single mode characteristic over the entire wavelength change width cannot be obtained. Therefore, the shift amount Ω _C of the phase shift 20 is preferably in the range of the following equation (9).

Accordingly, when considered together with the first embodiment, the range of the shift amount Ω _C of the phase shift 20 is expressed by the following expression (10).

  It should be noted that if the region where the refractive index difference is negative is used, it can be easily inferred that the phase shift 20 should be larger than λ / 4.

  The present invention is suitable for application to a wavelength tunable semiconductor laser.

11 n-type InP lower cladding layer 12 GaInAsP active waveguide layer 13 GaInAsP inactive waveguide layer (wavelength control layer)
14 p-type InP upper cladding layer 15 diffraction grating 16 p-type InGaAs contact layer 17 active layer electrode 18 wavelength control electrode 19 electrode 20 phase shift between laser parts

Claims (3)

A wavelength tunable semiconductor laser having a structure in which an active waveguide layer having gain and an inactive waveguide layer for controlling wavelength are alternately and repeatedly formed on a semiconductor substrate,
A diffraction grating is formed over the entire length of the active waveguide layer and the inactive waveguide layer,
At least a phase shift Omega _C the refractive index of the active waveguide layer and the inactive waveguide layer is set to satisfy the phase condition so co oscillator if it is uniform along the resonance direction in the resonator Have more than one,
The wavelength tunable operation for performing the wavelength tunable operation for the refractive index of the inactive waveguide layer by injecting the current into the initial refractive index difference ΔNs between the inactive waveguide layer and the active waveguide layer before the current is injected. The refractive index change amount of the inactive waveguide layer when changed to a value corresponding to the shortest wavelength of the region is set to satisfy ΔNt so as to satisfy the following expression (1): Tunable semiconductor laser.

The wavelength tunable semiconductor laser according to claim 1, wherein the active waveguide layer and the inactive waveguide layer have at least two or more different repetition periods. 3. The wavelength tunable semiconductor laser according to claim 1, wherein the phase shift Ω _C satisfies the following expression (2).

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