Laser light source

Abstract

To obtain stable optical output by suppressing unwanted oscillation, in a laser light source using a semiconductor laser as its light source.SOLUTION: A laser source 1 using a semiconductor laser 10 as its light source includes: an optical detector 23 for detecting optical output of the semiconductor laser 10; a feedback circuit 22 for setting drive current of the semiconductor laser 10 by receiving a signal outputted from the optical detector 23 so that optical output indicated by the signal falls in a target region; and a control circuit 31 for setting the target region in a region between optical output P2 and optical output P1 which is lower than that.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a laser light source, and more particularly to a laser light source using a semiconductor laser as a light source.

Background Art

[0002] Conventionally, as shown in Patent Documents 1 and 2, for example, a laser light source using a semiconductor laser (laser diode) as a light source is known. This type of laser light source is often used as a measurement light source for various measuring devices, and in particular, in such cases, it is required to keep the light output within a certain range.

[0003] Conventionally, in order to meet the above requirements, APC (Automatic Power Control) shown in Patent Document 1 has been widely implemented. This APC detects the light output of the laser light source with a photodetector such as a PD (Photo Diode), and increases or decreases the drive current of the semiconductor laser according to the detected light output value to set the light output within a certain range. Patent Document 1 also describes providing an external resonator that selects the wavelength of the laser light emitted from the semiconductor laser and feeds it back to the semiconductor laser in addition to the resonator that originally constitutes the semiconductor laser.

[0004] As a technique for setting the light output of a semiconductor laser within a certain range, in addition to the above APC, a technique called ACC (Automatic Current Control) shown in paragraph

[0010] of Patent Document 2 is also known. This ACC is a control for keeping the current flowing through the semiconductor laser constant. In this case, in order to prevent light output fluctuations due to temperature, it is also necessary to precisely control the temperature of the semiconductor laser.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] Laser light sources using semiconductor lasers that perform APC are generally widely used because they usually have the favorable characteristic that the optical output value corresponds one-to-one with the drive current value. However, when a circuit loop is formed, there is a problem that oscillation occurs in high-speed control. This problem can be avoided and stable optical output control can be achieved as long as a high-speed circuit loop is not formed. However, especially when the laser light source has the external resonator described above, the optical output value does not correspond one-to-one with the drive current value, and there is a current region that has two optical outputs for one drive current, so stable optical output control cannot be achieved and unnecessary oscillation may occur. When unnecessary oscillation occurs, the optical noise increases, leading to the problem that a stable laser light source cannot be obtained.

[0007] Hereinafter, this problem will be described with reference to FIGS. 5 and 6. FIG. 5 shows the relationship between the semiconductor laser drive current value (horizontal axis) and the optical output (vertical axis) when, for example, APC is being performed. If APC is being performed properly, in this relationship, the optical output should increase monotonically as the drive current value increases, but in reality, it may not and may oscillate. As shown in FIG. 6, this oscillation is due to the phenomenon that for a certain drive current value lop1, the optical output changes stepwise between P1 and P2.

[0008] In Patent Document 1, this phenomenon is also described with reference to FIG. 12 thereof. In FIG. 12, it is shown that when the drive current value shown on the horizontal axis is around 200 mA, the optical output shown on the vertical axis can change stepwise between 45 mW and values greater than that (when the wavelength width Δλ of the output light is 2.0 nm). In this example, around the drive current value lop1 = 200 mA, the optical outputs are approximately P1 = 40 mW and P2 = 50 mW. The above-described stepwise change is due to the mode-hop phenomenon of the semiconductor laser, and it appears more prominently when an external resonator is provided for the semiconductor laser, which becomes an inhibiting factor in obtaining a stable laser light source.

[0009] On the other hand, a laser light source applying a semiconductor laser that performs ACC does not oscillate because no loop circuit is formed. However, due to deterioration caused by energization of the semiconductor laser itself or the entire laser light source, a problem is recognized that the optical output gradually decreases.

[0010] The present invention has been made in view of the above circumstances, and an object thereof is to obtain a laser light source that suppresses unnecessary oscillation and can obtain a stable optical output.

Means for Solving the Problems

[0011] The laser light source according to the present invention is a laser light source using a semiconductor laser as a light source, a photodetector that detects the optical output of the semiconductor laser, a feedback circuit that receives a signal output from this photodetector and sets the drive current of the semiconductor laser so that the optical output indicated by the signal falls within a target region, a control circuit that sets the target region to a region between an optical output P2 and an optical output P1 smaller than it, is provided.

[0012] In addition, in the above configuration, it is desirable that (P2 - P1) / (P2 + P1) = 0.01 to 0.1. In the above configuration, it is also desirable to provide an external resonator separate from the resonator of the semiconductor laser. Furthermore, in the above configuration, it is also desirable to apply a GaN-based semiconductor laser having an oscillation wavelength in the range of 375 nm to 530 nm as the semiconductor laser.

[0013] In the above laser light source, it is desirable to have an external resonator composed of a mirror that returns part of the laser light emitted from the semiconductor laser to the semiconductor laser side while transmitting the remaining laser light and one end face of the semiconductor laser. Furthermore, when having the above external resonator, it is more desirable to provide a band-pass filter that is inserted into the laser optical path between the mirror and the semiconductor laser and selects the wavelength of the laser light to a narrow band.

Effect of the Invention

[0014] According to the laser light source of the present invention having the above configuration, unnecessary oscillation can be suppressed and a stable light output can be obtained.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

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Figure 8

Figure 9

Embodiments for Carrying Out the Invention

[0016] Hereinafter, embodiments of the present invention will be described with reference to the drawings. ≪First Embodiment≫ FIG. 1 shows a schematic configuration of a laser light source 1 according to the first embodiment of the present invention. This laser light source 1 includes a semiconductor laser 10 as a light source, a box-shaped housing 11 in which the semiconductor laser 10 is fixed inside, and a collimating lens 12. The collimating lens 12 is fixed inside the housing 11 in the same manner as the semiconductor laser 10. The housing 11 is fixed to a Peltier element 15 fixed on a base plate 14 via an adhesive layer 16. And on one side surface (the left side surface in the figure) of the housing 11, a light emission window 17 made of a light transmissive member is formed.

[0017] Note that it is more desirable to use the above adhesive 16 with beads mixed in for adhesion. The beads are preferably, for example, those made of ceramics, having a perfect circular shape, and a diameter of 20 to 70 μm. When the Peltier element 15 is driven when the environmental temperature of the laser light source 1 changes, strain occurs in the Peltier element 15. However, when beads are mixed, it becomes difficult for the strain to be transmitted to the housing 11 through the adhesive 16. That is, it is estimated that by mixing the beads, the beads absorb the strain of the Peltier element 15 and the strain transmitted to the housing 11 is reduced. When the housing 11 is distorted, the amount of return light from the resonator mirror 28 to the semiconductor laser 10 changes, so the optical output of the laser light source 1 changes. Thus, by mixing the beads, the optical output fluctuation of the laser light source 1 can be suppressed lower.

[0018] The semiconductor laser 10 is driven by being supplied with a drive current Cdr from a drive circuit 20. The drive circuit 20 is connected to a control circuit 21, and the control circuit 21 is connected to a feedback circuit 22. A photodiode 23 is connected to this feedback circuit 22. The photodiode 23 detects the optical output of the semiconductor laser 10 and is arranged outside the housing 11 as an example. Inside the housing 11, there are arranged a collimating lens 12 facing the semiconductor laser 10, and a narrow-band band-pass filter 26, a condensing lens 27, and a resonator mirror 28 through which the laser light 25 passes sequentially after passing through the collimating lens 12.

[0019] Outside the housing 11, there are arranged a collimating lens 29 that allows the laser light 25 emitted from the light output window 17 to pass through after passing through the resonator mirror 28, a beam splitter 30 that reflects and allows the laser light 25 passing through this collimating lens 29 to pass through, a diffusing plate 31 that diffuses the laser light 25 reflected by the beam splitter 30, a prism pair composed of prisms 32 and 33 into which the laser light 25 passing through the beam splitter 30 is incident, and a pinhole plate 34 having a pinhole 34a. The diffusing plate 31 is arranged in front of the light receiving surface of the photodiode 23 and diffuses the laser light 25 received by the photodiode 23.

[0020] Next, the operation of the above configuration will be described. As the semiconductor laser 10, for example, a GaN-based semiconductor laser (laser diode) with an oscillation wavelength of 488 nm is used. In this semiconductor laser 10, the cleavage surfaces of the laser medium that respectively constitute the front end face and the rear end face of the optical waveguide extending in the left-right direction in the figure are such that the front end face has a reflectivity of approximately 0% and the rear end face has a reflectivity of approximately 100%. The light emitted from the above optical waveguide made of the laser medium is naturally reflected at the rear end face of the laser medium and is also reflected at the front end face where the reflectivity is not completely 0%. Therefore, reflection is repeated between these two ends, causing laser oscillation. That is, the semiconductor laser 10 is capable of laser oscillation by itself.

[0021] The laser beam 25 generated in this way is emitted from the semiconductor laser 10 in a diverging state forward (to the left in the figure). After this laser beam 25 is collimated by the collimating lens 12, the wavelength is selected to be in a narrow band by the narrow-band band-pass filter 26, and then focused by the focusing lens 27 and then incident on the resonator mirror 28. A coating for partially reflecting the laser beam 25 is applied to the front end face of this resonator mirror 28, and an external resonator is formed by this front end face and the rear end face of the semiconductor laser 10 described above. That is, in this embodiment, a composite resonator is formed by this external resonator and the resonator of the semiconductor laser 10 itself.

[0022] Since the laser light source 1 of this embodiment having this composite resonator structure has a wavelength locking function, the number of longitudinal modes of the oscillation wavelength of the semiconductor laser 10 is reduced, and it becomes a stable laser light source with an oscillation wavelength that hardly fluctuates. To confirm this, experiments were conducted using resonator mirrors 28 with different reflectivities of 20%, 30%, 50%, 65%, and 80%. When the reflectivities are small, such as 20% and 30%, the amount of light returning to the semiconductor laser 10 is small, so the effect of the external resonator is small, the wavelength locking function for suppressing fluctuations in the oscillation wavelength becomes weak, and the optical output increases. On the other hand, when the reflectivities of the resonator mirror 28 are large, such as 50%, 65%, and 85%, the wavelength locking effect by the external resonator becomes large, but conversely, there is a drawback that the optical output from the resonator mirror 28 becomes small.

[0023] The laser light source 1 can be realized regardless of the reflectivity of the resonator mirror 28. However, considering the balance between the optical output and wavelength locking, when the resonator mirror 28 with a reflectivity of 65% is used, as the band-pass filter 26, if those with a half-value width of 1.0 nm and 0.5 nm are used, the state where the longitudinal mode of the oscillation wavelength is single can be stably maintained. Here, the "stable" means that even when the current value of the semiconductor laser 10 changes or the environmental temperature of the laser light source 1 changes, etc., the longitudinal mode always maintains the oscillation in the single mode. On the other hand, the "unstable" refers to the fact that the number of longitudinal modes is not stable, being single or multiple, and it becomes unstable when the half-value width is greater than 2.0 nm. In the laser light source 1 of this embodiment, as the band-pass filter 26, the one with a half-value width of 1.0 nm where the longitudinal mode becomes single as described above is used. For a laser light source with a single longitudinal mode of the oscillation wavelength, in measuring instruments with strict wavelength accuracy requirements, such as Raman scattering measuring instruments and semiconductor wafer inspection devices, etc., since such requirements are high, for these laser light sources for precision measurement, it is desirable that the half-value width of the band-pass filter 26 is 1.0 nm or less.

[0024] The laser light 25 that passes through the resonator mirror 28 and then exits the housing 11 through the light output window 17 is partially reflected by the beam splitter 30 and received by the photodiode 23, and the remainder passes through the beam splitter 30. The laser light 25 that passes through the beam splitter 30 passes through the prism pair composed of the prisms 32 and 33 and the pinhole 34a of the pinhole plate 34, and is used, for example, as measurement light in various measuring devices.

[0025] The prisms 32 and 33 are shown as simple rectangles in FIG. 1. However, when viewed from the vertical direction in FIG. 1, they are actually a pair of wedge-shaped prisms, that is, an anamorphic prism pair, such that a "wedge shape" is shown. By using this type of anamorphic prism pair, when the beam cross-sectional shape of the incident laser beam 25 is elliptical, it becomes possible to emit the laser beam 25 with a circular cross-sectional shape. In this embodiment, as an example, the beam diameter in the horizontal direction (the direction perpendicular to the plane of FIG. 1) of the laser beam 25 is 1500 μm before passing through the prisms 32 and 33. By passing the laser beam 25 through the prisms 32 and 33, the beam diameter could be converted to 500 μm, which is equal to the beam diameter in the vertical direction (the up-and-down direction in FIG. 1).

[0026] When the laser beam 25 is used particularly as the measurement light or the like described above, it is required to keep the light output within a certain range. Hereinafter, the points for satisfying this requirement will be described. The photodiode 23 that receives a part of the laser beam 25 outputs a received light signal S1. This received light signal S1 corresponds to the light intensity of the laser beam 25, that is, it corresponds to the light output of the semiconductor laser 10, and is input to the feedback circuit 22. The feedback circuit 22, together with the control circuit 21, sets the drive current Cdr of the semiconductor laser 10 so that the light output of the semiconductor laser 10 indicated by the received light signal S1 falls within the target region.

[0027] That is, the control circuit 21 sets the above target region between the light output P2 and the smaller light output P1, and sends a drive control signal S3 corresponding to the light intensity signal S2 (the received light signal S1 subjected to processing such as amplification) input from the feedback circuit 22 to the drive circuit 20. This drive control signal S3 decreases the drive current Cdr if the light output of the semiconductor laser 10 indicated by the light intensity signal S2 is greater than the light output P2, while increasing the drive current Cdr if the light output of the semiconductor laser 10 is less than the light output P1.

[0028] The above is clearly shown in Fig. 2. This figure shows the change of the above light intensity signal S2 over time in a coordinate system where the light output is taken on the vertical axis and the energization time of the laser light source 1 is taken on the horizontal axis. However, this change is shown schematically for the purpose of explanation. The above energization time is, in other words, also the elapsed time indicating the change of the light intensity signal S2. In the parts marked H1 and H2 in the figure, the light output of the semiconductor laser 10 indicated by the light intensity signal S2 increases over time and finally exceeds the preset light output P2. Then, the drive control signal S3 sent from the control circuit 21 to the drive circuit 20 decreases the drive current Cdr of the semiconductor laser 10, and the light output of the semiconductor laser 10 turns to decrease and is maintained below the light output P2.

[0029] On the other hand, in the part marked L in Fig. 2, the light output of the semiconductor laser 10 indicated by the light intensity signal S2 decreases over time and finally falls below the preset light output P1. Then, the drive control signal S3 sent from the control circuit 21 to the drive circuit 20 increases the drive current Cdr of the semiconductor laser 10, and the light output of the semiconductor laser 10 turns to increase and is maintained above the light output P1. As described above, the effect that the light output of the semiconductor laser 10 is always maintained in the range above the light output P1 and below the light output P2 can be obtained. As described above, when beads are mixed in the adhesive 16, the fluctuation of the light output of the laser light source 1 can be suppressed lower. Therefore, it is less likely that the light output of the laser light source 1 exceeds the light output P2 or falls below the light output P1 in Fig. 2. Thus, it can be said that mixing beads is desirable for maintaining the light output in the range above P1 and below P2.

[0030] Regarding the results of confirming the above effects, an explanation will be given with reference to FIG. 3. This figure shows the results of actually measuring the relationship between the optical output and the energization time in the laser light source 1 of the present embodiment. In this figure, in the coordinate system where the horizontal axis represents the energization time of the laser light source 1 and the left vertical axis represents the optical output, the temperature of the constant temperature bath for each energization time (unit: hour) is indicated by a number of white circles arranged continuously, and the optical output for each energization time is indicated by a number of black circles arranged continuously. When actually measuring the above relationship, the entire laser light source 1 is placed in a constant temperature bath and the optical output is measured, and the above constant temperature bath temperature refers to the temperature of the constant temperature bath. This constant temperature bath temperature is shown on the right vertical axis in this figure. In this example, the temperature of the constant temperature bath as the ambient temperature of the laser light source 1 was changed between 17°C and 37°C. Also, the optical output on the left vertical axis is shown as a relative value with respect to the optical output when the temperature of the constant temperature bath is 20°C being set to 1.00. The actual optical output when the optical output is 1.00 is approximately 50 mW.

[0031] As shown in this figure, when the temperature of the constant temperature bath changes, the optical output changes accordingly. However, even if the temperature of the constant temperature bath is changed significantly in a stepwise manner over 20°C, the optical output remains relatively small, with a minimum relative value of 0.97 and a maximum relative value of 1.03.

[0032] In the present embodiment, since the control range is set to ±3%, the fluctuation range of the optical output of the laser light source 1 also becomes ±3%, but the setting can be arbitrarily changed. Specifically, ±1%, ±5%, and ±10% are possible. In the case of ±1% or less, it is possible in terms of setting, but when the output fluctuation amount of the laser light source 1 is large among the variations, if the set value is too small, the fluctuation amount may conversely increase. For example, at ±0.1%, control will frequently operate. Since the control range is as small as 0.1%, overshoot and undershoot will occur repeatedly, and the fluctuation range may rather increase. As a result, the fluctuation range will not fall within 0.1% of the set value. An appropriate value is generally ±1% or more. Regarding the upper limit, as a practical performance, it is ±10%. Beyond that, it will become a laser light source with large fluctuations. The relational expression is as follows. (P2 - P1) / (P2 + P1)=0.01~0.1 Explaining along this formula, 0.01 corresponds to a variation of ±1% and 0.1 corresponds to a variation of ±10% respectively.

[0033] Figure 4 also shows the relationship between the optical output and the generated optical noise in the laser light source 1 of this embodiment. The optical output is shown on the horizontal axis with the unit of mW, and the optical noise is shown on the vertical axis with the unit of %rms. In this relationship, even when the optical output changes in the range of 10 mW to 50 mW, the change in the optical noise at that time remains relatively small, about 0.8%rms to 0.13%rms.

[0034] Here, the above optical noise will be described in detail. This type of optical noise can be roughly classified into those caused by the controller (driver) that drives the semiconductor laser 10 and those caused by the laser light source. The former optical noise is actually electrical noise, and the latter optical noise is the noise of the laser light itself. Since the former depends on the controller, it is not directly related to the present invention. However, since it is impossible to distinguish what causes the measurement data itself, the electrical noise of the former is included as optical noise. The latter optical noise is profound and consists of various noises such as thermal noise, return light noise, mode-hop noise, interference noise, and quantum noise, etc.

[0035] What the present invention attempts to reduce is mainly mode-hop noise. This mode-hop noise is caused by an external resonator and by a semiconductor laser. In the laser light source according to the present invention, the mode-hop noise caused by the external resonator has no influence on the optical noise and does not cause practical problems.

[0036] Resonators are formed at both cleaved end faces of the laser medium that constitutes the semiconductor laser, and resonators are formed from these both end faces. In a non-reflective semiconductor laser, an anti-reflection coating is applied to the output end face so that the reflectivity is approximately 0%, but since it is not exactly 0%, a small amount of return light is generated and a resonator is formed. Therefore, mode-hop noise of the semiconductor laser occurs. Since the semiconductor laser is made of a semiconductor, the refractive index changes due to temperature and drive current, so it is very difficult to form a coating film with a reflectivity of exactly 0%.

[0037] Next, a comparative example to which the present invention is not applied will be described. ≪First Comparative Example≫ In this first comparative example, the drive circuit 20, the control circuit 21, and the feedback circuit 22 shown in FIG. 1 that perform the actions as described above are not provided. Instead, an APC circuit (not shown) that executes a conventionally known APC (Automatic Power Control) is provided. That is, in the laser light source which is this first comparative example, for example, a photodetector similar to the photodiode 23 is provided, and the photodetection signal output from the photodetector is input to the APC circuit. This photodetection signal corresponds to the optical output of the laser light emitted by a semiconductor laser (not shown), and based on this photodetection signal, the APC circuit inputs a semiconductor laser drive control signal for setting the optical output to a predetermined value to the semiconductor laser drive circuit. By the above processing, the semiconductor laser is driven and controlled so that the optical output of the laser light emitted therefrom becomes a predetermined value. Thus, the laser light source driven by APC can obtain a constant optical output, but on the other hand, since a circuit loop is formed, there is a problem that it oscillates in high-speed control.

[0038] The problem of the above oscillation occurring in an APC-driven laser light source will be described with reference to FIGS. 5 and 6. When this type of laser light source is APC-driven, as shown in FIG. 5, a region where this relationship repeats partially (the region surrounded by a circle in the figure) may occur in the relationship between the drive current value and the light output value of the laser light source (for example, when scanning at a frequency = 50 Hz). As schematically shown in FIG. 6, it is presumed that oscillation occurs in the vicinity of a certain current value lop, and the light output repeatedly takes values between P1 and P2. Therefore, it is considered that the oscillation itself remains even when APC-driven under such circumstances.

[0039] <<Second Comparative Example>> In this second comparative example, the drive circuit 20, control circuit 21, and feedback circuit 22 shown in FIG. 1 are not provided. Instead, an ACC circuit (not shown) that executes a conventionally known ACC (Automatic Current Control) is provided. That is, in the laser light source which is this second comparative example, for example, a photodetector similar to the photodiode 23 is provided, and the photodetection signal output from the photodetector is input to the ACC circuit. Based on this photodetection signal, the ACC circuit inputs a semiconductor laser drive control signal that sets the drive current to a predetermined value to the semiconductor laser drive circuit. By the above processing, the semiconductor laser is controlled to be driven at a constant current value. The laser light source using a semiconductor laser thus driven by ACC does not have a problem of oscillation because it does not form a loop circuit, but there is a problem that the light output gradually decreases due to deterioration caused by energization of the semiconductor laser itself or energization of the light source including the semiconductor laser. Furthermore, this type of laser light source cannot suppress sudden fluctuations due to mode hopping.

[0040] FIG. 7 shows the result of actually measuring the relationship between the optical output and the energization time in the laser light source driven by ACC as described above, similar to FIG. 3. The display method of FIG. 7 is the same as that of FIG. 3. As shown in the figure, when any change occurs in the laser light source, the output suddenly changes. FIG. 7 shows the measurement results when the ambient temperature of the laser light source is changed between 17°C and 37°C. The optical output is stable until 8 hours have elapsed since the start of energization, but suddenly the mode hop of the semiconductor laser occurs, and a sudden output fluctuation of about 8% occurs. As such, it is difficult to obtain a stable optical output with ACC drive.

[0041] <<Second Embodiment>> Next, referring to FIG. 8, the laser light source 2 according to the second embodiment of the present invention will be described. FIG. 8 shows the schematic configuration of this laser light source 2. The laser light source 2 is basically configured by removing the elements constituting the external resonator from the laser light source 1 shown in FIG. 1. That is, a semiconductor laser 10 as a light source that can oscillate alone is applied. In FIG. 8, the same reference numerals as those in FIG. 1 are given to the elements equivalent to those in FIG. 1, and duplicate explanations thereof are omitted.

[0042] Also in the laser light source 2 shown in FIG. 8, the mode hop of the semiconductor laser described above may occur, and thereby there is a possibility of generating optical noise. That is, in FIG. 9, the horizontal axis represents the drive current of the semiconductor laser, and the vertical axis represents the optical output of the semiconductor laser. In the region circled in the figure, the relationship between the drive current and the optical output may change suddenly. Considering that no external resonator is provided in the configuration of FIG. 8, it is considered that the mode hop occurs in the semiconductor laser itself.

[0043] Also in the laser light source 2 having the above basic configuration, a drive circuit 20, a control circuit 21, a feedback circuit 22, and a photodiode 23 similar to those shown in FIG. 1 are provided, and the semiconductor laser 10 is driven in the same manner as in the laser light source 1. Therefore, also in this laser light source 2, the same effects as those exhibited by the laser light source 1 can be obtained.

[0044] In the above-described embodiments, a GaN-based semiconductor laser with an oscillation wavelength of 488 nm is used as the semiconductor laser 10. However, the semiconductor laser 10 is not limited thereto, and a GaN-based semiconductor laser having an oscillation wavelength within the range of 375 nm to 530 nm can also be preferably used.

Explanation of Reference Numerals

[0045] 1, 2 Laser light source 10 Semiconductor laser 11 Housing 12, 24, 29 Collimating lens 14 Base plate 15 Peltier element 16 Adhesive layer 17 Light exit window 20 Drive circuit 21 Control circuit 22 Feedback circuit 23 Photodiode 25 Laser light 26 Narrow-band bandpass filter 27 Condensing lens 28 Resonator mirror 30 Beam splitter 31 Diffusion plate 32, 33 Prism 34 Pinhole plate 34

Claims

1. In a laser light source using a semiconductor laser as a light source, a photodetector that detects the light output of the semiconductor laser; a feedback circuit that receives the signal output by this photodetector and sets the drive current of the semiconductor laser so that the light output indicated by the signal falls within a target region; a control circuit that sets the target region to a region between a light output P2 and a smaller light output P1; A laser light source provided with the above is provided.

2. The laser light source according to claim 1, wherein (P2 - P1) / (P2 + P1) = 0.01 to 0.

1.

3. The laser light source according to claim 1 or 2, having an external resonator different from the resonator of the semiconductor laser.

4. The laser light source according to any one of claims 1 to 3, wherein the semiconductor laser is a GaN-based semiconductor laser having an oscillation wavelength in the range of 375 nm to 530 nm.

5. The laser light source according to any one of claims 1 to 4, having an external resonator composed of a mirror that returns a part of the laser light emitted from the semiconductor laser to the semiconductor laser side while transmitting the remaining laser light and one end face of the semiconductor laser.

6. The laser light source according to any one of claims 1 to 5, wherein a band-pass filter is inserted into the laser optical path between the mirror and the semiconductor laser to select the wavelength of the laser light in a narrow band.

7. The laser light source according to claim 6, wherein the half-value width of the band-pass filter is 1.0 nm or less.

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