WAVELENGTH BEAM COMBINING SYSTEM

A wavelength beam combining system includes: at least one laser diode bar that includes a plurality of emitters arranged in a row from a first end side to a second end side, and a heating element placed on the second end side with respect to the plurality of emitters; an optical element that condenses beams emitted from the plurality of emitters; a diffraction grating; an external resonance mirror; and a controlling apparatus that controls power supplied to the plurality of emitters and the heating element. The laser diode bar is placed so that a locked wavelength for an emitter located on the second end side is longer than the locked wavelength for an emitter located on the first end side. The controlling apparatus controls the power supplied to the heating element so that the heating element has a higher temperature than the plurality of emitters.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is entitled and claims the benefit of Japanese Patent Application No. 2021-112783 filed on Jul. 7, 2021, the disclosure of which including the specification, drawings and abstract is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to a wavelength beam combining system.

BACKGROUND ART

A wavelength beam combining (WBC) system has been known as a system to obtain a high-power laser beam by combining a plurality of beams with different wavelengths to a single point. The wavelength beam combining system is disclosed in Patent Literature 1, for example.

The wavelength beam combining system includes, for example, a laser diode (LD) bar, a beam twister lens unit (BTU), a diffraction grating, and an external resonance mirror.

The laser diode bar emits beams from a plurality of emitters. The plurality of beams emitted from the laser diode bar are focused onto the diffraction grating through the beam twister lens unit. The diffraction grating diffracts the incident beam at a diffraction angle determined by the wavelength and emits the beam. The beam emitted from the diffraction grating is incident on the external resonance mirror. The external resonance mirror is a partially transparent mirror, and vertically reflects some of the incident beam in the direction of the diffraction grating. The diffraction grating feeds back the beam reflected by the external resonance mirror to the laser diode bar. Oscillation then occurs between the laser diode bar and the external resonance mirror due to external resonance, and a laser beam is emitted from the wavelength beam combining system.

The oscillation due to external resonance occurs only for a beam with a wavelength (locked wavelength) uniquely determined by the positional relationship of individual emitters of the laser diode bar, the diffraction grating, and the external resonance mirror.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

Incidentally, a laser diode bar with a plurality of emitters tends to have a temperature difference in the array direction of the emitters. The wavelengths of the beams emitted from the plurality of emitters vary with temperature. No oscillation occurs due to external resonance without the locked wavelength in a range of the wavelengths of the beams. When no oscillation occurs due to external resonance in some of the plurality of emitters, the intensity of a laser beam emitted from the wavelength beam combining system is reduced.

An objective of the present disclosure is to prevent reduction in the intensity of a laser beam emitted from a wavelength beam combining system.

Solution to Problem

A wavelength beam combining system according to an aspect of the present disclosure includes: at least one laser diode bar that includes a plurality of emitters arranged in a row from a first end side to a second end side, and a heating element placed on the second end side with respect to the plurality of emitters; an optical element that condenses a beam emitted from each of the plurality of emitters; a diffraction grating that diffracts the beam condensed by the optical element; an external resonance mirror that causes external resonance by feeding back, to the laser diode bar, a part of the beam diffracted by the diffraction grating; and a controlling apparatus that controls power to be supplied to the plurality of emitters and the heating element. The laser diode bar is placed in such a posture that a locked wavelength for at least one of the plurality of emitters located on the second end side is longer than the locked wavelength for at least one of the plurality of emitters located on the first end side, the locked wavelength causing oscillation due to the external resonance, and the controlling apparatus controls the power to be supplied to the heating element so that a temperature of the heating element is higher than temperatures of the plurality of emitters.

Advantageous Effects of Invention

According to a wavelength beam combining system of the present disclosure, it is possible to prevent reduction in the intensity of a laser beam emitted from the wavelength beam combining system.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. First, the background that led to the present invention will be explained.

FIG.1is a schematic diagram of wavelength beam combining system (hereinafter, referred to as WBC system)10. WBC system10actually includes components other than the components illustrated inFIG.1, of course, although not illustrated.

WBC system10includes a plurality of laser diode bars (hereinafter, referred to as LD bars)100, optical elements200, diffraction grating300, and external resonance mirror400. Note that, althoughFIG.1illustrates four LD bars, the number is not limited to this obviously. The number of LD bars100may be one.

FIG.2illustrates a configuration of one LD bar100. LD bar100includes a plurality of emitters101and heating element102.

The plurality of emitters101are arranged in stripes in a row along the long-side direction of LD bar100. The long-side direction is the array direction in which the plurality of emitters101are aligned. The plurality of emitters101are arranged so that emitters101adjacent to each other have equal intervals. Note that, althoughFIG.2illustrates seven emitters101, the number is not limited to seven obviously.

The plurality of emitters101are composed of first conductive cladding layer103, active layer104, and second cladding layer105stacked together. The plurality of emitters101are placed to substrate106(e.g., nitride semiconductor substrate).

A plurality of P-type electrodes107are placed on the upper surface of LD bar100so as to correspond to the plurality of emitters101. N-type electrode108is placed on the lower surface of LD bar100with substrate106therebetween. When current is supplied in parallel from a power supply unit (not illustrated) to the plurality of P-type electrodes107, laser oscillation occurs in the plurality of emitters101, and beams are respectively emitted from the plurality of emitters101. The beams are emitted simultaneously and parallel to each other from the plurality of emitters101along the short-side direction from a surface on one side of the short-side direction of LD bar100. The short-side direction is the external resonance direction in which external resonance occurs.

Heating element102will be described later.

The description continues with reference toFIG.1again. The plurality of LD bars100are arranged in a row so that their surfaces from which the beams are emitted face diffraction grating300.

Optical elements200condense the beams emitted from the plurality of emitters101. A plurality of optical elements200are arranged between LD bars100and diffraction grating300so as to correspond to the plurality of LD bars100. Optical elements200are, for example, beam twister lens units. Note that optical elements200may be cylindrical lenses, spherical lenses, or mirrors, for example.

Optical elements200focus the beams emitted from the plurality of emitters101to a single point on the surface of diffraction grating300. That is, the angle of incidence of the beam emitted from each of the plurality of emitters101with respect to diffraction grating300changes depending on optical element200.

Diffraction grating300diffracts the beams combined by optical elements200. Diffraction grating300is transmission diffraction grating300. Note that diffraction grating300may be reflection diffraction grating300.

External resonance mirror400is a partially transparent mirror. External resonance mirror400vertically reflects some of the incident beam back to diffraction grating300. Diffraction grating300feeds back the beam reflected by external resonance mirror400to LD bar100. Oscillation then occurs between LD bar100and external resonance mirror400due to external resonance, and a laser beam is emitted from WBC system10.

WBC system10has the following three characteristics.The oscillation due to external resonance occurs only when the beams emitted from the plurality of emitters101meet a diffraction condition of diffraction grating300and have a wavelength reflected by external resonance mirror400.The wavelength at which the oscillation due to external resonance occurs (hereinafter referred to as a locked wavelength) is uniquely determined by the arrangement of diffraction grating300and LD bar100.No oscillation due to external resonance occurs when the locked wavelength is not in a range of the wavelengths of the beams emitted from emitters101by laser oscillation of emitters10. When no oscillation due to external resonance occurs in any of the plurality of emitters101, the output of the laser beam emitted from WBC system10is reduced. Note that the range of the wavelengths of the beams emitted from emitters101is determined by the configuration of LD bar100.

The diffraction condition of diffraction grating300can be expressed by d (sin α+sin β=mλ, where the period of diffraction grating300is d, the angle of incidence is α, the emission angle is β, the wavelength is λ, and the degree is m, in diffraction grating300. Note that it is common to select diffraction grating300that sets 1 as actual effective degree m.

To meet this diffraction condition in conventional WBC system10, LD bar100is designed and manufactured so that LD bar100with larger angle of incidence a to diffraction grating300emits a beam with a longer wavelength, as illustrated inFIG.1.

The same applies to emitters101aligned on one LD bar100illustrated inFIG.3. As described above, even in one LD bar100, angle of incidence a of the beam emitted from each of the plurality of emitters101with respect to diffraction grating300changes depending on optical element200. That is, to sufficiently meet an external resonance condition, one LD bar100should be designed and manufactured so that emitter101with larger angle of incidence a to diffraction grating300emits a beam with a longer wavelength.

FIG.4illustrates exemplary locked wavelengths for respective emitters101in one LD bar100.FIG.4illustrates an example in which38emitters101are formed on one LD bar100, and the length from first emitter101to 38th emitter101(length W of LD bar100inFIG.3) is 10 [mm]. Note that, in WBC system10, LD bar100is configured so that emitter101with a greater emitter number has larger angle of incidence α. As described above, the larger angle of incidence α is, the longer the locked wavelength is. That is, the greater the emitter number of emitter101is, the longer the locked wavelength is.

According to calculations, when length W of LD bar100is 10 [mm] and the distance from LD bar100to diffraction grating300is 2.6 [m], the difference 4-EC bar in the locked wavelength between emitters101at both ends of LD bar100is approximately 1.0 [nm]. Also, according to calculations, when length W of LD bar100is 10 [mm] and the distance from LD bar100to diffraction grating300is 1.3 [m], the difference 4-EC bar in the locked wavelength between emitters101at both ends of LD bar100is approximately 2.0 [nm].

FIG.5illustrates a relationship between the wavelengths and intensity of beams emitted from the plurality of emitters101. The curved line in the drawing indicates the gain of laser light generated in emitter101. In emitter101, only the laser light with a wavelength within a predetermined range corresponding to a predetermined value or more of the gain is emitted as a beam by laser oscillation. That is, the predetermined range is a range of the wavelengths at which laser oscillation can be performed in emitter101. The wavelengths within the predetermined range include a gain peak wavelength at which the gain intensity is maximized.

As described above, the locked wavelength is uniquely determined by the arrangement of diffraction grating300and the like. Thus, when the locked wavelength is within the predetermined range, oscillation due to external resonance occurs at the locked wavelength. In the example ofFIG.5, the oscillation due to external resonance occurs at locked wavelength1, which is within the predetermined range. In contrast, no oscillation due to external resonance occurs at locked wavelength2, which is outside the predetermined range.

Further, the greater the difference between the locked wavelength and the gain peak wavelength is, the smaller the intensity of the gain corresponding to the locked wavelength is. The intensity of the laser beam obtained by external resonance is reduced accordingly. In other words, the smaller the difference between the locked wavelength and the gain peak wavelength is, the greater the intensity of the gain corresponding to the locked wavelength is, thereby preventing the reduction in the intensity of the laser beam emitted from the wavelength beam combining system.

FIGS.6and7each illustrate a relationship between the gain peak wavelength and the locked wavelength of each emitter101of LD bar100. As described above, the locked wavelength has a slope according to angle of incidence α to diffraction grating300.

FIG.6illustrates an example where distributions of the gain peak wavelength to emitters101of LD bar100, which are indicated by the thin solid line, have a slope in the same direction as that of the locked wavelength indicated by the thick solid line. In the example ofFIG.6, the locked wavelength is within a predetermined range for all emitters101. Thus, the oscillation due to external resonance occurs in all emitters101.

In contrast,FIG.7illustrates an example where distributions of the gain peak wavelength to emitters101of LD bar100have a slope opposite to that of the locked wavelength. In the examples ofFIG.7, the locked wavelength is outside the predetermined range for some of emitters101. Thus, no oscillation due to external resonance occurs in some emitters101.

Incidentally, the gain peak wavelength is conventionally almost equal for emitters101of the same LD bar100. Accordingly, in the graphs as illustrated inFIGS.6and7, the gain peak wavelength distributions are represented as a straight line with no slope. Thus, considering the relationship with the locked wavelength having a slope, there is a possibility that no oscillation due to external resonance occurs in emitters101around both end parts of the long-side direction of LD bar100.

Further, a plurality of emitters101generate heat in LD bar100and the plurality of emitters101are arranged along the long-side direction; accordingly, the heat is easily built up at the center part of the long-side direction and both end parts of the long-side direction tend to radiate the heat. Thus, in LD bar100, the temperature at both end parts of the long-side direction is lower than the temperature at the center part of the long-side direction. That is, the center part of the long-side direction has the highest temperature in LD bar100. In addition, the higher the temperature of emitter101is, the longer the wavelength of the beam emitted from emitter101is, as described above. In other words, the lower the temperature of emitter101is, the shorter the wavelength of the beam emitted from emitter101is.

That is, when the temperature at both end parts of the long-side direction is lower than the temperature at the center part of the long-side direction in LD bar100, the gain peak wavelengths at both end parts of the long-side direction become shorter than the gain peak wavelength at the center part of the long-side direction, as indicated by the thick broken line inFIG.6. As a result, the larger angle of incidence α is, the greater the difference between the gain peak wavelength and the locked wavelength tends to be, as indicated by the thick broken line inFIG.6.

The greater the difference between the gain wavelength and the locked peak wavelength is, the smaller the intensity of the gain corresponding to the locked wavelength is (FIG.5). Accordingly, the intensity of the laser beam emitted from WBC system10is reduced.

In addition, when the difference between the gain peak wavelength and the locked wavelength is large and the locked wavelength is outside the predetermined range, no oscillation due to external resonance occurs in emitter101corresponding to the locked wavelength outside the predetermined range. In this case, the intensity of the laser beam emitted from WBC system10is further reduced.

One of the features of the present disclosure performed under such considerations is to change the temperature distribution in LD bar100so that the difference between the gain peak wavelength and the locked wavelength is small. To be more specific, the temperature of LD bar100is controlled so that the distribution of the gain peak wavelength follows the distribution of the locked wavelength by utilizing the fact that the higher the temperature of emitter101is, the longer the wavelength of the beam emitted from emitter101is.

Next, heating element102will be described.FIG.8is a plan view of one LD bar100. InFIG.8, n emitters101are aligned along the long-side direction (array direction). In the drawing, the subscripts of the reference sings of emitters101indicate the numbers of emitters101.

N emitters101are aligned with equal intervals. First emitter1011of n emitters101is located at the end part of LD bar100on the first end side in the array direction. N-th emitter101nof n emitters101is located at the end part of LD bar100on the second end side in the array direction. In WBC system10, LD bar100is placed so that angle of incidence α of the beam is larger from the first end side to the second end side. In addition, the larger angle of incidence α is, the longer the locked wavelength is, as described above. That is, in WBC system10, LD bar100is placed so that the locked wavelength of emitter101located on the second end side is longer than the locked wavelength of emitter101located on the first end side.

Heating element102is located on the upper surface of LD bar100on the second end side with respect to the plurality of emitters101. Heating element102is a resistor (e.g., chip resistor) that generates heat with voltage applied. Heating element102is supplied with power from an independent power source (e.g., external power source) different from the power supply unit for supplying power to the plurality of emitters101.

FIG.9Ais a side view illustrating LD bar100mounted. LD bar100is mounted to sub-mount109. Sub-mount109is, for example, a ceramic substrate. Sub-mount109includes first conductive layer109aand second conductive layer109b.First conductive layer109aand second conductive layer109bare insulated from each other.

First conductive layer109ais electrically connected to a plurality of P-type electrodes107. Voltage is applied to the plurality of emitters101via first conductive layer109aand N-type electrode108. Second conductive layer109bis electrically connected to heating element102. Voltage is applied to heating element102via second conductive layer109band N-type electrode108.

Note that, as illustrated inFIG.9B, sub-mount109may be configured to include single conductive layer109cinstead of first and second conductive layers109aand109b.Conductive layer109cis electrically connected to the plurality of P-type electrodes107and heating element102. In this case, N-type electrode108is configured to include first N-type electrode108aand second N-type electrode108b.First N-type electrode108aand second N-type electrode108bare insulated from each other. First N-type electrode108ais located on the lower surface of LD bar100at a position corresponding to the plurality of emitters101. Voltage is applied to the plurality of emitters101via conductive layer109cand first N-type electrode108a.Second N-type electrode108bis located on the lower surface of LD bar100at a position corresponding to heating element102. Voltage is applied to heating element102via conductive layer109cand second N-type electrode108b.

Further, copper block110composing a water cooling jacket is located so as to make contact with LD bar100via sub-mount109. Heat of LD bar100is released mainly through sub-mount109and block110.

WBC system10further includes controlling apparatus500that controls power supplied to the plurality of emitters101and heating element102. Controlling apparatus500controls power supplied to heating element102so that the temperature of heating element102is higher than the temperatures of the plurality of emitters101. To be more specific, controlling apparatus500controls power supplied to heating element102so as to satisfy Expression 1.

In Expression 1, Wtotis the sum of the power supplied to the plurality of emitters101. The n is the number of emitters101. ΔTchipis the difference between the temperatures of the center part and the end part on the second end side of LD bar100. The temperature of the center part of LD bar100corresponds to the temperature of emitter101located at the center part of LD bar100. The temperature of emitter101is calculated by converting the wavelength of the beam emitted from emitter101into the temperature using a temperature coefficient of the beam. The temperature coefficient is a coefficient indicating a relationship between the temperature and wavelength of the beam. The wavelength of the beam is pre-measured before operating WBC system10. The temperature of the end part of LD bar100on the second end side corresponds to the temperature of emitter101located at the end part of LD bar100on the second end side.

Rthis thermal resistance of a heat dissipation path of heating element102. The heat dissipation path of heating element102is provided to sub-mount109and block110. That is, Rthis the sum of the thermal resistance of sub-mount109and the thermal resistance of block110. Wcontis the power supplied to heating element102. ΔTECis a temperature difference obtained by dividing the difference in the locked wavelengths of emitters101at both ends by the temperature coefficient of the beam.

As indicated by the thick broken line inFIG.6above, the gain peak wavelength distribution has a slope in the same direction as the slope of the locked wavelength on the side where angle of incidence α decreases from the center part of the long-side direction (on the side where the locked wavelength decreases). Meanwhile, the gain peak wavelength distribution has a slope opposite to the slope of the locked wavelength on the side where angle of incidence α increases from the center part of the long-side direction (on the side where the locked wavelength increases). With this regard, setting power Wcontsupplied to heating element102within the range indicated by Expression 1 makes it possible to change the gain peak wavelength distribution so as to have a slope with an appropriate angle in the same direction as the slope of the locked wavelength over the long-side direction, as described below.

In Expression 1, the first term Wtot/n on the left side corresponds to the power supplied to one emitter101. The second term ΔTchip/Rthon the left side corresponds to the power supplied to heating element102in order for emitter101on the second end side (the side where angle of incidence α is larger, the side where the locked wavelength is longer, and the side where heating element102is provided) to raise the temperature for the temperature difference between emitter101located at the center part of LD bar100and emitter101located at the end part on the second end side of LD bar100.

Thus, setting power Wcontsupplied to heating element102larger than Wtot/n+ΔTchip/Rthmakes it possible for the temperatures of emitters101located on the second end side to be higher than the temperature of emitter101located at the center part of LD bar100. That also increases the gain peak wavelengths of emitters101located on the second end side and the upper and lower limits of the wavelength ranges of the beams oscillated from emitters101. Accordingly, the locked wavelengths of emitters101can be included within the range (predetermined range) of the wavelengths of the beams oscillated from emitters101, as illustrated inFIG.10.

Note that, when power Wcontsupplied to heating element102is equal to or less than Wtot/n+ΔTchip/Rth, the temperature of heating element102is not sufficiently raised, and the upper limit of the predetermined range remains below the locked wavelength for at least some of emitters101located on the second end side. In some cases, the gain peak wavelength distribution still has a slope opposite to the slope of the locked wavelength on the side where angle of incidence α increases from the center part of the long-side direction. When the locked wavelength is outside the predetermined range for some of emitters101located at the end part on the second end side, possibly no oscillation occurs due to external resonance.

The right side of Expression 1 includes the term 2×ΔTEC/Rth, compared to the left side. ΔTEC/Rthcorresponds to the power supplied to heating element102in order for emitter101non the second end side to raise the temperature for the temperature difference, which is based on the difference in the locked wavelength between emitter1011located at the end part on the first end side and emitter101nlocated at the end part on the second end side. When heating element102is supplied with power less than Wtot/n+(2×ΔTEC+ΔTchip)/Rthobtained by adding, to the left side, 2×ΔTEC/Rth, where an experimentally determined coefficient 2 is multiplied by the above ΔTEC/Rth, it is possible to increase the gain peak wavelengths of emitters101located on the second end side while preventing the lower limit of the predetermined range from exceeding the locked wavelength. Note that the coefficient for ΔTEC/Rthis set to an appropriate value that is experimentally determined. The coefficient is not limited to 2, and may be set to a value that is 2 or more and 3 or less.

Note that, when power Wcontsupplied to heating element102is equal to or greater than Wtot/n+(2×ΔTEC+ΔTchip)/Rth, the temperature of heating element102is excessively raised and the lower limit of the predetermined range exceeds the locked wavelength for at least some of emitters101located on the second end side. Thus, when the locked wavelength is outside the predetermined range for some of emitters101located at the end part on the second end side, possibly no oscillation occurs due to external resonance.

Prior to operating WBC system10, an operator measures the wavelengths of emitters101associated with Expression 1 for each of LD bars100and calculates the power to be supplied to heating element102based on Expression 1. When controlling WBC system10, controlling apparatus500supplies the power calculated based on Expression1to heating element102. This causes heating element102to generate heat.

The heat generated by heating element102is transferred to emitters101through each of the layers of LD bar100. Heating element102is located on the second end side with respect to the plurality of emitters101. Thus, the greatest amount of heat is transferred to n-th emitter101nlocated on the most second end side, and the amount of heat transferred to emitter101becomes less from n-th emitter101ntoward the first end side.

In other words, the amount of heat generated from heating element102and transferred to emitter101gradually increases from the first end side to the second end side, that is, from the side where angle of incidence α is smaller to the side where it is larger. Thus, the temperature rise of emitter101gradually increases from the side where angle of incidence α is smaller to the side where it is larger, and the gain peak wavelength becomes longer in accordance with the temperature rise of emitter101.

In addition, the power supplied to heating element102is set as in Expression 1. This causes the gain peak wavelength distribution to change so as to have a slope with an appropriate angle in the same direction as the slope of the locked wavelength, as illustrated inFIG.10, so that all the locked wavelengths fall within the predetermined range. Thus, the oscillation due to external resonance occurs in all of the plurality of emitters101.

Further, the difference between the gain peak wavelength and the locked wavelength is reduced for the plurality of emitters101, thereby increasing the intensity of the gain corresponding to the locked wavelength and even the intensity of the beam emitted by the oscillation due to external resonance. Accordingly, it is possible to prevent the reduction of the intensity of the laser beam emitted from WBC system10.

Next, WBC system10according to Embodiment 2 of the present disclosure will be described. The description will be mainly for a part different from Embodiment 1.FIG.11is a plan view of LD bar100according to Embodiment 2. LD bar100according to Embodiment 2 includes second heating element111in addition to the configuration of LD bar100according to Embodiment 1.

Second heating element111is located on the upper surface of LD bar100on the first end side (on the side where angle of incidence α is smaller and the side where the locked wavelength is shorter) with respect to the plurality of emitters101. Second heating element111is a resistor (e.g., chip resistor) that generates heat with voltage applied. Second heating element111is supplied with power from an independent power source different from the power supply unit for supplying power to the plurality of emitters101and the power source for supplying power to heating element102.

FIG.12is a side view illustrating LD bar100mounted. Second heating element111is mounted to sub-mount109. Note that the conductive layers are not illustrated inFIG.12. LD bar100according to Embodiment 2 is applied in a case where the difference between the gain peak wavelength and locked wavelength is greater than that of LD bar100according to Embodiment 1 as illustrated inFIG.13.

Controlling apparatus500controls power supplied to second heating element111so as to meet Expression 2.

In Expression 2, Wtotis the sum of the power supplied to the plurality of emitters101. The n is the number of emitters101. ΔTchip2is the difference between the temperatures of the center part and the end part on the first end side of LD bar100. The temperature of the end part of LD bar100on the first end side corresponds to the temperature of emitter101located at the end part of LD bar100on the first end side.

Rth2is thermal resistance of a heat dissipation path of second heating element111. The heat dissipation path of second heating element111is provided to sub-mount109and block110. That is, Rth2is the sum of the thermal resistance of sub-mount109and the thermal resistance of block110. Wcont2is the power supplied to second heating element111.

Power Wcontsupplied to heating element102is set within the range indicated by Expression 1, and power Wcont2supplied to second heating element111is set within the range indicated by Expression 2. This allows the gain peak wavelength distribution to be changed so as to have a slope with an appropriate angle in the same direction as the slope of the locked wavelength over the long-side direction.

In Expression 2, Wtot/n on the left side corresponds to the power supplied to one emitter101. Thus, setting power Wcont2supplied to second heating element111larger than Wtot/n makes it possible to increase the temperature of emitters101located on the first end side. That also increases the gain peak wavelengths of emitters101located on the first end side and the upper and lower limits of the wavelength ranges of the beams oscillated from those emitters101. Accordingly, the locked wavelengths of emitters101can be included within the range (predetermined range) of the wavelengths of the beams oscillated from emitters101, as illustrated inFIG.13.

Note that, when power Wcont2supplied to second heating element111is equal to or less than Wtot/n, the temperature of second heating element111is not sufficiently raised, and the upper limit of the predetermined range remains below the locked wavelength for at least some of emitters101located on the first end side. When the locked wavelength is outside the predetermined range for some of emitters101located at the end part on the first end side, possibly no oscillation occurs due to external resonance.

The right side of Expression 2 is configured in the same manner as the left side of Expression 1. That is, the first term Wtot/n on the right side corresponds to the power supplied to one emitter101. The second term ΔTchip2/Rth2on the right side corresponds to the power supplied to second heating element111in order for emitter101on the first end side (the side where angle of incidence α is smaller, the side where the locked wavelength is shorter, and the side where second heating element111is provided) to raise the temperature for the temperature difference between emitter101located at the center part of LD bar100and emitter101located at the end part on the first end side of LD bar100.

Thus, setting power Wcont2supplied to second heating element111less than Wtot/n+ΔTchip2/Rth2makes it possible for the temperatures of emitters101located on the first end to be lower than the temperature of emitter101located at the center part of LD bar100. Thus, it is possible to increase the gain peak wavelengths of emitters101located on the first end side while preventing the lower limit of the predetermined range from exceeding the locked wavelengths.

Note that, when power Wcont2supplied to second heating element111is equal to or greater than Wtot/n+ΔTchip2/Rth2, the temperature of second heating element111is excessively raised and the lower limit of the predetermined range exceeds the locked wavelengths for at least some of emitters101located on the first end side. When the locked wavelength is outside the predetermined range for some of emitters101located at the end part on the first end side, possibly no oscillation occurs due to external resonance.

When the power set by an operator based on Expression 2 is supplied to second heating element111, the heat generated by second heating element111is transferred to emitters101through each of the layers of LD bar100. Second heating element111is located on the first end side with respect to the plurality of emitters101. Thus, the greatest amount of heat is transferred to first emitter101located on the most first end side, and the amount of heat transferred to emitter101becomes less from first emitter101toward the second end side.

Meanwhile, as in Embodiment 1, the operator sets the power between the power calculated from the left side of Expression 1 and the power calculated from the right side to the power supplied to heating element102. When the power set by the operator based on Expression 1 is supplied to heating element102, the greatest amount of heat is transferred to n-th emitter101nlocated on the most second end side, and the amount of heat transferred to emitter101becomes less from n-th emitter101ntoward the first end side, as described above.

Further, the power supplied to heating element102is set as in Expression 1, and the power supplied to second heating element111is set as in Expression 2. This causes the gain peak wavelength distribution to change so as to have a slope with an appropriate angle in the same direction as the slope of the locked wavelength, as illustrated inFIG.13, so that all the locked wavelengths fall within the predetermined range. Thus, the oscillation due to external resonance occurs in all of the plurality of emitters101. In addition, the difference between the gain peak wavelength and the locked wavelength is reduced for the plurality of emitters101, thereby increasing the intensity of the gain corresponding to the locked wavelength and even the intensity of the beam emitted by the oscillation due to external resonance. Thus, it is possible to prevent the reduction in the intensity of the laser beam emitted from the wavelength beam combining system.

Note that above-described Embodiments 1 and 2 are based on the assumption that the distance (hereinafter, referred to as the optical path length) from LD bar100to diffraction grating300is fixed. As described above, when length W (FIGS.3and4) of LD bar100is 10 [mm] and the optical path length is 2.6 [m], difference ALEC bar in the locked wavelength between emitters101at both ends of LD bar100is approximately 1.0 [nm], and ΔTECis approximately 13 [K].

When the optical path length is changed from 1.0 to 4.0 [m] while ALEC bar is kept constant at approximately 1.0 [nm], length W of LD bar100is changed from 3.8 to 15 [mm]. When length W of LD bar100is less than 3.8 [mm], the difference between the temperatures at the center part and at both ends of LD bar100in the long-side direction is small, and the heat generated by heating element102is easily transferred to LD bar100over the long-side direction.

Meanwhile, when length W of LD bar100is greater than 15 [mm], the heat generated by heating element102is not easily transferred to the center part of LD bar100, and only the end part of LD bar100on the second end side in the long-side direction is heated. Thus, the temperature distribution in LD bar100can be accurately controlled by controlling the power supplied to heating element102when length W of LD bar100is 3.8 mm to 15 mm. That is, length W of LD bar100is preferably in the range from 3.8 to 15 [mm].

Next, WBC system10according to Embodiment 3 of the present disclosure will be described. The description will be mainly for a part different from Embodiment 1. In Embodiment 3, heating element102is placed on substrate106instead of the upper surface of LD bar100.

The long-side direction of substrate106corresponds to the long-side direction of LD bar100(FIG.2). In the long-side direction, the first end side of substrate106corresponds to the first end side of LD bar100, and the second end side of substrate106corresponds to the second end side of LD bar100. That is, LD bar100is placed on substrate106so that the plurality of emitters101are arranged from the first end side to the second end side of substrate106. In WBC system10, LD bar100is placed so that angle of incidence α of the beam emitted from emitter101is larger from the first end side to the second end side of substrate106. In other words, LD bar100is placed so that the locked wavelength of emitter101located on the second end side is longer than the locked wavelength of emitter101located on the first end side, in WBC system10.

As illustrated inFIG.14, heating element102is placed on the surface of substrate106on which LD bar100is placed, on the second end side with respect to LD bar100. Note that the conductive layers are not illustrated inFIG.14. Placing heating element102on substrate106instead of the upper surface of LD bar100prevents deterioration of LD bar100. In addition, it is possible to eliminate the impact of a change in the characteristics of LD bar100(e.g., change over time) on the characteristics of heating element102.

Next, WBC system10according to Embodiment 4 of the present disclosure will be described. The description will be mainly for a part different from Embodiment 1. LD bar100according to Embodiment 4 is mounted by being divided into a plurality of LD bars100as illustrated inFIG.15. Each of the divided LD bars100is referred to as a chip. That is, LD bar100is divided into a plurality of chips120. Substrate106and N-type electrode108are also divided in accordance with the plurality of chips120. Note that the conductive layers are not illustrated inFIG.15.

The interval between adjacent chips120is an interval where heat dissipation at both ends of one chip120is reduced. Thus, in each of chips120located at both ends, the end part that does not face adjacent chip120releases heat more than the other end part that faces adjacent chip120does.

With this regard, heating element102is placed at the end part on the second end side of chip120located on the most second end side in the long-side direction. Controlling apparatus500controls power supplied to heating element102so as to meet Expression 3.

In Expression 3, Wtot2is the sum of the power supplied to the plurality of emitters101in chip120located on the most second end side. Then is the number of emitters101in chip120located on the most second end side. ΔTchip3is the difference between the temperatures of the center part and the end part on the second end side of chip120located on the most second end side. The temperature of the center part of chip120corresponds to the temperature of emitter101located at the center part of chip120. The temperature at the end part on the second end side of chip120corresponds to the temperature of emitter101located at the end part on the second end side of chip120.

Rth3is thermal resistance of a heat dissipation path of heating element102. Wcont3is the power supplied to heating element102. ΔTEC2is a temperature difference obtained by dividing the difference between the locked wavelengths of emitters101at both ends of chip120located on the most second end side by a temperature coefficient of a beam.

The power supplied to heating element102is set as in Expression 3. Expression 3 is configured in the same manner as Expression 1 described above. This causes the gain peak wavelength distribution to change so as to have the same slope as the locked wavelength, so that all the locked wavelengths fall within the predetermined range. Thus, the oscillation due to external resonance occurs in all of the plurality of emitters101.

As described above, LD bar100according to Embodiment 4 is provided by being divided into a plurality of chips120. Further, LD bar100is provided by being cut out from a wafer (not illustrated) having a layered structure of first conductive cladding layer103, active layer104, and second conductive cladding layer105. Thus, dividing LD bar100into a plurality of chips120makes it possible to use more parts of the wafer as LD bar100, thereby improving the yield rate of LD bar100.

The present disclosure is not limited to the embodiments described above. Aspects in which variations are applied to the present embodiments or aspects constructed by combining components in different embodiments may also be included within the scope of the present disclosure without departing from the spirit or scope of the present disclosure.

For example, the power supplied to heating element102and second heating element111is calculated based on the above expressions, but the power may be predetermined. The predetermined power is derived in advance, for example, by experiments so that the temperature of heating element102is higher than the temperatures of the plurality of emitters101.

In the above embodiments, transmission diffraction grating300has been used for the description. Here is a case of reflection diffraction grating300. Regardless of transmission diffraction grating300or reflection diffraction grating300, in WBC system10, LD bar100is placed so that the locked wavelength of emitter101located on the second end side is longer than the locked wavelength of emitter101located on the first end side. However, the direction in which angle of incidence α is larger for the reflection diffraction grating is opposite to that for the transmission diffraction grating. That is, in the case of using reflection diffraction grating300in WBC system10, the smaller angle of incidence α is, the longer the locked wavelength is, which is opposite to the case of using transmission diffraction grating300. Thus, in the case of using reflection diffraction grating300in WBC system10, LD bar100is placed so that angle of incidence α of a beam is smaller from the first end side to the second end side.

Accordingly, in the case of using reflection diffraction grating300in WBC system10, LD bar100is designed and manufactured so that LD bar100with smaller angle of incidence α to diffraction grating300emits a beam with a longer wavelength. Also, in the case of using reflection diffraction grating300, one LD bar100is designed and manufactured so that emitter101with smaller angle of incidence α to diffraction grating300emits a beam with a longer wavelength.

INDUSTRIAL APPLICABILITY

The present invention is widely applicable to wavelength beam combining systems.

Reference Signs List

111Second heating element

α Angle of incidence