CURRENT CONTROL DEVICE AND LASER DEVICE

Abstract

A current control device (42) that supplies current to a semiconductor laser (1) to output laser light from the semiconductor laser (1), the current control device (42) comprising: a processor (5) configured to output a setpoint value corresponding to a current value by increasing the setpoint value over time until it reaches a target setpoint value corresponding to the current value to output the laser light of a predetermined intensity; and a circuit (6, 7) configured to supply current of a magnitude corresponding to the setpoint output by the processor (5) to the semiconductor laser (1), wherein: the processor (5) is configured to increase an increment of the setpoint value when an ambient temperature of the semiconductor laser (1) is lower than a predetermined temperature, and to reduce the increment of the setpoint value when the ambient temperature is higher than the predetermined temperature; and the processor (5) is further configured to increase the setpoint value step by step, wherein the number of steps is equal to or greater than 15.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from Japanese application No. JP 2016-126370 A, filed on June 27, 2016, the disclosure of which is hereby expressly incorporated by reference in its entirety. BACKGROUND OF THE INVENTIONField of the invention

[0002] The present invention relates to a current control device that supplies current to a semiconductor laser and a laser device. Description of related technology

[0003] Conventionally, laser devices are known in which a resonator length changes based on a saturated absorption line included in a light output obtained by irradiating laser light onto an absorption cell, and an oscillation frequency of the laser light is stabilized to a specific saturated absorption line (see, for example, Japanese Patent Application No. JP 2012-134371 A).

[0004] In conventional laser devices, an arrangement of each optical element provided inside a resonator is adjusted to achieve a desired wavelength, and the final emission power of the laser light is adjusted by an optical system connected to a later stage of the resonator.

[0005] In a transient state immediately after starting a semiconductor laser, the laser light emitted from the semiconductor laser and the laser light reflected inside the resonator collide with a KTP crystal provided inside the resonator, which converts the wavelength of the laser light to a predetermined wavelength. Therefore, the heat of the laser light is externally disturbed, and the temperature of the KTP crystal changes. Since a refractive index of the KTP crystal changes based on temperature, the conversion efficiency of the laser light to the predetermined wavelength changes according to the temperature change. Therefore, in the transient state immediately after starting the semiconductor laser, the emission power of the laser light emitted from the resonator increases compared to a state where enough time has passed after starting.This can also increase the emission power of the laser light emitted from the laser device.

[0006] Fig. Figure 7 shows a relationship between a current command value input immediately after starting the semiconductor laser and the emission power of the laser light in the conventional laser device. As shown in Fig. As shown in Figure 7, when the set value is given in a square waveform, a pulse-like overshoot occurs on the emission power in the laser light and the emission power increases rapidly.

[0007] Based on the safety standard IEC 60825-1, laser products are classified according to emission power and the like, and safety measures must be taken according to the respective class. For laser products that have the characteristics specified in Fig. As shown in Figure 7, the emission power is approximately 2.5 mW, corresponding to Class 3R, when the temperature of the KTP crystal is stable after startup. However, in the transient state immediately after startup, the emission power is approximately 6.6 mW, corresponding to Class 3B. Class 3B requires more stringent safety measures than Class 3R, and thus, the product cost may increase. Furthermore, Class 3B also increases the burden on a user, as a safety officer must be present during use. Given this fact, in the transient state immediately after startup, overshoot of the emission power of the laser device into the Class 3R range must be suppressed.

[0008] Other current control devices or laser devices of the generic type are disclosed, for example, in JP 2001-267 669 A, US 2009 / 185 587 A1, US 5,151,910 A and US 8,907,999 B2. SUMMARY OF THE INVENTION

[0009] In view of these circumstances, the present invention provides a current control device according to claim 1, which is capable of suppressing overshoot of the emission power of a laser device in a transient state immediately after startup. Furthermore, the present invention provides a laser device according to claim 3, which is capable of suppressing overshoot of the emission power in the transient state immediately after startup.

[0010] According to one aspect of the present invention, the current control device supplies current to a semiconductor laser to output laser light to the semiconductor laser. The current control device includes a current control unit and a power supply. The current control unit outputs a set value corresponding to a current value by increasing the set value over time until it reaches a target set value corresponding to the current value to output the laser light at a predetermined intensity. The power supply supplies current of a magnitude corresponding to the set value output by the current control unit to the semiconductor laser.

[0011] The current control unit can increase the setpoint gradually. The current control unit can increase the setpoint continuously. The current control unit can decrease the setpoint increment over time. The current control unit can increase the setpoint increment over time. The current control device further includes a detector that detects the current value, and the current control unit can change the setpoint increment based on the detected current value.

[0012] According to another aspect of the present invention, a laser device comprises the current control device; the semiconductor laser; a nonlinear optical crystal that converts the laser light output from the semiconductor laser into laser light having a different frequency than the frequency of the laser light; and a temperature control device that controls the temperature of the nonlinear optical crystal. The current control unit outputs the increased set value over time for a period longer than the time required for the temperature control device to control the temperature of the nonlinear optical crystal.

[0013] According to the present invention, overshoot of the emission power of the laser device in the transient state immediately after starting can be suppressed. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The present invention will be further described in the following detailed description with reference to the designated plurality of drawings by way of non-limiting examples of exemplary embodiments of the present invention, in which like reference numerals represent similar parts throughout the several views of the drawings. In the drawings: Fig. 1 shows a configuration of a laser device; Fig. 2 shows a configuration of a current control device; Fig. 3 shows a relationship between a current set value and the emission power of laser light when the current set value is increased stepwise; Fig. 4 a relationship between the number of steps and overshoot of the current setpoint; Fig. 5 shows a relationship between the current set value and the emission power of the laser light when the current set value is continuously increased; Fig. 6 a relationship between the current setpoint according to another aspect and the elapsed time; and Fig. 7 shows a relationship between the current command value input immediately after starting a semiconductor laser and the emission power of the laser light in a conventional laser device. DETAILED DESCRIPTION OF THE INVENTION

[0015] The details shown herein are exemplary and are intended only to illustrate embodiments of the present invention and are presented to provide what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention. In this regard, no attempt is made to show structural details of the present invention in more detail than necessary for a basic understanding of the present invention, but the description, taken in conjunction with the drawings, will readily advise those skilled in the art how to practice the forms of the present invention. Laser device configuration 100

[0016] Fig. 1 depicts a configuration of a laser device 100 according to the present embodiment. The laser device 100 is an iodine-stabilized 532 nm laser. The laser device 100 is a solid-state laser that continuously oscillates in the 532 nm region and uses an Nd:YVO4 crystal (as a gain medium) excited by a semiconductor laser as a length scale. The laser device 100 uses a spectroscopic technique for iodine molecule absorption lines, and a high degree of frequency stability can be achieved by controlling an oscillation frequency centered around a saturated absorption line of the iodine molecule.

[0017] The laser device 100 includes an excitation semiconductor laser 1, a laser resonator housing 2, an iodine-stabilized optical system 3, and a controller 4. The excitation semiconductor laser 1 has a semiconductor laser 11 and allows excitation laser light in an 808 nm region, which is output from the semiconductor laser 11, to be incident on the laser resonator housing 2.

[0018] The laser resonator housing 2 includes an Nd:YVO4 crystal 21, a KTP crystal 22, an etalon 23, a reflecting mirror 24, and a piezoelectric element 25. The excitation laser light emitted from the excitation semiconductor laser 1 is incident on the Nd:YVO4 crystal 21, and the Nd:YVO4 crystal 21 emits laser light with a wavelength of 1064 nm. A coating is applied to one end of the Nd:YVO4 crystal 21 to reflect the 1064 nm light. The 1064nm laser light output by the Nd:YVO4 crystal 21 is converted into a laser light with a wavelength of 532nm, which is a second harmonic in the KTP crystal 22 (nonlinear optical crystal).

[0019] The laser light emitted by the KTP crystal 22 hits the reflecting mirror 24 via the etalon 23, which is a wavelength filter. The reflecting mirror 24 reflects the laser light and configures a laser resonator with the Nd:YVO4 crystal 21 by reflecting the incident light.

[0020] The piezo element 25 is a piezoelectric element that shifts the position of the reflecting mirror 24 through a distortion caused by the application of voltage from the controller 4. By applying a sinusoidal AC voltage to the piezo element 25, a distance between the Nd:YVO 4 crystal 21 and the reflecting mirror 24 shifts in a cycle synchronized with the AC voltage, and thus, modulated laser light is output by applying frequency modulation to the laser light.

[0021] The iodine-stabilized optical system 3 includes a beam splitter 31 and a stabilized signal detector 32. The beam splitter 31 separates the laser light input from the laser resonator housing 2 into emission laser light L1 and wavelength-regulating laser light L2. The wavelength-regulating laser light L2 is input to the stabilized signal detector 32. The stabilized signal detector 32 includes an iodine cell, which is an absorption cell, and inputs a light output signal based on the wavelength-regulating laser light L2 that has passed through the iodine cell to the controller 4.

[0022] The controller 4 includes a current control device (controller or current regulator) 42, a temperature control device (temperature regulator) 43, and a voltage control device 44. The temperature control device 43 activates a temperature changing device (a Peltier element or the like) provided on a fixed holder of each control subject based on the temperature detected by a temperature sensor provided besides the semiconductor laser 11, the KTP crystal 22, the etalon 23, and the stabilized signal detector 32 to be controlled, and controls each control subject to achieve a target temperature.

[0023] The voltage control device 44 stabilizes the laser wavelength by controlling the voltage applied to the piezoelectric element 25 and the resonator length (distance between the Nd:YVO4 crystal 21 and the reflecting mirror 24). Specifically, the voltage control device 44 detects a signal component based on the oscillation frequency from the light output signal output from the stabilized signal detector 32, for example, through a lock-in amplifier, and controls the displacement amount of the piezoelectric element 25 attached to the reflecting mirror 24 by controlling the resonator length by changing the position of the reflecting mirror 24.

[0024] The current control device 42 supplies laser current to the semiconductor laser 11 to cause the semiconductor 11 to output the laser light. Fig. Figure 2 illustrates a configuration of the current control device 42. The current control device 42 includes a current control unit (processor or current processor) 5, a current regulator 6, and a current driver 7.

[0025] The current control unit 5 includes a microcomputer 51 and a D / A converter 52. The microcomputer 51 outputs the current command value, which is a digital signal, to the D / A converter 52. The D / A converter 52 converts the current command value input from the microcomputer 51 into a control command value, which is an analog signal. The D / A converter 52 applies the control command voltage (the voltage value converted from the control command value) to the current controller 6. In the following description, the control command value is simply referred to as the command value.

[0026] The current regulator 6 and the current driver 7 serve as a power supply (circuit or supply circuit) and supply current of a magnitude corresponding to the command value output from the current control unit 5 to the semiconductor laser 11. The current regulator 6 includes an operational amplifier 61 and a current detection circuit 62. The current detection circuit 62 detects the current value of the laser current supplied to the semiconductor laser 11 and inputs the laser current detection voltage corresponding to the current value to the operational amplifier 61. The operational amplifier 61 controls the laser current so that the control command voltage and the laser current detection voltage match.

[0027] The current driver 7 is, for example, a transistor 71 and supplies the control current output from the operational amplifier 61 as a base current to the semiconductor laser 11, and the current value of the base current is amplified by a predetermined amplification rate as a laser current.

[0028] In the present embodiment, the current control unit 5 of the current regulating device 42 outputs the set value corresponding to the current value of the laser current by increasing the set value over time until the semiconductor laser 11 reaches a target set value corresponding to the current value of the laser current, so as to output the laser light at a predetermined intensity. Specifically, the current control unit 5 outputs the set value gradually (stepwise).

[0029] Fig. Figure 3 shows a relationship between the set value and the emission power of the laser light when the set value output by the current control unit 5 is gradually increased. In particular, Fig. 3 shows a relationship between the setpoint and the emission power of the laser light in the case where the output of the setpoint starts at a time ‘T’ and the setpoint is increased in increments of 100 mV over time.

[0030] In the Fig. In the example shown in Figure 3, by gradually increasing the set value, it can be confirmed that overshoot can be suppressed to a small degree immediately after the set value increases, and the maximum emission power value is suppressed to approximately 4 mW. Since, in this example, a range of emission power regulated in Class 3R is 5 mW or less, the emission power range of the laser device 100 is suppressed within the emission power range regulated by Class 3R.

[0031] As the setpoint increases, the laser light output from the semiconductor laser 11 and the laser light reflected inside the resonator case 2 collide with the KTP crystal 22. As a result, the heat of the laser light is externally disturbed, and the temperature of the KTP crystal 22 changes. Because the refractive index changes with temperature, the KTP crystal 22 changes its conversion efficiency to the 532nm laser light. This also increases the emission power of the laser light output from the laser device 100, causing overshoot. The time required to settle the overshoot corresponds to the time required for the temperature of the KTP crystal 22 to converge to the target temperature through the temperature control device 43.Therefore, the current control unit 5 outputs the increased set value over time for a period of time longer than the time required for the temperature control of the KTP crystal 22 by the temperature control device 43.

[0032] Furthermore, when the set value is increased stepwise, and especially when the set value is increased in each step before the overshoot settles down, the overshoot increases greatly due to the repeated occurrence of the overshoot. For this reason, the current control unit 5 increases the set value stepwise for a period longer than the product of the time required for the overshoot in each step to settle down with the set value and the number of steps, and then causes the set value to reach the target set value. Thus, the current control device 42 can reduce the overshoot of the emission power by stepwise increasing the set value, compared to outputting a rectangular wave set value by the conventional laser device.

[0033] In the Fig. In the example shown in Figure 3, when the setpoint is at least 1100 mV, the emission power is reduced even though the setpoint is increased. This is because the wavelength of laser light is converted by the laser current, changing the excitation efficiency of the resonator.

[0034] Next, a relationship between the number of steps and the overshoot of the emission power is described. Fig. 4 shows a relationship between the number of steps and the overshoot. The vertical axis of Fig. 4 shows a maximum value of the overshoot in each step. Fig. In the example shown in Figure 4, the relationship between the number of steps and the overshoot in four cases is shown with “◆”. As shown in Fig. As shown in Figure 4, the overshoot is reduced when the number of steps is increased. variant

[0035] In the above-described embodiment, the current control unit 5 outputs the increased set point gradually. However, the present invention is not limited to this. The current control unit 5 may output the set point that is continuously increased. For example, if the target set point is set to I1 and an arrival time is set to T1, the current control unit 5 outputs the continuously increased set point such that the amount of increase of the set point per second remains at I1 / T1. In this example, the arrival time T1 is longer than the time required for the temperature control of the KTP crystal 22 by the temperature control device 43.

[0036] Fig. Figure 5 shows a relationship between the setpoint and the emission power of the laser light when the setpoint is continuously increased. As shown in Fig. As shown in Figure 5, even if the set value is continuously increased, a rapid increase in the emission power of the laser light is suppressed and the overshoot is reduced.

[0037] Furthermore, the current control unit 5 outputs the setpoint value, keeping the increment of the setpoint value at a certain value. However, the present invention is not limited to this. Fig. Figure 6 shows a relationship between the setpoint according to another aspect and the elapsed time. For example, the current control unit 5 may reduce the increment of the setpoint over time, as shown in Fig. 6A, or can increase the increment of the setpoint over time, as shown in Fig. 6B shown.

[0038] Furthermore, the current control unit 5 can change the increment of the set value based on the current value of the laser current detected by the current detection circuit 62. For example, information showing the relationship between the increment of the set value and the amount of overshoot, and information showing the relationship between the current value of the laser current and the emission power of the laser light can be stored in advance in a memory (not shown in the drawings) provided for the controller 4. With reference to the information stored in the memory, the increment of the set value increment can be changed so that the emission power of the laser light does not exceed the previously set emission power.In addition, the current control unit 5 monitors an overshoot condition and can perform control so that the emission power of the laser light does not exceed the previously specified emission power.

[0039] If the laser device 100 can select the emission power of the laser light, the current control unit 5 can also change the increment of the set value according to the selected emission power. For example, if the selected emission power is relatively low and does not exceed the specified value even with overshoot, the current control unit 5 can increase the increment of the set value and cause the emission power to quickly reach the target value. If the emission power is relatively high, the current control unit 5 can also decrease the increment of the set value and prevent the emission power from exceeding the specified value with overshoot.

[0040] Furthermore, the current control unit 5 can change the increment of the setpoint based on the ambient temperature of the laser device 100. For example, the current control unit 5 can increase the increment of the setpoint when the ambient temperature of the laser device 100 is lower than the predetermined temperature and decrease the increment of the setpoint when the ambient temperature is higher than the predetermined temperature. Advantages of the present embodiment

[0041] As described above, the current control device 42 according to the present embodiment increases the set value corresponding to the current value of the laser current over time until it reaches the target set value corresponding to the current value of the laser current to output the laser light with the predetermined intensity, and supplies current of an amount corresponding to the set value to the semiconductor laser 11. Thus, the current control device 42 can suppress overshoot of the emission power in the transient state immediately after the laser device 100 is started, and therefore the emission power of the laser device 100 can be suppressed within the predetermined class.

[0042] The present invention will be described using an embodiment, but the technical scope of the present invention is not limited to that described in the above embodiment. It will be apparent to those skilled in the art that various changes and improvements could be added to the above embodiment. The scope of the claims makes it clear that the addition of such changes and improvements is also included in the technical scope of the present invention. For example, the configuration of the current control device 42 may be designed differently as long as it achieves the function described in the above embodiment. For example, the current control unit 5 of the current control device 42 may be configured with an integrated circuit of the operational amplifier and the like, and the set value may be increased over time. Further, the current control unit 5 may be digitally controlled.Furthermore, the current control unit 5 and the current driver 7 may be configured from a single element using a power operational amplifier.

[0043] It should be noted that the foregoing examples are provided for the purpose of illustration only and are in no way limiting the present invention. Although the present invention has been described with reference to exemplary embodiments, it is to be understood that the language used herein is descriptive and explanatory rather than restrictive. Changes may be made within the scope of the appended claims as herein pointed out and modified without departing from the spirit and scope of the present invention in its aspects. Although the present invention has been described herein with reference to particular structures, materials, and embodiments, the present invention is not intended to be limited to the details disclosed herein; rather, the present invention covers all functionally equivalent structures, methods, and uses falling within the scope of the appended claims.

[0044] The present invention is not limited to the embodiments described above, and various variations and modifications may be possible without departing from the scope of the present invention.

Claims

[1] A current control device (42) that supplies current to a semiconductor laser (1) to output laser light from the semiconductor laser (1), the current control device (42) comprising: a processor (5) configured to output a setpoint value corresponding to a current value by increasing the setpoint value over time until it reaches a target setpoint value corresponding to the current value to output the laser light of a predetermined intensity; and a circuit (6, 7) configured to supply current of a magnitude corresponding to the setpoint output by the processor (5) to the semiconductor laser (1), wherein: the processor (5) is configured to increase an increment of the setpoint value when an ambient temperature of the semiconductor laser (1) is lower than a predetermined temperature, and to reduce the increment of the setpoint value when the ambient temperature is higher than the predetermined temperature; and the processor (5) is further configured to increase the setpoint value step by step, wherein the number of steps is equal to or greater than 15. [2] The current control device (42) of claim 1, further comprising a detector configured to detect the current value, wherein the processor (5) is further configured to change the increment of the setpoint value based on the detected current value. [3] Laser device (100) comprising: a semiconductor laser (1); a controller (4) configured to supply current to the semiconductor laser (1) to output laser light from the semiconductor laser (1), the controller (4) comprising: a processor (5) configured to output a setpoint value corresponding to a current value by increasing the setpoint value over time until it reaches a target setpoint value corresponding to the current value to output the laser light of a predetermined intensity; and a circuit (6, 7) configured to supply current of a magnitude corresponding to the setpoint value output by the processor (5) to the semiconductor laser (1); and a non-linear optical crystal (22) configured to convert the laser light output from the semiconductor laser (1) into the laser light having a different frequency than the frequency of the laser light; and a temperature controller (43) configured to control a temperature of the non-linear optical crystal (22), wherein the processor (5) is further configured to output the increased setpoint over time in a time period longer than the time required for the temperature controller (43) to regulate the temperature of the non-linear optical crystal (22), wherein: the processor (5) is configured to increase an increment of the setpoint value when an ambient temperature of the semiconductor laser (1) is lower than a predetermined temperature, and to reduce the increment of the setpoint value when the ambient temperature is higher than the predetermined temperature; and the processor (5) is further configured to increase the setpoint value step by step, wherein the number of steps is equal to or greater than 15. [4] Laser device (100) according to claim 3, wherein: the controller (4) further comprises a detector (62) configured to detect the current value, and the processor (5) is further configured to change the increment of the setpoint based on the detected current value.

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