JP2007049051A - Semiconductor laser light source - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent a variation of a light output created when high frequency overlay-driving in a semiconductor laser light source which has a plurality of semiconductor lasers and combines laser beams emitted from there. <P>SOLUTION: In the semiconductor laser beam source which combines laser beams B1, B2, B3, and B4 emitted from a plurality of semiconductor lasers LD1, LD2, LD3, and LD4 into one laser beam B, high frequency currents RF1, RF2 having common frequency are overlaid on driving currents Id1, Id2, Id3, Id4 of a plurality of semiconductor lasers, in a state that phases are different each other among each groups where the plurality of semiconductor lasers are divided into N groups (2≤N). The means performing such an overlapping is composed of, e.g., a high frequency oscillator 50 to produce the high frequency current RF1, and a phase inverter 51 which inverts the phase of the high frequency current RF1 to change to the high frequency current RF2. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a semiconductor laser light source, and more particularly to a semiconductor laser light source that includes a plurality of semiconductor lasers and multiplexes laser beams emitted from them.

  Conventionally, semiconductor lasers have been put to practical use in many fields, and Patent Document 1 discloses a combination of a plurality of semiconductor lasers, and a laser beam emitted from them is combined to obtain a high-power laser. A wave laser source is shown.

  By the way, in a semiconductor laser, a so-called mode hopping phenomenon in which the oscillation wavelength fluctuates due to a change in ambient temperature or the like may occur. When this mode hopping occurs, the optical output of the semiconductor laser fluctuates. Therefore, for example, when this mode hopping occurs in an apparatus that performs image exposure using laser light emitted from a semiconductor laser, problems such as density unevenness occur in the exposed image.

Conventionally, in order to suppress the occurrence of this mode hopping, for example, as disclosed in Patent Document 2, a method of superposing a high-frequency current on a driving current of a semiconductor laser is known. When the high-frequency current is superimposed on the driving current of the semiconductor laser in this way, the semiconductor laser oscillates in multiple longitudinal modes, so that mode hopping is prevented from occurring.
JP 2004-77779 A Japanese Patent Laid-Open No. 5-167169

  However, when the semiconductor laser is driven at a high frequency as described above, the laser light emitted from the semiconductor laser is modulated by the high frequency, so that the light output fluctuates. Since this light output fluctuation is a high frequency, there is often no particular problem. However, for example, in the case of exposing a very high-definition image using laser light from a semiconductor laser, the image quality of the exposed image is impaired. May cause problems.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a semiconductor laser light source capable of preventing the influence of fluctuations in light output due to high frequency superposition.

  The semiconductor laser light source according to the present invention pays particular attention to the configuration in which a plurality of laser beams are combined as described above, and the phase of the high-frequency modulation is shifted between the plurality of laser beams before combining. Optical output fluctuations cancel each other out by multiplexing.

That is, more specifically, the semiconductor laser light source according to the present invention is:
A plurality of semiconductor lasers;
In a semiconductor laser light source comprising a multiplexing optical system that combines laser beams emitted from these semiconductor lasers,
High-frequency superposition in which the plurality of semiconductor lasers are divided into N groups (2 ≦ N) and the phases are different from each other, and high-frequency currents having a common period are superimposed on the drive currents of the plurality of semiconductor lasers Means is provided.

  Here, in the semiconductor laser light source of the present invention having the above configuration, it is particularly desirable that the phase of the high-frequency current is sequentially shifted by 2π / N between the N groups. In that case, the value of N preferably matches the number of semiconductor lasers.

  Further, it is desirable that the number of semiconductor lasers forming the above group be the same among the groups.

  The high-frequency superimposing means includes, for example, a single high-frequency oscillator, a means for branching a high-frequency current generated from the high-frequency oscillator into N systems and superimposing them on the driving currents of the N group semiconductor lasers, A delay circuit that gives different amounts of delay to each branch system with respect to the generated high frequency can be configured.

  Alternatively, in particular, if N = 2, the high frequency superimposing means splits the high frequency current generated from the single high frequency oscillator into two systems and superimposes them on the driving currents of the two groups of semiconductor lasers. And means for converting one of the branched high frequencies into an opposite phase.

  In the semiconductor laser light source according to the present invention, the plurality of semiconductor lasers are divided into N groups (2 ≦ N) and the phases between the groups are mutually different for the drive currents of the plurality of semiconductor lasers that emit the combined laser beams. Since high-frequency superimposing means for superposing high-frequency currents having a common period in different states is provided, the high-frequency modulation components of the laser light from each group of semiconductor lasers cancel each other in the combined laser light Therefore, a stable combined laser beam with little output fluctuation can be obtained.

  When the phase of the high-frequency current is sequentially shifted by 2π / N between the N groups, the optical output fluctuations due to the high-frequency modulation of the laser light from the semiconductor lasers of each group cancel each other better, so that the output A combined laser beam with very little fluctuation can be obtained. In this case, particularly when the number of N matches the number of semiconductor lasers, it is possible to avoid multiple modulation components due to high-frequency currents from being superimposed in the same phase when the laser beams are combined. The output peak value of the laser light can be kept low.

  In addition, even when the number of semiconductor lasers forming the above group is the same among the groups, the fluctuations in output due to the high frequency modulation of the laser light from the semiconductor lasers in each group cancel each other better. Thus, a combined laser beam with very little can be obtained.

  In particular, when the phase of the high-frequency current is sequentially shifted by 2π / N among the N groups and the number of semiconductor lasers is the same between the groups, in principle, lasers from the semiconductor lasers of the groups Since optical output fluctuations due to high frequency modulation of light cancel each other out, it is possible to obtain a combined laser beam in which output fluctuations are suppressed to the limit.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a schematic plan view showing a semiconductor laser light source according to an embodiment of the present invention. As shown in the figure, this semiconductor laser light source includes four chip-shaped laser diodes (semiconductor lasers) LD1, LD2, LD3, and LD4 as an example, and a submount 11 that fixes these laser diodes LD1, LD2, LD3, and LD4, respectively. 12, 13 and 14, the heat block 10 in which these submounts 11 to 14 are fixed and the temperature is adjusted to a predetermined temperature, and laser light emitted from each of the laser diodes LD1, LD2, LD3 and LD4 in a divergent light state It has collimator lenses 21, 22, 23 and 24 for collimating B1, B2, B3 and B4, and an airtight package 30 in which the above elements are accommodated.

  The package 30 is provided with a transparent window glass 31 that transmits the collimated laser beams B1 to B4. A condensing lens 32 for condensing the laser beams B1 to B4 transmitted through the window glass 31 and an optical fiber 33 are provided outside the package 30. The optical fiber 33 is arranged in a state where one end surface 33a serving as an incident end surface is at a convergence position of the laser beams B1 to B4 by the condenser lens 32.

  From the other end face (outgoing end face) 33b of the optical fiber 33, laser light B, which is a combination of laser lights B1 to B4, is emitted in a divergent light state as described later. A collimator lens 34 that collimates the laser beam B is disposed at a position where the laser beam is incident. Further, a half mirror 35 that branches a part of the laser beam B that has become parallel light and a photodetector 36 that detects a part of the laser beam B reflected by the half mirror 35 are provided.

  Next, the operation of the semiconductor laser light source having the above configuration will be described. The four laser diodes LD1 to LD4 are driven as will be described later, and the laser beams B1 to B4 emitted therefrom are collimated by collimator lenses 21 to 24, and are emitted from the window glass 31 to the outside of the hermetic package 30. Exit. These laser beams B1 to B4 are condensed by the condenser lens 32 so as to converge on the incident end face 33a of the optical fiber 33, enter the core 33c of the optical fiber 33 from the end face 33a, and are guided there. Then, it is multiplexed into one laser beam B.

  The combined laser beam B is emitted in a divergent light state from the emission end face 33b of the optical fiber 33, and then is collimated by the collimator lens 34 and used for a predetermined application. At this time, a part of the collimated laser beam B is reflected by the half mirror 35 and branched, and the branched laser beam B is detected by the photodetector 36.

  Next, driving of the laser diodes LD1 to LD4 will be described with reference to FIG. FIG. 2 shows an electric circuit for driving the laser diodes LD1 to LD4. As shown, the laser diodes LD1, LD2, LD3, and LD4 are driven by constant current sources 41, 42, 43, and 44, respectively. Driven by currents Id1, Id2, Id3 and Id4. In this embodiment, a part of the combined laser beam B is detected by the photodetector 36, and the output signal S1 of the photodetector 36 indicating the optical output of the laser beam B is an APC (Automatic Power Control) circuit. Entered in 40. The APC circuit 40 controls the operation of the constant current sources 41, 42, 43 and 44 based on this signal S1, thereby increasing or decreasing the values of the drive currents Id1, Id2, Id3 and Id4, and the output of the laser beam B is increased. It is maintained at a predetermined target value.

  Here, the driving currents Id1 and Id3 are superimposed with the high-frequency current RF1 emitted from the high-frequency oscillator 50, and the driving currents Id2 and Id4 are phase-shifted by the phase inverter 51 after being emitted from the high-frequency oscillator 50. The inverted high frequency current RF2 is superimposed. By superimposing the high-frequency current on the drive currents Id1 to Id4 of the laser diodes LD1 to LD4 in this way, the laser diodes LD1 to LD4 oscillate in multiple longitudinal modes, so that mode hopping in them is prevented from occurring.

  In the present embodiment, since the high-frequency currents RF1 and RF2 are inverted in phase as described above, the drive currents Id1 and Id3 and the drive currents Id2 and Id4 after the high-frequency superposition have the former waveforms. FIG. 3A shows the latter waveform as shown in FIG. 3B, in which the modulation phases are shifted from each other by 2π / 2 = π. Therefore, the laser beams B1 and B3 emitted from the laser diodes LD1 and LD3 and the laser beams B2 and B4 emitted from the laser diodes LD2 and LD4 are also similarly shifted in the phase of modulation due to high-frequency superposition. Therefore, in the laser beam B obtained by combining these four laser beams B1 to B4, the modulation components due to the high-frequency superimposition cancel each other between the laser beams B1 and B3 and the laser beams B2 and B4. The fine fluctuation of light output due to is suppressed.

  As described above, in the combined laser beam B, if a fine light output fluctuation due to high frequency superposition is suppressed, as has been considered in the past, in order to make this light output fluctuation inconspicuous, It is not necessary to set a very high frequency. Specifically, in the conventional apparatus, the frequency of the high-frequency current is set to approximately 100 MHz or more. However, in the semiconductor laser light source of the present invention, it is possible to apply a high-frequency current having a frequency of about 1 to 100 MHz.

  In the present embodiment, the four laser diodes LD1 to LD4 are divided into two groups (N = 2), and the high-frequency currents RF1 and RF2 superimposed on the drive currents of the two groups of laser diodes are phase-shifted with each other. Is shifted by 2π / 2 = π, but the four laser diodes LD1 to LD4 are divided into four groups (N = 4) one by one and each high frequency superimposed on the drive current of the four groups of laser diodes. The currents may be sequentially shifted in phase by 2π / 4 = π / 2.

  Even if the four laser diodes LD1 to LD3 are divided into two groups or divided into four groups, the average light output of the combined laser beams B is common. However, when divided into two groups, in the combined laser beam B, two modulated components due to the high-frequency current are superimposed on each other in the same phase, so that the output peak value of the combined laser beam becomes high, but divided into four groups. In such a case, this is not the case, and it is advantageous for keeping the output peak value of the combined laser beam low.

  In the above embodiment, the present invention is applied to a semiconductor laser light source that performs APC driving of the four laser diodes LD1 to LD4. However, the present invention is applied to a semiconductor laser light source that does not perform such APC driving. Of course it is also possible. However, if the present invention is applied when APC driving is performed, it is possible to prevent the optical output fluctuation of the combined laser beam due to the high frequency superimposition, so that an APC photodetector 36 capable of high-speed response can be applied. It becomes. On the other hand, when the optical output fluctuation caused by the high frequency superposition remains in the combined laser beam, the APC photodetector 36 having a slow response must be used so as not to pick up the optical output fluctuation. I do not get.

  Furthermore, in the above-described embodiment, the phase inversion unit 51 is used to invert the phase of the high-frequency current RF1, thereby generating the high-frequency current RF2 whose phase is shifted by π. Instead of using it, it is possible to generate a high-frequency current RF2 having a phase shifted by π from the high-frequency current RF1 by using a delay circuit that delays the high-frequency current RF1.

  In the above-described embodiment, the sinusoidal high-frequency currents RF1 and RF2 are superimposed on the laser diode drive current. However, as shown in FIGS. 4A, 4B, and 4C, the laser diode is When driving with a high-frequency pulsed drive current, the drive currents themselves may be set out of phase with each other. For example, in the example of FIG. 4, the three types of high-frequency pulsed drive currents may be shifted in phase by 2π / 3 from each other.

  In the above description, the number of semiconductor lasers in each of the N groups is the same between the groups. However, in the semiconductor laser light source of the present invention, the number of semiconductor lasers in each of the N groups is mutually different. Even if they are different from each other, fluctuations in the optical output of the combined laser beam due to high frequency superposition can be suppressed. However, when the number of semiconductor lasers is the same among the groups, optical output fluctuations due to high-frequency modulation of the laser light from the semiconductor lasers of each group cancel each other better, so that the output fluctuations are extremely small. Laser light can be obtained.

  In addition, it is not always necessary to set the phase shift amount of the high-frequency current between the N groups to 2π / N, and the optical output fluctuation of the combined laser beam caused by the high-frequency superposition is suppressed even if the phase shift amount is other than that. An effect can be obtained. However, when the amount of phase shift of the high-frequency current between the N groups is 2π / N, the fluctuations in the optical output due to the high-frequency modulation of the laser light from the semiconductor lasers of each group cancel each other better. A combined laser beam with very little fluctuation can be obtained.

  Further, the number of semiconductor lasers used in the semiconductor laser light source of the present invention is not limited to the number exemplified above, and the present invention can be applied even when other numbers of semiconductor lasers are used. Light output fluctuation due to high frequency modulation of laser light can be suppressed.

1 is a schematic plan view showing a semiconductor laser light source according to an embodiment of the present invention. 1 is a block diagram showing the electrical configuration of the semiconductor laser light source of FIG. Graph showing the waveform of the high-frequency current in the semiconductor laser light source of FIG. The graph which shows the waveform of another high frequency current which can be applied in this invention

Explanation of symbols

LD1 to LD4 Laser diode (semiconductor laser)
B1 to B4 Laser light B Laser light after combining
21, 22, 23, 24 Collimator lens
32 condenser lens
33 Optical fiber
35 half mirror
36 photodetectors
40 APC circuit
41, 42, 43, 44 Constant current source
50 high frequency oscillator
51 Phase inversion section

Claims (6)

A plurality of semiconductor lasers;
In a semiconductor laser light source comprising a multiplexing optical system that combines laser beams emitted from these semiconductor lasers,
High-frequency superposition in which the plurality of semiconductor lasers are divided into N groups (2 ≦ N) and the phases of the groups are different from each other, and high-frequency currents having a common period are superimposed on the drive currents of the plurality of semiconductor lasers A semiconductor laser light source provided with means.   2. The semiconductor laser light source according to claim 1, wherein the phase of the high-frequency current is sequentially shifted by 2π / N between the N groups.   3. The semiconductor laser light source according to claim 2, wherein the value of N coincides with the number of semiconductor lasers.   4. The semiconductor laser light source according to claim 1, wherein the number of semiconductor lasers forming the group is the same among the groups.   The high-frequency superimposing means includes a single high-frequency oscillator, a means for branching a high-frequency current generated from the high-frequency oscillator into N systems and superimposing them on the driving currents of the N group semiconductor lasers, and 5. The semiconductor laser light source according to claim 1, wherein the semiconductor laser light source comprises a delay circuit that gives different amounts of delay for each branch system.   The high-frequency superimposing means includes a single high-frequency oscillator, means for branching a high-frequency current generated from the high-frequency oscillator into two systems and superimposing them on the driving currents of the two groups of semiconductor lasers, and one of the branched high-frequency oscillators 5. The semiconductor laser light source according to claim 1, wherein the semiconductor laser light source is constituted by means for converting the light into an opposite phase.

Images