Semiconductor laser element and light emitting device

Abstract

To provide a DFB laser capable of stable optical output against temperature changes.SOLUTION: A distributed feedback semiconductor laser element has a lamination structure comprising a GaN substrate, a first conductivity type semiconductor layer, an active layer, and a second conductivity type semiconductor layer. A diffraction grating 150 is adjacent to the current constriction structure. The diffraction grating is configured such that the oscillation wavelength λDFB of the semiconductor laser element is longer than the peak wavelength λGAINPEAK of the gain spectrum at room temperature.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a semiconductor laser device. [Background technology]

[0002] Semiconductor lasers made of gallium nitride (GaN)-based materials are widely used. Distributed feedback semiconductor laser diodes (DFB-LDs) are also known, in which a diffraction grating is formed on the side of a mesa structure (also called a ridge portion) for the purpose of controlling the oscillation mode of nitride-based semiconductor lasers (see Patent Documents 1 and 2, etc.).

[0003] In conventional DFB-LDs, the period of the diffraction grating is designed according to the purpose with respect to the peak wavelength (referred to as the gain peak wavelength) of the gain spectrum at room temperature (for example, 25° C.) (Patent Document 3, Patent Document 4). [Prior art documents] [Non-patent literature]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2023-039519 [Patent Document 2] Japanese Patent Application Laid-Open No. 2018-037495 [Patent Document 3] Patent No. 5043880 [Patent Document 4] Japanese Patent Application Publication No. 9-129970 Summary of the Invention [Problem to be solved by the invention]

[0005] However, as the temperature of a DFB-LD rises, the deviation of the DFB oscillation wavelength from the gain peak wavelength changes, and generally, the threshold current increases and the slope efficiency decreases, resulting in deterioration of characteristics. As a result, a temperature control mechanism is required to maintain a constant temperature of the DFB-LD, which leads to problems such as an increase in module size and power consumption. Alternatively, it has become necessary to design modules and systems that take into account the temperature change of the optical output characteristics (IL characteristics) of the DFB-LD.

[0006] The present disclosure has been made in view of the above-mentioned problems, and one exemplary purpose of an embodiment thereof is to provide a DFB laser capable of outputting a stable optical output against temperature changes. [Means for solving the problem]

[0007] One aspect of the present disclosure relates to a distributed feedback semiconductor laser device. The semiconductor laser device has a layered structure including a GaN substrate, a first conductivity type semiconductor layer, an active layer, and a second conductivity type semiconductor layer. The layered structure is provided with a current confinement structure extending in a first direction perpendicular to the stacking direction, and a diffraction grating formed along the current confinement structure. The diffraction grating is configured so that the oscillation wavelength of the semiconductor laser device is longer than the peak wavelength of the gain spectrum at room temperature.

[0008] Any combination of the above elements, or mutual substitution of elements or expressions between methods, devices, systems, etc., are also valid aspects of the present invention or the present disclosure. Furthermore, the description in this section (Means for Solving the Problems) does not explain all essential features of the present invention, and therefore, subcombinations of the described features may also constitute the present invention. [Effects of the Invention]

[0009] According to a semiconductor laser device according to an aspect of the present disclosure, the optical output can be stabilized against temperature changes. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a perspective view of a distributed feedback semiconductor laser device according to an embodiment; [Figure 2] 10 is a diagram showing the relationship between the gain spectrum and the oscillation wavelength λDFB at room temperature of the semiconductor laser element according to the embodiment. FIG. [Figure 3] FIG. 10 is a diagram showing the relationship between the gain spectrum and the oscillation wavelength λDFB at room temperature of a semiconductor laser device according to a comparative technique. [Figure 4A] 10 is a diagram illustrating the temperature dependence of the gain spectrum and oscillation wavelength of the semiconductor laser device according to Comparative Technique 1. FIG. [Figure 4B] 10 is a diagram illustrating the temperature dependence of the gain spectrum and oscillation wavelength of the semiconductor laser device according to Comparative Technique 3. FIG. [Figure 5] 5A and 5B are diagrams illustrating the temperature dependence of the semiconductor laser element according to the embodiment. [Figure 6A] FIG. 1 is a diagram illustrating the stop band of a DFB-LD having a uniform diffraction grating with no phase shift. [Figure 6B] FIG. 1 is a diagram illustrating the stop band of a DFB-LD having a diffraction grating with a phase shift. [Figure 7] FIG. 10 is a diagram showing the relationship between the detuning amount ΔG and the IL characteristics in a 410 nm band DFB-LD. [Figure 8A] FIG. 10 is a diagram showing the temperature dependence of the threshold current ITH in a 410 nm band DFB-LD with different detuning amounts ΔG. [Figure 8B] FIG. 10 is a diagram showing the temperature dependence of the slope efficiency in a 410 nm band DFB-LD with different detuning amounts ΔG. [Figure 9A] FIG. 9A is a graph showing the detuning amount ΔG dependency of the normalized threshold current ITH in a 410 nm band DFB-LD. [Figure 9B] FIG. 9B is a diagram showing the detuning amount ΔG dependency of the normalized slope efficiency SE in a 410 nm band DFB-LD. [Figure 10] 1A and 1B are diagrams illustrating a light emitting device according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] (Outline of the embodiment) A summary of some exemplary embodiments of the present disclosure is provided below. This summary is intended as a prelude to the more detailed description that follows, or to provide a basic understanding of the embodiments. This summary is intended to briefly explain some concepts of one or more embodiments and is not intended to limit the scope of the invention or disclosure. Furthermore, this summary is not an exhaustive overview of all possible embodiments, nor does it limit essential elements of the embodiments. For convenience, the term "one embodiment" may refer to one embodiment (example or variant) or multiple embodiments (examples or variants) disclosed herein.

[0012] (Outline of the embodiment) A distributed feedback (DFB) semiconductor laser device according to one embodiment has a layered structure including a GaN substrate, a first conductivity type semiconductor layer, an active layer, and a second conductivity type semiconductor layer. The layered structure is provided with a current confinement structure extending in a first direction perpendicular to the layering direction, and a diffraction grating formed along the current confinement structure. The diffraction grating is configured so that the oscillation wavelength of the semiconductor laser device is longer than the peak wavelength of the gain spectrum at room temperature.

[0013] The peak wavelength (gain peak wavelength) and oscillation wavelength of the gain spectrum of a semiconductor laser element shift to the longer wavelength side as the temperature rises. In a DFB-LD, the oscillation wavelength is determined by the pitch of the diffraction grating. In general, the temperature change of the gain peak wavelength is larger than the temperature change of the oscillation wavelength. Therefore, the oscillation wavelength λ at room temperature DFB and gain peak wavelength λ GAINPEAK The difference (λ DFB -λ GAINPEAK By optimizing the detuning amount, λ DFB This can reduce the change in gain in the laser diode, thereby reducing the temperature dependency of the optical output.

[0014] In this specification, the gain spectrum refers to the wavelength dependence of the gain obtained in a quantum well when a current is injected. This corresponds to the spectrum of the emitted light observed when the drive current of a semiconductor laser device is increased, which changes gradually with wavelength. In particular, when the threshold current is increased to I TH When the drive current I OP =0.9×I TH The gain spectrum at this temperature is sometimes referred to as the gain spectrum immediately before oscillation or the radiation spectrum immediately before oscillation. The room temperature can typically be selected to a predetermined value in the range of 25°C to 30°C, such as 25°C, 27°C, or 30°C. The laser oscillation caused by the periodicity of the diffraction grating characteristic of DFB-LD is called DFB oscillation, and the oscillation wavelength at this time is sometimes called the DFB oscillation wavelength, but is also sometimes simply called oscillation or oscillation wavelength.

[0015] In one embodiment, the difference (detuning amount) between the oscillation wavelength of the semiconductor laser element and the peak wavelength of the gain spectrum at room temperature is 1.8×10 as a function of wavelength λ. ―5 λ 2 nm or more (for example, at an oscillation wavelength λ=410 nm, the detuning amount may be 3.0 nm or more). Although different from the configuration of the present disclosure, in an electroabsorption modulator integrated DFB-LD (EA-DFB-LD) for 1.55 μm band communications (Patent Document 3), a detuning amount of 10 to 25 nm (equivalent to -12.7 to -5.1 meV in energy) is considered desirable. In addition, in a 1.3 μm band communications DFB-LD, a negative detuning amount is generally considered desirable (Patent Document 4). 1.8×10 ―5 λ 2 The detuning amount of over 100 nm appears small in wavelength difference compared to the communication LDs, but when converted into an energy difference, it is -22 meV, which is a larger absolute value than the energy-equivalent value of the detuning amount that has been considered desirable in the communication infrared lasers.

[0016] In one embodiment, the grating has a stop band width Δλ that appears in the gain spectrum. SB but 1.6×10 -6 λ 2≦Δλ SB ≦4.0×10 -6 λ 2 If the detuning amount is determined so as to reduce the temperature dependence of the optical output, the gain at the oscillation wavelength will be significantly lower than the gain at the peak wavelength at room temperature. Therefore, to achieve stable laser oscillation, it is necessary to increase the coupling coefficient between the guided light and the diffraction grating. The stop band width has a positive correlation with the coupling coefficient, and designing the stop band width to satisfy the above formula makes it easier to obtain the coupling coefficient required for laser oscillation.

[0017] In one embodiment, the width W of the current confinement structure may be 1 μm≦W≦3 μm. When the current confinement structure is a ridge structure, the width W is the mesa width.

[0018] In one embodiment, the diffraction grating may be configured such that the change in threshold current over the temperature range of 10° C. to 80° C. is within ±10% of the threshold current at room temperature.

[0019] In one embodiment, the diffraction grating may be configured such that the change in slope efficiency over the temperature range of 10° C. to 80° C. is within ±10% of the slope efficiency at room temperature.

[0020] In one embodiment, at least one facet of the laser element may be coated with a facet coating film having a reflectance of 2% or less at the oscillation wavelength.

[0021] A light emitting device according to one embodiment may include a distributed feedback semiconductor laser element, a nonlinear optical element that generates a second harmonic of light emitted from the distributed feedback semiconductor laser element, and a filter that cuts the emitted light.

[0022] (Embodiment) Preferred embodiments will be described below with reference to the drawings. The same or equivalent components, parts, and processes shown in each drawing will be designated by the same reference numerals, and redundant descriptions will be omitted where appropriate. Furthermore, the embodiments are merely examples and do not limit the disclosure or invention, and all features and combinations thereof described in the embodiments are not necessarily essential to the disclosure or invention.

[0023] In addition, the dimensions (thickness, length, width, etc.) of each component shown in the drawings may be enlarged or reduced as appropriate for ease of understanding. Furthermore, the dimensions of multiple components do not necessarily represent their relative sizes, and even if a component A is depicted as being thicker than another component B in the drawings, it is possible that component A is thinner than component B.

[0024] (Embodiment) 1 is a perspective view of a distributed feedback semiconductor laser device 100 according to an embodiment. The semiconductor laser device 100 includes an n-type substrate 110, an n-type semiconductor layer 120, an active layer 130, a p-type semiconductor layer 140, and a film made of an insulating material 160.

[0025] The substrate 110 is a nitride semiconductor, and x Al y Ga 1-x-y The substrate 110 may have a composition of GaN (0≦x≦1, 0≦y≦1, 0≦x+y≦1). To generate blue laser light at 444 nm, the material of the substrate 110 may be GaN (x=y=0). The substrate 110 is not limited to this, and any substrate that has a similar effect may be used, for example, a Si substrate or a sapphire substrate.

[0026] An n-type semiconductor layer 120, an active layer 130, and a p-type semiconductor layer 140 are formed in this order on an n-type substrate 110 by epitaxial growth, and form a stacked structure 102.

[0027] The n-type semiconductor layer 120 may include an n-type cladding layer and an n-type guide layer. For example, the material of the n-type cladding layer is n-Al 0.05 Ga 0.95N, and the material of the n-type guide layer is n-GaN.

[0028] An active layer (light emitting layer) 130 having a quantum well structure is formed on the n-type semiconductor layer 120. When the oscillation wavelength is set to 444 nm, the material of the quantum well structure includes In as a barrier layer. 0.01 Ga 0.99 N as the well layer and In as the 0.15 Ga 0.85 You can choose N.

[0029] In order to suppress the diffusion of impurities from the n-type semiconductor layer 120 to the active layer 130, an undoped nitride guide layer (not shown) In is provided between them. 0.02 Ga 0.98 N can be inserted.

[0030] The p-type semiconductor layer 140 may include a carrier block (electron block EB) layer, a p-type guide layer, a p-type cladding layer, and a contact layer. For example, the material of the p-type guide layer is p-In 0.02 Ga 0.98 N, and the material of the p-type cladding layer is p-Al 0.04 Ga 0.96 The material of the contact layer is p-GaN.

[0031] In order to suppress the diffusion of impurities from the p-type semiconductor layer 140 to the active layer 130, an undoped nitride guide layer (not shown) can be inserted between them.

[0032] A ridge portion (also called a mesa portion) 142 is formed in the p-type semiconductor layer 140 (p-type cladding layer) as a current confinement structure 141. The height of the ridge portion 142 can be several hundred nm, and the width of the ridge portion 142 (mesa width) can be several microns. For example, the height may be 500 nm and the width may be 2 μm.

[0033] The coupling coefficient κ, which will be described later, varies depending on the mesa width and the position where the diffraction grating is formed within the chip. The narrower the mesa width, the larger the coupling coefficient κ. However, when the mesa width is less than 1 μm, the lateral optical confinement weakens, and the laser characteristics begin to deteriorate. On the other hand, when the mesa width exceeds 3 μm, the waveguide mode (lateral mode) of the emitted light tends to become unstable. Changes in temperature or drive current cause the lateral single mode to change to transverse multimode, resulting in degradation of characteristics such as kinks in the IL characteristics. Therefore, to obtain a high coupling coefficient κ in a stable transverse single mode, it is preferable that the mesa width Wm be 1 μm≦Wm≦3 μm.

[0034] The p-type cladding layer 140 having the ridge portion 142, together with the active layer 130 and the n-type cladding layer 120, forms a ridge-type waveguide 144. The ridge-type waveguide 144 extends in a first direction (the z-axis direction in the figure). For example, the waveguide length may be 400 μm to 3000 μm.

[0035] The n-side electrode 171 is formed on the back surface of the substrate 110 , and the p-side electrode 172 is formed on the top surface of the ridge portion 142 .

[0036] Diffraction gratings 150_1 and 150_2 are formed on both sides of the mesa adjacent to the ridge waveguide 144 in the second direction (x-axis direction). Each of the diffraction gratings 150_1 and 150_2 has a plurality of grooves 152 formed in the p-type semiconductor layer 140 on the sides of the mesa. The plurality of grooves 152 are adjacent to each other in the first direction, and each groove extends in the second direction.

[0037] An insulating material 160 is formed on the p-type semiconductor layer 140. The insulating material 160 is formed so as to fill the grooves 152 of the diffraction grating 150.

[0038] The pitch of the diffraction grating 150 is set to the desired oscillation wavelength λ of the DFB-LD. DFB is designed to obtain

[0039] FIG. 2 shows the gain spectrum and the oscillation wavelength λ of the semiconductor laser device 100 according to the embodiment at room temperature (for example, 25° C.). DFB FIG.

[0040] In this embodiment, the oscillation wavelength λ DFB is the peak wavelength λ of the gain spectrum at room temperature GAINPEAK The pitch of the diffraction grating 150 is designed so that the detuning amount ΔG=λ is longer than DFB -λ GAINPEAK is positive (>0). Preferably, the oscillation wavelength λ DFB and the peak wavelength λ of the gain spectrum at room temperature GAINPEAK The difference (detuning amount) ΔG is 1.8×10 -5 λ 2 nm or more.

[0041] The above is the configuration of the semiconductor laser device 100. The advantages of the semiconductor laser device 100 will become clear when compared with a comparative technology, so the comparative technology will be explained first.

[0042] (Comparative Technology) FIG. 3 shows the gain spectrum and oscillation wavelength λ of a semiconductor laser device according to a comparative technique at room temperature (for example, 25° C.). DFB FIG.

[0043] (Comparative Technology 1, 2) The semiconductor laser element according to comparative technique 1 or 2 is an InP-based DFB-LD that oscillates in the infrared communication wavelength band (1.3 μm band). Lasers in this wavelength band are mainly used for short-distance communications, so high speed in direct modulation is required. In comparative technique 2, the oscillation wavelength λ DFB2 is the peak wavelength λ of the gain spectrum GAINPEAK Since the shorter wavelength side is advantageous for high-speed operation, the oscillation wavelength λ DFB2 is the peak wavelength λ of the gain spectrum at room temperature (for example, 25°C). GAINPEAK The diffraction grating 150 is designed so that: That is, the detuning amount ΔG of Comparative Technique 1 is 0, and the detuning amount of Comparative Technique 2 is negative.

[0044] (Comparative Technology 3) The semiconductor laser element according to Comparative Technology 3 is an InP-based electroabsorption modulator integrated DFB-LD (EA-DFB-LD) that oscillates in the infrared communication wavelength band (1.55 μm band). Lasers in this wavelength band are primarily used for long-distance communications, so modulation is performed by the electroabsorption modulator section. Therefore, it is preferable that the temperature change of the optical output characteristics in the DFB-LD section is relatively small, and for this purpose, the oscillation wavelength λ DFB3 is the peak wavelength λ of the gain spectrum GAINPEAK Therefore, in Comparative Technique 3, the oscillation wavelength λ DFB3 is the peak wavelength λ of the gain spectrum at room temperature (for example, 25°C). GAINPEAK The diffraction grating 150 is designed so that the detuning amount ΔG is positive.

[0045] Next, the operation of the semiconductor laser device 100 will be described in comparison with the comparative technology.

[0046] FIG. 4A is a diagram illustrating the temperature dependence of a semiconductor laser device according to Comparative Technology 1. FIG. 4B is a diagram illustrating the temperature dependence of a semiconductor laser device according to Comparative Technology 3. Here, because the temperature dependence of the gain peak wavelength is generally greater than that of the DFB oscillation wavelength, the temperature dependence of the oscillation wavelength has been omitted to avoid complexity. The gain spectrum has temperature dependence, and as the temperature increases, its peak (gain peak wavelength) shifts toward longer wavelengths, and its absolute value (gain) decreases at the same drive current value (left graphs in FIG. 4A and FIG. 4B). Here, gain spectra are shown at room temperature (25°C), as well as at 60°C and 80°C.

[0047] Referring to FIG. 4A, in Comparative Technique 1, λ DFB1 =λ GAINPEAK @25℃, so the oscillation wavelength λ DFB1 The gain is highest at λ , and as the temperature rises, it shifts relatively to the long wavelength side. DFB1The gain at the gate must be increased until it reaches the threshold gain (Fig. 4A, right). As a result, the threshold current increases and the slope efficiency decreases due to heat generation and carrier overflow caused by the increased drive current.

[0048] In comparative technology 2, the oscillation wavelength λ DFB2 Since the time is shorter, the decrease in gain with increasing temperature is more significant than in Comparative Technique 1.

[0049] See Figure 4B. In Comparative Technique 3, λ GAINPEAK @25℃<λ DFB3 Therefore, the temperature change of the threshold current is smaller than that of Comparative Technique 1 and Comparative Technique 2. However, λ DFB3 In order for the gain at λ to reach the threshold gain and to reach oscillation, as in Comparative Technique 1 and Comparative Technique 2, a current is injected and λ DFB3 It is necessary to increase the gain at the temperature up to the threshold gain (right diagram in Figure 4B). This reduces the change in threshold current, but the slope efficiency decreases due to heat generation and carrier overflow caused by the increase in drive current. It is expected that the temperature change of the threshold current can be reduced by further increasing the detuning amount in the positive direction. However, in the InP-based DFB-LDs that have been the focus of research and development to date, due to the inherent material properties of InP, it is not possible to obtain a gain that reaches the threshold gain even with a larger current injection. Therefore, even if oscillation is achieved, the slope efficiency is significantly reduced due to heat generation and carrier overflow caused by the large current injection. Therefore, there are limitations to the method of increasing the detuning amount in the positive direction.

[0050] Therefore, it is necessary to incorporate a temperature control mechanism to stabilize the device temperature in the semiconductor laser elements according to Comparative Techniques 1 to 3. Alternatively, it is necessary to design a module or system that takes into account the temperature change of the optical output characteristics (IL characteristics) of the DFB-LD.

[0051] 5 is a diagram illustrating the temperature dependence of the GaN-based semiconductor laser device according to the embodiment. GAINPEAK @25℃<λ DFBHere, the temperature dependence of the gain peak wavelength is generally greater than that of the DFB oscillation wavelength, so the temperature dependence of the oscillation wavelength has been omitted to avoid complexity. As mentioned above, the gain spectrum is temperature dependent, and as the temperature increases, its peak (gain peak wavelength) shifts toward longer wavelengths, and its absolute value (gain) decreases at the same current value. Here, gain spectra are shown at room temperature (25°C), as well as at 60°C and 80°C. Furthermore, although this is thought to be a characteristic unique to GaN materials, the gain spectrum appears to be broader in terms of energy than InP-based materials (the horizontal axis in Figure 5 represents wavelength, but the gain spectrum appears broader for easier qualitative understanding). The spatial composition distribution of In in the active layer is presumed to be the reason, but further research is needed to clarify this. For GaN-based DFB-LDs, the amount of detuning has not been clearly understood due to the lack of publicly available information. However, it is believed that the wide energy gain spectrum inherent to GaN-based materials allows for oscillation without requiring a large current injection amount, even when the energy detuning amount is larger than the detuning amount previously considered in the research and development of InP-based DFB-LDs. Therefore, in the embodiment of Figure 5, oscillation is achieved at a constant current injection amount, i.e., the same threshold current, at each temperature. Furthermore, due to the unique characteristics of GaN-based materials, such as the difference in thermal conductivity between InP-based and GaN-based materials, a decrease in slope efficiency due to heat generation, carrier overflow, etc. can be suppressed. According to this embodiment, the temperature dependence of the optical output of the semiconductor laser device 100 can be reduced compared to Comparative Techniques 1 to 3.

[0052] In the semiconductor laser element according to the embodiment, the temperature control mechanism for stabilizing the device temperature can be simplified compared to that required in Comparative Techniques 1 to 3, or the temperature control mechanism can be omitted, thereby reducing the cost of a light-emitting device incorporating the semiconductor laser element. Furthermore, the temperature change of the slope efficiency is small, making module and system design easier.

[0053] From another perspective, the semiconductor laser device 100 according to this embodiment has an oscillation wavelength λ DFB can be considered to be set to coincide with the peak wavelength of the gain spectrum at a predetermined temperature T (60°C in the example of FIG. 5) that is sufficiently higher than room temperature. The predetermined temperature is lower than the upper limit (e.g., 80°C) of the temperature range in which the semiconductor laser device 100 is guaranteed to operate.

[0054] In this embodiment, the detuning amount ΔG is determined so as to reduce the temperature dependency of the optical output. DFB The gain at the peak wavelength is lower than the gain at room temperature. Therefore, in order to achieve stable laser oscillation, it is necessary to increase the coupling coefficient. However, since the coupling coefficient is an index that is difficult to evaluate directly and quantitatively, the stop band width Δλ, which has a positive correlation with the coupling coefficient, is used. SB It is convenient to use as a parameter for design. SB can be easily and directly measured from the gain spectrum.

[0055] Figure 6A is a diagram explaining a DFB-LD with a uniform diffraction grating with no phase shift. Figure 6A shows the spectrum of emitted light just before oscillation in a DFB-LD with a uniform diffraction grating with no phase shift in the 410 nm band (Violet). In a DFB-LD, a region where emitted light decreases is formed in the vicinity of a wavelength determined by the pitch (period) of the diffraction grating and the effective refractive index, and this region is called a stop band. Stop band width Δλ SB =λ2-λ1, and the greater the coupling between the diffraction grating and the propagating light, the smaller Δλ SB In a DFB-LD without a phase shift, DFB oscillation generally occurs at either λ1 or λ2, but in Figure 6A, λ2 = λ DFB It is as follows.

[0056] Figure 6B is a diagram illustrating a DFB-LD with a diffraction grating with a phase shift. In the case of a diffraction grating with a phase shift, such as a λ / 4 shift, laser oscillation occurs at a wavelength near the center of the stop band. As shown in the part of the gain spectrum just before oscillation in Figure 6B, the stop band width in such a case is also Δλ. SB =λ2-λ1, where λ2≠λ DFB It is as follows.

[0057] Generally, equation (1) holds between the wavelength and energy of light. E [eV] = 1240 × λ [nm] … (1) Therefore, the stop band width Δλ in the gain spectrum SB If we express =λ2-λ1 in terms of the energy width ΔE, we obtain equation (2). ΔE=1240 / λ1-1240 / λ2…(2) λ1=λ-Δλ SB / 2, λ2=λ+Δλ SB / 2, we obtain the approximate formula (3). ΔE=1240·Δλ SB / λ 2 …(3)

[0058] It is known that when the reflectivity of both end faces is not zero, the stop band width changes depending on the phase of the grating at the end face (end face phase). The inventors have investigated this case and found that, if the reflectivity of either end face is 2% or less, the energy width ΔE SB It was concluded that the stop band width Δλ is 2.2 to 5.0 meV. SB The appropriate range of is expressed by equation (4). 1.8×10 -6 λ 2 ≦Δλ SB ≦4.0×10 -6 λ 2 …(4) Stop band width Δλ SB If is designed within the range of equation (4), an appropriate coupling coefficient can be obtained, and stable oscillation is possible despite the gain reduction caused by detuning.

[0059] Appropriate stopband width Δλ at a typical wavelength SB is as follows: Blue-violet λ=410nm Δλ SB =0.30~0.67nm Blue λ=460nm Δλ SB =0.38~0.85nm Green λ=520nm Δλ SB =0.49~1.09nm Red λ=640nm Δλ SB =0.74~1.64nm

[0060] In this embodiment, the detuning amount ΔG is determined so as to reduce the temperature dependency of the optical output. DFB The gain at the DFB oscillation wavelength is lower than the gain at the peak wavelength at room temperature. Therefore, for stable DFB oscillation, at least one facet must have a reflectivity of 2% or less at the oscillation wavelength. This makes the threshold gain required for Fabry-Perot (FP) mode oscillation sufficiently higher than the threshold gain required for DFB oscillation. Therefore, even if the gain at the DFB oscillation wavelength is significantly lower than the gain at the gain peak wavelength, oscillation in the FP mode (FP oscillation) can be suppressed, enabling stable DFB oscillation.

[0061] Fig. 7 shows the relationship between the detuning amount ΔG and the IL characteristics in a 410 nm band DFB-LD. detuned =0 meV], ΔG=1.9 nm [ΔE detuned =-13.8 meV], ΔG=5.2 nm [ΔE detuned The IL characteristics are shown in three cases: the threshold current I TH And, I>I TH The detuning amount ΔG can be specified by two parameters: the slope of the optical output in the region (slope efficiency) and the detuning amount ΔG = 5.2 nm [ΔE detuned =-37.6 meV], the threshold current I TH It can be seen that the fluctuations in both the IL characteristics and the slope efficiency are suppressed, and the temperature dependence of the IL characteristics is very small.

[0062] Figure 8A shows the threshold current I TH 8A and 8B show the temperature dependence of the slope efficiency in a 410 nm band DFB-LD with different detuning amounts ΔG. The characteristics of several samples with different detuning amounts ΔG are shown in Figs. 8A and 8B, and the threshold current I TH The slope efficiency SE is normalized to the value at room temperature of 25°C. The threshold current I TH and the slope efficiency change with the threshold current I TH It is preferable that the deviation is within ±10% of the value of the slope efficiency SE, and therefore it is advisable to optimize the detuning amount ΔG so that it falls within the range of 0.9 to 1.1.

[0063] Figure 9A shows the normalized threshold current I TH 9B is a graph showing the dependence of the normalized slope efficiency SE on the detuning amount ΔG in a 410 nm band DFB-LD. As in FIGS. 8A and 8B, the threshold current I TH (80℃), I TH (60℃) is the threshold current at room temperature of 25℃ ITH The slope efficiencies SE(80°C) and SE(60°C) at 80°C and 60°C are normalized by the slope efficiency SE(25°C) at room temperature of 25°C. From Figure 9A, the normalized threshold current I TH It can be seen that in order for the normalized slope efficiency SE to fall within the range of 0.9 to 1.1, the detuning amount ΔG is preferably +3.0 nm or more. Similarly, from FIG. 9B, it can be seen that in order for the normalized slope efficiency SE to fall within the range of 0.9 to 1.1, the detuning amount ΔG is preferably -0.5 nm or more. Therefore, the range of ΔG that satisfies these simultaneously is ΔG≧3.0 nm. Using equation (1), the preferable range of detuning amount can be expressed in terms of energy as follows: ΔE detuned≦-22 meV. In GaN-based DFB-LDs, the value on the right side of this equation is constant regardless of the wavelength band. Furthermore, when this is expressed as a function of wavelength λ, it becomes as shown in equation (5). ΔG≧1.8×10 -5 λ 2 …(5) When ΔG is designed within the range of equation (5), the threshold current I TH The change in the threshold current I TH Similarly, the change in slope efficiency SE in the temperature range of 10° C. to 80° C. can be kept within ±10% of the slope efficiency SE at room temperature.

[0064] The range of the appropriate detuning amount ΔG at a typical wavelength is as follows: Blue-violet λ=410nm ΔG≧3.0nm Blue λ=460nm ΔG≧3.7nm Green λ=520nm ΔG≧4.8nm Red λ=640nm ΔG≧7.4nm

[0065] 10 is a diagram showing a light emitting device 200 according to an embodiment. The light emitting device 200 includes the above-described semiconductor laser device 100, a nonlinear optical element 210, and a filter 220.

[0066] The semiconductor laser element 100 oscillates in a single longitudinal mode at λ=444 nm. The nonlinear optical element 210 is a wavelength conversion element that generates the second harmonic λ / 2 (wavelength 222 nm) of the light emitted from the semiconductor laser element 100. The nonlinear optical element 210 may be made of, for example, β-BaB2O4 (BBO) or CsLiB6O 10 (CLBO) or the like is used. By irradiating the fundamental wave (444 nm) from an appropriate direction onto this wavelength conversion element, a second harmonic wave (222 nm) can be generated. The filter 220 is a short-pass filter that removes the fundamental wave λ and emits the second harmonic wave λ / 2.

[0067] The embodiments merely illustrate the principles and applications of the present invention, and many modifications and changes in arrangement are permitted to the embodiments as long as they do not deviate from the spirit of the present invention as defined in the claims. [Explanation of symbols]

[0068] 100 Semiconductor laser element 102 Laminated structure 110 GaN substrate 120 n-type semiconductor layer 130 Active layer 140 p-type semiconductor layer 141 Current confinement structure 142 Ridge 144 Ridge Waveguide 150 Diffraction Grating 152 Groove 160 Insulating Materials 200 Light-emitting device 210 Nonlinear Optical Elements 220 filters

Claims

1. A distributed feedback semiconductor laser element, a laminated structure including a substrate, a first conductivity type semiconductor layer, an active layer, and a second conductivity type semiconductor layer; The laminated structure includes: a current confinement structure extending in a first direction perpendicular to the stacking direction; a diffraction grating formed along the current confinement structure; is provided, The diffraction grating is configured so that the oscillation wavelength of the semiconductor laser element is longer than the peak wavelength of the gain spectrum at room temperature.

2. When the difference ΔG between the oscillation wavelength of the semiconductor laser element and the peak wavelength of the gain spectrum at room temperature is expressed as a function of wavelength λ [nm], ΔG≧1.8×10 －５ ・λ ２ [nm] 2. The semiconductor laser device according to claim 1, wherein

3. The diffraction grating has a threshold current of I ＴＨ When the driving current I ＯＰ = 0.9 × I ＴＨ The stop band width Δλ that appears in the gain spectrum at ＳＢ but, 1.8 × 10 －６ ・l ２ ≦Dl ＳＢ ≦4.0×10 －６ ・l ２ 3. The semiconductor laser device according to claim 1, wherein the semiconductor laser device is configured such that:

4. The active layer is In ｘ Al ｙ Ga １－ｘ－ｙ 3. The semiconductor laser device according to claim 1, wherein N (0≦x≦1, 0≦y≦1) is included.

5. 3. The semiconductor laser device according to claim 1, wherein the width W of the current confinement structure is in the range of 1 [mu]m≦W≦3 [mu]m.

6. 3. The semiconductor laser device according to claim 1, wherein the diffraction grating is configured so that a change in threshold current in a temperature range of 10° C. to 80° C. is within ±10% of the threshold current at room temperature.

7. 3. The semiconductor laser device according to claim 1, wherein the diffraction grating is configured so that a change in slope efficiency in a temperature range of 10° C. to 80° C. is within ±10% of the slope efficiency at room temperature.

8. 3. The semiconductor laser device according to claim 1, wherein at least one facet is coated with a facet coating film having a reflectance of 2% or less at the oscillation wavelength.

9. a distributed feedback semiconductor laser element according to claim 1 or 2; a nonlinear optical element that generates a second harmonic of the light emitted from the distributed feedback semiconductor laser element; a filter that cuts the emitted light; A light emitting device comprising:

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