JP5286198B2 - Distributed feedback semiconductor laser - Google Patents

Description

  The present invention relates to a distributed feedback semiconductor laser, and more particularly to a distributed feedback semiconductor laser for communication having high-speed modulation characteristics, excellent oscillation single mode characteristics, and a wide operating temperature range.

  In general, as a communication semiconductor laser emitting in a single mode, a distributed feedback (DFB) semiconductor laser is used for an intermediate or long-distance optical communication system. In particular, in recent years, there has been a demand for cost reduction and speedup due to the explosive increase in transmission capacity in optical communications accompanying the development of the Internet. In order to reduce the cost, it is necessary not to use a cooling means. In this case, when the operating environment temperature of the semiconductor laser is 85 ° C., the element temperature of the semiconductor laser is 95 ° C. Therefore, a direct modulation DFB laser that operates at a modulation rate of 10 Gbps and an element temperature of 95 ° C. or higher without using a cooling means is desired.

  The basic requirement of the semiconductor laser as described above is that the current-light output characteristics do not deteriorate at the maximum operating temperature. Furthermore, stable single mode oscillation is possible from the low temperature range to the high temperature range. In addition, high-speed modulation characteristics are required not to deteriorate in a high temperature region. However, it is very difficult to satisfy the stability of the single mode oscillation and the high-speed modulation characteristics at the same time because they are contradictory items as the design parameters of the laser structure.

  For example, semiconductor lasers have been proposed that obtain high efficiency and high output characteristics and high single mode oscillation stability (see, for example, Patent Documents 1 and 2). As shown in FIG. 12, the conventional semiconductor laser has a guide layer 505 in which a diffraction grating 501 is formed in a part in the resonator direction, an active layer 504 formed in the entire resonator direction, and a highly reflective film. A high reflectivity end face 502 formed and a low reflectivity end face 503 provided with a low reflection film are provided, and laser light is emitted from the low reflectivity end face 503.

  The above semiconductor laser is intended to obtain high efficiency and high output characteristics and high stability of single mode oscillation, and cannot achieve high-speed characteristics with an element temperature of 95 ° C. and 10 Gbps or more. The reason is that the guide layer 505 remains in a region where no valleys are formed, so that the remaining guide layer 505 does not contribute to DFB oscillation, but the guide layer 505 has a composition having a higher refractive index than the cladding layer 508. Therefore, the light absorption loss increases. When the absorption loss is large, the oscillation threshold value is increased and the light emission efficiency is decreased, so that heat generation is increased and the temperature characteristics are deteriorated.

  In the region where the guide layer 505 in which no valley is formed remains, light is pulled toward the guide layer 505 compared to the region in which the valley is formed. That is, in the region where the guide layer 505 remains, the optical confinement coefficient of the active layer 504 becomes small. Due to the deterioration of the optical confinement factor and the increase of the absorption loss, the relaxation oscillation frequency at the time of modulation decreases and the high-speed modulation characteristics deteriorate.

  For the reasons described above, high-speed characteristics of 10 Gbps or more could not be achieved in a high operating temperature range.

  On the other hand, the conventional semiconductor laser described in Patent Document 3 discloses a structure in which a guide layer does not remain in a region where peaks and valleys are not formed. This laser is a CW (continuous) oscillation, and has a high output and a narrow spectral line. This is intended to be wide (1 MHz or less), and since the resonator length is 500 μm or more, it does not have high-speed modulation characteristics.

JP-A-5-102597 Japanese Patent Laid-Open No. 11-68220 JP 2004-31402 A

  As described above, it has been difficult for a conventional semiconductor laser to realize a semiconductor laser having high-speed modulation characteristics, excellent oscillation single mode characteristics, and a wide operating temperature range. An object of the present invention is to realize a semiconductor laser that has a high-speed modulation characteristic of 10 Gbps or more and a high single mode stability even in an element temperature range of 95 ° C. or more, which has a wide operating temperature range.

In order to achieve the above object, a distributed feedback semiconductor laser according to the present invention includes an active layer, a guide layer disposed below or above the active layer, a low reflection film formed on one end surface, a high reflection film formed on the other end surface, a diffraction grating formed in said one end face of a distributed feedback semiconductor laser for have a length between the both end faces in the following 280μm or 170μm And the diffraction grating is formed by removing all of the guide layer at a constant period and embedding the removed portion of the guide layer in a cladding layer, and the length of the region forming the diffraction grating is The entire length of the guide layer in the region where the diffraction grating is not formed on the other end surface side is 60% of the length between the both end surfaces, and the distributed feedback semiconductor laser is relaxed. Vibration frequency f _r is 10 GHz or more .

  The above inventions can be combined as much as possible.

  According to the present invention, it is possible to realize a distributed feedback semiconductor laser having high-speed modulation characteristics of 10 Gbps or more and excellent high single mode stability even in an element temperature range of 95 ° C. or more, which has a wide operating temperature range. Furthermore, low threshold characteristics can be obtained.

It is sectional drawing which shows the structure of one Embodiment of this invention. 1 is a structural cross-sectional view of a distributed feedback semiconductor laser according to the present invention. 1 is a perspective view of a distributed feedback semiconductor laser according to the present invention. FIG. It is a figure which shows the relationship between the resonator length and relaxation oscillation frequency of the distributed feedback semiconductor laser which concerns on this embodiment. The relationship between the ratio of the length Lg of the diffraction grating formation region to the resonator length L and the relaxation oscillation frequency of the distributed feedback semiconductor laser according to this example is shown. The relationship between the ratio of the length Lg of the diffraction grating formation region to the resonator length L and the oscillation threshold value of the distributed feedback semiconductor laser according to this example is shown. The relationship between the ratio of the length Lg of the diffraction grating formation region to the resonator length L and the series resistance value of the distributed feedback semiconductor laser according to this example is shown. It is a figure for demonstrating the effect of the distributed feedback semiconductor laser which concerns on a present Example, and is a figure which shows the relationship between the electric current sent through a distributed feedback semiconductor laser, and the luminous efficiency of a distributed feedback semiconductor laser. It is a figure which shows the relationship between the electric current sent through a distributed feedback semiconductor laser, and the light emission efficiency of a distributed feedback semiconductor laser in the distributed feedback semiconductor laser of a comparative example. It is a figure explaining the effect of the distributed feedback semiconductor laser which concerns on a present Example, and is a figure which shows the relationship between the electric current sent through a distributed feedback semiconductor laser, and the relaxation oscillation frequency of a distributed feedback semiconductor laser. It is a figure which shows the relationship of the electric current sent through a distributed feedback semiconductor laser, and the relaxation oscillation frequency of a distributed feedback semiconductor laser in the distributed feedback semiconductor laser of a comparative example. It is sectional drawing which shows the structure of the conventional semiconductor laser.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. Note that members having the same function are denoted by the same reference numerals, and repeated description thereof is omitted. Moreover, the form and effect of the Example of this invention are demonstrated, referring drawings.

  FIG. 1 is a cross-sectional view showing the configuration of an embodiment of the present invention. The distributed feedback semiconductor laser according to the present embodiment includes an active layer 504, a guide layer 505 disposed above the active layer 504, a low reflectivity end face 503 having a low reflection film formed on one end face, and the other. A high reflectivity end face 502 having a high reflection film formed on one end face thereof, and a diffraction grating 10 formed on one end face side. The resonator length L, which is the length between both end faces, is 170 μm or more and 280 μm or less.

  The diffraction grating 10 is formed by removing all of the guide layer 505 at a constant period and embedding the removed portion of the guide layer 505 in the cladding layer 508. In the diffraction grating 10, all of the guide layer 505 is removed on the other high reflectance end face 502 side. That is, in the distributed feedback semiconductor laser according to the present embodiment, the diffraction grating 10 is formed only on the low reflectance end surface 503 side on the light emission side, and the guide layer 505 is completely on the rear high reflectance end surface 502 side. The configuration does not exist.

  In the distributed feedback semiconductor laser according to this embodiment, the length Lg of the region forming the diffraction grating 10 is preferably 40% or more and 80% or less of the resonator length L. As a result, high-speed operation of 10 Gbps can be realized at an element temperature of 95 ° C.

  FIG. 2 is a structural sectional view of the distributed feedback semiconductor laser according to this embodiment. As an example, a 1.3 μm band ridge waveguide type distributed feedback semiconductor laser is shown. In this embodiment, an n-type InP substrate 1, an n-type InP clad layer 2, an n-type InGaAlAs SCH layer 3, and an InGaAlAs-based strained quantum well layer as the active layer 504 shown in FIG. Quantum well (MQW) layer 4, p-type InGaAlAs SCH layer 5, p-type InAlAs carrier stop layer 6, p-type InGaAsP ridge forming etching layer 7, p-type InP spacer layer (8 below 505 The P-type InGaAsP guide layer 505 is sequentially grown in multiple layers by metal organic vapor phase epitaxy.

  Thereafter, the guide layer 505 is removed at a constant period on one end face side in the cavity direction by using a photolithography technique, an electron beam drawing technique, and an etching technique, and the structure of the diffraction grating 10 is formed at the same time. The guide layer 505 is completely removed on the end face side. In the diffraction grating structure on one end face side, a portion where the p-type InGaAsP guide layer 505 is completely removed and a portion where the p-type InGaAsP guide layer 505 is completely removed are periodically formed in the direction of the resonator.

  Next, the metal-organic vapor phase epitaxy method is used again to re-grow the p-type InP cladding layer 8 in a structure in which the guide layer 505 is embedded on the multilayer substrate on which the diffraction grating structure is formed. Thereby, the removed portion of the guide layer 505 is filled with the cladding layer, and the diffraction grating 10 is formed on one end face side in the resonator direction. Then, a p-type InGaAs contact layer 9 is regrown on the p-type InP cladding layer 8.

  Next, as shown in FIG. 3, using an insulating film forming technique such as a thermal CVD (Chemical Vapor Deposition) method, a photolithography technique, and an etching technique, a p-type InGaAsP etching stop layer 7 is used to form a p-type. The InP clad layer 8 is etched to form a ridge-shaped optical waveguide structure.

Next, a p-type electrode 12 electrically connected to the optical waveguide structure through a p-type InGaAs contact layer 9 and an n-type electrode 11 electrically connected to the n-type InP substrate 1 are formed. . A SiO ₂ insulating film is disposed on the surface excluding the top of the ridge, and the p-type electrode 12 and the semiconductor layer excluding the top of the ridge are insulated. In FIG. 3, the p-type electrode other than the SiO ₂ insulating film and the top of the ridge is omitted. The n-type electrode 11 is formed after the p-type electrode 12 is formed and then the back side of the n-type InP substrate 1 is polished to a thickness capable of cleavage in a bar shape.

  After that, cleaved into a bar shape on the emission end face side and the rear end face side, a low reflectance film is formed on the emission end face side where the diffraction grating layer of the semiconductor laser is formed, and a high reflectance film is formed on the rear end face side of the semiconductor laser After forming, the semiconductor laser device is chipped.

ｆ_ｒは緩和振動周波数、Γはダンピングレート、Ｃはｎ型電極１１とｐ型電極１２の間の寄生容量、Ｒはｎ型電極１１とｐ型電極１２の間の直列抵抗を示す。 <Examination of laser modulation characteristics>
The modulation frequency characteristic Rm is expressed by the following equation.

f _r is a relaxation oscillation frequency, Γ is a damping rate, C is a parasitic capacitance between the n-type electrode 11 and the p-type electrode 12, and R is a series resistance between the n-type electrode 11 and the p-type electrode 12.

Considering the 3dB bandwidth limitation, the modulation frequency characteristics Rm when changing the relaxation oscillation frequency _{f r} is 3dB change is when the frequency f is 1.55f _r. The modulation frequency characteristic Rm changes by 3 dB when the parasitic capacitance C and the series resistance R are changed when the frequency f is 1 / (2πCR). Therefore, in order to increase the high-speed characteristics, increasing the relaxation oscillation frequency f _r, it is necessary to reduce the parasitic capacitance C or the series resistor R.

  In order to reduce the parasitic capacitance C, the resonator length L may be shortened. On the other hand, in order to reduce the series resistance R, the width of the multiple quantum well layer 4 may be widened and the resonator length L may be increased, but the latter goes against reducing the parasitic capacitance C.

<Examination of laser temperature characteristics>
There are three possible methods for improving the temperature characteristics of the laser.
The first method is to increase luminous efficiency. For this purpose, lowering the threshold value of the laser is effective. This is because by lowering the laser oscillation threshold, the operating current is reduced and the amount of heat generation is reduced. Absorption loss is reduced by reducing the amount of heat generation.
The second method is to reduce the series resistance R. This is because the amount of heat generated by the series resistor R decreases.
The third method is to increase thermal diffusion. For this purpose, it is effective to increase the resonator length L or to use a Peltier element. Increasing the resonator length L also decreases the series resistance R. However, if the resonator length L is increased, the parasitic capacitance C increases, and the modulation frequency characteristic Rm deteriorates. One Peltier element cannot be used in the case of an uncooled laser.

  From the above examination, the distributed feedback semiconductor laser according to the present embodiment employs the first method, and improves the laser temperature characteristics by lowering the laser oscillation threshold. Specifically, by adopting the configuration of the diffraction grating 10 shown in FIG. 1, the absorption loss in the guide layer 505 is reduced and the laser oscillation threshold is lowered.

<Examination of Cavity Length L of Distributed Feedback Semiconductor Laser>
Figure 4 shows the resonator length L of the distributed feedback semiconductor laser according to the present embodiment and the relationship between the relaxation oscillation frequency f _r. The element temperature of the laser is 95 ° C., the current value is 60 mA, and Lg / L is 60%.
The resonator length L is the relaxation oscillation frequency f _r in the vicinity of 220μm showed the maximum value. When the resonator length L is large, the threshold current is increased and the parasitic capacitance C of the laser is increased, so that the relaxation oscillation frequency _fr is deteriorated. On the contrary, when the resonator length L is small, the threshold current is small and the parasitic capacitance C of the laser is small, but the temperature characteristic of the laser emission efficiency is deteriorated due to heat generation, so the relaxation oscillation frequency _fr Deteriorates.

Relaxation oscillation frequency _{f r} in order to satisfy the high-speed modulation operation of 10Gbps is need more than satisfy 10 GHz, as shown in FIG. 4, the cavity length L is 170μm or more 280μm or less in the range of relaxation oscillation frequency f _{Since r} is 10 GHz or more, the resonator length L is preferably 170 μm or more and 280 μm or less.

<Examination of the ratio of the length Lg of the diffraction grating formation region to the resonator length L>
FIG. 5 shows the relationship between the ratio of the length Lg of the diffraction grating formation region to the resonator length L and the relaxation oscillation frequency of the distributed feedback semiconductor laser according to this example. The horizontal axis represents the ratio of the length Lg of the diffraction grating formation region to the resonator length L, that is, Lg / L, and the vertical axis represents the relaxation oscillation frequency f _r (GHz). The element temperature is 95 ° C., the relaxation oscillation frequency _{f r} is the value of a current value 60 mA.

When Lg / L is 100%, that is, when the diffraction grating region is the entire cavity length L, the oscillation threshold value of the distributed feedback semiconductor laser increases, the light emission efficiency decreases, and the absorption loss increases. The relaxation oscillation frequency _{fr is} also reduced. As Lg / L decreases from 100%, the oscillation threshold decreases and the relaxation oscillation frequency _fr also increases. When Lg / L is around 60%, the relaxation oscillation frequency _fr is maximum, and when Lg / L is further decreased, the relaxation oscillation frequency _fr is decreased. Lg / L that is the relaxation oscillation frequency f _r in accordance smaller than 60% becomes small, a decrease in the diffraction grating region is because the differential gain decreases.

Here, in order to realize high-speed operation of 10Gbps at element temperature 95 ° C., it is necessary to 10GHz or more relaxation oscillation frequency _{f r.} In the distributed feedback semiconductor laser according to this example, since the relaxation oscillation frequency _fr is 10 GHz or more when the value of Lg / L is in the range of 40% to 80%, the length of the region where the diffraction grating is formed. Lg is preferably 40% or more and 80% or less of the resonator length L.

  FIG. 6 shows the relationship between the ratio of the length Lg of the diffraction grating formation region to the resonator length L and the oscillation threshold value of the distributed feedback semiconductor laser according to this example. The horizontal axis indicates the ratio of the length Lg of the diffraction grating formation region to the resonator length L, that is, Lg / L, and the vertical axis indicates the oscillation threshold value (mA) of the distributed feedback semiconductor laser. The element temperature is 95 ° C.

  When Lg / L is large, the oscillation threshold is increased due to an increase in the absorption loss of the diffraction grating 10, and the oscillation characteristics are deteriorated. On the other hand, when Lg / L is small, the oscillation threshold is increased due to a decrease in the differential gain of the DFB, so that the oscillation characteristics deteriorate.

  FIG. 7 shows the relationship between the ratio of the length Lg of the diffraction grating formation region to the resonator length L and the series resistance value of the distributed feedback semiconductor laser according to this example. The horizontal axis indicates the ratio of the length Lg of the diffraction grating formation region to the resonator length L, that is, Lg / L, and the vertical axis indicates the series resistance value of the distributed feedback semiconductor laser. The element temperature is 95 ° C., and the series resistance value is a current value of 60 mA.

  If the ratio of the diffraction grating formation region to the resonator length L is large, the hetero interface region between the guide layer 505 and the cladding layer 8 constituting the diffraction grating 10 increases, and the series resistance R increases due to the increase in resistance due to this hetero interface. When the series resistance R is large, the amount of heat generated by the resistance increases, so that the laser characteristics (oscillation efficiency, high speed characteristics) in a high temperature region deteriorate.

<Effect of distributed feedback semiconductor laser according to this embodiment>
Next, the effect of the distributed feedback semiconductor laser according to the present embodiment will be described with reference to the drawings.
FIG. 8 is a diagram for explaining the effect of the distributed feedback semiconductor laser according to the present embodiment, and is a diagram showing the relationship between the current flowing through the distributed feedback semiconductor laser and the light emission efficiency of the distributed feedback semiconductor laser. The number of multiple quantum well layers (reference numeral 4 shown in FIG. 2) is 8, the resonator length L is 200 μm, the reflectance of the low reflectance film disposed on the low reflectance end face 503 side is 1%, and the high reflectance end face The product of the reflectivity of the high reflectivity film disposed on the 502 side is 80%, the diffraction grating region is 67% of the resonator length L, 134 μm, the coupling coefficient κ of the diffraction grating 10 and the length Lg of the diffraction grating formation region. 1.2 shows a case where the element temperature is 95 ° C. The vertical axis represents the luminous efficiency (W / A), and the larger this value, the better the light output characteristics of the distributed feedback semiconductor laser. A current value at which the luminous efficiency increases rapidly from zero is a so-called threshold current.

  FIG. 9 is a graph showing the relationship between the current flowing through the distributed feedback semiconductor laser and the light emission efficiency of the distributed feedback semiconductor laser in the distributed feedback semiconductor laser of the comparative example. The structure manufactured in the comparative example is different only in the structure of the diffraction grating 10 shown in FIG. 2, and the other structure is the same as that of the distributed feedback semiconductor laser according to the present embodiment shown in FIG. Specifically, in the structure manufactured in the comparative example, the diffraction grating 10 is different from the p-type InGaAsP guide layer 505 shown in FIGS. 12 is a shape in which peaks and valleys such as the diffraction grating 501 shown in FIG.

  The threshold current of the distributed feedback semiconductor laser according to this example was 15.7 mA as shown in FIG. On the other hand, the threshold current of the distributed feedback semiconductor laser according to the comparative example was 21.1 mA as shown in FIG. Thus, the threshold current of the distributed feedback semiconductor laser according to this example is significantly smaller. This is because in this embodiment, there is no absorption loss due to the region where the guide layer 505 in which the valleys and valleys are not formed remains. It can also be seen that the luminous efficiency of the distributed feedback semiconductor laser according to this example is larger than that of the comparative example.

  Further, as the current value is increased, the light emission efficiency eventually becomes zero, that is, the current value at which the laser light output does not increase even if the current is increased further is about 92 mA in the comparative example shown in FIG. On the other hand, in this embodiment shown in FIG. 8, it was much larger than 100 mA. Thus, it can be seen from the current-luminous efficiency characteristics that the distributed feedback semiconductor laser according to this example has achieved a significant improvement in the temperature characteristics of the semiconductor laser.

  FIG. 10 is a diagram for explaining the effect of the distributed feedback semiconductor laser according to the present embodiment, showing the relationship between the current flowing through the distributed feedback semiconductor laser and the relaxation oscillation frequency of the distributed feedback semiconductor laser. 2 shows current-relaxation vibration frequency characteristics under the same conditions as in FIG. 2, that is, at an element temperature of 95 ° C. FIG. FIG. 11 is a diagram showing the relationship between the current flowing through the semiconductor laser and the relaxation oscillation frequency of the semiconductor laser in the distributed feedback semiconductor laser of the comparative example.

As it is clear from FIG. 10 and FIG. 11, in this embodiment, when the current value 60 mA, the relaxation oscillation frequency _{f r} was 11.1GHz. In the comparative example, when the current value 60 mA, the relaxation oscillation frequency f _r is 9.0GHz, it can be seen that the relaxation oscillation frequency f _r of this embodiment is made much larger. This is because in this embodiment, there is no deterioration of the optical confinement coefficient and increase of absorption loss due to the region where the guide layer 505 in which no valley is formed is left.

  2 and 3, the diffraction grating 10 is formed on the upper side of the multiple quantum well layer 4; however, it is formed on the lower side of the multiple quantum well layer 4, that is, on the so-called substrate 1 side. The effect is also the same. In this embodiment, an n-type InP substrate is used as the substrate 1, but a p-type InP substrate can also be used.

  In this embodiment, the ridge waveguide type is used as the light confinement structure in the lateral direction, but a buried hetero structure type can also be used. Furthermore, in this embodiment, the InGaAlAs system is used as the material of the multiple quantum well layer 4, but an InGaAsP system can also be used.

  Since the distributed feedback semiconductor laser of the present invention is used for a communication semiconductor laser, it can be applied to the information communication industry.

1: n-type InP substrate 2: n-type InP cladding layer 3: n-type InGaAlAs SCH layer 4: InGaAlAs-based multiple quantum well layer 5: p-type InGaAlAs SCH layer 6: p-type InAlAs carrier stop layer 7 : P-type InGaAsP etching stop layer 8: p-type InP cladding layer 9: p-type InGaAs contact layer 10: diffraction grating 11: n-type electrode 12: p-type electrode 501: diffraction grating 502: high reflectivity end face 503: Low reflectance end face 504: active layer 505: guide layer 506: n-type electrode 507: p-type electrode 508: cladding layer

Claims (1)

An active layer,
A guide layer disposed below or above the active layer;
A low reflection film formed on one end surface;
A highly reflective film formed on the other end face;
A distributed feedback semiconductor laser for chromatic and a diffraction grating formed in said one end face of
The length between the both end faces is 170 μm or more and 280 μm or less,
The diffraction grating is formed by removing all of the guide layer at a constant period and embedding the removed portion of the guide layer in a cladding layer,
The length of the region forming the diffraction grating is 60% of the length between the both end faces,
On the other end face side, all of the guide layer in a region where the diffraction grating is not formed is removed ,
Relaxation oscillation frequency _{f r} of the DFB semiconductor laser is equal to or greater than 10 GHz,
A distributed feedback semiconductor laser characterized by that.

Images