JP2017034034A - Distribution feedback lateral multimode semiconductor laser element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a distribution feedback lateral multimode semiconductor laser element capable of increasing the dynamic range of a driving current that can output laser light of single wavelength, in the longitudinal mode.SOLUTION: Assuming the wavelength of the peak value of gain spectrum of the light generated in an active layer is λp, and the wavelength of the peak value of selection spectrum selected by a diffraction grating layer GR is λ, following relation is satisfied; λp≤λ≤λp+5 nm. In the case of this condition, even if the driving current is changed significantly, oscillation based on Fabry-Perot is suppressed significantly, while generating oscillation based on the DFB, in the spectrum of laser light, and laser light of single wavelength can be outputted. In other words, dynamic range of the driving current can be increased.SELECTED DRAWING: Figure 9

Description

  The present invention relates to a distributed feedback (DFB) lateral multimode semiconductor laser device.

  The DFB semiconductor laser element is described in, for example, Patent Document 1 and Patent Document 2 below. A DFB semiconductor laser device includes a lower cladding layer, an active layer formed on the lower cladding layer, an upper cladding layer formed on the active layer, and a diffraction grating layer provided on the lower cladding layer or the upper cladding layer. The laser beam having a wavelength defined by the diffraction grating layer is emitted from the active layer.

  The intensity distribution of the laser light along the width direction of the cross section of the laser light (the width direction of the active layer) indicates a horizontal transverse mode as the waveguide mode. In the horizontal mode, the fundamental mode is located at the center in the width direction of the active layer, but when the dimension in the width direction of the active layer is longer than the intensity distribution length of the fundamental mode, multiple modes are used. Of the laser beam exists in the active layer and becomes a transverse multimode having a plurality of intensity distribution peaks.

  The transverse mode, that is, the waveguide mode, becomes a single mode when there is a single solution that satisfies the wave equation and the boundary condition at the same time (cutoff condition), and becomes a multimode when there are multiple solutions. As a semiconductor laser element in which the horizontal transverse mode is a single mode, the one described in Patent Document 3 is known, and a semiconductor laser element having a narrow resonator width is disclosed.

  The intensity distribution of the laser beam along the thickness direction (active layer thickness direction) of the transverse cross section of the laser beam indicates the vertical transverse mode. In the vertical transverse mode, the thickness of the active layer can be set so that the peak position of the laser light intensity distribution is set to one.

  The resonance mode in the cavity length direction of the laser element (the cavity length direction of the active layer), that is, the light traveling direction is the longitudinal mode. In a Fabry-Perot laser device, there are a plurality of fundamental mode wavelengths that satisfy the resonance condition and a plurality of integral multiples of wavelengths in the longitudinal mode, so that longitudinal multimode oscillation occurs, and laser light having a plurality of wavelengths is generated in the active layer. Is output from. Further, as disclosed in Patent Document 4, in a DFB semiconductor laser, a laser beam having a specific wavelength component among laser beams generated in an active layer is selected by a diffraction grating layer, and in a longitudinal mode, a single mode is selected. Oscillation can be performed.

  The above-described DFB multi-mode semiconductor laser element is used for, for example, a pumping light source of a laser medium or an optical pickup in a laser processing machine.

Japanese Patent No. 3204474 Japanese Patent No. 3707847 JP-A-63-222487 JP 2000-357841 A

  However, although the output and wavelength stability of the DFB lateral multimode semiconductor laser element described above are increased, laser light having a selected wavelength is output in the longitudinal mode even if the dynamic range of the drive current is widened. There is expected. The present invention has been made in view of such problems, and a DFB lateral multimode semiconductor laser element capable of widening a dynamic range of a drive current capable of outputting laser light of a selected wavelength in a longitudinal mode. The purpose is to provide.

In order to solve the above-mentioned problems, a distributed feedback lateral multimode semiconductor laser device according to the present invention includes an active layer, a cladding layer sandwiching the active layer, and a diffraction grating layer overlapping the laser light generated in the active layer. Where Ep is the energy band gap corresponding to the peak value of the gain spectrum of the light generated in the active layer, and E _DFB is the energy band gap corresponding to the peak value of the selected spectrum selected by the diffraction grating layer. Ep−6.4 meV ≦ E _DFB ≦ Ep.

When the energy band gap is converted into a wavelength, the distributed feedback lateral multimode semiconductor laser device according to the present invention overlaps the active layer, the cladding layer sandwiching the active layer, and the laser light generated in the active layer. A wavelength of a peak value of a gain spectrum of light generated in the active layer is λp, and a wavelength of a peak value of a selected spectrum selected by the diffraction grating layer is λ _DFB , λp ≦ λ _DFB ≦ λp + 5 nm is satisfied.

  In the case of the above conditions, even when the drive current is greatly changed, oscillation based on Fabry-Perot is greatly suppressed while oscillation based on DFB occurs in the spectrum of the laser light, and single wavelength laser light is Can be output. That is, the dynamic range of the drive current can be widened.

  According to the distributed feedback lateral multimode semiconductor laser device of the present invention, it is possible to widen the dynamic range of the drive current that can output the laser beam having the selected wavelength.

It is a perspective view of a semiconductor laser device. It is a perspective view of a DFB semiconductor laser device. It is a longitudinal cross-sectional view of a DFB semiconductor laser element. It is a longitudinal cross-sectional view of a diffraction grating layer. It is a block diagram of the laser apparatus which used the semiconductor laser element as LD for excitation (laser diode). It is a graph which shows the relationship between a wavelength (nm) and optical output (dB). Graph (A) showing the relationship between light output (W) and wavelength (nm) at LD temperature -8 ° C. Graph (B) showing the relationship between light output (W) and wavelength (nm) at LD temperature 0 ° C. 5 is a graph (C) showing the relationship between light output (W) and wavelength (nm) at an LD temperature of 10 ° C. It is a graph which shows the conditions of the optical output (W) in which only the laser oscillation of DFB was performed, and temperature. Graph (A) showing the relationship between the wavelength (nm) of the laser beam and the optical output (au) at an LD temperature of −8 ° C. and an optical output of 6 W, and the wavelength (nm) of the laser beam at an LD temperature of 0 ° C. and an optical output of 6 W. ) And optical output (au), a graph (B) showing the relationship between the wavelength (nm) of laser light and optical output (au) at an LD temperature of 10 ° C. and an optical output of 7 W ( C). It is a graph which shows the relationship between (lambda) p and (lambda) _DFB in LD temperature -8 degreeC. It is a graph which shows the relationship between (lambda) p and (lambda) _DFB in LD temperature 0 degreeC. It is a graph which shows the relationship between (lambda) p and (lambda) _DFB in LD temperature of 10 degreeC. It is a graph which shows wavelength (lambda) and energy band gap Eg. It is a graph which shows the relationship between the electric current (A) supplied to a laser element, and optical output (W). It is a graph which shows the relationship between a wavelength (nm) and optical output (dB). It is a graph which shows the relationship between mode gain (GAMMA) g and photon energy. It is a graph which shows the relationship between mode gain (GAMMA) g and photon energy. It is a chart which shows various relational expressions.

  Hereinafter, a distributed feedback (DFB) lateral multimode semiconductor laser device according to an embodiment will be described. Note that the same reference numerals are used for the same elements, and redundant description is omitted.

  FIG. 1 is a perspective view of a semiconductor laser device.

  The semiconductor laser device 100 includes a base 30 having a step on the upper surface, a submount 20 fixed on the lower surface of the base 30, and a DFB semiconductor laser element whose lower surface is fixed on the submount 20 with a conductive paste or the like. (Hereinafter, semiconductor laser device 10). A plurality of wires W are connected between the upper surface of the base 30 and the upper electrode E1 of the semiconductor laser element 10. The base 30 includes an upper-stage conductor member 31 and a lower-stage conductor member 32, and a ceramic insulator is provided between the upper-stage conductor member 31 and the lower-stage conductor member 32.

  The length of the upper electrode E1 that defines the resonator length is 4 mm, and the width (light emission width) of the upper electrode E1 is 100 μm.

  FIG. 2 is a perspective view of the semiconductor laser device.

  The semiconductor laser element 10 includes a substrate 1, a lower cladding layer 2 formed on the substrate 1, an active layer 3 formed on the lower cladding layer 2, and an upper cladding layer 4 ( 4a, 4b), a diffraction grating layer GR provided on the upper cladding layer 4, a contact layer 5 formed on the upper cladding layer 4, and an upper electrode E1 formed on the contact layer 5. . A lower electrode is formed on the lower surface of the substrate 1 with a conductive paste such as gold.

  The upper clad layer 4 is composed of a first upper clad layer 4a and a second upper clad layer 4b, and the diffraction grating layer GR is sandwiched therebetween. The diffraction grating layer GR has a striped concavo-convex structure, and the second upper cladding layer 4b is embedded in a concave portion of the concavo-convex structure. The upper cladding layer 4 and the diffraction grating layer GR have different refractive indexes, and regions having different refractive indexes exist alternately along the resonator length direction.

  Further, the diffraction grating layer GR may be provided in the lower cladding layer 2 in the same manner as that provided in the upper cladding layer 4. This is because these cladding layers are equivalent if they are turned upside down. The upper clad layer and the lower clad layer have different conductivity types. When a diffraction grating layer is provided in the lower clad layer 2, the lower clad layer 2 can be divided into two like the upper clad layer 4.

  FIG. 3 is a longitudinal sectional view of the semiconductor laser element.

  This figure shows a more detailed structure. The semiconductor laser device 10 includes a first light guide layer 3G1 and a first graded composition layer 3C1 between the active layer 3 and the lower cladding layer 2 in order from the outside. The adjacent light guide layer 3g1 is provided, and the second light guide layer 3G2, the second composition gradient layer 3C2, and the adjacent light guide layer 3g are arranged between the active layer 3 and the first upper cladding layer 4a in order from the outside. It has. The adjacent light guide layers 3g1 and 3g2 adjacent to the active layer 3 are thicker near the substrate 1, and the composition gradient layers 3C1 and 3C2 gradually decrease as the Al composition approaches the active layer 3. It is a layer to do. Further, in the same figure, the lower electrode E2 is shown on the back side of the substrate 1.

The material / thickness of each of the above layers is as follows.

  Note that the preferable range of the impurity concentration is 1/10 to 10 times the above concentration, and the thickness of each layer operates, for example, even when an error of about ± 50% is included. In addition, the laser beam is sufficiently emitted. Further, the composition of Al in the diffraction grating layer is preferably greater than 0% and 10% or less. This is because, when the surface is once exposed to the atmosphere in the diffraction grating manufacturing process and contains more than 10% of highly active Al, device characteristics are significantly deteriorated due to significant natural oxidation. When Al is contained, there is an advantage that a change in the shape of the diffraction grating is suppressed in the heating process of regrowth. Zn can be used as the P-type impurity in the contact layer, the upper cladding layer, and the diffraction grating layer, and Si can be used as the N-type impurity in the substrate, the lower cladding layer, the first light guide layer, and the first composition gradient layer. Further, even if the P-type and N-type conductivity types are switched, the laser element operates.

The second optical guiding layer _3G2: between _{(Al X Ga 1-X As} X = 0.25) and the active layer 3 adjacent optical guide layer _{3g2 (Al X Ga 1-X} As: X = 0. 18) and a composition gradient layer 3C2 (Al _X Ga _1-X As: X = 0.25 to 0.18) having a composition between them can be inserted, and the first light guide layer 3G1 (Al _X Between Ga _1-X As: X = 0.25) and the active layer 3, the adjacent light guide layer 3g1 (Al _X Ga _1-X As: X = 0.18) and the composition between them are composition gradient layer _3C1 having _{(Al X Ga 1-X As} : X = 0.25~0.18) can be inserted, these layers may be omitted. All the light guide layers are set to have an energy band gap larger than that of the active layer, but the energy band gap of the light guide layer closer to the active layer is set to be smaller than that of the farther (ie, the Al composition ratio). X is small).

If the Al composition in each layer is described in detail, the Al composition of the clad layer is different from the Al composition of the diffraction grating layer and has a different refractive index. In Al _X Ga _1-X As, the higher the Al composition ratio X, the larger the energy band gap and the lower the refractive index. For example, the Al composition in the cladding layer is set to X = 0.45 in the upper cladding layer on the P side and X = 0.33 (33%) in the lower cladding layer on the N side, and the light guide closer to the active layer. The Al composition of the layers is set to, for example, both X = 0.15 (15%), and the Al composition of the light guide layers 3G1 and 3G2 farther from the active layer is, for example, both X = 0.25 (25%). ). The Al composition ratio X of each layer other than the diffraction grating layer can operate even with an error of ± 0.05 (= 5%).

  Note that a MOCVD (metal organic chemical vapor deposition) method can be used as a formation method. TMA (trimethylaluminum) can be used as a raw material when Al is contained, TMG (trimethylgallium) can be used as a raw material when Ga is contained, and arsine can be used as a raw material when As is contained. The growth temperature of AlGaAs is around 700 ° C., and the growth method of these materials is well known.

As an example of the manufacturing method of the above-mentioned structure, an N-type Al _0.33 Ga _0.67 As cladding layer, an AlGaAs undoped light guide layer including an InGaAs quantum well layer, a P-type Al _0.45 on an N-type GaAs substrate. A Ga _0.55 As first cladding layer (about 40 nm) and a P-type Al _0.1 Ga _0.9 As diffraction grating layer (about 50 nm) are grown sequentially. Next, a diffraction grating is produced by interference exposure and wet etching. The period Λ of the diffraction grating is set to be in the range of λp ≦ λ _DFB ≦ λp + 5 nm. The coupling coefficient is uniquely determined by the structure, and in the case of the structure, κ = 2.0 to 2.5 (cm ⁻¹ ). Next, the wafer from which the diffraction grating has been cut is subjected to surface treatment and promptly introduced into a crystal growth furnace to grow a p-type Al _0.45 Ga _0.55 As second cladding layer and a p-type GaAs contact layer. An electrode is formed on the completed laser crystal. The electrode has a width of 100 μm, and a dielectric multilayer film having a desired reflectivity is formed on each of the emission surface and the reflection surface of the wafer cleaved to a resonator length of 4 mm. The semiconductor laser element is a junction-down type. Can be soldered to the submount.

  FIG. 4 is a longitudinal sectional view of the diffraction grating layer GR. This is a cross section obtained by cutting the diffraction grating layer GR along a plane including the resonator length and the thickness direction.

  As described above, the diffraction grating layer GR is made of a zinc-blende structure III-V group compound semiconductor, and has a concavo-convex structure including a plurality of convex portions and concave portions extending in the width direction. The structure can be formed by etching, but the etching prevents the recess from reaching the bottom surface of the AlGaAs layer constituting the diffraction grating layer.

  FIG. 5 is a block diagram of a laser device using the semiconductor laser device 100 as an excitation LD.

  Laser light emitted from the semiconductor laser device 100 as an excitation LD is incident on a laser medium 101 such as a YAG crystal to excite the laser medium 101. When seed light is incident on the laser medium 101, or by the excitation laser light itself, the laser light reciprocating between the reflection mirrors 102 and 102 sandwiching the laser medium 101 is amplified, and the reflection mirror 103 having the lower reflectivity is amplified. The laser beam is emitted from. The emitted laser light is input to a scanning device 106 such as a one-dimensional or two-dimensional galvanometer through the reflecting mirror 104 and the reflecting mirror 105, and the object on the table 108 is passed through the fθ lens 107 as an objective lens. 109 is incident. The wavelength of the excitation light is 976 nm in this example.

  FIG. 6 is a graph showing the relationship between wavelength (nm) and light output (dB).

This graph shows the gain spectrum of the laser beam generated in the active layer of the semiconductor laser element, and the peak of the spectrum is located at the position of wavelength λp = 976 nm. The wavelength λ _DFB selected by the diffraction grating layer is set to a peak wavelength λp or more and λp + 5 (nm) or less.

That is, the wavelength of the peak value of the gain spectrum of the light generated in the active layer 3 .lambda.p, if the wavelength of the peak value of the selected spectral selecting the diffraction grating layer GR was lambda _DFB, satisfy _{λp ≦ λ DFB ≦ λp + 5nm} .

  In the case of the above conditions, even when the drive current is greatly changed, oscillation based on Fabry-Perot is greatly suppressed while oscillation based on DFB occurs in the spectrum of the laser light, and single wavelength laser light is Can be output. That is, the dynamic range of the drive current can be widened.

  The details will be described below.

  FIG. 7 is a graph (A) showing the relationship between the light output (W) and the wavelength (nm) at an LD (laser diode) temperature of −8 ° C., and the light output (W) and the wavelength (nm) at an LD temperature of 0 ° C. The graph (B) which shows the relationship of this, and the graph (C) which shows the relationship between light output (W) and wavelength (nm) in LD temperature of 10 degreeC. In addition, the material of a component is as above-mentioned.

When a diffraction grating having a period Λ and an order m is engraved inside the resonator, the wavelength selected by the diffraction grating layer is λ _DFB = (2ΛN _eff ⁰ ) / m. N _eff ⁰ is the equivalent refractive index corresponding to the waveguide mode, and the superscript 0 means the fundamental mode.

Incidentally, FIG. 10, in the LD temperature -8 ° C., Chart, Figure 11 shows the relationship between λp and lambda _DFB, in LD temperature 0 ° C., table showing the relationship between λp and lambda _DFB, 12 LD temperature 10 ° C. FIG. 13 is a chart showing the relationship between λp and λ _DFB , and FIG. 13 is a chart showing the wavelength λ and the energy band gap Eg.

  As shown in FIG. 7A, the oscillation with the selected wavelength was obtained at the optical power of 7 to 9 W at the LD temperature of −8 ° C.

  As shown in FIG. 7C and FIG. 12, the oscillation with the selected wavelength was obtained at the light output of 1 W to 4 W at the LD temperature of 10 ° C.

As shown in FIG. 7B and FIG. 11, the oscillation with the selected wavelength was obtained at the optical output of 1 W to 9 W at the LD temperature of 0 ° C. The maximum value of Δλ at this time is 5.0 nm, and the DFB oscillation range can be maximized by setting the gain relationship during LD oscillation to λ _DFB = λp + 5 nm.

Specifically, as shown in FIG. 13, λp = 969.1 nm and λ _DFB = 974.1 nm at 273 K = 0 ° C., optical output 1 W, and the refractive index period Λ in the diffraction grating layer is 432.4 nm. Set to.

  As shown in FIG. 7A, when the LD temperature is −8 ° C., the laser beam (DFB) selected by the diffraction grating layer is distributed in the wavelength range of 973.7 nm to 975.1 nm, but is generated in the active layer. The laser light itself (Fabry-Perot oscillation: FP) is distributed in the wavelength range of 967.4 nm to 972 nm. That is, when the drive current is greatly changed, laser light (DFB, FP) having a plurality of peak wavelengths is output from the semiconductor laser element.

  As shown in FIG. 7C, when the LD temperature is 10 ° C., the laser beam (DFB) selected by the diffraction grating layer is distributed in the wavelength range from 974.8 nm to 976.2 nm, while it is generated in the active layer. The laser light itself (Fabry-Perot oscillation: FP) is distributed in the wavelength range of 970.7 nm to 981.5 nm. That is, when the drive current is greatly changed, laser light (DFB, FP) having a plurality of peak wavelengths is output from the semiconductor laser element.

  As shown in FIG. 7B, when the LD temperature is 0 ° C., only the laser beam (DFB) selected by the diffraction grating layer is distributed in the wavelength range from 974.1 nm to 975.5 nm, but is generated in the active layer. The laser beam itself (Fabry-Perot oscillation: FP) is not observed when the optical output is in the range of 1 W to 9 W. That is, even if the driving current is greatly changed, only the laser beam having the selected peak wavelength is output from the semiconductor laser element.

  FIG. 8 is a chart showing conditions of optical output (W) and temperature in which only DFB laser oscillation is performed. When only a laser beam having a single peak wavelength based on DFB is output, a circle is indicated, and when not, an X is indicated.

  As shown in FIGS. 7A, 8 and 10, at 265 K (−8 ° C.), even if the optical output is changed from 7 W to 9 W, only the laser beam having a single peak wavelength based on DFB is used. It was not output. As shown in FIGS. 7B, 8, and 11, at 273 K (0 ° C.), only a laser beam having a single peak wavelength based on DFB is output even if the optical output is changed from 1 W to 9 W. Was not. As shown in FIGS. 7C, 8 and 12, at 283K (10 ° C.), even if the optical output is changed from at least 1 W to 4 W, only laser light having a single peak wavelength based on DFB is used. It was not output. At 303 K (20 ° C.), only the laser light having a single peak wavelength based on DFB was output even when the optical output was changed from 1 W to 3 W. At 313 K (30 ° C.), when the optical output was 1 W, only laser light having a single peak wavelength based on DFB was output. In the case of other light output (drive current), FP and other modes of oscillation were observed.

  When the wavelength is λ and the optical output is I, the function λ = f (I) connecting the data in FIG. 7 is given by the following equation.

The linear approximation formula connecting the FP data (−8 ° C.) in FIG. 7A is λ = 0.8296I + 967.15, and the square of the correlation coefficient R is. R ² = 0.9666.

The linear approximation formula connecting the DFB data (−8 ° C.) in FIG. 7A is λ = 0.27I + 972.6, and the square of the correlation coefficient R is. R ² = 0.961.

The linear approximation formula connecting the DFB data (0 ° C.) in FIG. 7B is λ = 0.175I + 973.84, and the square of the correlation coefficient R is. R ² = 0.9643.

  The linear approximation formula of FP (0 ° C.) in FIG. 7B inferred from the FP data in FIGS. 7A and 7C is λ = 1.1I + 968.

The linear approximation formula connecting the FP data (10 ° C.) in FIG. 7C is λ = 1.4I + 968.87, and the square of the correlation coefficient R is. R ² = 0.99983.

The linear approximation formula connecting the DFB data (10 ° C.) in FIG. 7C is λ = 0.1648I + 974.57, and the square of the correlation coefficient R is. R ² = 0.9343.

  FIG. 9 is a graph (A) showing the relationship between the wavelength (nm) of laser light and the optical output (au) at an LD temperature of −8 ° C. and an optical output of 6 W, and the laser beam at an LD temperature of 0 ° C. and an optical output of 6 W. (B) showing the relationship between the wavelength (nm) of the laser beam and the optical output (au), the relationship between the wavelength (nm) of the laser beam and the optical output (au) at an LD temperature of 10 ° C. and an optical output of 7 W. It is a graph (C) which shows.

  When the LD temperature is 0 ° C. and the optical output is 6 W, a single peak laser beam is output. When the LD temperature is −8 ° C., the optical output is 6 W, the LD temperature is 10 ° C., and the optical output is 7 W, two peak laser beams are output. I understand that

  Note that the demerits in the case of outside the range of 0 nm ≦ Δλ ≦ 5 nm are as follows. That is, when using the above-mentioned DFB semiconductor laser with wavelength stabilization for fiber laser excitation or solid-state laser excitation, if there is a spectrum submode, the light output of the spectrum submode does not contribute as excitation light (offset from the absorption spectrum peak). Therefore, it becomes a cause of lowering the excitation efficiency. Furthermore, the light output after excitation becomes unstable due to interference of the excitation light mode by the spectral submode.

  FIG. 14 is a graph showing the relationship between the current (A) supplied to the laser element and the optical output (W).

  It can be seen that when the drive current is increased, the optical output of the laser light increases linearly. In the figure, the spectrum was observed as follows when the current value was small (A), medium (B), and large (C).

  FIG. 15 is a graph showing the relationship between wavelength (nm) and light output (dB).

  This graph shows an emission spectrum (DFB in the graph) mainly including a gain spectrum (GAIN in the graph) of the laser light generated in the active layer of the semiconductor laser element and a selected spectrum of the wavelength selected in the diffraction grating layer. ing.

  The graphs (A), (B), and (C) in FIG. 15 show the spectra in the small drive current region (A), the middle drive current region (B), and the high drive current region (C) in FIG. 14, respectively. ing. As the drive current increases, the temperature of the semiconductor laser element increases.

In the graph of (A), it can be seen that the relationship of λp ≦ λ _DFB ≦ λp + 5 nm is satisfied, and oscillation occurs only at the wavelength (λ _DFB ) selected by the diffraction grating layer. For the corresponding drive current, a graph similar to that in FIG. 7B was obtained. Specifically, λ _DFB = λp + 2.5.

In still graph temperature rises (B), it has a relationship of λp = λ _DFB, it can be seen that the oscillating selected wavelength of the diffraction grating layer (lambda _DFB) only. A graph similar to that in FIG. 7B is obtained for the corresponding driving current.

In the graph of (C) where the temperature further increased, the relationship of λ _DFB <λp is established, and in addition to the spectrum generated due to the wavelength selected in the diffraction grating layer, the spectrum of the laser light generated in the active layer That is, the Fabry-Perot oscillation spectrum (FP in the graph) is also generated. That is, the laser light output based on the Fabry-Perot oscillation is suppressed under the conditions of the temperature (A) and (B).

As described above, in the DFB lateral multimode semiconductor laser device according to the embodiment, the relationship between the wavelength λ _DFB selected by the diffraction grating layer and the peak wavelength λp satisfies λp ≦ λ _DFB ≦ λp + 5 nm at 0 ° C. Yes. In this case, as the temperature increases, the gain peak wavelength increases at 0.3 nm / K, and the band edge wavelength selected by the diffraction grating layer increases at 0.07 nm / K. Therefore, when the wavelength λ _DFB selected by the diffraction grating layer is larger than the peak wavelength λp of the gain spectrum in a range of +5 nm or less, the respective peak positions are close to each other, so that stable output can be obtained. When it exceeds λp + 5 nm, spectra due to both distributed feedback and Fabry-Perot occur simultaneously.

  Further, in order for the transverse multimode laser light to be generated and the spectrum to be broadened, the width of the region through which the current of the active layer 3 flows needs to satisfy 10 μm or more.

  The above-described wavelength relationship can be explained in terms of a semiconductor energy band gap.

That is, referring to FIG. 3 by converting the above wavelength into an energy band gap, the above-described DFB lateral multimode semiconductor laser device includes an active layer 3, cladding layers 2 and 4 sandwiching the active layer 3, and an active layer 3. And a diffraction grating layer GR on which the laser light is incident, and the energy band gap corresponding to the peak value of the gain spectrum of the light generated in the active layer 3 is Ep, and the selection of the diffraction grating layer GR when the energy band gap corresponding to the peak value of the selected spectrum with a _{E DFB,} at 0 ° C., and satisfies the _{Ep-6.4meV ≦ E DFB ≦ Ep} .

  FIG. 16 is a graph showing the relationship between the mode gain Γg and the photon energy, and shows the gain spectrum of the multimode Fabry-Perot laser.

The oscillation condition of the Fabry-Perot type semiconductor laser is expressed by Expression (3-1) (see FIG. 18). Here, R _f and R _b are the power reflectivities of the front and rear facet mirrors, Γg is the mode gain, α _int is the coefficient of light propagation loss in the waveguide when the semiconductor laser element is a waveguide, and L is It is the resonator length. The gain spectrum before and after the oscillation threshold is shown conceptually. The formula of threshold gain in the figure is a modification of formula (3-1).

  When the drive current is increased and the output of the laser light generated in the active layer is increased, the photon energy is increased, and when the threshold gain Ep is exceeded, laser oscillation is performed, and a photon having a plurality of energies ( Multi-longitudinal mode) laser light is generated.

  The oscillation conditions of the Fabry-Perot semiconductor laser basically do not depend on the multimode or single mode. When a current is injected into the active layer and the gain exceeds the loss (when the left side> the right side in the threshold gain equation shown in FIG. 16), oscillation starts centering on the top Ep of the gain spectrum. When oscillation starts at Ep, so-called spectral hole burning occurs, in which the gain spectrum is recessed due to carrier consumption in the vicinity thereof. As a result, the gains at the photon energy points above and below Ep become relatively high, and oscillation at Ep ± ΔE (ΔE is an appropriate constant) starts. When this phenomenon occurs continuously, the photon energy of the stimulated emission light becomes a spectrum having a width composed of a plurality of bright lines. The important point is that in the Fabry-Perot type semiconductor laser, the behavior near the top of the gain spectrum dominates the oscillation characteristics.

  FIG. 17 is a graph showing the relationship between the mode gain Γg and the photon energy, and shows the gain spectrum of the transverse multimode DFB laser.

The oscillation condition of the DFB laser is expressed by Expression (3-2) (see FIG. 18). Here, r _f and r _b are the reflection coefficients of the front and rear facet mirrors, respectively, and R _f and R _b are as shown in equations (3-3) and (3-4) (see FIG. 18), respectively. There is a relationship. m is the order of the diffraction grating, and Λ is the period of the diffraction grating.

In Expression (3-2), r ₊ and r ₋ are functions proportional to the coupling coefficient κ between the active layer and the diffraction grating and inversely proportional to the deviation Δ from the Bragg condition, and both are 0 when κ = 0. It becomes. γ is a function related to γ = −i (g ² / 4-κ ² ) ^1/2 by the gain g and the coupling coefficient κ. When there is no diffraction grating, that is, when Λ = ∞ and κ = 0, Equation (3-2) matches Equation (3-1).

The formula for the DFB type laser is similar to that of the Fabry-Perot type, but r ₊ and r ₋ depend not only on the coupling coefficient κ but also on the deviation Δ and the gain g from the Bragg condition, and on the effective This is different from a Fabry-Perot laser in that iγ depends on κ and Δ in addition to g. This indicates that, in the case of a DFB laser, not only the vicinity of the top of the gain spectrum in FIG. 16 affects the oscillation, but depending on the conditions, the bottom part is also important.

Further, in the lateral multimode type DFB laser element, due to the wide injection area, spatial unevenness of the injected carriers occurs, and a local high injection region is generated. As a result, the oscillation condition of the DFB mode can be obtained even at a mode gain point (this point is referred to as _EDFB ) that is about an order of magnitude lower than the gain spectrum peak. FIG. 17 shows a conceptual diagram of the mechanism.

  FIG. 17 shows that the threshold gain Th (DFB) point of the DFB laser is lower than the threshold gain Th (FP) of the Fabry-Perot laser.

Design of the refractive index period Λ of the diffraction grating layer, whether it is higher energy (E _DFB ⁽⁺⁾ ) or lower energy (E _DFB ⁽⁻⁾ ) than Ep determined by the band gap energy of the active layer Thus, in principle, it is possible to adjust the oscillation mode. However, in order to obtain a wide region in which DFB mode oscillation is dominant, it is preferable to set to a low energy (E _DFB ⁽⁻⁾ ). This mechanism is derived from the fact that the transition of the excited carrier is easy to select the transition to the low energy side.

In order to maintain the DFB mode dominant state in the entire optical output region from 1 W to 10 W, the above-described wavelength relationship may be changed to an energy relationship. In this case, E _DFB is E _p −6.4 ( It is desirable that meV) ≦ E _DFB ≦ E _p . This effective range is a range that focuses on the relative relationship between the energy state of injected carriers and the band gap energy of the active layer, and thus does not depend on the material or the oscillation wavelength. In addition, a wide dynamic range can be obtained by setting this range in other wavelength bands such as the 800 nm band and the visible band. Note that λ _DFB and E _DFB can be adjusted by changing the period Λ of the diffraction grating layer.

  In the semiconductor laser device described above, the peak of the spectrum peak is broken into a plurality of peaks. This is because the diffraction grating width perceived by the laser light is slightly different depending on the wavelength.

  In the above-described semiconductor laser element, the output level is 10 W, but it is desirable that the output wavelength is as wide as possible (for example, from near the threshold to 10 W) with a single wavelength. When a single wavelength is described, generally speaking, a single wavelength often refers to a line width sub-pm order (for example, a communication laser). Such a laser is a so-called single mode distributed feedback laser having a light emission width of several μm, and emits laser light of only one spatial mode in accordance with the cutoff condition of the diffraction grating layer. In a single-mode distributed feedback laser, since it is a laser beam only in the fundamental mode, there is only one wavelength. However, in the case of a multimode (broad rear) DFB laser element, there are many spatial modes. Wavelengths will be displayed in their linear combination and will not be a strict single wavelength. The single wavelength property in a multimode DFB laser element is a characteristic in which (1) the line width is in the sub-nm order, and (2) the wavelength shift amount associated with the element temperature change due to the change of the driving condition is about 0.07 nm / K. Point to.

  In the semiconductor laser device described above, it is expected that the maximum output of 10 W or more and the single wavelength property in a wide output region are compatible. The output range from 4W to 6W has been a single wavelength so far, but the range has been expanded from 1W to 9W in the other output ranges which have become multi-wavelength. The semiconductor laser element is preferably a large laser device made of an InGaAs / AlGaAs-based material, having an emission width of 100 μm and an element length of 4 mm.

As described above, the period Λ of the diffraction grating layer composed of the AlGaAs-based material and the DFB oscillation wavelength λ _DFB (= (2ΛN _eff ⁰ ) / m) determined by the equivalent refractive index N _eff ⁰ of the constituent material are Λ is set so as to be longer than the oscillation gain peak λp determined by the band gap of the quantum well layer. In principle, the smaller the order m, the higher the coupling coefficient, and it is better because no extra radiation mode occurs. However, considering the balance between the corresponding wavelength and the fine processing capability of the device, it is easy to make the order. It is realistic to choose. In this embodiment, m = 3 is adopted. The coupling coefficient κ is set to be significantly smaller than the conventional DFB laser because the resonator length L is large. κL = 0.8 to 1.0 and κ = 2.0 to 2.5 (cm ⁻¹ ). In a general DFB laser, κ = 20 (cm ⁻¹ ).

  A multi-mode laser (oscillation in the transverse multi-mode) will be described. In a semiconductor laser, a difference in refractive index (more precisely, equivalent refractive index) occurs between the current injection region and the non-injection region due to the layer structure and current injection. In a semiconductor laser, a clad layer sandwiching an active layer can be approximated to a simple three-layer slab structure, that is, a structure in which a layer having a large refractive index is sandwiched between layers having a small refractive index. When there is a ridge structure, the non-injection region has a shape in which the upper low refractive index layer is cut. As a result, it has a three-stage structure of air (refractive index is 1), high refractive index layer, and low refractive index layer. When there is no shaving, that is, the equivalent refractive index is equivalent to the amount of air layer entering compared to the current injection region. Becomes smaller. When the epitaxial crystal growth direction is defined as the x-axis, the direction parallel to the emission width W as the y-axis, and the direction parallel to the resonator length L as the z-axis, the three-layer slab bridge structure has a low equivalent in the y-axis direction. A vertical three-layer structure having a refractive index region, a high equivalent refractive index region, and a low equivalent refractive index region is formed. This can be extended not only to a three-layer slab structure but also to a more complicated structure.

  Moreover, when current is injected, the refractive index of the injection region is lowered due to the injected carrier plasma effect, and the refractive index of the injection region is increased due to heat generation. Actually, since the latter contribution is increased by an order of magnitude, the current injection region has a higher refractive index. Even in the case where the ridge structure is not formed, the equivalent refractive index in the current injection region is large, and therefore the equivalent refractive index is formed in the same order of low, high, and low in the y-axis direction. In this y-axis three-layer structure, the solution satisfying the wave equation and the boundary condition at the same time is the guided mode.

  Single mode oscillation only when the width of the central high-refractive index layer is as narrow as several μm and the equivalent refractive index difference with the peripheral layer is sufficient, resulting in a single solution that satisfies the two conditions. (= Cut-off condition), and a laser manufactured under such a condition is referred to as a transverse single mode laser, with “horizontal” being related to the y-axis direction as an acronym. If there are two or more solutions, all are classified as transverse multimode lasers. Even if the ridge width is narrow, if the digging is shallow and sufficient equivalent refractive index difference is not obtained, it becomes multimode, and if the ridge width is wide but the digging is deep, it enters the cut-off condition and single It shows that it can be made into a mode.

  Based on the above contents, if the transverse multimode laser according to the present invention is defined again, the emission width is 10 μm or more, and the semiconductor laser sufficiently deviates from the above-mentioned cutoff condition. A ridge structure may be formed. Further, in order to obtain a 10 W class light output, the light emission width is desirably in the range of 50 μm to 200 μm.

As described above, at 273 K = 0 ° C., the distributed feedback lateral multimode semiconductor laser element of the embodiment suppresses Fabry-Perot (FP) oscillation when λp ≦ λ _DFB ≦ λp + 5 nm is satisfied.

That is, in the case of λ _DFB = λp (FIG. 15B), the output of the laser beam based on the Fabry-Perot oscillation was not confirmed, and only the oscillation of the selected laser beam wavelength of the diffraction grating layer was observed.

In the case of λ _DFB = λp + α (α = 2.5) (FIG. 15A), the output of the laser beam based on the Fabry-Perot oscillation is not confirmed, and only the oscillation of the selected laser beam wavelength of the diffraction grating layer is not confirmed. Was observed.

In the case of λ _DFB = λp + α (α = 5 nm) (FIG. 7B), the laser beam output based on Fabry-Perot oscillation is not confirmed, and only the oscillation of the selected laser beam wavelength of the diffraction grating layer is observed. It was.

  In addition, at 0 ° C. (= 273 K), diffraction is performed even when α = 0 nm, α = 0.19 nm, α = 1.29 nm, α = 2.0 nm, α = 3.14 nm, and α = 3.89 nm. Only oscillation of the selected laser beam wavelength of the lattice layer was observed.

  In FIG. 11, the oscillation of only DFB was selectively observed in the range of optical output 1 W to 6 W and Δλ = 0.2 to 5.0 nm at 0 ° C. When the optical output is 6.25 W, Even when Δλ = 0, only DFB oscillation was selectively observed. Note that when Δλ <0 when the optical output is increased, the spectral line width may be widened, which is excluded.

Further, in order to achieve Δλ = 0 to 5.0 nm at 0 ° C., if the initial values of λp and λ _DFB at 0 ° C. with an optical output of 1 W are λp ₀ = 969.1 nm and λ _DFB0 = 974.1 nm, The former can be determined by the composition of the active layer (energy band gap), and the latter can be determined by the period of the diffraction grating layer. _{λp = λp 0 + 0.3nm / K} , because it is _{λ DFB = λ DFB0 + 0.07nm /} K, the increase in drive current, temperature rises slightly. Therefore, the LD temperature satisfying the above condition is the ambient temperature of the laser element, that is, the temperature of the submount 20 in which the semiconductor laser element shown in FIG. 1 is disposed. It was temperature.

Further, if the relative relationship between the initial values λp ₀ = 969.1 nm and λ _DFB0 = 974.1 nm at 0 ° C. and an optical output of 1 W is out of the above-described condition of Δλ, there is a possibility that DFB and FP will oscillate simultaneously. This is inferior to the case where the conditions are met.

  DESCRIPTION OF SYMBOLS 1 ... Board | substrate, 2 ... Lower clad layer, 3 ... Active layer, 4 ... Upper clad layer, 5 ... Contact layer.

Claims (2)

In the distributed feedback type lateral multimode semiconductor laser element,
An active layer,
A clad layer sandwiching the active layer;
A diffraction grating layer overlapping the laser light generated in the active layer;
With
An energy band gap corresponding to a peak value of a gain spectrum of light generated in the active layer is represented by Ep,
The energy band gap corresponding to the peak value of the selected spectrum selected by the diffraction grating layer is represented by _EDFB ,
If
Ep−6.4 meV ≦ E _DFB ≦ Ep,
A distributed feedback lateral multimode semiconductor laser device characterized by satisfying: In the distributed feedback type lateral multimode semiconductor laser element,
An active layer,
A clad layer sandwiching the active layer;
A diffraction grating layer overlapping the laser light generated in the active layer;
With
The wavelength of the peak value of the gain spectrum of light generated in the active layer is λp,
The wavelength of the peak value of the selected spectrum selected by the diffraction grating layer is λ _DFB ,
If
λp ≦ λ _DFB ≦ λp + 5 nm,
A distributed feedback lateral multimode semiconductor laser device characterized by satisfying:

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