JP7486119B2 - Laser diode bar, wavelength beam combining system using laser diode bar, and method for manufacturing laser diode bar - Google Patents

Description

The present disclosure relates to wavelength laser diode bars, wavelength beam combining systems using laser diode bars, and methods for manufacturing laser diode bars.

Wavelength beam combining systems (WBC (Wavelength Beam Combining) systems) are known as systems that obtain a high-power laser beam by combining multiple beams with different wavelengths at a single point. For example, the system described in Patent Document 1 is an example of a WBC system.

The WBC system includes a laser diode bar (LD (Laser Diode) bar), a beam twister unit (BTU (Beam Twister Lens Unit)), a diffraction grating, and an external resonator mirror, etc.

The LD bar has multiple emitters, each of which emits a beam. The multiple beams emitted from the LD bar are individually rotated 90 degrees by the BTU. This prevents the individual spots from interfering with each other. The beam emitted from the BTU is incident on a transmission or reflection type diffraction grating, which diffracts the incident beam at a diffraction angle determined by its wavelength and emits it. The beam emitted from the diffraction grating is incident on an external resonator mirror. The external resonator mirror is a partially transmitting mirror that reflects part of the incident beam vertically in the direction of the diffraction grating. As a result, a wavelength (called the lock wavelength), which is uniquely determined by the positional relationship between the individual emitters of the LD bar, the diffraction grating, and the external resonator mirror, is fed back between the rear mirror of the LD bar and the external resonator mirror, and a laser beam is output by external resonant oscillation.

Since each emitter in the LD bar is positioned differently relative to the diffraction grating, they oscillate through external resonance at slightly different wavelengths, but because they are combined at a single point by an external resonance mirror, a high-power laser beam can be output.

JP 2015-106707 A

However, in a WBC system, if the difference between the gain peak wavelength of the LD bar (i.e. the oscillation wavelength of the LD bar resulting from the configuration of the LD bar itself, also known as the ASE (Amplified Spontaneous Emission) wavelength) and the lock wavelength due to external resonance becomes large, there is a risk that the beam will not oscillate. If only some of the multiple emitters in the LD bar can oscillate due to external resonance, the WBC system will be an inefficient system.

The present disclosure has been made in consideration of the above points, and aims to provide a laser diode bar that can improve the oscillation performance of a wavelength beam combining system, a wavelength beam combining system using the same, and a method for manufacturing the laser diode bar.

The present disclosure mainly solves the above-mentioned problems.
1. A laser diode bar for use in a wavelength beam combining system, comprising:
a nitride semiconductor substrate having a substrate surface whose surface orientation is the (0001) plane and which is provided with an off angle of greater than 0° from the (0001) plane to at least one of the m-axis and the a-axis;
a laminated structure including a first conductive type cladding layer, an active layer, and a second conductive type cladding layer formed on the nitride semiconductor substrate;
a plurality of emitters formed in a stripe shape in the laminate structure such that a waveguide direction of the emitters is perpendicular to a principal axis direction of the off-angle;
A laser diode bar comprising:

In other respects,
The laser diode bar;
a diffraction grating that diffracts the laser beams emitted from the emitters of the laser diode bar;
an external resonator mirror that reflects a part of the laser light diffracted by the diffraction grating back to the laser diode bar side and causes external resonance between the external resonator mirror and a reflective film of the laser diode bar.
Wavelength beam combining system.

In other respects,
1. A method for manufacturing a laser diode bar for use in a wavelength beam combining system, comprising:
preparing a nitride semiconductor substrate having a substrate surface oriented in the (0001) plane and having an off-angle of greater than 0° from the (0001) plane toward at least one of the m-axis and the a-axis;
forming a laminated structure including a first conductive type cladding layer, an active layer, and a second conductive type cladding layer on the nitride semiconductor substrate;
forming a plurality of emitters arranged in a stripe pattern in the laminate structure such that a principal axis direction of an off-angle of the nitride semiconductor substrate is perpendicular to a waveguide direction of the emitters;
cutting the laser diode bar having the plurality of emitters from the nitride semiconductor substrate;
The method for producing a laser diode bar includes the steps of:

The present disclosure can improve the oscillation performance of a wavelength beam combining system.

FIG. 1 is a schematic diagram of a wavelength beam combining system according to one aspect of the present disclosure. FIG. 1 is a schematic diagram of a wavelength beam combining system according to one aspect of the present disclosure. FIG. 1 is a perspective view of an LD bar according to one embodiment of the present disclosure. FIG. 1 illustrates the relationship between an LD bar and a diffraction grating according to an embodiment of the present disclosure. A diagram showing the lock wavelength of each emitter in the LD bar. A diagram showing the range of wavelengths that an LD bar can oscillate. FIG. 1 shows an example of an ASE spectrum. A diagram showing the relationship between the gain peak wavelength and the lock wavelength. A diagram showing the relationship between the gain peak wavelength and the lock wavelength. FIG. 1 is a diagram showing the principal axis direction of the off-angle of a substrate according to one embodiment of the present disclosure. FIG. 1 is a schematic diagram showing LD bars on a wafer in a method for manufacturing LD bars according to an embodiment of the present disclosure. FIG. 1 is a schematic diagram showing LD bars on a wafer in a method for manufacturing LD bars according to an embodiment of the present disclosure. FIG. 1 is a schematic diagram showing LD bars on a wafer in a method for manufacturing LD bars according to an embodiment of the present disclosure. FIG. 1 is a schematic diagram showing LD bars on a wafer in a prior art LD bar manufacturing method. FIG. 1 is a schematic diagram showing LD bars on a wafer in a prior art LD bar manufacturing method. FIG. 1 is a schematic diagram showing LD bars on a wafer in a method for manufacturing LD bars according to an embodiment of the present disclosure. FIG. 1 is a schematic diagram showing LD bars on a wafer in a method for manufacturing LD bars according to an embodiment of the present disclosure. FIG. 1 is a schematic diagram showing LD bars on a wafer in a method for manufacturing LD bars according to an embodiment of the present disclosure. FIG. 1 is a schematic diagram showing LD bars on a wafer in a method for manufacturing LD bars according to an embodiment of the present disclosure. FIG. 1 is a schematic diagram showing LD bars on a wafer in a method for manufacturing LD bars according to an embodiment of the present disclosure. FIG. 1 is a schematic diagram showing LD bars on a wafer in a method for manufacturing LD bars according to an embodiment of the present disclosure.

Embodiments of the present disclosure are described below with reference to the drawings.

Figure 1 is a schematic diagram of a wavelength beam combining system 10. An actual wavelength beam combining (WBC) system has components other than those shown in Figure 1, but these are omitted.

The WBC system 10 has a laser diode bar array (hereinafter also referred to as LD bar array) 100A including one or more LD bars 100, a diffraction grating 200, and an external cavity mirror 300. Note that an optical system such as a BTU (not shown) may be provided between the LD bar 100 and the diffraction grating 200. The LD bar 100 constitutes the laser diode bar array 100A.

In this embodiment, a transmissive diffraction grating is used as the diffraction grating 200, but the technology disclosed herein can also be applied to a WBC system 10' that uses a reflective diffraction grating 200' as shown in FIG. 2.

Figure 3 shows a perspective view of a laser diode bar (hereinafter also referred to as an LD bar) 100. The LD bar 100 has multiple emitter injection regions (hereinafter also referred to as emitters) 101 formed at intervals from each other. The multiple emitters are arranged in a stripe shape in a line along the longitudinal direction of the LD bar 100. When a voltage is supplied in parallel from a power supply unit (not shown) to all the emitters 101 in the LD bar 100, laser beams are simultaneously emitted from the laser elements corresponding to each emitter 101 in the waveguide direction (i.e., the external resonance direction).

The emitter 101 of the LD bar 100 is formed, for example, on a nitride semiconductor substrate as a laminate of a first conductive cladding layer, an active layer, and a second conductive cladding layer. The LD bar 100 has a P-side electrode on its upper surface and an N-side electrode on its lower surface (not shown in FIG. 3). A transparent film is provided on the entire end surface of the LD bar 100 on the side from which light is emitted, and a reflective film is provided on the entire end surface opposite the transparent film on the side from which light is not emitted (not shown in FIG. 3).

In FIG. 3, a laser structure is shown having a current injection region in the shape of a protruding stripe called a ridge stripe, but other laser structures may be used. The LD bar 100 may have a buried structure, in which case the current injection region 101 may be selectively formed.

In the WBC system 10, 10', among the beams emitted from each emitter of the LD bar 100, the wavelengths that satisfy the diffraction conditions of the diffraction grating 200 and are vertically reflected by the external resonator mirror 300 are returned to the original emitter section, causing external resonance and enabling laser oscillation.

The oscillation wavelength of each LD bar 100 and each emitter 101 is uniquely determined by the arrangement of the diffraction grating 200 and the LD bar 100. This wavelength is called the locking wavelength.

When the difference between the gain peak wavelength of the LD bar 100 (i.e., the oscillation wavelength of the LD bar 100 resulting from the configuration of the LD bar 100) and the locking wavelength due to external resonance becomes large, it becomes difficult for the beam to oscillate.

Here, in the diffraction grating 200, if the period of the diffraction grating is d, the incident angle is α, the exit angle is β, the wavelength is λ, and the order is m, then the diffraction condition of the diffraction grating 200 can be expressed by the following formula (1).
d(sinα+sinβ)=mλ...Equation (1)

Note that we choose a diffraction grating arrangement where the only effective order is m=1.

Figure 4 shows the relationship between one LD bar 100 and the diffraction grating 200. Figure 5 shows an example of the locking wavelength of the WBC system 10 for each emitter 101 in one LD bar 100. In the example of Figure 5, 50 emitters 101 are formed in one LD bar 100, and the length from the first emitter 101 to the 50th emitter 101 (length W of the LD bar 100 in Figure 4) is 10 mm.

When a diffraction grating (d=0.333 μm) with a groove period of 3000 lines/mm is used and light with a wavelength of about 400 to 500 nm is set to an incident angle α of 45°, when the length W of LD bar 100 is 10 mm and the distance L from LD bar 100 to the diffraction grating is 2.6 m, the difference in lock wavelength Δλ _{EC_bar} between emitters 101 present at both ends of LD bar 100 (hereinafter referred to as "lock wavelength difference Δλ _{EC_bar} between both ends of LD bar 100") is calculated to be about 1.0 nm. Similarly, when the length W of LD bar 100 is 10 mm and the distance L from LD bar 100 to the diffraction grating is 1.3 m, the difference in lock wavelength Δλ _{EC_bar} between both ends of LD bar 100 is about 2.0 nm.

When the LD bar 100 is applied to the WBC system 10, the angle of incidence of the laser light emitted from the emitter 101 on the diffraction grating 200 changes depending on the position of the emitter 101 in the LD bar 100 in the longitudinal direction of the LD bar 100. Here, the lock wavelength of each emitter 101 in the LD bar 100 typically gradually decreases or increases along the longitudinal direction of the LD bar 100. For example, within a 10 mm LD bar 100, the lock wavelength of the emitter 101 on one end differs from the lock wavelength of the emitter 101 on the other end by about 1 to 2 nm.

Figure 6 shows the range of wavelengths at which one emitter 101 of the LD bar 100 can oscillate. The curve in Figure 6 is the gain spectrum (hereinafter also referred to as the ASE spectrum) of the emitter 101, and shows the wavelength dependency of the gain of the LD bar 100. The LD bar 100 can oscillate only at wavelengths where the gain is equal to or greater than a certain value. In other words, the LD bar 100 will oscillate at lock wavelengths within a certain range from the gain peak wavelength, and will not oscillate at lock wavelengths outside the certain range. In the example of Figure 6, lock wavelength 1 oscillates because it is within the range of wavelengths that can be oscillated, but lock wavelength 2 does not oscillate because it is outside the range of wavelengths that can be oscillated.

7 shows a typical example of the ASE spectrum of the emitter 101 of the LD bar 100. The ASE spectrum is defined as the EL spectrum (electroluminescence, current injection emission spectrum) of the LD bar 100 before oscillation, and in this disclosure, is defined as the EL spectrum when the injection current value I=0.8×I _th , where I _th is the threshold current of the internal resonant oscillation of the LD bar 100. λ _ASE is the peak wavelength of the ASE spectrum.

Based on the peak intensity of the ASE spectrum, the bandwidth B2W _{ASE_bar} at slice level SL value of 0.8 or more is 3.2 nm, and the bandwidth B1W _{ASE_bar} at SL value of 0.9 or more is 1.2 nm. To realize laser oscillation by external resonance in the WBC system 10, 10', it is important that the lock wavelength by external resonance is within the range of λ _ASE ±(B2W _{ASE_bar} /2), i.e., λ _ASE ±1.6 nm. Here, if the lock wavelength by external resonance is located within the range of λ _ASE ±(B1W _{ASE_bar} /2), i.e., λ _ASE ±0.6 nm, the WBC system 10, 10' can have even higher performance.

Figures 8 and 9 show the relationship between the gain peak wavelength (hereinafter also referred to as the ASE wavelength) of each emitter 101 of the LD bar 100 and the locking wavelength by the WBC system 10. As described above, the locking wavelength has a slope corresponding to the change in the angle of incidence to the diffraction grating 200. In addition, in Figures 8 and 9, the distribution of the gain peak wavelengths of each emitter 101 of the LD bar 100 has different slopes due to differences in the manufacturing methods of the LD bar 100 (described later with reference to Figure 10).

As in the example of Figure 8, when the distribution of gain peak wavelengths in the LD bar 100 is in the same direction as the positive and negative slope of the lock wavelength, and the difference between the gain peak wavelength and the lock wavelength in all emitters 101 is within a specified range, all emitters 101 can oscillate. Also, in Figure 8, the slope of the distribution of gain peak wavelengths in the LD bar 100 is close to the slope of the distribution of lock wavelengths at each position along the longitudinal direction of the LD bar 100. Therefore, at each position in the LD bar 100, the gain peak wavelength of the emitter 101 is close to the lock wavelength, making it possible to maximize the output of each emitter 101.

In contrast, as in the example of Figure 9, if part of the distribution of gain peak wavelengths in the LD bar 100 falls outside the range of the lock wavelengths at both ends of the LD bar 100, some of the emitters 101 that fall outside that range cannot oscillate.

In the example of FIG. 9, the gain peak wavelength of each emitter 101 of the LD bar 100 can be kept within the range of the lock wavelength at both ends of the LD bar 100 by raising the distribution of the gain peak wavelengths in the LD bar 100 as a whole. However, even with this configuration, the gradient of the distribution of the gain peak wavelengths in the LD bar 100 is smaller than the gradient of the distribution of the lock wavelengths at each position along the longitudinal direction of the LD bar 100. Therefore, among the multiple emitters 101 in the LD bar 100, there will be emitters 101 whose gain peak wavelength deviates from the lock wavelength and whose output is reduced.

From this perspective, the manufacturing method of the LD bar 100 according to the present disclosure adjusts the relationship between the formation direction of the emitter 101 and the principal axis direction of the off-angle of the nitride semiconductor substrate in order to adjust the distribution of the gain peak wavelengths of each emitter 101 in the LD bar 100 as shown in FIG. 8.

The manufacturing method of the LD bar 100 will be described with reference to FIG. 10. In manufacturing the LD bar 100, first, a semiconductor laser stack structure including a light emitting layer is formed on the wafer 400 by epitaxial growth, then a ridge stripe structure is formed as the emitter 101 portion on the stacked wafer 400, and then a P-side electrode and an N-side electrode are formed. Next, multiple LD bars 100 are cut out, and a high-reflection coating film is formed on the rear end face of the LD bar, and an anti-reflection coating film is formed on the front end face. Furthermore, the cut out multiple LD bars 100 are combined to produce a laser diode bar array 100A used in the WBC system 10.

A nitride semiconductor substrate is used as the wafer 400, and it is particularly preferable to use a GaN substrate. When manufacturing a semiconductor laser with a wavelength band of 350 nm or more and 550 nm or less, it is preferable to use a GaN substrate as the base wafer.

Normally, GaN substrates are tilted at a certain off-angle (greater than 0°, approximately 0.3° to 0.7°) with respect to a certain axis in order to improve the crystal quality of the active layer. In general, the substrate surface of a GaN substrate has a plane orientation of (0001), and the main axis direction of the off-angle (representing the tilt direction of the substrate surface with respect to a crystal plane with an off-angle of 0°; the same applies below) is set to the ±m-axis or ±a-axis directions. In FIG. 10, the main axis direction of the off-angle of the substrate surface of wafer 400 is set to the +a-axis direction.

In addition, since GaN substrates are formed by crystal growth using heterogeneous substrates with different thermal expansion coefficients, GaN substrates are generally more susceptible to crystal warping than Si or GaAs. According to the findings of the present inventors, when a GaN substrate is cut from an ingot with an off-angle on the (0001) plane of the GaN substrate due to the crystal warping in the GaN substrate, the off-angle gradually increases along the main axis of the off-angle within the substrate plane of the GaN substrate, as shown in FIG. 10. The emission wavelength of the semiconductor laser when the InGaN layer is epitaxially grown (i.e., the gain peak wavelength) changes at a rate of about 30 nm/° according to the change in the off-angle of the GaN substrate.

The off-angle distribution within the GaN substrate plane has a significant effect on the performance of the semiconductor laser's light-emitting layer, particularly on the emission wavelength. In general, when manufacturing a semiconductor laser with a wavelength band of 350 nm or more and 550 nm or less, it is preferable to use an InGaN layer containing In for the light-emitting layer. The In composition of the InGaN layer is affected by the off-angle distribution within the GaN substrate plane, and in areas with large off-angles, the In composition is small, and the emission wavelength tends to be short.

In other words, the emission wavelength of each of the multiple emitters 101 formed on the LD bar 100 varies depending on the off-angle of the wafer 400 at the position where the emitter 101 is formed (see, for example, Figures 8 and 9).

Based on this knowledge, the inventors of the present application have optimized the manufacturing method of the LD bar 100 and have come up with the technical idea of forming the LD bar 100 so that the waveguide direction of the LD bar 100 is perpendicular to the main axis direction of the off-angle of the substrate surface of the wafer 400.

This configuration allows the multiple emitters 101 of the LD bar 100 to be arranged along the direction in which the off-angle of the wafer 400 gradually decreases or increases, so that the distribution of the emission wavelengths (gain peak wavelengths) of the multiple emitters 101 can be gradually decreased or increased along the longitudinal direction of the LD bar 100. In other words, this allows the distribution of the gain peak wavelengths at each position in the longitudinal direction of the LD bar 100 to be inclined so as to match the inclination of the distribution of the lock wavelengths at each position in the longitudinal direction of the LD bar 100, and as shown in FIG. 8, it is possible to make the gain peak wavelength exist near the lock wavelength in each emitter 101 of the LD bar 100. This allows all the emitters 101 of the LD bar 100 to oscillate, and in addition, it is possible to maximize the output of each emitter 101.

In addition, this configuration makes it possible to make the gain peak wavelength uniform along the waveguide direction of the emitter 101. This makes it possible to maximize the output of each emitter 101 of the LD bar 100.

The main axis direction of the off-angle of the substrate surface of the LD bar 100 being perpendicular to the waveguide direction of the LD bar 100 means that the angle between the main axis direction of the off-angle of the substrate surface and the waveguide direction is approximately 90°. In reality, the main axis direction of the off-angle of the substrate surface is not exactly perpendicular to the waveguide direction, but has a certain angle from perpendicular.

Below, we will explain the results of verifying the performance of the LD bar 100 manufactured using the above manufacturing method.

Example 1
In this embodiment, the LD bar 100 was formed on a GaN substrate with the main axis direction of the off-angle: +a-axis direction, the off-angle gradient: 0.004°/mm, and the central off-angle: 0.46°, as shown in FIG. 10, so that the lock wavelength of each emitter 101 of the LD bar 100 is included in the oscillation range of the gain peak wavelength of the emitter 101 of the LD bar 100. The LD bar 100 has a bar length of 10 mm and a resonator length of 2 mm. As shown in FIG. 10, the main axis direction of the off-angle in the wafer 400 plane is the +a-axis direction, so that the main axis direction of the off-angle exists in the longitudinal direction of the LD bar 100 cut out from the wafer 400.

11 shows the distribution width of the peak wavelength λ _ASE of the ASE spectrum of each LD bar 100 in this embodiment in the longitudinal direction of the LD bar 100 (see the numerical values in each LD bar 100 in FIG. 11). In this embodiment, the lock wavelength difference Δλ _{EC_bar} between both ends of the LD bar 100 is 2.0 nm. The longitudinal size of the LD bar 100 is 10 mm. LD bars 100 whose lock wavelength falls within the λ _ASE ±1.6 nm distribution range of the emitter 101 of the LD bar 100 are shown in white, and LD bars 100 whose lock wavelength distribution is outside the λ _ASE ±1.6 nm distribution range of the emitter 101 at the center and both ends of each LD bar 100 are shown in black.

11, in all of the multiple LD bars 100 (60 bars/60 bars) formed on the surface of the wafer 400, the lock wavelength distribution is included within the range of λ _ASE ±1.6 nm distribution of the emitters 101 at the center and both ends of each LD bar 100. Therefore, all of the LD bars 100 manufactured from the wafer 400 are capable of laser oscillation by external resonance.

12 shows the distribution width (see the numerical values in each LD bar 100 in FIG. 12) of the peak wavelength λ _ASE of the ASE spectrum of each LD bar 100 formed on the wafer 400 in this embodiment in the longitudinal direction of the LD bar 100. In FIG. 12, in the same wafer 400 as in FIG. 11, LD bars 100 whose lock wavelength distribution is included within the range of λ _ASE ±0.6 nm distribution of the emitters 101 at the center and both ends of each LD bar 100 are shown in white, and LD bars 100 whose lock wavelength distribution is outside the range of λ _ASE ±0.6 nm distribution of the emitters 101 at the center and both ends of each LD bar 100 are shown in black. The lock wavelength difference Δλ _{EC_bar} between both ends of the LD bar 100 is 2.0 nm, and the LD bar length is 10 mm.

12, within the surface of the wafer 400 of this embodiment, although there are LD bars 100 whose lock wavelength distribution is outside the range of λ _ASE ±0.6 nm of the emitters 101 at the center and both ends of each LD bar 100, 81.7% (49 bars/60 bars) of the LD bars 100 are white, that is, the LD bars 100 whose lock wavelength distribution is included within the range of λ _ASE ±0.6 nm distribution of the emitters 101 at the center and both ends of each LD bar 100. Therefore, it can be seen that a relatively high proportion of LD bars 100 capable of realizing higher performance WBC systems 10, 10' can be obtained.

In this embodiment, the main axis direction of the off-angle of the GaN wafer 400 is the +a-axis direction, and there is almost no off-angle in the waveguide direction, i.e., the m-axis direction (0.0005°/mm or less). Therefore, as shown in FIG. 10, the LD bar 100 cut out from the wafer 400 has a uniform ASE spectrum peak wavelength λ _ASE in the waveguide direction. The LD bar 100 having a uniform λ _ASE in the waveguide direction can maximize the gain of laser oscillation. Therefore, by using such an LD bar 100, it is possible to increase the efficiency and output of the WBC system 10, 10'.

13 shows the distribution width in the waveguide direction of the peak wavelength λ _{ASE of the ASE spectrum of each LD bar 100 formed on the wafer 400 in this embodiment (see the numerical values in each LD bar 100 in FIG. 13). In FIG. 13, the λ ASE distribution} in the waveguide direction of the 2 mm-long LD bar 100 on the wafer 400 surface is shown as white for LD bars 100 that are within the range of λ _ASE ±0.6 nm of the emitter 101 at the center and both ends of the LD bar 100, and as black for LD bars 100 that are outside the range.

13, 98% (59 bars/60 bars) of the LD bars have a λ _ASE distribution in the waveguide direction within the plane of the wafer 400 that is within the range of λ _ASE ±0.6 nm of the emitters 101 at the center and both ends of the LD bar 100. This shows that a high percentage of the LD bars 100 have λ _ASE within a certain range in the waveguide direction, and that high-quality LD bars 100 can be manufactured with a high yield when the LD bars 100 are manufactured from the wafer 400.

As a product target, it is particularly preferable to use the LD bars 100 shown in white in both Figures 12 and 13 among the LD bars 100 formed in the wafer 400 surface. That is, it is particularly preferable that the lock wavelength distribution of the LD bar 100 is within the range of λ _ASE ±1.6 nm distribution, and the peak wavelength λ _ASE distribution of the ASE spectrum in the waveguide direction is within the range of λ _ASE ±0.6 nm of the emitters 101 at the center and both ends of the LD bar 100. It is noted from Figures 12 and 13 that 80% (48 bars/60 bars) of the LD bars 100 satisfy both conditions in the wafer surface. Therefore, it is understood that it is possible to obtain very high quality LD bars 100 capable of maximizing the output of the WBC system with a high yield.

Fig. 14 shows the distribution width (see the numerical values in each LD bar 100 in Fig. 14) of the peak wavelength λ _ASE of the ASE spectrum of each LD bar formed on a wafer in the longitudinal direction of the LD bar 100 in a conventional LD bar manufacturing method. In Fig. 14, an LD bar is formed on a GaN substrate with a main axis direction of the off angle: +m-axis direction, an off angle gradient: 0.009°/mm, and a central off angle: 0.38°. The bar length of each LD bar is 10 mm, and the resonator length is 2 mm.

In Fig. 14, LD bars on the wafer whose lock wavelength distribution is within the λ _ASE ±1.6 nm distribution range of the emitters 101 at the center and both ends of each LD bar are shown in white, and LD bars whose lock wavelength distribution is outside the λ _ASE ±1.6 nm distribution range of the emitters 101 at the center and both ends of each LD bar are shown in black. The lock wavelength difference Δλ _{EC_bar} between both ends of the LD bar 100 is 2.0 nm. Within the wafer surface in Fig. 14, only 65% (39 bars/60 bars) of the LD bars have a lock wavelength within the λ _ASE ±1.6 nm distribution range of the emitters 101 at the center and both ends of each LD bar.

Fig. 15 shows the distribution width (see the numerical values in each LD bar 100 in Fig. 15) in the waveguide direction of the peak wavelength λ _ASE of the ASE spectrum of each LD bar formed on a wafer in a conventional LD bar manufacturing method, similar to Fig. 14. In Fig. 15, LD bars 100 in which the peak wavelength λ _ASE distribution of the ASE spectrum in the waveguide direction of a 2 mm length of LD bars on the wafer is within the range of λ _ASE ±0.6 nm of the emitter 101 at the center and both ends of each LD bar 100 is shown as white, and LD bars outside that range are shown as black.

In the wafer of Fig. 15, the main axis direction of the off-angle is the +m-axis direction, so that a wavelength distribution due to the off-angle distribution occurs in the manufactured LD bar. Therefore, sufficient uniformity of the peak wavelength λ _ASE of the ASE spectrum in the waveguide direction cannot be obtained, and only 81% (49 bars/60 bars) of the LD bars in the wafer plane of Fig. 15 have a λ _ASE distribution related to the peak wavelength of the ASE spectrum in the waveguide direction within the range of λ _ASE ±0.6 nm of the center and both ends of the LD bar 100.

14 and 15, only 55% (34 bars/60 bars) of the LD bars 100 have a lock wavelength distribution within the range of λ _ASE ±1.6 nm and a λ _ASE distribution in the waveguide direction within the range of λ _ASE ±0.6 nm of the emitters 101 at the center and both ends of the LD bar 100. Therefore, the manufacturing method according to the conventional technology reduces the yield in manufacturing high-quality LD bars 100.

As described above, by comparing the LD bar 100 formed by the manufacturing method of this embodiment with the LD bar formed by the manufacturing method of the prior art, it is possible to manufacture a high-output LD bar 100 by setting the main axis direction of the off-angle of the substrate surface of the LD bar 100 perpendicular to the waveguide direction of the LD bar 100.

Example 2
In this example, the LD bar 100 is formed on a GaN wafer 400 with the off-angle main axis direction: +a-axis, the off-angle gradient: 0.009°/mm, and the central off-angle: 0.44° in the same manner as in Example 1. As in the first embodiment, the LD bar 100 has a bar length of 10 mm and a resonator length of 2 mm. This example differs from Example 1 in that the off-angle gradient is set to 0.009°/mm.

16 shows the distribution width of λ _ASE of each LD bar 100 formed on the wafer 400 in this embodiment in the longitudinal direction of the LD bar 100 (see the numerical values within each LD bar 100 in FIG. 16). The lock wavelength difference Δλ _{EC_bar} between both ends of the LD bar 100 is 2.0 nm. In FIG. 16, LD bars 100 whose lock wavelength distribution is included within the range of λ _ASE ±0.6 nm distribution of the emitters 101 at the center and both ends of each LD bar 100 are shown in white, and LD bars 100 whose lock wavelength distribution is outside the range of λ _ASE ±0.6 nm distribution of the emitters 101 at the center and both ends of each LD bar 100 are shown in black.

16, within the surface of the wafer 400 of this embodiment, there are LD bars 100 whose lock wavelength distribution is outside the range of λ _ASE ±0.6 nm of the emitters 101 at the center and both ends of each LD bar 100, but 73.3% (44 bars/60 bars) of the LD bars 100 are white, that is, the LD bars 100 whose lock wavelength distribution is included within the range of λ _ASE ±0.6 nm distribution of the emitters 101 at the center and both ends of each LD bar 100. Therefore, it can be seen that even when the off-angle gradient is set to 0.009°/mm, it is possible to obtain LD bars 100 capable of realizing higher performance WBC systems 10, 10' at a relatively high rate.

17 shows the distribution width in the waveguide direction of λ _ASE (see the numerical values in each LD bar 100 in FIG. 17) of each LD bar 100 formed on the wafer 400 in this embodiment. In FIG. 17, the λ _ASE distribution in the waveguide direction of the 2 mm-long LD bars 100 on the wafer 400 surface is shown as white for LD bars that are within the λ _ASE range of ±0.6 nm of the emitter 101 at the center and both ends of each LD bar 100, and as black for LD bars 100 that are outside that range.

17, 86.7% (52 bars/60 bars) of the LD bars 100 have a λ _ASE distribution in the waveguide direction within the plane of the wafer 400 that is within the range of λ _ASE ±0.6 nm of the emitters 101 at the center and both ends of the LD bar 100. This shows that a high percentage of the LD bars 100 have λ _ASE within a certain range in the waveguide direction, and that high-quality LD bars 100 can be manufactured with a high yield when manufacturing the LD bars 100 from the wafer 400.

16 and 17, LD bars 100 whose lock wavelength distribution is within the range of λ _ASE ±1.6 nm and whose λ _ASE distribution in the waveguide direction is within the range of λ ASE ±0.6 _nm of the emitters 101 at the center and both ends of the LD bar 100 account for 61.7% (38 bars/60 bars) of the LD bars 100 formed on the surface of the wafer 400. It can therefore be seen that very high quality LD bars 100 capable of maximizing the output of the WBC systems 10, 10' can be obtained with a high yield.

In Examples 1 and 2, the main axis direction of the off-angle of the wafer 400 forming the LD bar 100 is the +a-axis direction, the off-angle gradient is 0.004 to 0.009°/mm, and the bar length and resonator length of the LD bar 100 are 10 mm and 2 mm, respectively, but it is possible to use various LD bar 100 lengths by optimizing the off-angle gradient.

In order to optimize the WBC systems 10 and 10', it is preferable to set the off-angle gradient ΔD in the longitudinal direction of the LD bar 100 so as to be within the range of the following formula (2).
0<ΔD≦((Δλ _{EC_bar} + 3.2)/Lt)/30...Equation (2)
(where Lt (mm) is the bar length of the LD bar 100, and Δλ _{EC_bar} is the lock wavelength difference (nm) between both ends of the LD bar 100)

Furthermore, in order to construct a high-performance WBC system 10, 10', it is preferable to set the off-angle gradient ΔD with respect to the longitudinal direction of the LD bar 100 so as to be within the range of the following formula (3).
((Δλ _{EC_bar} −1)/Lt)/0.3≦ΔD≦(Δλ _{EC_bar} +1)/Lt)/30...Equation (3)

The above formulas (2) and (3) are suitable conditions when the following formula (4) is satisfied.
0>((Δλ _{EC_bar} −1.2)/Lt)/30 ... Equation (4)

However, in the state of the following formula (5), the WBC systems 10 and 10' may be set so that the lock wavelength difference between both ends of the LD bar 100 falls within the range of the following formula (6).
0>((Δλ _{EC_bar} −1.2)/Lt)/30...Equation (5)
0<ΔD≦((Δλ _{EC_bar} + 1.2)/Lt)/30...Equation (6)

In addition, from the viewpoint of obtaining a high-quality LD bar 100 capable of maximizing the output of the WBC system 10, 10', it is preferable that the off-angle gradient ΔD satisfies both of the above formulas (2) and (3), or both of the above formulas (2) and (4).

Example 3
In this embodiment, the LD bar 100 is formed on a GaN wafer 400 with an off-angle in the main axis direction plus the a-axis direction, an off-angle gradient of 0.004°/mm, and a central off-angle of 0.46°, in the same manner as in embodiment 1. Also, like embodiment 1, the bar length of the LD bar 100 is 10 mm. This embodiment differs from embodiment 1 in that the resonator length of the LD bar 100 is set to 1 mm.

Fig. 18 shows the distribution width in the waveguide direction of λ _ASE (see the numerical values in each LD bar 100 in Fig. 18) of each LD bar formed on the wafer 400 in this embodiment. In Fig. 18, the λ _ASE distribution in the waveguide direction of the 2 mm-long LD bars 100 on the wafer 400 surface is shown in white for LD bars 100 that are within the range of λ _ASE ±0.6 nm of the emitter 101 at the center and both ends of each LD bar 100, and LD bars 100 that are outside that range are shown in black.

18, the λ _ASE distribution in the waveguide direction is small for LD bars 100 having a resonator length of 1 mm, and 100% (120 bars/120 bars) of the LD bars have a λ _ASE distribution in the waveguide direction within the plane of the wafer 400 that is within the range of λ _ASE ±0.6 nm of the center and the emitters 101 at both ends of the LD bar 100. From this, it is found that even when the resonator length is 1 mm, a high percentage of LD bars 100 can be obtained in which the λ _ASE in the waveguide direction is within a certain range, and that high-quality LD bars 100 can be manufactured with a high yield when manufacturing the LD bars 100 from the wafer 400.

Example 4
In this example, the LD bar 100 is formed on the GaN wafer 400 with an off-angle in the main axis direction plus the a-axis direction, an off-angle gradient of 0.004°/mm, and a central off-angle of 0.46°, in the same manner as in Example 1. As in embodiment 1, the bar length of the LD bar 100 is 10 mm. This example differs from Example 1 in that the resonator length of the LD bar 100 is set to 4 mm.

Fig. 19 shows the distribution width in the waveguide direction of λ _ASE (see the numerical values in each LD bar 100 in Fig. 19) of each LD bar formed on a wafer 400 in this embodiment. In Fig. 19, the λ _ASE distribution in the waveguide direction of a 2 mm length of the LD bar 100 on the surface of the wafer 400 is shown in white for LD bars that are within the range of λ _ASE ±0.6 nm of the emitter 101 at the center and both ends of each LD bar, and shown in black for LD bars that are outside the range.

19, even for LD bars 100 having a resonator length of 4 mm, 90% (27 bars/30 bars) of the LD bars have a λ _ASE distribution in the waveguide direction within the plane of the wafer 400 that is within the range of λ _ASE ±0.6 nm of the λ ASE of the emitters 101 at the center and both ends of the LD bar 100. From this, it is found that even when the resonator length is 4 mm, a high percentage of LD bars 100 can be obtained in which the λ _ASE in the waveguide direction is within a certain range, and that high-quality LD bars 100 can be manufactured with a high yield when manufacturing LD bars 100 from wafer 400.

Example 5
In this example, the LD bar 100 is formed on the GaN wafer 400 with an off-angle in the main axis direction plus the a-axis direction, an off-angle gradient of 0.004°/mm, and a central off-angle of 0.46°, in the same manner as in embodiment 1. As in example 1, the bar length of the LD bar 100 is 10 mm. This example differs from example 1 in that the resonator length of the LD bar 100 is set to 6 mm.

Fig. 20 shows the distribution width in the waveguide direction of λ _ASE (see the numerical values in each LD bar 100 in Fig. 20) of each LD bar formed on the wafer 400 in this embodiment. In Fig. 20, the λ _ASE distribution in the waveguide direction of the 2 mm-long LD bars 100 on the surface of the wafer 400 is shown in white for LD bars 100 that are within the range of λ _ASE ±0.6 nm of the emitter 101 at the center and both ends of each LD bar 100, and LD bars 100 that are outside that range are shown in black.

20, even for LD bars 100 having a resonator length of 6 mm, 78% (14 bars/18 bars) of LD bars 100 have a λ _ASE distribution in the waveguide direction within the plane of wafer 400 that is within the range of λ _ASE ±0.6 nm of the λ ASE of the emitters 101 at the center and both ends of the LD bar 100. From this, it is found that even when the resonator length is 4 mm, a high percentage of LD bars 100 can be obtained in which λ _ASE in the waveguide direction is within a certain range, and that high-quality LD bars 100 can be manufactured with a high yield when manufacturing LD bars 100 from wafer 400.

Example 6
In this embodiment, the LD bar 100 is formed on the GaN wafer 400 with an off-angle in the main axis direction plus the a-axis direction, an off-angle gradient of 0.004°/mm, and a central off-angle of 0.46°, in a manner similar to that of embodiment 1. As in embodiment 1, the bar length of the LD bar 100 is 10 mm. This embodiment differs from embodiment 1 in that the resonator length of the LD bar 100 is set to 8 mm.

Fig. 21 shows the distribution width (see the numerical values within each LD bar 100 in Fig. 21) of λ _ASE within each LD bar 100 (waveguide direction) of each LD bar 100 formed on a wafer 400 in this embodiment. In Fig. 21, the λ _ASE distribution in the waveguide direction of a 2 mm length of the LD bar 100 within the surface of the wafer 400 is shown in white for LD bars that are within the λ _ASE range of ±0.6 nm of the emitter 101 at the center and both ends of each LD bar, and shown in black for LD bars that are outside that range.

21, even for LD bars 100 having a resonator length of 8 mm, 64% (9 bars/14 bars) of the LD bars have a λ _ASE distribution in the waveguide direction within the plane of the wafer 400 that is within the range of λ _ASE ±0.6 nm of the emitters 101 at the center and both ends of the LD bar 100. This shows that even when the resonator length is 4 mm, a high percentage of LD bars 100 can be obtained in which λ _ASE in the waveguide direction is within a certain range, and that high-quality LD bars 100 can be manufactured with a high yield when manufacturing LD bars 100 from wafer 400.

In the above-mentioned Examples 1 to 6, LD bars with a resonator length of 1 mm to 8 mm are created, and laser oscillation is realized by external resonance to construct the WBC systems 10, 10'. Considering that the typical resonator length used in a normal internal resonance type single LD is about 0.3 mm to 1 mm, laser oscillation can be realized by external resonance if it is 0.3 mm or more. Also, from the viewpoint of yield, it is desirable for the resonator length to be 1 mm or more and 6 mm or less according to Examples 1 to 6.

The laser diode bar and wavelength beam combining system disclosed herein can improve the oscillation performance of the wavelength beam combining system and can therefore be applied to high power processing systems.

10, 10' Wavelength beam combining system 100 Laser diode bar 100A Laser diode bar array 101 Injection region 200, 200' Diffraction grating 300 External cavity mirror 400 Wafer

Claims (11)

1. A laser diode bar for use in a wavelength beam combining system, comprising:
a nitride semiconductor substrate having a substrate surface whose surface orientation is the (0001) plane and which is provided with an off angle of greater than 0° from the (0001) plane to at least one of the m-axis and the a-axis;
a laminated structure including a first conductive type cladding layer, an active layer, and a second conductive type cladding layer formed on the nitride semiconductor substrate;
a plurality of emitters formed in a stripe shape in the laminate structure such that a waveguide direction of the emitters is perpendicular to a principal axis direction of the off angle;
Equipped with
A gradient of the distribution of the off-angle in the principal axis direction satisfies the following formula (1) with respect to a gradient of the distribution of the lock wavelengths of the multiple emitters:
Laser diode bar.
0<ΔD≦((Δλ _{EC_bar} + 3.2) / Lt) / 30 ... Formula (1)
(wherein ΔD (angle/mm) represents the gradient of the distribution of the off-angle in the principal axis direction, Lt (mm) represents the length of the laser diode bar in the longitudinal direction, and Δλ _{EC_bar} (nm) represents the lock wavelength difference between both ends of the laser diode bar in the longitudinal direction.) The gradient of the distribution of the off-angle in the principal axis direction and the distribution of the lock wavelengths of the plurality of emitters satisfy the following formula (2) :
2. The laser diode bar according to claim 1.
((Δλ _{EC_bar} −1)/Lt)/0.3≦ΔD≦((Δλ _{EC_bar} +1)/Lt)/30...Equation (2)
(wherein ΔD (angle/mm) represents the gradient of the distribution of the off-angle in the principal axis direction, Lt (mm) represents the length of the laser diode bar in the longitudinal direction, and Δλ _{EC_bar} (nm) represents the lock wavelength difference between both ends of the laser diode bar in the longitudinal direction.) The gradient of the distribution of the off-angle in the principal axis direction and the distribution of the lock wavelengths of the plurality of emitters satisfy the following formula (3) :
2. The laser diode bar according to claim 1.
0<ΔD≦((Δλ _{EC_bar} + 1.2)/Lt)/30...Equation (3)
(wherein ΔD (angle/mm) represents the gradient of the distribution of the off-angle in the principal axis direction, Lt (mm) represents the length of the laser diode bar in the longitudinal direction, and Δλ _{EC_bar} (nm) represents the lock wavelength difference between both ends of the laser diode bar in the longitudinal direction.) The slope of the distribution of the lock wavelengths of the plurality of emitters is in the same positive and negative directions as the slope of the distribution of the ASE (Amplified Spontaneous Emission) wavelengths of the plurality of emitters.
2. The laser diode bar according to claim 1. The length of the laser diode bar in the waveguide direction is 0.3 mm or more and 8 mm or less.
The laser diode bar according to any one of claims 1 to 4 . The length of the laser diode bar in the waveguide direction is 1 mm or more and 6 mm or less.
The laser diode bar according to any one of claims 1 to 5. The main axis direction of the off angle is the a-axis direction.
The laser diode bar according to any one of claims 1 to 6 . A laser diode bar according to any one of claims 1 to 7 ;
a diffraction grating that diffracts the laser beams emitted from the emitters of the laser diode bar;
an external resonator mirror that reflects a part of the laser light diffracted by the diffraction grating back to the laser diode bar side and causes external resonance between the external resonator mirror and a reflective film of the laser diode bar.
Wavelength beam combining system. 1. A method for manufacturing a laser diode bar for use in a wavelength beam combining system, comprising:
preparing a nitride semiconductor substrate having a substrate surface oriented in the (0001) plane and having an off-angle of greater than 0° from the (0001) plane toward at least one of the m-axis and the a-axis;
forming a laminated structure including a first conductive type cladding layer, an active layer, and a second conductive type cladding layer on the nitride semiconductor substrate;
forming a plurality of emitters arranged in a stripe pattern in the laminate structure such that a principal axis direction of an off-angle of the nitride semiconductor substrate is perpendicular to a waveguide direction of the emitters;
cutting the laser diode bar having the plurality of emitters from the nitride semiconductor substrate;
Including,
A gradient of the distribution of the off-angle in the principal axis direction satisfies the following formula (4) with respect to a gradient of the distribution of the lock wavelengths of the multiple emitters:
A method for manufacturing laser diode bars.
0<ΔD≦((Δλ _{EC_bar} + 3.2)/Lt)/30...Equation (4)
(wherein ΔD (angle/mm) represents the gradient of the distribution of the off-angle in the principal axis direction, Lt (mm) represents the length of the laser diode bar in the longitudinal direction, and Δλ _{EC_bar} (nm) represents the lock wavelength difference between both ends of the laser diode bar in the longitudinal direction.) The nitride semiconductor substrate is a GaN substrate.
A method for producing a laser diode bar according to claim 9 . The main axis direction of the off angle is the a-axis direction.
A method for producing a laser diode bar according to claim 9 or 10 .