JP6985018B2 - External resonator type semiconductor laser device - Google Patents

Description

The present invention relates to a semiconductor laser device, and more particularly to an external resonator type semiconductor laser device including a semiconductor light emitting device that does not oscillate by itself and an external resonator that oscillates light emitted from the semiconductor light emitting device. It is a thing.

Conventionally, for example, a semiconductor laser device is widely used as a recording light source of various image recording devices and a measurement light source of various measuring devices. As one type of such a semiconductor laser device, for example, as shown in Patent Document 1, an external resonance that selects the wavelength of a semiconductor laser and the laser beam emitted from the semiconductor laser and feeds it back to the semiconductor laser. A semiconductor laser device including a device is known. In this type of semiconductor laser device, a so-called composite resonator is composed of a resonator structure of a semiconductor laser that oscillates by itself and an external resonator.

In the above-mentioned type of semiconductor laser apparatus, a wavelength control element composed of a narrow band band pass filter or the like is arranged in the optical path of the external resonator, and the wavelength of the laser beam is selected in a desired wavelength range. There is. Further, in this type of semiconductor laser apparatus, for example, a semiconductor laser is driven by high-speed modulation by supplying a high-frequency superimposed drive current to the semiconductor laser. Patent Document 1 also describes that wavelength selection is performed by a wavelength control element (referred to as “wavelength selection element” in Patent Document 1), and that a semiconductor laser is driven by high-speed modulation.

On the other hand, as another type of a semiconductor laser device provided with an external resonator, for example, as shown in Patent Document 2, a semiconductor light emitting device that does not oscillate by itself and light emitted from the semiconductor light emitting device are oscillated. A semiconductor laser device including an external resonator is also known. In Patent Document 2, the semiconductor light emitting device is referred to as a "laser diode".

Also in this type of semiconductor laser device, a wavelength control element consisting of a narrow band band pass filter or the like is arranged in the optical path of the external resonator to select the wavelength of the oscillating laser light in a desired wavelength range. It has been. Patent Document 2 also describes this wavelength selection.

In Patent Document 1, when the wavelength control element is arranged in the external resonator constituting the composite resonator as described above and the semiconductor laser is driven by high-speed modulation, the longitudinal mode of the laser beam emitted from the semiconductor laser apparatus is multi. It has been shown to be in mode. On the other hand, as shown in Patent Document 2, when the wavelength control element is arranged in the external cavity which is the only resonator, the longitudinal mode of the laser beam emitted from the semiconductor laser apparatus is usually single. It is considered to be in mode. In fact, also in Patent Document 2, there is a description that the wavelength of the laser beam is variable by rotating the bandpass filter, but there is no description or suggestion that a plurality of wavelengths exist at the same time.

Japanese Unexamined Patent Publication No. 2001-242500 Japanese Unexamined Patent Publication No. 11-17286

By the way, when the semiconductor laser device is applied to various measuring devices and inspection devices, it is required that the oscillation state of the semiconductor laser device is particularly stable. That is, if the oscillation state of the semiconductor laser device is not stable, the oscillation wavelength fluctuates due to a slight return light from the end face of the optical component such as the lens end face, and at that time, the output of the semiconductor laser device also changes rapidly. Because it will end up. A semiconductor laser device in which the wavelength of the laser beam fluctuates in this way is unsuitable for use in measurement or inspection of an object having wavelength dependence. Further, a semiconductor laser device in which the output of laser light fluctuates is disadvantageous in ensuring high accuracy in measurement, inspection, and the like.

However, it is hard to say that the oscillation state of the conventional external resonator type semiconductor laser device is particularly stable. Hereinafter, this point will be described in detail.

First, in the external resonator type semiconductor laser device of the type shown in Patent Document 1, since there is a mode in which the semiconductor laser alone oscillates and a mode in which the semiconductor laser oscillates, the drive current of the semiconductor laser is increased. If it increases or decreases, the oscillation wavelength becomes unstable. This is considered to be due to the fact that the Fabry-Perot mode of the semiconductor laser hops as the drive current increases or decreases.

Further, the external resonator type semiconductor laser device of the type shown in Patent Document 2 has a structure in which the oscillation mode by the external resonator is not controlled, which may cause the following problems. That is, in this type of semiconductor laser device, the drive current and temperature are controlled to be constant (for example, within 0.01 ° C for temperature control error), and the resonator length of the external resonator having several tens of mm is kept constant. However, since the wavelength width that a semiconductor laser can oscillate is very wide, the oscillation wavelength cannot be controlled reliably during long-term operation, and the oscillation wavelength changes, or the longitudinal mode changes from single mode to multimode. Therefore, it becomes difficult to maintain stable oscillation.

The present invention has been made in view of the above circumstances, and provides an external resonator type semiconductor laser device that stably oscillates in the longitudinal mode even if the drive current fluctuates, and the oscillation wavelength is also stable. With the goal.

One external resonator type semiconductor laser device according to the present invention is
A semiconductor light-emitting device having a pair of end faces with a light-emitting part sandwiched between them, and
An external resonance that is composed of a resonator mirror arranged outside the semiconductor light emitting element and one end surface of the pair of end faces on the side far from the resonator mirror, and oscillates light emitted from the semiconductor light emitting element. In a semiconductor laser device having a device
As the semiconductor light emitting device, a semiconductor light emitting device having a structure that does not oscillate the above light by itself, that is, by itself, is used.
A wavelength control element that is arranged in the optical path in the external resonator and selects the wavelength range of the light, and
A drive circuit that drives a semiconductor light emitting device with high-speed modulation,
Is characterized by being provided.

Further, another external resonator type semiconductor laser device according to the present invention is
A semiconductor light-emitting device having a pair of end faces with a light-emitting part sandwiched between them, and
An external resonance that is composed of a resonator mirror arranged outside the semiconductor light emitting element and one end surface of the pair of end faces on the side far from the resonator mirror, and oscillates light emitted from the semiconductor light emitting element. In a semiconductor laser device having a device
A confocal optical system that is arranged in the optical path of the external cavity and focuses the light on the other end surface of the pair of end faces on the side closer to the resonator mirror and on the reflection surface of the resonator mirror. ,
A wavelength control element that is arranged in the optical path in the external resonator and selects the wavelength range of the light, and
A drive circuit that drives a semiconductor light emitting device with high-speed modulation,
Is characterized by being provided.
In this other external resonator type semiconductor laser device, a semiconductor light emitting device having a structure for oscillating light by itself may be used. Further, the present invention is not limited to this, and a semiconductor light emitting device having a structure that does not oscillate light by itself may be used.

The above-mentioned "high-speed modulation" refers to modulation having a modulation frequency of 20 MHz or more and 500 MHz or less. The frequency of this high-speed modulation is more preferably 100 MHz or more and 500 MHz or less, and further preferably 200 MHz or more and 500 MHz or less.

Further, as the above-mentioned drive circuit, for example, a circuit in which a drive current obtained by superimposing a high frequency on a direct current is applied to the semiconductor light emitting device is preferably used.

Alternatively, as the above-mentioned drive circuit, a circuit that applies a pulse-shaped drive current in which the on period and the off period are alternately repeated to the semiconductor light emitting device can also be preferably used.

On the other hand, in the external resonator type semiconductor laser device of the present invention, an external modulation circuit that controls the amount of light emitted from the semiconductor light emitting element based on the external modulation signal is further provided, and the frequency of the external modulation signal is relative to the frequency of the external modulation signal. It is desirable that the frequency of high-speed modulation be set to twice or more.

Further, as the wavelength control element described above, for example, a transmission type narrow band bandpass filter is preferably used. Further, the present invention is not limited to this, and an element made of a prism can be preferably used as the transmission type wavelength control element. More specifically, as a wavelength control element composed of prisms, an anamorphic prism pair is particularly preferably used.

However, the present invention is not limited to this, and a reflection type wavelength control element composed of a diffraction grating or the like may be used. When such a reflection type wavelength control element is used, the wavelength control element is often made to function as a resonator mirror. Even when the wavelength control element also serves as a resonator mirror as described above, it is considered that the wavelength control element is "arranged in the optical path in the external resonator" as described above.

Further, as the semiconductor light emitting device, a nitride semiconductor light emitting device that emits light having a wavelength of 370 nm to 530 nm is preferably used.

As described above, one external resonator type semiconductor laser apparatus according to the present invention is the type of external resonator type semiconductor laser apparatus shown in Patent Document 1, which oscillates by itself instead of the semiconductor laser that oscillates by itself. It is in the form of applying a semiconductor light emitting element that does not. Conventionally, an external resonator type semiconductor laser device having such a configuration has not been proposed at all. As described above, the oscillation mode is not present in the external resonator type semiconductor laser device of the type shown in Patent Document 1 and also in the external resonator type semiconductor laser device of the type shown in Patent Document 2. It is thought that this is because the common problem of stability is recognized. That is, conventionally, when trying to stabilize the oscillation mode to a high degree, it has been common knowledge of those skilled in the art to avoid the combination of these two types, both of which tend to be unstable in the oscillation mode.

However, when the present inventor actually tried to combine the above two types, contrary to expectations and overturning the conventional wisdom, an external resonator type in which the oscillation mode is highly stable even if the drive current increases or decreases. It turned out that a semiconductor laser device can be obtained. The stability of this oscillation mode will be specifically described later according to an embodiment.

On the other hand, it has been found that another external resonator type semiconductor laser device according to the present invention can also have a highly stable oscillation mode regardless of the increase or decrease in the drive current. In another external cavity type semiconductor laser device according to the present invention, a cofocal optical system for focusing light on the reflective surface of the cavity mirror is provided, and the focal position thereof depends on the wavelength of light. As it changes, light of a specific wavelength resonates strongly. That is, the confocal optical system also has the function of selecting the wavelength range of the resonating light, similarly to the wavelength control element.

Schematic block diagram of an external cavity type semiconductor laser device according to the first embodiment of the present invention. A graph showing the transmission characteristics of the narrowband bandpass filter used in the device of FIG. Waveform diagram (A to C) of current at each point in the apparatus of FIG. 1 and waveform diagram (D) of output light. The figure which shows the spectrum of the output light when the device of FIG. 1 is driven by changing the drive current. A graph showing the relationship between the optical output of the output light, the center wavelength, and the wavelength width when the device of FIG. 1 is driven by changing the drive current. A graph showing the relationship between the optical output of output light, the center wavelength, and the wavelength width when an external resonator type semiconductor laser device other than the present invention is driven by changing the drive current. A graph showing the relationship between the drive current and the optical output in the device of FIG. A graph showing the relationship between the drive current and the optical output in an external resonator type semiconductor laser device outside the present invention. A graph showing the relationship between the optical output and the amount of noise in the device of FIG. A graph showing the relationship between the optical output and the amount of noise in an external resonator type semiconductor laser device outside the present invention. Schematic block diagram of an external cavity type semiconductor laser device according to a second embodiment of the present invention. The waveform diagram (A to C) of the current at each point in the apparatus of FIG. 11 and the waveform diagram (D) of the output light. Schematic block diagram of an external cavity type semiconductor laser device according to a third embodiment of the present invention. The waveform diagram (A to C) of the current at each point in the apparatus of FIG. 13 and the waveform diagram (D) of the output light. Schematic block diagram of an external cavity type semiconductor laser device according to a fourth embodiment of the present invention. The waveform diagram (A to C) of the current at each point in the apparatus of FIG. 15 and the waveform diagram (D) of the output light. Schematic block diagram of an external cavity type semiconductor laser device according to a fifth embodiment of the present invention. Waveform diagram (A to C) of current at each point in the apparatus of FIG. 17 and waveform diagram (D) of output light. A graph showing an example of the results of measuring the coherent length and the oscillation wavelength width of the external resonator type semiconductor laser device of the present invention. A graph showing another example of the result of measuring the coherent length and the oscillation wavelength width for the external resonator type semiconductor laser device of the present invention. Schematic block diagram of an external cavity type semiconductor laser device according to a sixth embodiment of the present invention. The figure which shows the spectrum of the output light when the apparatus of FIG. 21 is driven with an optical output of 10 mW. The figure which shows the spectrum of the output light when the apparatus of FIG. 21 is driven with an optical output of 20 mW. The figure which shows the spectrum of the output light when the apparatus of FIG. 21 is driven by an optical output of 30 mW. The figure which shows the spectrum of the output light when the apparatus of FIG. 21 is driven by an optical output of 40 mW. The figure which shows the spectrum of the output light when the apparatus of FIG. 21 is driven with an optical output of 50 mW. The figure which shows the spectrum of the output light when the apparatus of FIG. 21 is driven with an optical output of 60 mW. The figure which shows the spectrum of the output light when the apparatus of FIG. 21 is driven with an optical output of 70 mW. The figure which shows the spectrum of the output light when the apparatus of FIG. 21 is driven by the optical output 80mW. A graph showing the relationship between the optical output of the output light, the center wavelength, and the wavelength width when the device of FIG. 21 is driven by changing the drive current. A graph showing the relationship between the optical output and the amount of noise in the device of FIG. 21. The figure which shows the interference intensity and the coherent length when the apparatus of FIG. 21 is driven with an optical output of 20 mW. The figure which shows the interference strength when the apparatus of FIG. 21 is driven with an optical output of 30 mW. The figure which shows the interference strength when the apparatus of FIG. 21 was driven with an optical output of 40 mW. The figure which shows the interference strength when the apparatus of FIG. 21 was driven with an optical output of 50 mW. The figure which shows the interference strength when the apparatus of FIG. 21 is driven with an optical output of 60 mW. The figure which shows the interference strength when the apparatus of FIG. 21 is driven with an optical output of 70 mW. The figure which shows the interference strength when the apparatus of FIG. 21 was driven with an optical output of 80 mW. Schematic block diagram of an external cavity type semiconductor laser device according to a seventh embodiment of the present invention. Schematic diagram for explaining the characteristics of the prism pair A graph showing the relationship between the optical output of the output light, the center wavelength, and the wavelength width when the device of FIG. 39 is driven by changing the drive current. Schematic block diagram of an external cavity type semiconductor laser device according to an eighth embodiment of the present invention. The figure which shows the coherent length in the external resonator type semiconductor laser apparatus as a comparative example with respect to the apparatus of FIG. 42, and the spectrum of output light. The figure which shows the coherent length in the apparatus of FIG. 42, and the spectrum of an output light. A graph showing the relationship between the optical output of the output light, the center wavelength, and the wavelength width when the device of FIG. 42 is driven by changing the drive current.

Hereinafter, embodiments of the external cavity type semiconductor laser device according to the present invention will be described in detail with reference to the drawings.

<< First Embodiment >>
FIG. 1 is a schematic configuration diagram showing an external resonator type semiconductor laser device 1 according to the first embodiment of the present invention. As shown here, the external resonator type semiconductor laser device 1 of the present embodiment is basically arranged between the semiconductor light emitting element 11, the resonator mirror 12, the semiconductor light emitting element 11 and the resonator mirror 12 that emit light 10. It has a collimator lens 13, a narrow band band pass filter 14 as a wavelength control element arranged between the collimator lens 13 and the resonator mirror 12, and a drive circuit 15 for driving the semiconductor light emitting element 11. There is.

Further, a beam splitter 17 that partially reflects and branches the output light 16 is arranged in the optical path of the output light 16 emitted from the resonator mirror 12. The amount of light of a part of the branched output light 16 is detected by a photodetector 18 made of, for example, a photodiode. The photodetector 18 outputs a light amount detection signal S1 indicating the detected light amount, and the light amount detection signal S1 is input to the microcomputer 19.

As an example, the semiconductor light emitting device 11 is made of a laser diode. This laser diode has the configuration described later, and does not oscillate by itself, that is, by itself, but the semiconductor layer configuration as a light emitting element is basically the same as that of a normal laser diode. It will be referred to as a "laser diode". In this embodiment, as an example, a laser diode that emits light 10 having a wavelength in the vicinity of 488 nm, which is made of a nitride semiconductor, that is, a GaN-based compound semiconductor, is applied.

The semiconductor light emitting device 11 has a channel-shaped optical waveguide 11a serving as a light emitting portion, a front end surface 11b including one end surface of the optical waveguide 11a, and a rear end surface 11c including the other end surface of the optical waveguide 11a. ing. The front end surface 11b is coated with a non-reflective coating 11d having a reflectance of approximately 0% with respect to the wavelength of the light 10 emitted by the semiconductor light emitting device 11. On the other hand, the rear end surface 11c is coated with a highly reflective coating 11e having a reflectance of 99.9% or more with respect to the wavelength.

As described above, in the semiconductor light emitting device 11, of the pair of end faces 11b and 11c sandwiching the optical waveguide 11a which is a light emitting portion, the front end face 11b is coated with the non-reflective coating 11d, so that the light 10 is emitted. There is no resonance between the end faces 11b and 11c of. In this way, the semiconductor light emitting device 11 is not oscillated by itself.

On the other hand, the resonator mirror 12 has a front end surface (end surface opposite to the semiconductor light emitting device 11) 12b and a rear end surface 12c. The front end surface 12b is coated with a non-reflective coating 12d having a reflectance of about 0% with respect to the wavelength. On the other hand, the rear end surface 12c is coated with a partially reflective coating 12e having a reflectance of about 65% with respect to the wavelength.

Since the semiconductor light emitting element 11 and the resonator mirror 12 are formed as described above, the rear end surface 11c of the semiconductor light emitting element 11 (the end surface on the side far from the resonator mirror 12) and the rear end surface 12c of the resonator mirror 12 are formed. Therefore, an external cavity that resonates the light 10 emitted from the semiconductor light emitting element 11 is configured.

The narrow band bandpass filter 14 selects and transmits only light in a predetermined narrow wavelength range from the light 10 emitted from the semiconductor light emitting device 11. FIG. 2 shows an example of the transmission characteristics of the narrow band bandpass filter 14. In the characteristics shown here, the transmission center wavelength λcf = 488 nm and the transmission width Δλf = 2.0 nm indicated by the full width at half maximum (FWHM), but the transmission center wavelength λcf can be changed as described later. be.

The drive circuit 15 includes a DC current source 21 that supplies a direct current to the semiconductor light emitting element 11 via the coil 20, an oscillator 23 connected between the coil 20 and the semiconductor light emitting element 11 via a capacitor 22, and a control circuit. Has 24. The control circuit 24 controls the operations of the DC current source 21 and the oscillator 23 based on the control signal S2 output by the above-mentioned microcomputer 19.

Hereinafter, the operation of the external resonator type semiconductor laser device 1 having the above configuration will be described. A direct current of a predetermined value is emitted from the direct current source 21. A schematic waveform of this direct current flowing through point A in FIG. 1 is shown in A in FIG. Further, as an example, the oscillator 23 emits a high-frequency current having a sine waveform whose frequency is in the range of about 100 MHz to 350 MHz. The schematic waveform of this high frequency current flowing through point B in FIG. 1 is shown in B in FIG. After passing through the capacitor 22, this high-frequency current is superimposed on the direct current that has passed through the coil 20. Therefore, a drive current on which a high-frequency current is superimposed is applied to the semiconductor light emitting device 11. A schematic waveform of this drive current flowing through point C in FIG. 1 is shown in C in FIG.

The semiconductor light emitting device 11 receives the driving current and emits light 10 in a divergent light state. The light 10 is made into parallel light by the collimator lens 13, passes through the narrow band bandpass filter 14, and is incident on the resonator mirror 12. As described above, the light 10 resonates in the external resonator composed of the rear end surface 11c of the semiconductor light emitting device 11 and the rear end surface 12c of the resonator mirror 12. By increasing the energy in the resonator in this way, stimulated emission is performed in the semiconductor light emitting element 11, and oscillated laser light is obtained. A part of this laser beam passes through the resonator mirror 12 and is taken out of the resonator as output light 16.

The schematic waveform of the output light 16 which is a laser beam is shown in D of FIG. As shown here, the optical output of the output light 16 periodically increases or decreases in response to the drive current of the semiconductor light emitting device 11 on which the high frequency is superimposed as described above. In this way, the semiconductor light emitting device 11 is driven by high-speed modulation. The optical output waveform of the output light 16 shown in FIG. 3D is basically the same as the drive current waveform shown in FIG. 3C. To be more precise, the optical output waveform of the output light 16 also reflects the optical output characteristics of the semiconductor light emitting device 11, so that it does not exactly match the drive current waveform, but it is shown in the matched state in FIG. There is.

A part of the output light 16 is reflected and branched by the beam splitter 17, and the remainder is transmitted through the beam splitter 17 and used as the light to be used. The reflectance of the beam splitter 17 is set to, for example, about 10%, but the reflectance is not limited to that. The branched output light 16 is incident on the photodetector 18, and the amount of the light is detected by the photodetector 18. The light amount detection signal S1 output by the photodetector 18 is input to the microcomputer 19. The microcomputer 19 controls the operation of the control circuit 24 based on the light amount detection signal S1. That is, if the light amount detection signal S1 is larger than the set value (that is, if the detected light amount is larger than the target value), the value of the DC current generated by the DC current source 21 is lowered, and conversely, the light amount detection signal S1 If is smaller than the set value (that is, if the detected light amount is smaller than the target value), the operation of the control circuit 24 is controlled so as to increase the value of the direct current generated by the direct current source 21. As a result, the optical output (average value) of the output light 16 is maintained at a desired constant value.

Here, the wavelength of the output light 16 which is the laser light is selected by the narrow band bandpass filter 14. That is, in the present embodiment, the wavelength of the light 10 is selected by the narrow band band pass filter 14 having the transmission characteristics as shown in FIG. 2, and the selected light 10 returns from the external resonator to the semiconductor light emitting element 11. Will be done. As a result, the wavelength of the oscillating light 10, that is, the output light 16, is limited to a relatively narrow range.

The transmission characteristic of the narrowband bandpass filter 14 is that the narrowband bandpass filter 14, which is arranged diagonally with respect to the optical axis in the external resonator, is rotated in a direction in which the incident angle changes with respect to the optical axis. It can be changed by. By changing the transmission characteristics of the narrowband bandpass filter 14 and changing the selection wavelength of the narrowband bandpass filter 14, the wavelength of the output light 16 can be set to a desired value. If the narrowband bandpass filter 14 is rotated as described above and then the narrowband bandpass filter 14 is fixed so as to be non-rotatable, the wavelength of the output light 16 can be stabilized.

Next, the effect of the external resonator type semiconductor laser device 1 of the present embodiment will be described. In order to confirm this effect, an external resonator type semiconductor laser device having the following specifications was created in this embodiment. This will be referred to as Example 1 below.
(Example 1)
(1) Specifications of semiconductor light emitting device 11 ・ Band in which gain exists: 470 nm to 495 nm
-Reflectance of front end surface 11b: Approximately 0%
-Reflectance of the rear end surface 11c: 99.9% or more (2) Specifications of the resonator mirror 12-Reflectance of the front end surface 12b: Approximately 0%
-Reflectance of the rear end surface 12c: 65.0%
(3) Specifications and transmission characteristics of the narrow band bandpass filter 14: Basically as shown in FIG. However, the transmission center wavelength λcf can be changed by rotation.
(4) Specifications of beam splitter 17 ・ Reflectance: 10.0%
(5) Drive current of semiconductor light emitting device 11 ・ High frequency superimposition on direct current. High frequency frequency = 100MHz.

Further, in order to compare with the external cavity type semiconductor laser device 1 of Example 1, the external resonator type semiconductor laser device of Comparative Examples 1 to 4 was prepared. The main configurations of these Comparative Examples 1 to 4 are shown in Table 1 in comparison with the configurations of the external resonator type semiconductor laser device 1.

なお、表１に特記してある点以外の比較例１〜４の構成は、基本的に実施例１の外部共振器型半導体レーザ装置１と同じである。上記表１において、「駆動」の欄の「ＤＣ」は、高周波重畳されていない直流電流で半導体発光素子１１を駆動していることを示している。また「波長制御素子」の欄の「なし」は、波長選択する波長制御素子が設けられていないことを、「ＢＰＦ」は波長選択する波長制御素子として狭帯域バンドパスフィルターが適用されていることを、「プリズム」は波長選択する波長制御素子としてアナモルフィックプリズム対が適用されていることを示している。なお、このアナモルフィックプリズム対を用いるレーザ光の波長選択については、例えば特開２０１５−５６４６９号公報に記載がなされている。また、「外部共振器」の欄の「なし」は、外部共振器を設けずに、半導体発光素子として単体で発振可能なレーザダイオードを用いていることを示している。

The configurations of Comparative Examples 1 to 4 other than those specified in Table 1 are basically the same as those of the external resonator type semiconductor laser device 1 of the first embodiment. In Table 1 above, "DC" in the "Drive" column indicates that the semiconductor light emitting device 11 is driven by a direct current that is not superposed with high frequency. Further, "None" in the "Wavelength control element" column means that a wavelength control element for selecting a wavelength is not provided, and "BPF" means that a narrow band bandpass filter is applied as a wavelength control element for selecting a wavelength. , "Prism" indicates that an anamorphic prism pair is applied as a wavelength control element for wavelength selection. The wavelength selection of the laser beam using this anamorphic prism pair is described in, for example, Japanese Patent Application Laid-Open No. 2015-56469. Further, "None" in the column of "External resonator" indicates that a laser diode capable of oscillating alone is used as a semiconductor light emitting element without providing an external resonator.

First, the longitudinal mode in the external resonator type semiconductor laser device 1 of the first embodiment will be described. FIG. 4 shows the spectrum of the output light 16 when the semiconductor light emitting element 11 of the external resonator type semiconductor laser device 1 is driven by changing the value (average value) of the drive current in seven ways. FIG. 4 is a photograph of the display screen of the optical spectrum analyzer. In measuring this spectrum, the transmission characteristics of the narrowband bandpass filter 14 were set so that the transmission center wavelength λcf was around 482 nm by rotating the filter 14 as described above.

In the screens 101, 102, 103, 104, 105, 106 and 107 shown in FIG. 4, the optical outputs of the semiconductor light emitting device 11 are 3 mW, 9.8 mW, 15.4 mW, 26 mW, 39 mW, 49 mW and 61.6 mW, respectively. As described above, the spectrum in each case where the drive current of the semiconductor light emitting device 11 is changed is shown. In each screen, the horizontal axis indicates the wavelength and the vertical axis indicates the optical output. From FIG. 4, it can be seen that in the external resonator type semiconductor laser device 1 of the first embodiment, the longitudinal mode is set to the multimode. Further, in the external resonator type semiconductor laser device 1 of the first embodiment, it was confirmed that the transverse mode is also a single mode by, for example, a mode field measuring instrument provided with a CCD image pickup element.

FIG. 5 is a graph showing the center wavelength λc of the output light 16 and the wavelength width Δλ of the spectrum having the center wavelength λc in each of the above seven cases. The center wavelength λc shown here is defined as X = 10 by the so-called Pk-XdB method. That is, the wavelength width Δλ is defined as the wavelength width Δλ between the points a and b of the spectrum cut at the baseline attenuated by 10 dB from the maximum peak value of the spectrum, and the wavelength at the midpoint between the points a and b is defined as the center wavelength λc. The wavelength width Δλ is the full width of the wavelength generally called FWTM (full width at tenth maximum).

As shown in FIG. 5, in Example 1, it can be seen that the center wavelength λc of the output light 16 is stable in the vicinity of 482.3 nm even if the drive current of the semiconductor light emitting device 11 increases or decreases in the above range. .. Specifically, this fluctuation of the center wavelength λc is within the range of ± 0.1 nm. Further, the wavelength width Δλ (FWTM) is within the range of 0.35 nm to 0.6 nm.

On the other hand, the external resonance of Example 1 except that the external resonator type semiconductor laser device of Comparative Example 1 (the semiconductor light emitting element is driven by the DC current in which the high frequency is not superimposed and does not have the wavelength control element and the external resonator). In the device having the same configuration as that of the device-type semiconductor laser device 1, the center wavelength λc of the laser beam fluctuated by a maximum of 1.5 nm when the drive current was increased or decreased in the same manner as in the case of the first embodiment. In this case, the wavelength width Δλ (FWTM) was 0.98 nm to 1.6 nm.

Further, in the external resonator type semiconductor laser device of Comparative Example 2 (a device having the same configuration as the external resonator type semiconductor laser device 1 of Example 1 except that it does not have a wavelength control element and an external resonator). When the drive current was increased or decreased in the same manner as in the case of Example 1, the center wavelength λc of the laser beam fluctuated by 1.2 nm at the maximum. In this case, the wavelength width Δλ (FWTM) took a value of 2.4 nm to 3.13 nm.

Further, the external resonator type semiconductor laser device 1 of Comparative Example 3 has the same configuration as the external resonator type semiconductor laser device 1 of the first embodiment except that the anamorphic prism pair is applied as the wavelength control element. By changing the drive current of the semiconductor light emitting element, the light output is driven to change in 17 ways between 5.8 mW and 89 mW, and in each of these cases, the center wavelength λc of the laser beam and The wavelength width Δλ (FWTM) was measured. The wavelength width Δλ (FWTM) is also 1/10 of the width in the spectrum having the center wavelength λc. The result of this measurement is shown in FIG.

As shown in FIG. 6, in the external cavity type semiconductor laser device of Comparative Example 3, the center wavelength λc of the laser beam fluctuated by 1.0 nm or more according to the increase or decrease of the drive current. In this case, the wavelength width Δλ (FWTM) was a value of 0.01 nm to 0.7 nm. The value of 0.01 nm is the measurement limit of the wavelength measuring instrument.

From the above, the external resonator type semiconductor laser device 1 of Example 1 has a fluctuation in the center wavelength λc of the laser light when the drive current increases or decreases, as compared with each of the external resonator type semiconductor laser devices of Comparative Examples 1 to 3. It can be seen that they are small, the wavelength width Δλ (FWTM) is small, and their fluctuations are also small.

Next, the optical output characteristics of the external resonator type semiconductor laser device 1 of the first embodiment will be described. The result of measuring this light output characteristic is shown in FIG. In FIG. 7, the total current I on the horizontal axis indicates the total current value of the direct current and the high frequency current (converted by the average current). From FIG. 7, it can be seen that when the total current I increases or decreases, the light output changes smoothly accordingly.

On the other hand, the external resonator type semiconductor laser apparatus of Comparative Example 4, that is, the external one similar to the external resonator type semiconductor laser apparatus 1 of the first embodiment except that it is driven by a DC current without performing high frequency superimposition. FIG. 8 shows the results of measuring the optical output characteristics of the resonator type semiconductor laser device. Current I _DC of the horizontal axis in FIG. 8 is a value of the DC current applied to the semiconductor light emitting element 11. From FIG. 8, it can be seen that in Comparative Example 4, the light output does not change smoothly when _{the DC current I DC increases or decreases.}

An external resonator type semiconductor laser device in which the center wavelength λc of the laser beam fluctuates greatly due to an increase or decrease in the drive current, such as the devices of Comparative Examples 1 to 3, is used to measure and inspect an object having wavelength dependence. When applied as a light source, the measurement result and inspection result will change due to the fluctuation of the center wavelength λc. Therefore, such an external resonator type semiconductor laser device is unsuitable for ensuring high accuracy in measurement and inspection, etc., in order to apply it to a device for measuring and inspecting an object having wavelength dependence. It becomes. On the other hand, in the external resonator type semiconductor laser device 1 of the first embodiment, the fluctuation of the center wavelength λc of the laser beam can be suppressed to be small even if the drive current increases or decreases. It can also be suitably used for an apparatus for performing inspection and the like.

Next, the amount of noise in the external resonator type semiconductor laser device 1 of the present embodiment will be described. FIG. 9 shows the noise characteristics of the output light 16 emitted by the external resonator type semiconductor laser device 1 of the first embodiment. In the figure, the horizontal axis shows the optical output of the output light 16, and the vertical axis shows the ratio of noise to the optical output (% rms: percentage of the root mean square). In measuring this characteristic, the output light 16 was detected by a photodetector having a low-pass filter characteristic having a cutoff frequency of 20 MHz. From FIG. 9, it can be seen that in the external resonator type semiconductor laser device 1 of the first embodiment, the noise continuously increases or decreases as the optical output increases or decreases. It is considered that this is because the external resonator type semiconductor laser device 1 of the first embodiment performs high frequency superimposition at a frequency of 100 MHz, so that the optical spectrum oscillates stably even if the optical output increases or decreases. ..

On the other hand, FIG. 10 shows the results of performing the same measurement on the external resonator type semiconductor laser device of Comparative Example 1. From FIG. 10, it can be seen that in the external resonator type semiconductor laser device of Comparative Example 1, the amount of noise greatly fluctuates as the optical output increases or decreases in a region where the optical output is approximately 40 mW or less. It is considered that this is because the spectrum is unstable and the number of longitudinal modes fluctuates greatly when high frequency superimposition is not performed, and as a result, noise fluctuates greatly.

In the first embodiment described above, a laser diode made of a GaN-based compound semiconductor and emitting light 10 having a wavelength (gain peak wavelength) in the vicinity of 488 nm is applied. Unlike, for example, a GaAs-based red laser diode, this laser diode has a wide oscillation spectrum when driven by a direct current, and oscillates in a multi-longitudinal mode. Further, when the high frequency superimposition drive is performed to reduce the noise of the return light, the above becomes more remarkable. Therefore, when a laser diode made of a GaN-based compound semiconductor is used, it is very difficult to construct a light source having a stable oscillation wavelength and a narrow oscillation wavelength width. This tendency is also recognized in laser diodes made of GaN-based compound semiconductors that emit light having wavelengths other than 488 nm.

Therefore, a laser diode made of a GaN-based compound semiconductor that emits light of, for example, 370 nm, 405 nm, 445 nm, 473 nm, and 530 nm is used as a wavelength other than 488 nm, and the other configurations are the same as those of the first embodiment. When a resonator type semiconductor laser device was manufactured and the stability of the oscillation wavelength was examined, it was found that basically the same effect as in the first embodiment could be obtained. From the above, it can be said that applying the present invention when using a laser diode made of a GaN-based compound semiconductor is particularly effective in stabilizing the oscillation wavelength regardless of the wavelength.

Further, in the present embodiment, a laser diode that does not oscillate by itself is applied as the semiconductor light emitting device 11, but when a semiconductor light emitting element that oscillates by itself is applied, it is basically the same as in the first embodiment. It was confirmed that the effect of was obtained. Specifically, the front end surface 11b of the semiconductor light emitting device 11 shown in FIG. 1 is coated with a reflectance of 10%, 20%, and 30% with respect to the wavelength of the light emitted by the semiconductor light emitting device 11. When the stability of the oscillation wavelength was examined by manufacturing three types of external resonator type semiconductor laser devices in which the element 11 alone can oscillate and the other configurations are the same as those of the first embodiment, basically. It was found that the same effect as in the first embodiment can be obtained. In the above case, a composite resonator is composed of a resonator composed of both end faces of a laser diode which is a semiconductor light emitting element and an external resonator.

In the present embodiment described above, the value of the direct current generated by the direct current source 21 is set to ½ of the amplitude of the high frequency current. As a result, the drive current of the semiconductor light emitting device 11 superposed with high frequency is periodically increased or decreased with the lowest value (bottom value) as 0 (zero) value. However, not limited to this, the value of the DC current generated by the DC current source 21 is set to a value larger than 1/2 of the amplitude of the high frequency current, and the minimum value of the drive current after high frequency superimposition is a + (plus) value. It doesn't matter if it becomes.

If the value of the DC current generated by the DC current source 21 is set to a value smaller than 1/2 of the amplitude of the high frequency current, the drive current after high frequency superimposition is a + (plus) value and a- (minus) value. Will be taken alternately. When a laser diode is applied as the semiconductor light emitting device 11 as in the present embodiment, it is necessary that a + or-always current flows through the laser diode, so that the drive current after high frequency superimposition is high. It should be avoided to alternate between + and-values as described above. This point needs special attention when the value of the direct current before high frequency superimposition is continuously changed as in the fourth or fifth embodiment described later.

If a Zener diode is connected in the direction opposite to that of the laser diode used as the semiconductor light emitting device 11, it will be safer in the unlikely event that a surge current is applied. The above-mentioned "connecting in the opposite direction" means connecting both in parallel with respect to the pn junction of the laser diode with the junction direction of the Zener diode set to np.

<< Second Embodiment >>
Next, a second embodiment of the present invention will be described. FIG. 11 is a schematic configuration diagram showing an external resonator type semiconductor laser device 2 according to a second embodiment of the present invention. In FIG. 11, the same reference numbers are assigned to the elements equivalent to those shown in FIG. 1, and the description thereof will be omitted unless otherwise specified (the same applies hereinafter).

The external resonator type semiconductor laser device 2 of the present embodiment basically has a control circuit 224 different from the control circuit 24 as compared with the external resonator type semiconductor laser device 1 of the first embodiment shown in FIG. It differs in that it is used. That is, the control circuit 224 outputs an external control signal S3 composed of a digital signal in which an on (ON) period in which the signal level rises to a predetermined level and an off (OFF) period in which the signal level is 0 (zero) level are repeated. It is supposed to be emitted. The schematic waveform of the external control signal S3 flowing through the Ao point of FIG. 11 is shown in A of FIG.

The external control signal S3 is input to the DC current source 21 and the oscillator 23. When the external control signal S3 is input, the DC current source 21 emits a current having the same waveform as the external control signal S3. That is, the waveform of the current flowing through the point A in FIG. 11 is basically the waveform shown in A in FIG.

On the other hand, when the external control signal S3 is input, the oscillator 23 emits a high frequency current having a sine waveform. The level of this high frequency current corresponds to the output level of the external control signal S3. That is, the waveform of the signal flowing through the point B in FIG. 11 is the waveform shown in B in FIG.

The current that has passed through the coil 20, that is, the current that flows through point A in FIG. 11, is added as a bias current to the high-frequency current that has passed through the capacitor 22. Therefore, the current flowing through the point C in FIG. 11 has a positive current waveform without the negative current component. A schematic waveform of this drive current flowing through point C in FIG. 11 is shown in C in FIG. Therefore, a drive current that periodically turns on and off is applied to the semiconductor light emitting device 11.

The semiconductor light emitting device 11 receives the driving current and emits light 10. Then, the oscillated laser beam is obtained in the same manner as in the external resonator type semiconductor laser device 1 of FIG. A part of this laser beam passes through the resonator mirror 12 and is taken out of the resonator as output light 16. The schematic waveform of the output light 16 is shown in D of FIG. As shown here, the output light 16 is intermittently emitted in a pulse shape corresponding to the ON-OFF waveform of the drive current of the semiconductor light emitting device 11. The light output of the output light 16 periodically increases or decreases in response to the waveform of the high-frequency current.

As described above, in the present embodiment, the optical output of the output light 16 can be digitally modulated based on the external control signal S3. Such digital modulation can be applied to, for example, pulse width modulation and pulse number modulation for image recording, image display, and the like. Further, also in this embodiment, since the output light 16 to be pulsed light is high-speed modulated by the high-frequency superimposition drive, the effect of this high-speed modulation can be obtained in the same manner as in the first embodiment.

Here, the relationship between the frequency of the high frequency current superimposed on the drive current of the semiconductor light emitting device 11 and the frequency of the external control signal S3 will be described. Here, the external control signal S3 will be described as a signal that has the same ON period and OFF period and is ON-OFF at a constant frequency. It is desirable that the frequency of the high frequency current is at least twice the frequency of the external control signal S3. That is, otherwise, one cycle of high-frequency signal cannot exist in one ON period of the external control signal S3, and by extension, the light output has at least one cycle within one emission period of the output light 16. This is because it becomes impossible to increase or decrease.

In particular, when the pulsed output light 16 emitted intermittently is used for image recording and one dot of a pixel is recorded by a plurality of pulsed output lights 16, the frequency of the high frequency current is an external control signal. It is desirable that the frequency is higher than twice the frequency of S3.

<< Third Embodiment >>
Next, a third embodiment of the present invention will be described. FIG. 13 is a schematic configuration diagram showing an external cavity type semiconductor laser device 3 according to a third embodiment of the present invention. The external resonator type semiconductor laser device 3 of the present embodiment basically uses an oscillator 323 different from the oscillator 23 as compared with the external resonator type semiconductor laser device 2 of the second embodiment shown in FIG. The difference is that the modulated signal source 321 is used instead of the DC current source 21, and the mixer 300 is arranged between the capacitor 22 and the semiconductor light emitting element 11. In this configuration, the coil 20 shown in FIG. 11 is unnecessary.

When the modulation signal source 321 receives the external control signal S3, it outputs a modulation signal which is a voltage signal, and this modulation signal is input to the mixer 300. The waveform of this modulated signal is shown in FIG. 14A. In this embodiment, the same control circuit 224 as that in the external resonator type semiconductor laser device 2 of the second embodiment shown in FIG. 11 is used. A schematic waveform of the external control signal S3 output from the control circuit 224 and flowing through the Ao point in FIG. 13 is shown in FIG. 14A. That is, the waveform of the modulated signal flowing through the point A in FIG. 13 is basically the same as the waveform of the external control signal S3.

On the other hand, the oscillator 323 is supposed to generate a pulse current in which an on (ON) period in which the signal level rises to a predetermined level and an off (OFF) period in which the signal level is 0 (zero) level are repeated. The schematic waveform of this pulse current flowing through the point B of FIG. 13 is shown in B of FIG.

After passing through the capacitor 22, the pulse current is modulated in the mixer 300 based on the modulation signal output from the modulation signal source 321. As a result, the drive current composed of the pulse current is applied to the semiconductor light emitting device 11 only during the period when the external control signal S3 is rising. A schematic waveform of this drive current flowing through point C in FIG. 13 is shown in C in FIG.

The semiconductor light emitting device 11 receives the driving current and emits light 10. Then, the oscillated laser beam is obtained in the same manner as in the external resonator type semiconductor laser device 1 of FIG. A part of this laser beam passes through the resonator mirror 12 and is taken out of the resonator as output light 16. The schematic waveform of the output light 16 is shown in D of FIG. As shown here, the output light 16 is emitted in a pulse shape corresponding to the pulse waveform of the drive current of the semiconductor light emitting device 11.

As described above, in the present embodiment, the optical output of the output light 16 can be digitally modulated based on the external control signal S3. Such digital modulation can be applied to, for example, pulse width modulation and pulse number modulation for image recording, image display, and the like. Further, also in this embodiment, since the output light 16 is pulsed at high speed, the effect of this high speed modulation can be obtained in the same manner as in the second embodiment.

<< Fourth Embodiment >>
Next, a fourth embodiment of the present invention will be described. FIG. 15 is a schematic configuration diagram showing an external cavity type semiconductor laser device 4 according to a fourth embodiment of the present invention. The external resonator type semiconductor laser device 4 of the present embodiment basically has a control circuit 424 different from the control circuit 24 as compared with the external resonator type semiconductor laser device 1 of the first embodiment shown in FIG. It differs in that it is used. That is, the control circuit 424 is supposed to emit an external control signal S4 composed of an analog signal having a sine waveform. The schematic waveform of the external control signal S4 flowing through the Ao point of FIG. 15 is shown in A of FIG.

The external control signal S4 is input to the DC current source 21 and the oscillator 23. When the external control signal S4 is input, the DC current source 21 emits a DC current having basically the same waveform as the external control signal S4. That is, the waveform of the direct current flowing through the point A in FIG. 15 is the waveform shown in A in FIG.

On the other hand, when the external control signal S4 is input, the oscillator 23 emits a high frequency current having a sine waveform. The level of this high frequency current corresponds to the output level of the external control signal S4. That is, the waveform of the signal flowing through the point B in FIG. 15 is the waveform shown in B in FIG.

The current signal that has passed through the coil 20, that is, the current that flows through point A in FIG. 15, is added as a bias current to the high-frequency current that has passed through the capacitor 22. Therefore, the current flowing through the point C in FIG. 15 has a positive current waveform without the negative current component. A schematic waveform of this drive current flowing through point C in FIG. 15 is shown in C in FIG. Therefore, a drive current composed of a high-frequency current, which is amplitude-modulated according to the external control signal S4, is applied to the semiconductor light emitting device 11. A schematic waveform of this drive current flowing through point C in FIG. 15 is shown in C in FIG.

The semiconductor light emitting device 11 receives the driving current and emits light 10. Then, the oscillated laser beam is obtained in the same manner as in the external resonator type semiconductor laser device 1 of FIG. A part of this laser beam passes through the resonator mirror 12 and is taken out of the resonator as output light 16. The schematic waveform of the output light 16 is shown in D of FIG. As shown here, the output light 16 emits light periodically corresponding to the waveform of the high frequency current, and in each light emission, the optical output changes according to the amplitude modulation waveform of the drive current of the semiconductor light emitting device 11. It has become.

As described above, in the present embodiment, the optical output of the output light 16 can be analog-modulated based on the external control signal S4. Such analog modulation can be applied to, for example, light intensity modulation for image recording, image display, and the like. Also in this embodiment, since the intensity-modulated output light 16 is high-speed modulated based on the high-frequency current, the effect of this high-speed modulation can be obtained in the same manner as in the first embodiment.

In the present embodiment, the external control signal S4 has a sine waveform, but other waveforms may be used.

<< Fifth Embodiment >>
Next, a fifth embodiment of the present invention will be described. FIG. 17 is a schematic configuration diagram showing an external cavity type semiconductor laser device 5 according to a fifth embodiment of the present invention. The external resonator type semiconductor laser device 5 of the present embodiment is basically a modulation signal source different from the modulation signal source 321 as compared with the external resonator type semiconductor laser device 3 of the third embodiment shown in FIG. It is different in that 521 is used and a control circuit 424 different from the control circuit 224 is used. The control circuit 424 is basically the same as that shown in FIG.

When the modulation signal source 521 receives the external control signal S4, it outputs a modulation signal which is a voltage signal, and this modulation signal is input to the mixer 300. The waveform of this modulated signal is shown in FIG. 18A.

On the other hand, the oscillator 323 is the same as that shown in FIG. 13, and the on (ON) period in which the signal level rises to a predetermined level and the off (OFF) period in which the signal level is 0 (zero) level are repeated. It is supposed to emit a pulse current. The schematic waveform of this pulse current flowing through the point B of FIG. 17 is shown in B of FIG. The schematic waveform of the external control signal S4 output from the control circuit 424 and flowing through the Ao point in FIG. 17 is shown in FIG. 18A. That is, the waveform of the current flowing through the point A in FIG. 17 is basically the same as the waveform of the external control signal S4.

After passing through the capacitor 22, the pulse current is modulated in the mixer 300 based on the modulation signal output from the modulation signal source 521. That is, this pulse current is amplitude-modulated so that the waveform of the modulated signal is an envelope. As a result, a drive current consisting of a pulse current, which is amplitude-modulated according to the external control signal S4, is applied to the semiconductor light emitting device 11. A schematic waveform of this drive current flowing through point C in FIG. 17 is shown in C in FIG.

The semiconductor light emitting device 11 receives the driving current and emits light 10. Then, the oscillated laser beam is obtained in the same manner as in the external resonator type semiconductor laser device 1 of FIG. A part of this laser beam passes through the resonator mirror 12 and is taken out of the resonator as output light 16. The schematic waveform of the output light 16 is shown in FIG. 18D. As shown here, the output light 16 emits light periodically corresponding to the waveform of the pulse current, and in each light emission, corresponds to the amplitude modulation waveform (the envelope) of the drive current of the semiconductor light emitting element 11. The light intensity changes.

As described above, in the present embodiment, the light intensity of the output light 16 can be modulated based on the external control signal S4. Then, since the output light 16 whose intensity is modulated in this way is high-speed modulated based on the pulse current, the effect of this high-speed modulation can be obtained in the same manner as in the first embodiment.

Next, the coherent length and the like in the external resonator type semiconductor laser device of the present invention will be described. 19 and 20 show the coherent length Lc and the oscillation wavelength width Δλ (FWTM) of the external cavity type semiconductor laser device according to the present invention, particularly specifically, the external cavity type semiconductor laser device as the first embodiment described above. It shows the result of measuring.

Note that FIG. 19 shows the measurement results when a narrow bandpass filter 14 having the transmission characteristics shown in FIG. 2 (transmission width Δλf = 2.0 nm) is used. On the other hand, FIG. 20 shows the measurement results when a narrow band bandpass filter 14 having a transmission width Δλf different from the transmission characteristics shown in FIG. 2 (transmission width Δλf = 5.0 nm) is used. In each figure, the measurement results when the high-speed modulation frequencies are set to 100 MHz, 200 MHz, and 290 MHz are shown, and for comparison, the measurement results relating to a normal semiconductor laser are also shown on the far right side. Here, the above-mentioned ordinary semiconductor laser is a semiconductor laser that can oscillate by itself without having an external resonator, is driven by direct current, has an oscillation wavelength (center wavelength) of 488 nm, and has a longitudinal mode of multimode.

Compared to the external resonator type semiconductor laser device of the present invention, a normal semiconductor laser has a wider oscillation wavelength width Δλ, that is, the center wavelength is liable to fluctuate, and the coherent length Lc is also long, resulting in high coherence. There is. From these points, it can be said that ordinary semiconductor lasers have a difficult aspect to handle. Specifically, due to interference and speckle, the value of the detection data when the laser beam is received by the detector fluctuates, and the beam intensity distribution pattern of the laser beam is disturbed unevenly.

On the other hand, the external resonator type semiconductor laser device of the present invention has a narrow oscillation wavelength width Δλ, and the oscillation wavelength is locked by the external resonator and the wavelength control element (narrowband bandpass filter 14). The wavelength is stable and does not fluctuate easily. Further, the external resonator type semiconductor laser device of the present invention has a short coherent length Lc of about several mm and low coherence. From these points, it can be said that the external resonator type semiconductor laser device of the present invention is easy to handle. Specifically, the value of the detection data when the laser beam is received by the detector is stable. Further, since the beam intensity distribution pattern of the laser beam becomes smooth, for example, the beam diameter can be measured more accurately.

As described above, the external resonator type semiconductor laser device of the present invention has a property like a light emitting diode (LED) in which defects due to interference are reduced because the coherent length Lc is as short as several mm. Further, since the external resonator type semiconductor laser device of the present invention has a narrow oscillation wavelength width Δλ and good monochromaticity, it has a feature that the output laser light can be narrowed down to the diffraction limit as in the case of a normal semiconductor laser.

Further, while the output of the light emitting diode stays at about 10 mW, the external resonator type semiconductor laser device of the present invention can obtain an output of 100 mW or more like a normal semiconductor laser.

As described above, in the first to fifth embodiments, a laser diode made of a nitride semiconductor is applied as the semiconductor light emitting device 11, but the semiconductor light emitting device used in the external resonator type semiconductor laser device of the present invention is a semiconductor light emitting device. It is not limited to this. Further, when a laser diode made of a nitride semiconductor is applied as the semiconductor light emitting device 11, the laser diode made of a nitride semiconductor that emits light 10 having a wavelength of about 370 nm to 530 nm is not limited to the one that emits the above-mentioned wavelength. It can be used as appropriate.

Further, as the laser diode as described above, it is also possible to use a relatively high output type laser diode whose transverse mode is multimode. In addition, a laser diode made of a nitride semiconductor that emits a laser beam having a wavelength of about 445 nm was used as an excitation source, and the laser beam was used to excite a solid laser medium such as a ^{Pr 3+ doped LiYF crystal.} Laser diode excitation In solid lasers and the like, it is also possible to apply the present invention to a laser diode as an excitation source.

On the other hand, the wavelength control element used in the external resonator type semiconductor laser device of the present invention is not limited to the above-mentioned narrow band band path filter, for example, VBG (Volume Bragg Grating), prism pair, and narrow band band. Other known elements, such as those in which a pass filter and a prism pair are arranged in series with the optical path in the resonator, can also be used as appropriate.

Further, in the first to fifth embodiments described above, the rear end surface 12c of the resonator mirror 12 is provided with the partial reflection coating 12e, and the rear end surface 12c and the rear end surface 11c of the semiconductor light emitting element 11 resonate externally. Although the cavity is configured, a partial reflection coating is applied to the front end surface 12b of the resonator mirror 12, and an external resonator is formed by the front end surface 12b and the rear end surface 11c of the semiconductor light emitting element 11. It is also good. The same applies to the sixth and subsequent embodiments described below.

<< 6th Embodiment >>
Next, a sixth embodiment of the present invention will be described. FIG. 21 is a schematic configuration diagram showing an external cavity type semiconductor laser device 6 according to a sixth embodiment of the present invention. Compared to the external resonator type semiconductor laser device 1 shown in FIG. 1, the external resonator type semiconductor laser device 6 of the present embodiment basically has a confocal optical system arranged in the optical path of the external resonator. Is different. Note that this basic difference is the same in the seventh and eighth embodiments described later.

That is, in the sixth embodiment, the light emitted from the semiconductor light emitting element 11 by the rear end surface 11c of the semiconductor light emitting element 11 (the end surface on the side far from the resonator mirror 12) and the rear end surface 12c of the resonator mirror 12. An external cavity that resonates 10 is configured. In the optical path of this external resonator, a confocal optical system including a collimator lens 13 and a condenser lens 61 similar to those described above is arranged. The collimator lens 13 converts the light 10 emitted from the semiconductor light emitting element 11 into parallel light, and also condenses the light 10 returning to the semiconductor light emitting element 11 from the resonator mirror 12 to condense the light 10 to the front end surface of the semiconductor light emitting element 11 (this). Focus on the other end surface) 11b in the invention. Further, the condenser lens 61 concentrates the light 10 and focuses it on the rear end surface 12c of the resonator mirror 12, and makes the light 10 reflected by the rear end surface 12c parallel. The light 10 that has passed through the rear end surface 12c of the resonator mirror 12 and is in a divergent light state is converted into parallel light by the collimator lens 62.

A narrowband bandpass filter 63 that is basically the same as the narrowband bandpass filter 14 shown in FIG. 1 is arranged in the optical path of the light 10 between the collimator lens 13 and the condenser lens 61. The action and effect of the narrowband bandpass filter 63 are basically the same as the action and effect of the narrowband bandpass filter 14.

As described above, the front end surface 12b of the resonator mirror 12 may be partially reflectively coated, and the front end surface 12b and the rear end surface 11c of the semiconductor light emitting device 11 may form an external resonator. In that configuration, the light 10 is reflected by the front end surface 12b of the resonator mirror 12 and resonates. In that case, the confocal optical system is configured such that the condenser lens 61 concentrates the light 10 and focuses it on the front end surface 12c. The fact that the confocal optical system may be configured as such is the same in the seventh and eighth embodiments described later.

Further, in the sixth embodiment, the semiconductor light emitting device 11 is driven by high frequency superimposition by the same configuration as that shown in FIG. However, in order to drive the semiconductor light emitting device 11 by high-speed modulation, the configuration is not limited to that shown in FIG. 1, and other configurations, for example, the configurations shown in FIGS. 11, 13, 15 or 17 may be applied. This point is the same in the seventh and eighth embodiments described later.

In the sixth embodiment, as the semiconductor light emitting device 11, as an example, a device having a structure in which the emitted light 10 does not resonate by itself is used. However, when the confocal optical system as described above is arranged, a semiconductor light emitting device having a structure that resonates the emitted light 10 by itself may be used.

Next, an example of the device 6 will be described with respect to the longitudinal mode of the external resonator type semiconductor laser device 6. The specifications of the semiconductor light emitting device 11, the resonator mirror 12, and the narrowband bandpass filter 63 in this example are basically the same as the specifications in the first embodiment described above (narrowband bandpass filter). For 63, contrast with the narrowband bandpass filter 14). Further, the drive current of the semiconductor light emitting device 11 is also a current superimposed on the direct current at a high frequency as in the first embodiment, but here, the high frequency frequency = 200 MHz.

22 to 29 show the spectrum of the output light 16 when the semiconductor light emitting element 11 of the external resonator type semiconductor laser device 6 is driven by changing the value (average value) of the drive current in eight ways. These figures are photographs of the display screen of the optical spectrum analyzer, as in FIG. 4 described above. Further, the screens 201, 202, 203, 204, 205, 206, 207 and 208 shown in these figures have the optical outputs of the semiconductor light emitting device 11 of 10 mW, 20 mW, 30 mW, 40 mW, 50 mW, 60 mW, 70 mW and 80 mW, respectively. The spectrum in each case where the drive current of the semiconductor light emitting device 11 is changed in eight ways is shown. On each screen, the horizontal axis indicates the wavelength and the vertical axis indicates the optical output (relative value). From FIGS. 22 to 29, it can be seen that in the external resonator type semiconductor laser device 6, even if the optical output is changed between 10 mW and 80 mW, the change in the spectrum of the output light 16 is small.

FIG. 30 is a graph showing the center wavelength λc of the output light 16 and the wavelength width Δλ of the spectrum having the center wavelength λc in each of the above eight cases. The display method in FIG. 30 is basically the same as the display method in FIG. 5 described above. As shown in FIG. 30, in this example, even if the light output increases or decreases between 10 mW and 80 mW, the center wavelength λc fluctuates only about 0.1 nm. Further, the change in the wavelength width Δλ (FWTM) is also about 0.15 nm at the maximum, and oscillation in a narrow wavelength range is maintained.

Next, the amount of noise in each of the above eight cases will be described. FIG. 31 shows the noise characteristics of the output light 16 in each of the above eight cases. The display method in FIG. 31 is basically the same as the display method in FIG. 9 described above. Further, the method for measuring the noise characteristic is basically the same as the measuring method when the noise characteristic shown in FIG. 9 is measured. As shown in FIG. 31, in the external resonator type semiconductor laser device 6 of this example, the fluctuation of noise is small even if the optical output increases or decreases between 10 mW and 80 mW, and the ratio of noise to the optical output is small. It converges to a low value of 0.4% rms.

Next, with reference to FIGS. 32 to 38, the results of measuring the coherent length Lc in each of the seven cases, excluding the case where the light output is 10 mW, will be described. In this measurement, the output light 16 from the external resonator type semiconductor laser device 6 is branched into two systems, the interference intensity between the two branched output lights 16 is obtained, and the coherent length Lc is obtained based on the interference intensity. It is a thing.

32 to 38 are photographs of the display screen of the interferometer for which the interference strength has been obtained. The screens 301, 302, 303, 304, 305, 306 and 307 shown in these figures emit light from the semiconductor so that the light outputs of the semiconductor light emitting device 11 are 20 mW, 30 mW, 40 mW, 50 mW, 60 mW, 70 mW and 80 mW, respectively. The interference strength measurement results in each case where the drive current of the element 11 is changed are shown. On each screen, the vertical axis shows the interference strength (relative value). Further, the horizontal axis indicates the distance between the two ports for detecting the output light 16 of the two systems (this corresponds to the optical path length difference of the output light 16 of the two systems). The distance between the ports is zero at the leftmost end on the horizontal axis and 41 mm at the rightmost end on the horizontal axis in each figure. As shown in FIG. 32, the envelope of the signal indicating the interference strength is obtained, and the distance between ports where the interference strength takes 20% of the maximum value in this envelope is defined as the coherent length Lc.
As shown in FIGS. 32 to 38, in the external resonator type semiconductor laser device 6 of this example, the coherent length Lc is maintained at a small value of about 20 mm to 12 mm in the range of the optical output of 20 mW to 80 mW. To.

As described above, the confocal optical system has the effect of selecting the wavelength range of the resonating light, similar to the wavelength control element. An experiment was conducted to confirm the above effects using convex lenses having focal lengths of 2.4 mm, 4.0 mm, and 6.2 mm, respectively, as the condenser lens 61 (see FIG. 21) constituting the confocal optical system. As a result, the wavelength range selection effect was confirmed for all lenses. In general, the shorter the focal length of the lens, the greater the wavelength range selection effect. The greater this effect, the smaller the changes in the center wavelength λc and the wavelength width Δλ with respect to the increase or decrease in the drive current of the semiconductor light emitting device.

<< Seventh Embodiment >>
Next, a seventh embodiment of the present invention will be described. FIG. 39 is a schematic configuration diagram showing an external cavity type semiconductor laser device 7 according to a seventh embodiment of the present invention. Compared with the external resonator type semiconductor laser apparatus 6 shown in FIG. 21, the external resonator type semiconductor laser apparatus 7 of the present embodiment basically has two wavelength control elements instead of the narrow band band pass filter 63. The difference is that the prisms 71 and 72 are arranged. These two prisms 71 and 72 form a so-called anamorphic prism pair. The effect of the prisms 71 and 72 as a wavelength control element is basically the same as the effect of the narrowband bandpass filter 63.

In addition, in order to obtain the effect as a wavelength control element, as shown in FIG. 40, only one prism 71 may be used. However, if the anamorphic prism pair is used, when the beam cross-sectional shape of the incident light 10 is elliptical, it is possible to emit the light 10 having a perfect circular cross-sectional shape. Therefore, when it is desired that the beam cross-sectional shape of the output light 16 is a perfect circle, it is preferable to apply an anamorphic prism pair. Further, as is clear from comparing FIGS. 40 and 39, when only one prism 71 is used, the traveling direction of the light 10 is deviated, but when an anamorphic prism pair is used, before and after that. The traveling directions of the light 10 can be aligned in parallel.

FIG. 41 shows the output light 16 in each case where the drive current of the semiconductor light emitting device 11 is changed in six ways so that the light output of the semiconductor light emitting device 11 becomes 20 mW, 30 mW, 40 mW, 50 mW, 60 mW, and 70 mW. It is a graph which shows the wavelength width Δλ (FWHM) of the spectrum which takes the center wavelength λc and the center wavelength λc. The display method in FIG. 41 is basically the same as the display method in FIG. 5 described above. As shown in FIG. 41, in this example, even if the light output increases or decreases between 20 mW and 70 mW, the center wavelength λc fluctuates only about 0.2 nm. Further, the change in the wavelength width Δλ (FWTM) is also about 0.11 nm at the maximum, and oscillation in a narrow wavelength range is maintained.

Here, a specific example of a prism as a wavelength control element will be described. First, the anamorphic prism pair will be described. An anamorphic prism pair was formed using two identical prisms having a glass material of SF10 and an apex angle of 27.5 °. By setting optical paths with different angles of incidence of light on the first and second prisms, it was possible to achieve an angular dispersion amount of 45 μrad / nm (refraction angle amount per 1 nm wavelength). An external resonator type semiconductor laser device 7 having a configuration as shown in FIG. 39 was obtained by arranging a prism pair having an angular dispersion amount in the resonator, and an experiment or the like showing the results in FIG. 41 was performed.

Since the emission angle from the prism differs depending on the wavelength of the light transmitted through the prism, it is possible to determine the resonance wavelength in the external resonator by configuring it so that only the light having a predetermined emission angle resonates. Further, when a prism pair is used, it is possible to give the prism pair a different amount of angular dispersion from the above by changing the relative angle between the first prism and the second prism. Further, for example, by changing the glass material of the first prism to SF10 and the glass material of the second prism to BK7 or the like, a desired angular dispersion amount can be obtained.

Next, an example in which only one prism is used as shown in FIG. 40 will be described. Using a prism with a glass material of BK7 and an apex angle of 30 °, an angular dispersion amount of 66 μrad / nm could be realized. Even when an external resonator as shown in FIG. 39 is configured by using this prism, the effect of the present invention can be obtained.

<< Eighth Embodiment >>
Next, an eighth embodiment of the present invention will be described. FIG. 42 is a schematic configuration diagram showing an external cavity type semiconductor laser device 8 according to an eighth embodiment of the present invention. Compared with the external resonator type semiconductor laser apparatus 6 shown in FIG. 21, the external resonator type semiconductor laser apparatus 8 of the present embodiment is basically a wavelength control element in addition to the narrow band band pass filter 63, and further. The difference is that the two prisms 71 and 72 are arranged. These two prisms 71 and 72 are basically the same as those shown in FIG. In the configuration of FIG. 42, the anamorphic prism pair including the prisms 71 and 72 and the narrow band bandpass filter 63 both function as wavelength control elements.

Here, with reference to FIGS. 43 and 44, the coherence and the longitudinal mode characteristic in the external resonator type semiconductor laser device 8 of the eighth embodiment will be described. In FIGS. 43 and 44, a photograph of the display screen of the interferometer is shown in the upper row as in FIGS. 32 to 38 described above, and the display of the optical spectrum analyzer is shown in the same manner as in FIG. 4 described above. The picture taken of the screen is shown in the lower row. Let the screen of FIG. 43 be 401, and the screen of FIG. 44 be 402.

FIG. 44 shows a measurement result in an example of the external resonator type semiconductor laser device 8 shown in FIG. 42. In this example, the semiconductor light emitting device 11 is driven by a current on which a high frequency of frequency = 200 MHz is superimposed. The light output in this example is 60 mW. On the other hand, FIG. 43 shows the measurement result in the external resonator type semiconductor laser apparatus as a comparative example with respect to the external resonator type semiconductor laser apparatus 8. In this comparative example, the semiconductor light emitting device 11 is driven by a direct current. The light output in this comparative example is 55 mW. In each example, as the narrow band bandpass filter 63, a transmission width Δλf (see FIG. 2) = 1.0 nm is used.

In both of the two examples, the longitudinal mode oscillates in single mode. In FIG. 44, the screen 402 shows the coherent length as Lc, but Lc = 12 mm. The coherent length Lc is a value specified in the same manner as the specified method described above with reference to FIG. 32. On the other hand, in the comparative example showing the measurement result in FIG. 43, the coherent length Lc exceeds 41 mm (the distance between the two ports for detecting the output light described above), and is estimated to be several hundred mm or more.

When the laser device is applied to the measurement light source of the precision position detection device, the coherent length Lc is preferably set to a preferable value, for example, less than 40 mm in order to keep the coherence low. In order to set the coherent length Lc to less than 40 mm, it is generally desirable to set the degree of modulation to a value higher than, for example, 80%. When the degree of modulation is 80% or less, the coherent length Lc is generally as long as 40 mm or more due to the relative increase in the DC component in the drive current. In that case, the coherence of the external resonator type semiconductor laser device increases, and the performance of the device deteriorates. Here, the degree of modulation = modulation component of the drive current / (DC component of the drive current + modulation component of the drive current).

As described above, the external resonator type semiconductor laser device 8 according to the present invention has both the performance of vertical single mode oscillation and short coherent length, while adopting the same resonance optical system as in the comparative example. A general laser has a characteristic that the coherent length becomes relatively long when the vertical single mode oscillation is performed, and the coherent length becomes shorter when the vertical multimode oscillation is performed. On the other hand, in the external resonator type semiconductor laser device 8 according to the present invention, both the non-interfering property having a short coherent length and the vertical single mode oscillation can be obtained. From this point, the external resonator type semiconductor laser device 8 according to the present invention can be applied to a wide range of applications in measuring instruments and the like.

The semiconductor light emitting element 11 of the external resonator type semiconductor laser apparatus 8 of the above example was driven by changing the drive current values (average values) to 10 mW, 20 mW, 30 mW, 40 mW, 50 mW, and 60 mW. FIG. 45 is a graph showing the center wavelength λc of the output light 16 at that time and the wavelength width Δλ (FWTM) of the spectrum having the center wavelength λc. The display method in FIG. 45 is basically the same as the display method in FIG. 5 described above. As shown in FIG. 45, in this example, even if the light output increases or decreases between 10 mW and 60 mW, the center wavelength λc fluctuates only about 0.1 nm. Further, the change in the wavelength width Δλ (FWTM) is also about 0.1 nm at the maximum, and oscillation in a narrow wavelength range is maintained. As described above, in the external resonator type semiconductor laser device 8, since the anamorphic prism pair composed of the prisms 71 and 72 and the narrow band band pass filter 63 both function as wavelength control elements, the center wavelength The changes in λc and the wavelength width Δλ are suppressed to be smaller.

1, 2, 3, 4, 5, 6, 7, 8 External cavity type semiconductor laser device 11 Semiconductor light emitting element 11a Optical light guide path of semiconductor light emitting element 11b Front end surface of semiconductor light emitting element 11c Rear end surface of semiconductor light emitting element 11d Non-reflective Coat 11e High-reflection coat 12 Resonator mirror 12b Front end surface of resonator mirror 12c Rear end surface of resonator mirror 12d Non-reflective coating 12e Partial reflection coat 13, 62 Collimeter lens 14, 63 Narrow band band pass filter (wavelength control element)
15 Drive circuit 16 Output light 19 Microcomputer 20 Coil 21 DC current source 22 Capacitor 23, 323 Oscillator 24, 224, 424 Control circuit 61 Condensing lens 71, 72 Prism 300 Mixer 321, 521 Modulated signal source

Claims (14)

A semiconductor light-emitting device having a pair of end faces with a light-emitting part sandwiched between them, and
It is composed of a resonator mirror arranged outside the semiconductor light emitting element and one end surface of the pair of end faces on the side far from the resonator mirror, and oscillates the light emitted from the semiconductor light emitting element. In a semiconductor laser device having an external cavity with a fixed cavity length,
As the semiconductor light emitting device, a semiconductor light emitting device having a structure that does not oscillate the light by itself is used, and then the semiconductor light emitting device is used.
A wavelength control element that is arranged in the optical path in the external resonator and selects the wavelength range of the light, and
A drive circuit that drives the semiconductor light emitting device with high-speed modulation,
Is provided,
Laser beam oscillation wavelength width Δλ (FWTM: full width at tenth maximum) and coherent length Lc are in the following ranges Δλ ≤ 0.6 nm, Lc ≤ 40 mm
An external cavity type semiconductor laser device characterized by this. A semiconductor light-emitting device having a pair of end faces with a light-emitting part sandwiched between them, and
It is composed of a resonator mirror arranged outside the semiconductor light emitting element and one end surface of the pair of end faces on the side far from the resonator mirror, and oscillates the light emitted from the semiconductor light emitting element. In a semiconductor laser device having an external cavity with a fixed cavity length,
A cofocal position arranged in the optical path of the external cavity to focus the light on the other end surface of the pair of end faces closer to the resonator mirror and on the reflection surface of the resonator mirror. Optical system and
A wavelength control element that is arranged in the optical path in the external resonator and selects the wavelength range of the light, and
A drive circuit that drives the semiconductor light emitting device with high-speed modulation,
Is provided,
Laser beam oscillation wavelength width Δλ (FWTM: full width at tenth maximum) and coherent length Lc are in the following ranges Δλ ≤ 0.6 nm, Lc ≤ 40 mm
An external cavity type semiconductor laser device characterized by this. The external resonator type semiconductor laser device according to claim 2, wherein as the semiconductor light emitting device, a semiconductor light emitting device having a structure for oscillating the light by itself is used. The external resonator type semiconductor laser device according to any one of claims 1 to 3, wherein the high-speed modulation frequency is 20 MHz or more and 500 MHz or less. The external resonator type semiconductor laser device according to any one of claims 1 to 3, wherein the high-speed modulation frequency is 100 MHz or more and 500 MHz or less. The external resonator type semiconductor laser device according to any one of claims 1 to 3, wherein the high-speed modulation frequency is 200 MHz or more and 500 MHz or less. The external resonator type semiconductor laser device according to any one of claims 1 to 6, wherein the drive circuit is a circuit for applying a drive current formed by superimposing a high frequency on a direct current to the semiconductor light emitting device. The external resonator type semiconductor laser device according to any one of claims 1 to 6, wherein the drive circuit applies a pulse-shaped drive current in which an on period and an off period are alternately repeated to the semiconductor light emitting device. An external modulation circuit that controls the amount of light emitted from the semiconductor light emitting device based on the external modulation signal is further provided.
The external resonator type semiconductor laser device according to any one of claims 1 to 8, wherein the frequency of the high-speed modulation is twice or more the frequency of the external modulation signal. The external resonator type semiconductor laser device according to any one of claims 1 to 9, wherein a transmission type element is used as the wavelength control element. The external cavity type semiconductor laser device according to claim 10, wherein a narrow band bandpass filter is used as the wavelength control element. The external cavity type semiconductor laser device according to claim 10, wherein a prism is used as the wavelength control element. The external cavity type semiconductor laser device according to claim 12, wherein an anamorphic prism pair is used as the prism. The external resonator type semiconductor laser device according to any one of claims 1 to 13, wherein the semiconductor light emitting device is a nitride semiconductor light emitting device that emits light having a wavelength of 370 nm to 530 nm.

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