JP3911002B2 - Semiconductor laser element - Google Patents

Description

The present invention includes a wavelength selection structure capable of selecting the oscillation wavelength λ _e independently of the optical gain distribution of the active layer in the vicinity of the active layer in the resonator structure, and emits laser light having the selected oscillation wavelength λ _e. More specifically, the semiconductor laser element is capable of stable single mode oscillation over a wide temperature range and has a large oscillation wavelength mode / submode suppression ratio (SMSR), and is particularly suitable as a light source for optical communication. The present invention relates to a semiconductor laser element.

A distributed feedback semiconductor laser (hereinafter referred to as a DFB laser) has a diffraction grating in which a real part or an imaginary part of a refractive index (complex refractive index) periodically changes, and emits light of a specific wavelength. By providing feedback only, the laser has wavelength selectivity. In a DFB laser having a diffraction grating composed of a compound semiconductor layer whose refractive index is periodically different from that of the surrounding, in the vicinity of the active layer, the oscillation wavelength λ _DFB of the DFB laser is such that the diffraction grating period Λ and the effective refractive index n of the waveguide _Since λ _DFB = 2n _eff Λ is determined based on _eff , the peak wavelength of the optical gain of the active layer can be determined by adjusting the period Λ of the diffraction grating and the effective refractive index n _{eff of the} waveguide. The oscillation wavelength λ _DFB can be set independently.

  For example, if the oscillation wavelength of the DFB laser is set to a wavelength shorter than the peak wavelength of the optical gain distribution of the active layer, the differential gain increases, so that the high-speed modulation characteristics of the DFB laser are improved. Further, when the oscillation wavelength of the DFB laser is set to about the peak wavelength of the optical gain distribution of the active layer, the threshold current at room temperature becomes small. In addition, when the oscillation wavelength of the DFB laser is set longer than the peak wavelength of the optical gain distribution of the active layer, the temperature characteristics are improved, the operating characteristics at high temperature, and the high light output characteristics at high temperature or large current injection. Will improve.

  By the way, in the conventional DFB laser, whether the oscillation wavelength is shorter than the peak wavelength of the optical gain distribution of the active layer or longer, (1) the threshold current can be kept low. (2) The oscillation wavelength is set to a near wavelength range within several tens of nanometers from the peak wavelength of the optical gain distribution of the active layer for the reason of maintaining the single mode operation. In the conventional DFB laser, the compound semiconductor layer constituting the diffraction grating has a band gap energy much larger than the band gap energy of the active layer and the energy of the oscillation wavelength. That is, the band gap wavelength of the compound semiconductor layer constituting the diffraction grating is usually on the short wavelength side of 100 nm or more from the oscillation wavelength, and the compound semiconductor layer is transparent to the oscillation wavelength and has almost no light absorption. It is a layer without loss. And the diffraction grating which shows the periodic change of a refractive index is produced by laminating | stacking the said compound semiconductor layer, and etching and forming the row | line | column of the layer which exists periodically in parallel.

Here, the conventional DFB laser will be described more specifically. As shown in FIG. 8A, the conventional DFB laser has a wavelength of λ _e of 1550 nm and a wavelength of λ _g of 1200 nm to 1300 nm. As shown in FIG. 8B, the first conventional example in which λ _g <λ _max <λ _e and λ _e is 1550 nm, λ _g is 1650 nm, and λ _max <λ _e <λ _g It can be roughly divided into a second conventional example. In the first conventional example, λ _e −λ _g = 300 nm, while in the second conventional example, λ _e −λ _g = −100 nm. Here, the solid curve in FIGS. 8A and 8B shows the optical gain distribution of the active layer with respect to the wavelength on the horizontal axis, and the broken curve shows the absorption (loss) amount of the diffraction grating layer with respect to the wavelength on the horizontal axis. It is a curve shown. Λ _e is the oscillation wavelength of the DFB laser determined by the period of the diffraction grating and the effective refractive index of the waveguide, λ _g is the band gap wavelength of the diffraction grating layer, λ _max is the peak wavelength of the optical gain distribution of the active layer, and the diffraction grating The band gap wavelength of the buried layer, usually the InP layer, is λ _InP (= 920 nm).

  However, in the conventional DFB laser, when the period of the diffraction grating is adjusted and the oscillation wavelength of the DFB laser is set apart from the peak wavelength of the optical gain distribution of the active layer, not the set oscillation wavelength of the DFB laser but the active layer A Fabry-Perot oscillation may occur at the peak wavelength of the optical gain distribution. In addition, even when oscillating at the set oscillation wavelength of the DFB laser, the submode suppression ratio (SMSR) between the oscillation mode of the set oscillation wavelength of the DFB laser and the mode near the peak wavelength of the optical gain distribution of the active layer is sufficiently high. There was a problem that it could not be taken big. For example, in a conventional DFB laser, the sub-mode suppression ratio (SMSR) is in the range of 35 dB to 40 dB, which is relatively small, although it varies depending on the amount of detuning of the oscillation wavelength of the DFB laser. As a result, the conventional DFB laser has a problem that the detuning amount of the oscillation wavelength of the DFB laser cannot be increased with respect to the peak wavelength of the optical gain distribution of the active layer.

With further explained, a large first conventional example than the bandgap wavelength lambda _g of the oscillation wavelength lambda _e in the DFB laser grating layer, the absorption loss of the oscillation wavelength lambda _e is small and hence the threshold current is small, the light output - injection Although it has the advantage of good current characteristics, the difference in refractive index between the diffraction grating layer and the InP buried layer is small, so the distance between the diffraction grating and the active layer needs to be reduced. As a result, the coupling coefficient varies greatly depending on the thickness of the diffraction grating layer and the duty ratio, and it is difficult to stably manufacture a DFB laser having the same characteristics. Further, if the absorption coefficient for the oscillation wavelength λ _e of the DFB laser is α _e , and the band gap wavelength of the active layer, that is, the absorption coefficient for the peak wavelength λ _max of the optical gain distribution of the active layer is α _max , α _e ≈α _max ≈ The suppression effect is small for both the Fabry-Perot oscillation mode near the peak wavelength of the optical gain distribution of the active layer and the oscillation mode of the DFB laser. Therefore, if the absolute value of the detuning amount | λ _e −λ _max | is increased, the single mode property of the longitudinal mode is deteriorated, so that the absolute value of the detuning amount | λ _e −λ _max | cannot be increased. there were.

The second conventional example in which the band gap wavelength λ _g of the diffraction grating layer is larger than the oscillation wavelength λ _{e of the} DFB laser has a large refractive index difference between the diffraction grating layer and the InP buried layer. The distance can be increased, and as a result, the coupling coefficient is difficult to vary depending on the film thickness and duty ratio of the diffraction grating layer, so that a DFB laser with the same characteristics can be stably manufactured. Has the advantage of high. On the other hand, a large absorption loss for the oscillation wavelength lambda _e, in order to be absorbed type diffraction grating, the threshold current is large, the light output - has a problem that injection current characteristics are not good. In addition, α _e ≈α _max > 0 and both Fabry-Perot oscillation mode and DFB laser oscillation mode have a suppression effect, but if the absolute value of the detuning amount | λ _e −λ _max | As a result, the absolute value of the detuning amount | λ _e −λ _max | cannot be increased.

In the above description, the problem related to the peak wavelength and oscillation wavelength of the optical gain distribution of the active layer has been described by taking the DFB laser as an example. However, this problem is not limited to the DFB laser, and the optical gain distribution of the active layer Has a wavelength selection structure that can independently select the oscillation wavelength λ _e in the vicinity of the active layer in the resonator structure, and is a problem that is universally associated with semiconductor laser elements that emit laser light of the selected oscillation wavelength λ _e. is there.

Therefore, the object of the present invention is to solve the above-mentioned problems. First, the absorption loss of the oscillation wavelength of the DFB laser is small, and the absorption loss of the peak wavelength of the optical gain distribution of the active layer is large. Accordingly, a semiconductor that has a small threshold current, good optical output-injection current characteristics, and can maintain good single mode characteristics in the longitudinal mode even when the absolute value of the detuning amount | λ _e −λ _max | is increased. The second object is to provide a semiconductor laser element having a structure in which the coupling coefficient hardly changes depending on the film thickness and the duty ratio of the diffraction grating layer of the DFB laser, and thus the product yield is high. That is.

In order to achieve the above object, a semiconductor laser device according to the present invention includes a wavelength selection structure capable of selecting an oscillation wavelength λ _e independently of the optical gain distribution of the active layer, in the vicinity of the active layer in the resonator structure, In a semiconductor laser device that emits laser light having a selected oscillation wavelength λ _e , a compound semiconductor in which the absorption coefficient α _max for the peak wavelength λ _max of the optical gain distribution of the active layer is larger than the absorption coefficient α _e for the oscillation wavelength λ _e An absorption region formed of a layer is provided in the vicinity of the active layer, and the absorption region selectively absorbs a mode in the vicinity of _λmax .

According to the present invention, the absorption region formed of the compound semiconductor layer in which the absorption coefficient α _max with respect to the peak wavelength λ _max of the optical gain distribution of the active layer is larger than the absorption coefficient α _{e with} respect to the oscillation wavelength λ _e is By providing in the vicinity, for example, in a DFB laser element, oscillation of the Fabry-Perot mode near the peak wavelength λ _max of the optical gain distribution of the active layer can be suppressed, and the mode of the set oscillation wavelength and the suppression of the submode are suppressed. The ratio (SMSR) can be increased. In addition, since a large amount of detuning can be taken, stable single mode oscillation can be maintained over a wide temperature range.

In the present invention, a wavelength selection structure capable of selecting the oscillation wavelength λ _e independently of the optical gain distribution of the active layer is provided in the vicinity of the active layer in the resonator structure, and a laser beam having the selected oscillation wavelength λ _e is emitted. The semiconductor laser element means, for example, a distributed feedback (DFB) semiconductor laser element, a distributed reflection (DBR) semiconductor laser element, a semiconductor laser module with fiber Bragg grading (FBG), or the like. Further, the vicinity of the active layer means that the light generated in the active layer is within a range that can be sensed.

In the present invention, an absorption region is provided such that the absorption coefficient with respect to the peak wavelength of the optical gain distribution of the active layer is larger than the absorption coefficient with respect to the oscillation wavelength of the semiconductor laser element, and a mode near the peak wavelength of the optical gain distribution of the active layer is provided. Only selectively absorbs and suppresses Fabry-Perot oscillation near the peak wavelength of the optical gain distribution of the active layer. Thereby, the wavelength selectivity of the oscillation wavelength can be improved, and the submode suppression ratio (SMSR) can be increased to improve the single mode property. Therefore, the product yield is improved. In other words, since Fabry-Perot oscillation near the peak wavelength of the optical gain distribution of the active layer is selectively suppressed, even if the detuning amount (λ _e −λ _max ) is increased, a good single mode can be obtained. Modality can be maintained. In addition, since the single mode can be maintained even at a high operating temperature, high output characteristics at high temperatures are good.

In the present invention, in the absorption region, the difference between the absorption coefficient α _max with respect to the peak wavelength λ _max of the optical gain distribution of the active layer and the absorption coefficient α _e with respect to the oscillation wavelength λ _e , α _max −α _e is larger. Although the effect of the present invention is preferable, practically, the effect of the present invention can be obtained if α _max −α _e ≧ 1 cm ⁻¹ in terms of waveguide loss. If α _max −α _e ≧ 5 cm ⁻¹ , a more remarkable effect can be obtained. In the absorption region, it is preferable that α _e is substantially 0, that is, the absorption region is transparent to the oscillation wavelength λ _e . Thereby, even if an absorption region is provided, the waveguide loss at the oscillation wavelength does not increase, and the threshold current value and the light emission efficiency do not deteriorate.

Furthermore, in the present invention, by providing an absorption region having a steep absorption edge due to the quantum effect, for example, by providing a quantum well layer or a quantum dot layer having a steep absorption edge as a selective absorption region, the active layer A large difference can be realized between the absorption coefficient α _max for the gain peak wavelength and the absorption coefficient α _e for the oscillation wavelength. Here, the term “quantized” means that the size of the compound semiconductor layer that constitutes the absorption region is as thin as the quantum mechanical wavelength of electrons and can exhibit a quantum effect.

  Furthermore, in the present invention, the wavelength selection structure may be configured as a diffraction grating, or a selective absorption layer functioning as an absorption region may be formed separately from the diffraction grating in the vicinity of the active layer. There is no problem even if the selective absorption layer is disposed on the opposite side of the diffraction grating with the active layer interposed therebetween or on the same side as the diffraction grating. However, the arrangement on the opposite side increases the degree of freedom of design because the distance to the active layer can be arbitrarily selected.

In the present invention, the absorption region is formed by a quantized compound semiconductor layer, and the peak wavelength λ _max of the optical gain distribution of the active layer is shown in FIG. 10 with respect to the oscillation wavelength λ _e of the DFB laser. Thus, by setting the relationship of λ _e <λ _max , the differential gain in the high frequency region is large, and the high-speed modulation characteristics can be improved.

Hereinafter, embodiments of the present invention will be described specifically and in detail with reference to the accompanying drawings.
Embodiment 1
The present embodiment is an example of an embodiment in which the semiconductor laser devices according to the first and second inventions are combined and applied to a DFB laser device. FIG. 1 shows the structure of the semiconductor laser device of the present embodiment. FIG. 2 is a sectional view of the semiconductor laser device taken along the line II of FIG. In addition, the composition, film thickness, and the like of the compound semiconductor layer shown in Embodiment Example 1 and Embodiment Example 2 described later are examples for understanding the present invention, and the present invention is not limited to these examples. . The semiconductor laser device 10 of the present embodiment is configured as a buried hetero DFB laser device with an oscillation wavelength set to 1550 nm.

The DFB laser device 10 includes an n-InP buffer layer 14 having a thickness of 1 μm, an MQW-SCH active layer 16 and a p-InP having a thickness of 200 nm, which are sequentially epitaxially grown on an n-InP substrate 12 by MOCVD or the like. The spacer layer 18 has a laminated structure of a diffraction grating 20 made of a GaInAsP layer having a period Λ of 240 nm and a film thickness of 20 nm, and a p-InP first cladding layer 22 in which the diffraction grating 20 is embedded. The peak wavelength λ _g of the optical gain distribution of the active layer 16 is about 1530 nm, and the band gap wavelength λ _g of the diffraction grating 20 is about 1510 nm.

  The upper portion of the n-InP substrate 12 and the n-InP buffer layer 14, the active layer 16, the p-InP spacer layer 18, the diffraction grating 20, and the p-InP first cladding layer in which the diffraction grating 20 is embedded are formed. 22 is etched into a mesa stripe having a width of the active layer 16 of 1.5 μm. A carrier block structure including a p-InP layer 24 and an n-InP layer 26 is formed on both sides of the mesa stripe. Further, the DFB laser device 10 includes a p-InP second cladding layer 28 having a film thickness of 2 μm and a p-GaInAs contact layer doped at a high concentration on the p-InP first cladding layer 22 and the n-InP layer 26. 30, a p-side electrode 32 made of a laminated metal film of Ti / Pt / Au on the contact layer 30, and an n-side electrode 34 made of AuGeNi on the back surface of the substrate 12.

In this embodiment, the diffraction grating 20 is formed of GaInAsP having a band gap wavelength λ _g of about 1510 nm, and the diffraction grating 20 has a peak wavelength of the optical gain distribution of the active layer 16 due to the influence of the band edge. Although some absorption occurs near 1530 nm, almost no absorption occurs at wavelengths near 1550 nm set as the oscillation wavelength. That is, the absorption coefficient α _max with respect to the peak wavelength of the optical gain distribution of the active layer is larger than the absorption coefficient α _{e with} respect to the oscillation wavelength. Note that by setting the band gap wavelength λ _g of the diffraction grating 20 between the peak wavelength 1530 nm of the optical gain distribution of the active layer and the oscillation wavelength 1550 nm of the DFB laser element 10, it is possible to realize a large difference in absorption coefficient. it can.

  In order to evaluate the DFB laser device 10 of the present embodiment, a wafer having the above-described laminated structure was chipped by cleavage and bonded to a can package type stem. When the front end face was coated with a non-reflective film and the rear end face was coated with a highly reflective film, the laser characteristics were measured, and the following measurement results were obtained. The DFB laser device 10 oscillated stably in a good single mode, and a large value of 45 dB to 50 dB was obtained as the submode suppression ratio. Incidentally, in a conventional type DFB laser element (hereinafter referred to as a conventional DFB laser element) in which the band gap wavelength of the diffraction grating is large, such a large sub-mode suppression ratio is difficult to obtain, and is approximately 35 dB to 40 dB.

Further, the threshold current was as low as 9 mA, and the light emission efficiency was not inferior to that of the conventional DFB laser device. Accordingly, it is considered that the absorption of the diffraction grating 20 with respect to the oscillation wavelength is sufficiently small. The diffraction grating 20 selectively absorbs only the mode near the peak wavelength of the optical gain distribution of the active layer 16, and the Fabry-Perot oscillation near the gain peak wavelength is suppressed. It is thought that it was improved. Furthermore, the variation of the coupling coefficient from element to element was small, and characteristics with good uniformity were obtained. In the present embodiment example, the band gap wavelength λ _g (1510 nm) of the diffraction grating 20 is set to be larger than the band gap wavelength λ _g of the diffraction grating of the conventional DFB laser element, for example, 1200 nm. Since the difference in refractive index between the surrounding InP layers becomes large, a sufficiently large diffraction grating coupling coefficient can be obtained even if the thickness of the p-InP spacer layer 18 is increased and the diffraction grating 20 is separated from the active layer 16. Therefore, the coupling coefficient is unlikely to vary depending on the thickness of the diffraction grating and the duty ratio. Accordingly, tolerance in the crystal growth process and manufacturing process is reduced, and stable manufacturing can be performed.

  A method for manufacturing the DFB laser device 10 of the first embodiment will be described below with reference to FIGS. FIGS. 3A to 3C and FIGS. 4D and 4E are cross-sectional views for each process in manufacturing the DFB laser device 10 of the first embodiment. 3 is a cross section taken along the arrow II in FIG. 1, and FIG. 4 is a cross section taken along the arrow II-II in FIG. First, an n-InP buffer layer 14 having a thickness of 1 μm, an MQW-SCH active layer 16 and a p-InP having a thickness of 200 nm are sequentially formed on an n-InP substrate 12 at a growth temperature of 600 ° C. using an MOCVD crystal growth apparatus. The spacer layer 18 and the GaInAsP diffraction grating layer 20 ′ having a thickness of 20 nm are epitaxially grown to form a stacked structure as shown in FIG.

  Next, an electron beam (EB) drawing resist is applied on the diffraction grating layer 20 ′ with a thickness of about 100 nm, and drawing is performed by an EB drawing apparatus to form a diffraction grating pattern 21 having a period Λ of 240 nm. Subsequently, etching is performed by a dry etching apparatus using the diffraction grating pattern 21 as a mask to form a groove 23 penetrating the diffraction grating layer 20 ′, and the p-InP spacer layer 18 is exposed at the groove bottom, and FIG. ), The diffraction grating 20 is formed. Next, the diffraction grating pattern 21 is removed, and then the p-InP first cladding layer 22 in which the diffraction grating 20 is embedded is regrown with an MOCVD crystal growth apparatus as shown in FIG.

Next, a SiN _X film is formed on the p-InP first cladding layer 22 using a plasma CVD apparatus, and then the SiN _X film is processed into a stripe shape by photolithography and a reactive ion etching apparatus (RIE). Then, the SiN _X film mask 25 is formed. Next, using the SiN _X film mask 25 as an etching mask, the p-InP first cladding layer 22 (diffraction grating 20), the p-InP spacer layer 18, the active layer 16, the n-InP buffer layer 14, and the n-InP substrate 12 are formed. The upper part is etched and processed into a mesa stripe having an active layer width of about 1.5 μm. Further, the p-InP layer 24 and the n-InP layer 26 are sequentially grown using the SiN _X film mask 25 as a selective growth mask, and carriers are formed on both sides of the mesa stripe as shown in FIG. A block structure is formed.

Next, after removing the SiN _X film mask 25, as shown in FIG. 4 (e), a high concentration is formed in order to make ohmic contact with the p-InP second cladding layer 28 having a thickness of 2 μm and the p-side electrode 32. The GaInAs layer doped in is epitaxially grown as the contact layer 30. The back surface of the n-InP substrate 12 is polished so that the substrate thickness becomes about 120 μm, and then a Ti / Pt / Au laminated metal film is formed on the contact layer 30 as the p-side electrode 32, and the AuGeNi film is formed on the back surface of the substrate. Is formed as the n-side electrode 34. By cutting the wafer having the above laminated structure into a chip by cleavage and bonding it to a can package type stem, the DFB laser device 10 having the laminated structure shown in FIG. 1 can be formed.

  In the present embodiment, the difference in refractive index between the diffraction grating and the surrounding InP layer is increased by bringing the band gap wavelength of the diffraction grating 20 close to the peak wavelength of the optical gain distribution of the active layer 16. Even if the diffraction grating 20 is separated from the active layer 16 as compared with the conventional DFB laser element, a desired refractive index coupling coefficient can be obtained. Accordingly, tolerance in the crystal growth process and manufacturing process is reduced, and stable manufacturing can be performed.

Embodiment 2
This embodiment is another example of the embodiment in which the semiconductor laser elements according to the first and second inventions are combined and applied to the DFB laser element. FIG. 5 shows the semiconductor laser element according to this embodiment. FIG. 6 is a partial sectional perspective view showing the configuration of FIG. 6 and FIG. 6 is a sectional view taken along the arrow III-III in FIG. In the DFB laser element 10 in the embodiment 1, independently selectively diffraction grating 20 formed as the wavelength selection structure which can select an oscillation wavelength lambda _e is the light of the gain peak wavelength mode and the optical gain distribution of the active layer It functions as an absorbing layer. On the other hand, the semiconductor laser device 40 of the present embodiment is similarly configured as a buried hetero DFB laser device with an oscillation wavelength set to 1550 nm. However, the light of the peak wavelength mode of the optical gain distribution of the active layer is used. A selective absorption layer for selectively absorbing is provided separately from the diffraction grating.

The DFB laser element 40 includes a selective absorption layer 45A composed of an n-InP buffer layer 44 having a thickness of 1 μm and an InGaAs layer having a thickness of 5 nm, which are sequentially epitaxially grown on an n-InP substrate 42 by MOCVD or the like. An n-InP spacer layer 45B having a thickness of 100 nm, an MQW-SCH active layer 46, a p-InP spacer layer 48 having a thickness of 100 nm, a diffraction grating 50 having a period Λ of 240 nm and a GaInAsP layer having a thickness of 30 nm, and A stacked structure of the p-InP first cladding layer 52 in which the diffraction grating 50 is embedded is provided. The selective absorption layer 45A has a band gap wavelength λ _g of 1540 nm, the active layer 46 has a band gap wavelength λ _g of 1530 nm, and the diffraction grating 50 has a band gap wavelength λ _g of about 1200 nm. Therefore, the diffraction grating 50 is sufficiently transparent to the peak wavelength of about 1530 nm of the optical gain distribution of the active layer 46 and to the wavelength 1550 nm set as the oscillation wavelength of the DFB laser element 40.

  The film thickness of the selective absorption layer 45A is 5 nm, which is sufficiently thin to exhibit the quantum effect, and the film thickness is adjusted so that the absorption edge wavelength (corresponding to the band gap wavelength) is about 1540 nm. It functions as a quantum well layer. Accordingly, the absorption coefficient of the selective absorption layer 45A is somewhat absorbed near 1530 nm of the peak wavelength of the optical gain distribution of the active layer 46, but almost no absorption occurs at the wavelength near 1550 nm set as the oscillation wavelength. Absent. By providing the selective absorption layer 45A having a steep absorption edge due to the quantum effect, absorption having a large difference between the absorption coefficient with respect to the peak wavelength of the optical gain distribution of the active layer 46 and the absorption coefficient with respect to the oscillation wavelength (almost zero). An area can be realized. In this embodiment, the single quantum well layer 45A is used as the selective absorption region. However, it is possible to realize a larger difference in absorption coefficient by forming a plurality of layers. Further, as in the first embodiment, the diffraction grating = selective absorption region = quantum well structure may be used.

  In the semiconductor laser device 40 according to the present embodiment, the selective absorption layer 45A is provided with a quantum well layer or a quantum dot layer having a steep absorption edge as a selective absorption region, whereby absorption at a gain peak wavelength and an oscillation wavelength is achieved. The difference in coefficients can be increased. In this embodiment, the selective absorption layer 45A is arranged on the side opposite to the diffraction grating with the active layer interposed therebetween, but there is no problem even on the same side as the diffraction grating. However, the degree of freedom of design is greater when it is arranged on the opposite side.

  An n-InP buffer layer 44, a selective absorption layer 45A, an n-InP spacer layer 45B, an active layer 46, a p-InP spacer layer 48, a diffraction grating 50, and an upper portion of the n-InP substrate 42, and a stacked structure. The p-InP first cladding layer 52 in which the diffraction grating 50 is embedded is etched into a mesa stripe having an active layer 46 width of 1.5 μm. A carrier block structure including a p-InP layer 54 and an n-InP layer 56 is formed on both sides of the mesa stripe.

  Further, the semiconductor laser device 40 includes a p-InP second cladding layer 58 having a film thickness of 2 μm and a p-GaInAs contact layer doped with a high concentration on the p-InP first cladding layer 52 and the n-InP layer 56. 60, a p-side electrode 62 made of a laminated metal film of Ti / Pt / Au, and an n-side electrode 64 made of AuGeNi on the back surface of the substrate 42.

  In order to evaluate the DFB laser device 40 of this embodiment, the wafer having the above-described laminated structure is chipped by cleavage, bonded to a can package type stem, a non-reflective film on the front end surface, and a rear end surface Measured the laser characteristics after coating a high reflectance film, and the following measurement results were obtained. The DFB laser device 40 sustained stable oscillation in a good single mode at an oscillation wavelength of 1550 nm as intended. The submode suppression ratio was a very good value of about 50 dB, and the threshold current was 8 mA, which was the same value as the conventional DFB laser. Therefore, it is considered that the absorption with respect to the oscillation wavelength by the selective absorption layer 45A can be almost ignored. Further, the ratio between the peak and valley of the Fabry-Perot mode near the peak wavelength of the optical gain distribution of the active layer 46 is smaller than that of the conventional DFB laser, and the mode near this causes a loss due to the selective absorption layer 45A. In response, Fabry-Perot mode oscillation was suppressed.

  In addition, even in a DFB laser element having the same structure as that of the DFB laser element 40 and having a longer period of the diffraction grating 50 and an oscillation wavelength set to 1570 nm, stable oscillation of a single mode at the oscillation wavelength 1570 is observed. It was. As described above, even when the tuning wavelength is largely detuned from the peak wavelength (1530 nm) of the optical gain distribution of the active layer, the mode near the gain peak is suppressed by the selective absorption layer 45A. Mode oscillation can be sustained.

  Next, with reference to FIG. 7, a method for manufacturing the semiconductor laser device 40 of the second embodiment will be described. FIGS. 7A to 7C are cross-sectional views for each process in fabricating the semiconductor laser device 40 of the second embodiment. First, a selective absorption layer comprising an n-InP buffer layer 44 and an InGaAs layer sequentially on an n-InP substrate 42 as shown in FIG. 7A using a MOCVD crystal growth apparatus at a growth temperature of 600 ° C. A diffraction grating layer 50 'composed of 45A, an n-InP spacer layer 45B, an MQW-SCH active layer 46, a p-InP spacer layer 48, and an InGaAsP layer is epitaxially grown.

  An electron beam (EB) drawing resist is applied to the diffraction grating layer 50 ′ with a thickness of 100 nm, and drawing is performed by an EB drawing apparatus to form a diffraction grating pattern 51 having a period Λ of 240 nm. Subsequently, etching is performed by a dry etching apparatus using the diffraction grating pattern 51 as an etching mask to form a groove 53 penetrating the diffraction grating layer 50 ', exposing the p-InP spacer layer 48 at the groove bottom, and FIG. As shown in b), a diffraction grating 50 is formed. As shown in FIG. 7C, the p-InP first cladding layer 52 in which the diffraction grating 50 is embedded is regrown.

Thereafter, in the same manner as in Embodiment 1, using the SiN _X film mask as an etching mask, the p-InP first cladding layer 52, the diffraction grating 50, the p-InP spacer layer 48, the active layer 46, and the n-InP spacer layer 45B. Then, the upper portions of the selective absorption layer 45A, the n-InP buffer layer 44, and the n-InP substrate 42 are etched to form a mesa stripe having an active layer width of about 1.5 μm. Subsequently, using the SiN _X film mask as a selective growth mask, the p-InP layer 54 and the n-InP layer 56 are sequentially grown to form carrier block structures on both sides of the mesa stripe. Next, after removing the SiN _X film mask, a 2 μm-thick p-InP clad layer 58 and a highly doped GaInAs contact layer 60 are epitaxially grown.

  The back surface of the n-InP substrate 42 is polished so that the substrate thickness becomes about 120 μm, and then a Ti / Pt / Au laminated metal film is formed as a p-side electrode 62 on the contact layer 60, and an AuGeNi film is formed on the back surface of the substrate Is formed as the n-side electrode 64. The semiconductor laser device 40 having the laminated structure shown in FIGS. 5 and 6 can be formed by cleaving the wafer having the above laminated structure into chips by cleaving and bonding the wafer to a can package type stem.

The first and second embodiments have been described by taking the DFB laser element in the 1550 nm band as an example, but the semiconductor laser element according to the present invention can be similarly applied to other wavelength bands. Further, in the embodiment example, the selective absorption layer 45A or the diffraction grating 20 is arranged in the entire resonator, but the same effect as in the present embodiment example can be obtained even if it is arranged in a part of the resonator. Furthermore, not only the DFB laser element but also a DBR laser element, an FBG laser element, etc., a wavelength selection structure capable of selecting the oscillation wavelength λ _e independently of the optical gain distribution of the active layer is provided in the vicinity of the active layer in the resonator structure. provided, it can be applied to a semiconductor laser device emitting a laser beam having a selected oscillation wavelength lambda _e.

2 is a partial cross-sectional perspective view showing the structure of the semiconductor laser device of Embodiment 1; FIG. It is sectional drawing of the semiconductor laser element of arrow II of FIG. 3A to 3C are cross-sectional views for each process in manufacturing the semiconductor laser device of the first embodiment. 4D and 4E are cross-sectional views for each process in manufacturing the semiconductor laser device of the first embodiment, following FIG. 3C. FIG. 6 is a partial cross-sectional perspective view showing the structure of a semiconductor laser device according to Embodiment 2. It is sectional drawing of the semiconductor laser element of arrow III-III of FIG. FIGS. 7A to 7C are cross-sectional views for each process in manufacturing the semiconductor laser device of the second embodiment. FIGS. 8A and 8B are schematic diagrams showing the relationship between λ _e , λ _max , and λ _g in the first conventional example and the second conventional example, respectively. FIGS. 9A and 9B are schematic diagrams showing the relationship between λ _e , λ _max , and λ _g of the second invention, respectively. It is a schematic diagram explaining the semiconductor laser element of (lambda) _max > (lambda) _e of the embodiment of the 1st and 2nd invention.

Explanation of symbols

10 DFB laser element 12 of Embodiment 1 n-InP substrate 14 n-InP buffer layer 16 MQW-SCH active layer 18 p-InP spacer layer 20 diffraction grating 20′GaInAs layer 21 diffraction grating pattern 22 p embedded with diffraction grating -InP first clad layer 23 Groove 24 p-InP layer 26 n-InP layer 28 p-InP second clad layer 30 p-GaInAs contact layer 32 p-side electrode 34 n-side electrode 40 DFB laser element 42 of Embodiment 2 n-InP substrate 44 n-InP buffer layer 45A Selective absorption layer 45B n-InP spacer layer 46 MQW-SCH active layer 48 p-InP spacer layer 50 'GaInAs layer 50 Diffraction grating 51 Diffraction grating pattern 52 Diffraction grating embedded p-InP first cladding layer 53 groove 54 p-InP layer 56 n-I P layer 58 p-InP second cladding layer 60 p-GaInAs contact layer 62 p-side electrode 64 n-side electrode

Claims (6)

In a semiconductor laser device comprising a wavelength selection structure capable of selecting an oscillation wavelength λ _e independently of the optical gain distribution of the active layer, in the vicinity of the active layer in the resonator structure,
An absorption region formed of a compound semiconductor layer in which the absorption coefficient α _max with respect to the peak wavelength λ _max of the optical gain distribution of the active layer is larger than the absorption coefficient α _{e with} respect to the oscillation wavelength λ _e is provided in the resonator structure; and A semiconductor laser device, wherein an absorption region selectively absorbs a mode in the vicinity of _λmax . 2. The semiconductor laser device according to claim 1, wherein α _max −α _e ≧ 1 cm ⁻¹ in terms of waveguide loss in the absorption region.   3. The semiconductor laser device according to claim 1, wherein the absorption region is formed of a quantized compound semiconductor layer. The absorption region is formed by a quantized compound semiconductor layer, and the peak wavelength λ _max of the optical gain distribution of the active layer has a relationship of λ _e <λ _max with respect to the oscillation wavelength λ _e The semiconductor laser device according to claim 1.   5. The semiconductor laser device according to claim 1, wherein the wavelength selection structure is configured as a diffraction grating, and the diffraction grating functions as an absorption region. 6.   5. The selective absorption layer having a wavelength selection structure configured as a diffraction grating and functioning as an absorption region is formed separately from the diffraction grating in the vicinity of the active layer. The semiconductor laser device according to any one of the above.

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