JP2017135315A - Wavelength tunable light source - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wavelength tunable light source in which variation in the change rate of wavelength can be suppressed.SOLUTION: A wavelength tunable light source 10 includes a semiconductor laser 1 emitting pulse light L1, a laser drive section 2 for driving the semiconductor laser 1, a diffraction grating 4 constituting an external resonator in conjunction with the semiconductor laser 1, receiving the pulse light L1 emitted from the semiconductor laser 1, and returning the diffraction light L2 of wavelength λ corresponding to the incident angle θ, out of the pulse light L1, to the semiconductor laser 1, and a step operation rotary drive section 5 for changing the incident angle θ by rotationally driving the diffraction grating 4 so that the angular speed thereof changes periodically. The laser drive section 2 drives the semiconductor laser 1 so as to emit the pulse light L1 in synchronization with the change of angular speed.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a wavelength tunable light source.

  In Patent Document 1, an optical gain medium that emits light, a wavelength selection element that selects the wavelength of light emitted from the optical resonator, and an optical resonator inside are arranged inside an optical resonator composed of two mirrors. An external resonator type wavelength swept light source including a light intensity modulation element for modulating the intensity of the light is described. In this wavelength swept light source, for example, a diffraction grating is used as a wavelength selection element, and the wavelength of emitted light is made variable by rotating the diffraction grating. Further, in this wavelength swept light source, the spectrum width of the emitted light is made constant by synchronizing the driving of the light intensity modulation element and the wavelength selection element.

JP 2014-42010 A

  A variable wavelength light source used for gas analysis or the like is required to have a constant wavelength change rate. However, in the wavelength swept light source as described above, when the diffraction grating is rotated using a step operation type actuator, the wavelength change speed varies as the rotation angle varies stepwise. When pulsed laser light is generated in this state, the amount of change in the wavelength of the laser light may vary from pulse to pulse even between adjacent pulses at the same time interval. For this reason, the absorption line width of the absorption curve obtained by gas analysis becomes unstable, and the accuracy of the analysis result cannot be improved.

  An object of the present invention is to provide a wavelength tunable light source capable of suppressing fluctuations in the wavelength change rate.

  A wavelength tunable light source according to the present invention is a semiconductor laser that emits pulsed light, a laser driving unit that drives the semiconductor laser, and a diffraction grating that forms an external resonator together with the semiconductor laser, and the pulse emitted from the semiconductor laser. Incident light is incident and the incident angle is changed by rotating the diffraction grating so that the angular velocity of the diffraction grating periodically changes, and a diffraction grating that returns light of a wavelength corresponding to the incident angle to the semiconductor laser. The step-operation-type rotation drive unit and the laser drive unit to be driven drive the semiconductor laser so as to emit pulsed light in synchronization with the change in angular velocity.

  In this wavelength tunable light source, a step operation type rotational drive unit is used for rotationally driving the diffraction grating that constitutes the external resonator together with the semiconductor laser, so that the angular velocity of the diffraction grating periodically changes. On the other hand, the semiconductor laser emits pulsed light in synchronization with the change in angular velocity. Thereby, the angular velocity when the pulsed light is emitted can always be regarded as constant. Therefore, the wavelength change rate of the pulsed laser light generated from this pulsed light can always be regarded as constant. As a result, fluctuations in the wavelength change rate can be suppressed.

  Further, the angular velocity periodically changes between the first angular velocity and the second angular velocity larger than the first angular velocity, so that the laser driving unit emits pulsed light when the angular velocity is the first angular velocity. A semiconductor laser may be driven. In this case, since the first angular velocity is the minimum angular velocity, when the angular velocity is the first angular velocity, the incident angle hardly changes with time. Therefore, when the angular velocity is the first angular velocity, the semiconductor laser emits pulsed light, thereby improving the resistance to jitter.

  A wavelength tunable light source according to the present invention constitutes an external resonator together with a semiconductor laser that emits pulsed light, a laser driving unit that drives the semiconductor laser, a mirror that reflects pulsed light emitted from the semiconductor laser, and the semiconductor laser. A diffraction grating that receives pulsed light reflected by a mirror and returns light having a wavelength corresponding to the incident angle of the pulsed light to the semiconductor laser via the mirror, and the angular velocity of the mirror is periodic. And a step operation type rotation drive unit that changes the incident angle by rotationally driving the mirror so as to change periodically, and the laser drive unit emits pulsed light in synchronization with the change in angular velocity Drive.

  In this wavelength tunable light source, a step operation type rotation drive unit is used for rotation drive of the mirror disposed between the semiconductor laser and the diffraction grating that forms the external resonator together with the semiconductor laser. Changes periodically. On the other hand, the semiconductor laser emits pulsed light in synchronization with the change in angular velocity. Thereby, the angular velocity when the pulsed light is emitted can always be regarded as constant. Therefore, the wavelength change rate of the pulsed laser light generated from this pulsed light can always be regarded as constant. As a result, fluctuations in the wavelength change rate can be suppressed.

  According to the present invention, fluctuations in the wavelength change rate can be suppressed.

It is a block diagram of the absorption spectrometer using the wavelength variable light source which concerns on this embodiment. It is a block diagram of the wavelength variable light source which concerns on this embodiment. It is a timing chart of the wavelength variable light source which concerns on this embodiment. It is an example of the amount of rotation in this embodiment, and the time waveform of optical output. It is an example of the time waveform of the amount of rotation and optical output in a comparison form. It is a block diagram of the wavelength variable light source which concerns on a modification.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In addition, in each figure, the same code | symbol is attached | subjected to the same or an equivalent part, and the overlapping description is abbreviate | omitted.

  FIG. 1 is a configuration diagram of an absorption spectrometer using a wavelength tunable light source according to the present embodiment. FIG. 2 is a configuration diagram of the wavelength tunable light source according to the present embodiment. As shown in FIG. 1, the absorption spectrometer 100 is an apparatus for analyzing a gas component, and includes a wavelength tunable light source 10, a rotation controller 12, a gas cell 16, a lens 18, a detector 20, and a lock. An in-amplifier 22 and an oscilloscope 24 are provided.

  As shown in FIGS. 1 and 2, the wavelength tunable light source 10 includes a semiconductor laser 1, a laser drive unit 2, a lens 3, a diffraction grating 4, a rotation drive unit 5, a lens 7, and a housing 8. And. Note that the housing 8 is omitted in FIG.

  The semiconductor laser 1 has a first end 1a and a second end 1b facing each other. In the present embodiment, the semiconductor laser 1 is a quantum cascade laser (QCL: Quantum Cascade Laser). The quantum cascade laser has a structure in which a plurality of active layers having different center wavelengths are stacked in a stack, and thus can emit broadband light in the mid-infrared region. The semiconductor laser 1 receives (applies) the drive current signal S2 from the laser drive unit 2 to generate (lights) the pulsed light L1 and emits the pulsed light L1 from the first end 1a. The semiconductor laser 1 is not limited to the quantum cascade laser.

  The first end 1a includes a reflection reducing film (not shown). Thereby, the reflectance when light is emitted from the first end 1a to the outside is reduced to, for example, 5% or less, and the reflectance when light is incident on the first end 1a from the outside is, for example, 5% or less. Reduced. The second end 1b includes a reflection control film (not shown) for obtaining an end face reflectance suitable for oscillation. Thereby, the reflectance when light is emitted from the second end 1b to the outside is set to 50% to 90%.

  The laser driving unit 2 drives the semiconductor laser 1. The laser drive unit 2 receives the pulsed movement command signal S1 output from the rotation control unit 12 as an external trigger, and supplies the pulsed driving current signal S2 to the semiconductor laser 1 based on the input movement command signal S1. This is a laser driving power source to output.

  The lens 3 collimates the pulsed light L1 emitted from the first end 1a. The lens 3 is made of a material (for example, ZnSe) that is transparent at the oscillation wavelength. The lens 3 may be an aspheric lens for reducing spherical aberration. The lens 3 may be provided on both sides with a reflection reducing film that reduces the reflectance to, for example, 5% or less at the oscillation wavelength.

  The diffraction grating 4 is, for example, a blazed diffraction grating having a sawtooth cross section. The pulsed light L1 emitted from the first end 1a and collimated by the lens 3 enters the diffraction grating 4. A reflection film 4 a having a high reflectance is provided on the grating surface of the diffraction grating 4. As a result, the diffraction grating 4 diffracts the light having the wavelength λ corresponding to the incident angle θ out of the incident pulsed light L1, and feeds it back to the first end 1a as the diffracted light L2. The incident angle θ is an angle formed by the optical axis A of the pulsed light L1 and the normal of the lattice plane.

If the grating period (step interval) d of the diffraction grating 4 is determined, the incident angle θ with respect to the wavelength λ to be fed back is uniquely determined by the following equation (1). Because of the Littrow arrangement, the diffraction order m is 1. The incident angle and the diffraction angle are equal.
mλ = 2d · sinθ (1)

  The diffraction grating 4 and particularly the second end 1 b of the semiconductor laser 1 constitute a Littrow external resonator (EC: ExternalCavity) E. That is, the semiconductor laser 1 is an external resonance type quantum cascade laser (EC-QCL), realizes a wide variable wavelength range, and can perform absorption analysis of a plurality of substances (particularly organic substances). The diffraction grating 4 and the second end 1b amplify the diffracted light L2 by multiple reflection. The diffraction grating 4 and the second end 1b emit the pulsed laser beam L3 obtained thereby from the second end 1b.

  The rotation drive unit 5 is a step operation type actuator having a large rotation range, and rotates the diffraction grating 4. In the step operation type actuator, the minimum movement step (minimum rotation amount) is discrete. The rotation angle of the diffraction grating 4 by the rotation driving unit 5 is an angle formed by the optical axis A of the pulsed light L1 and the normal line of the grating surface, and is equal to the incident angle θ.

  Examples of the step operation type actuator include an ultrasonic motor and a stepping motor. In the present embodiment, the rotation drive unit 5 is a stepping motor. The rotation driving unit 5 rotationally drives the diffraction grating 4 so that the angular velocity of the diffraction grating 4 periodically changes to change the incident angle θ. Details of the change in angular velocity will be described later.

  The lens 7 collimates the laser light L3 emitted from the second end 1b. The lens 7 is made of a material (for example, ZnSe) that is transparent at the oscillation wavelength. The lens 7 may be an aspheric lens for reducing spherical aberration. The lens 7 may be provided on both surfaces with a reflection reducing film that reduces the reflectance to, for example, 5% or less at the oscillation wavelength.

  The housing 8 accommodates the semiconductor laser 1, the lens 3, the diffraction grating 4, the rotation drive unit 5, and the lens 7 inside. The housing 8 has a window 8a. From the window 8a, the laser beam L3 emitted from the second end 1b and collimated by the lens 7 is emitted to the outside. The window 8a may be made of a material that is transparent at the oscillation wavelength (for example, ZnSe), and may be provided with a reflection reducing film that reduces the reflectance to, for example, 5% or less at the oscillation wavelength.

  The rotation control unit 12 outputs a movement command signal S <b> 1 to the rotation driving unit 5 and controls the rotation of the rotation driving unit 5. The rotation control unit 12 branches the movement command signal S1 and outputs it to the laser driving unit 2.

  The gas cell 16 is a cylindrical container and has a gas inlet 16a and an outlet 16b. The gas cell 16 is configured such that a gas to be subjected to gas analysis flows inside along the axial direction of the gas cell 16. The gas cell 16 has a first window 16c for allowing the laser light L3 to enter the gas cell 16 at one end in the axial direction, and a second window 16d for emitting the laser light L4 that has passed through the gas to the outside of the gas cell 16 in the other axial direction. Has at the end.

  The lens 18 condenses the laser light L4 and causes the detector 20 to receive the collected light. The detector 20 receives the light collected by the lens 18 and converts the intensity of the received light into an electric signal S3. The detector 20 is, for example, a photodiode. The detector 20 outputs the electric signal S3 to the lock-in amplifier.

  The lock-in amplifier 22 receives the electrical signal S3 from the detector 20 and also receives the drive current signal S2 from the laser driver 2 as a reference signal. The lock-in amplifier 22 removes unnecessary noise from the electrical signal S3 based on the drive current signal S2, and outputs the electrical signal S4 to the oscilloscope 24 as a signal indicating optical output (intensity).

  The oscilloscope 24 receives the electric signal S4 from the lock-in amplifier 22 and outputs, for example, a gas absorption line as an analysis result based on the electric signal S4. The gas absorption line is a graph with time (wavelength) on the horizontal axis and light output on the vertical axis.

  The operation of the absorption spectrometer 100 will be described with reference to FIGS. FIG. 3 is a timing chart of the wavelength tunable light source according to the present embodiment. In this timing chart, (a) a movement command signal S1, (b) a rotation amount (that is, a change amount of the incident angle θ), (c) an angular velocity of the diffraction grating (a differential value of the rotation amount), and (d) a drive current signal. S2 and (e) the light output (ie, electrical signal S4) is shown.

  First, the movement command signal S <b> 1 is output from the rotation control unit 12 to the rotation driving unit 5 and the laser driving unit 2. The movement command signal S1 is, for example, a pulse signal having a frequency of 100 kHz (cycle t is 0.010 msec). The movement command signal S1 is a rectangular wave signal that has a duty ratio of, for example, 50% and repeats rising and falling every t / 2.

  As described above, in the present embodiment, the rotation drive unit 5 is a stepping motor. The rotation drive unit 5 rotates by a minimum movement step every time one pulse of the movement command signal S1 is input. The minimum movement step of the rotation drive unit 5 is, for example, 0.0025 deg. The rotation drive unit 5 rotates using the movement command signal S1 as a trigger. In the present embodiment, the rotation driving unit 5 rotates using the rising edge of the movement command signal S1 as a trigger. As a result, the diffraction grating 4 is rotationally driven so that the angular velocity of the diffraction grating 4 periodically changes, and the incident angle θ changes. The period of change in angular velocity is equal to the period t of the movement command signal S1.

  The angular velocity at the rising edge of the movement command signal S1 is the minimum value ω1. The angular velocity after the elapse of about t / 2 from the rising edge of the movement command signal S1 is the maximum value ω2. The time waveform of the angular velocity is sinusoidal and changes continuously (smoothly) from the point where the angular velocity becomes ω1 to the point where the angular velocity next becomes ω1. In other words, the angular velocity periodically changes between ω1 and ω2 larger than ω1.

  By changing the angular velocity in this way, the amount of rotation changes stepwise. Here, the amount of rotation increases monotonously. The monotonic increase means not decreasing, and means a monotonic increase in a broad sense. In FIG. 3B, the amount of rotation in an ideal case where the angular velocity does not change (the angular velocity is constant) is indicated by a one-dot chain line.

  In an ideal case, the operation of the rotation drive unit 5 is a linear operation in direct proportion. Since the operation of the rotation drive unit 5 in the actual case is a step operation, the rotation amount by the rotation drive unit 5 repeatedly becomes smaller or larger than the rotation amount in the ideal case. The amount of rotation by the rotation drive unit 5 and the amount of rotation in an ideal case become equal after the rising of the movement command signal S1 and about t / 2 after the rising of the movement command signal S1.

  Subsequently, the drive current signal S <b> 2 is output from the laser drive unit 2 in synchronization with the movement command signal S <b> 1 and input to the semiconductor laser 1 and the lock-in amplifier 22. The drive current signal S2 is a pulse signal output in synchronization with the rotation of the rotation drive unit 5 every time one pulse of the movement command signal S1 is input. The drive current signal S2 is output using the movement command signal S1 as a trigger, similarly to the rotation of the rotation drive unit 5. In the present embodiment, the drive current signal S2 rotates using the rising edge of the movement command signal S1 as a trigger. The frequency and period of the drive current signal S2 are equal to the frequency and period t of the movement command signal S1. The duty ratio of drive current signal S2 is, for example, 1 to 10%.

  By inputting such a drive current signal S2 to the semiconductor laser 1, the pulsed light L1 is generated when the angular velocity is ω1 in synchronization with the change of the angular velocity (with the same cycle t as the cycle of the angular velocity change). . The generated pulsed light L1 is emitted from the first end 1a. For this reason, as long as the pulsed light L1 is emitted, it can be considered that the rotational drive unit 5 rotates at a constant speed and the angular velocity is constant.

  The pulsed light L 1 is collimated by the lens 3 and then enters the diffraction grating 4. In the diffraction grating 4, light having a wavelength λ corresponding to the incident angle θ of the pulsed light L1 is diffracted and returned to the first end 1a as diffracted light L2. The diffracted light L2 is amplified by multiple reflection between the diffraction grating 4 and the second end 1b. The diffracted light L2 oscillates when reaching the threshold current, and is emitted from the second end 1b as the laser light L3. The laser light L3 is collimated by the lens 7 and then emitted to the outside of the housing 8 through the window 8a.

  The laser beam L3 emitted from the wavelength tunable light source 10 enters the gas cell 16 through the first window 16c. The laser beam L4 that has passed through the gas in the gas cell 16 is emitted to the outside of the gas cell 16 through the second window 16d. The laser beam L4 is collected by the lens 18, then received by the detector 20, and converted into an electric signal S3. The electrical signal S3 is converted into an electrical signal S4 after unnecessary noise components are removed by the lock-in amplifier 22.

  The electrical signal S4 is output to the oscilloscope 24 as a signal indicating the optical output of the laser light L4. Since the electrical signal S4 is output in synchronization with the drive current signal S2, the cycle of the electrical signal S4 is equal to the cycle t similarly to the cycle of the drive current signal S2. The electric signal S4 is analyzed by the oscilloscope 24 to obtain a gas analysis result.

  Subsequently, in the present embodiment, it will be described that fluctuations in the wavelength change rate (non-uniformity in wavelength change) can be suppressed as compared with the comparative example. FIG. 4 is an example of the amount of rotation and the time waveform of the optical output in the present embodiment. FIG. 5 is an example of the rotation amount and the time waveform of the light output in the comparative embodiment. 4 and 5, the period t is 0.010 ms, and two periods are shown. In the regions I, III, and V, the amount of change of the rotation amount per time is small, and in the regions II and IV, the amount of change of the rotation amount per time is large. Therefore, when used for wavelength sweeping, the wavelength change is slow in the regions I, III, and V, and the wavelength change is fast in the regions II and IV.

  The comparative form is different from the present embodiment only in that the timing at which the laser driving unit drives the quantum cascade laser is different. In the comparative mode, the quantum cascade laser emits pulsed light with a period shorter than the period of change in angular velocity. As a result, as shown in FIG. 5, pulsed laser light is generated in each of the regions I to V. Accordingly, even between adjacent pulses at the same time interval, the wavelength change amount of the laser light varies between the pulses. In particular, in regions II and IV, the amount of change in the amount of rotation per time is large, so the amount of change in the wavelength of the laser light between pulses is also large.

  In general industrial products, step-like unevenness in the angular velocity of a step operation type actuator is hardly noticed and is often ignored. However, in the case of an external resonance type semiconductor laser, the angular velocity of the diffraction grating is directly linked to the wavelength change rate, so that the wavelength change rate becomes uneven as described above. When pulsed laser light is generated in this state, the wavelength change amount of the laser light fluctuates for each pulse even between adjacent pulses at the same time interval. For this reason, the absorption line width of the absorption curve obtained by gas analysis becomes unstable, and the accuracy of the analysis result cannot be improved.

  On the other hand, in the present embodiment, the laser driver 2 drives the semiconductor laser 1 so as to emit the pulsed light L1 in synchronization with the movement command signal S1 (as a result, in synchronization with the change in angular velocity). To do. Thereby, the semiconductor laser 1 emits the pulsed light L1 in synchronization with the change of the angular velocity. Therefore, the wavelength change rate of the pulsed laser light L3 generated from the pulsed light L1 can be always regarded as constant.

  The laser driving unit 2 is particularly in the regions I, III, and V where the amount of change per hour of the rotation amount is small, when the angular velocity is the minimum value ω1 (low rotation period, when the angular velocity time waveform is at the bottom). ), The semiconductor laser 1 is driven. Thereby, as shown in FIG. 4, the pulsed laser beam L3 is generated when the angular velocity is ω1. FIG. 4 shows only the light output of region III. When the angular velocity is ω1, the incident angle θ hardly changes with time, so that it is possible to improve resistance to jitter.

Here, when the angular velocity is ω1, the case where the angular velocity can be regarded as ω1 is included. The range of angular velocities in which the angular velocities can be regarded as ω1 can be calculated backward from the wave number accuracy to be achieved. For example, the number of steps of the diffraction grating 4 is 100 / mm, that is, the grating period d = 10 μm / line, the minimum rotation amount of the rotation driving unit 5 is 0.0025 deg, and the wavelength of the laser beam L3 is 5 μm, that is, the wave number Is 2000 cm ^−1, and the range in which the angular velocity can be regarded as ω1 is obtained by calculation when the wave number accuracy to be achieved is 1/100 of the minimum wave number increment.

In the above case, the initial angle of the diffraction grating 4 is 14.4775 deg, and the angle, wavelength, and wave number in the next step are 14.4800 deg, 5.00084 μm, and 1999.6612 cm ⁻¹ , respectively. Therefore, the change in wave number (i.e., the minimum wave number increment) is 0.3379Cm ^-1, and the wave number accuracy to be achieved is the 0.0034Cm ^-1 is one hundredth of this. Therefore, the upper limit of the allowable wavenumber 2000.0034Cm ^-1, the lower limit of the allowable wavenumber becomes 1999.9966cm ^-1.

  The lower limit of the allowable wavelength is 4.99999 μm, and the upper limit of the allowable wavelength is 5.00001 μm. At this time, the upper limit of the allowable angle is 14.47754 deg, and the lower limit of the allowable angle is 14.47749 deg. The difference between the upper limit and the lower limit of the allowable angle is 0.0500 mdeg. Therefore, the allowable angle error is 0.0250 mdeg.

  The frequency of the movement command signal S1 is 100 kHz and ω2 = 0.55 deg / ms. When the jitter amount of the laser driving unit 2 is 0.1 μs (corresponding to 1% of one cycle of 0.010 msec), the angular velocity threshold that can be regarded as constant is 0.05 deg / ms. Therefore, the ratio of the threshold of the angular velocity that can be regarded as constant with respect to ω2 is 9.1%. That is, the range in which the angular velocity can be regarded as ω1 is about 10% or less of ω2.

  Even when the wavelength of the laser beam L3 takes other values, the angular velocity can be regarded as ω1 as long as the wavenumber accuracy to be achieved is set to 1 / 100th of the wavenumber increment of one step (that is, the minimum wavenumber increment). Does not change and is about 10% or less of ω2.

  As described above, in the wavelength tunable light source 10, the angular velocity of the diffraction grating 4 is used because the step operation type rotation drive unit 5 is used to rotate the diffraction grating 4 that constitutes the external resonator E together with the semiconductor laser 1. Changes periodically. On the other hand, the semiconductor laser 1 emits the pulsed light L1 in synchronization with the change in angular velocity. Thereby, the angular velocity when the pulsed light L1 is emitted can always be regarded as constant. Therefore, the wavelength change rate of the pulsed laser light L3 generated from the pulsed light L1 can be always regarded as constant. As a result, fluctuations in the wavelength change rate can be suppressed.

  In the absorption analyzer 100, gas analysis is performed by the pulsed laser light L3 generated by the wavelength tunable light source 10. As described above, the laser light L3 is generated based on the pulsed light L1 emitted when the angular velocity can always be regarded as constant. For this reason, in the laser beam L3, the wavelength change amount is always constant between adjacent pulses at the same time interval. Thereby, the absorption line width of the absorption curve obtained by gas analysis is stabilized, and the accuracy of the analysis result can be improved.

  Further, the angular velocity periodically changes between ω1 and ω2, and the laser driver 2 drives the semiconductor laser 1 so as to emit the pulsed light L1 when the angular velocity is ω1. Since ω1 is the minimum angular velocity, when the angular velocity is ω1, the incident angle θ hardly changes with time. Therefore, when the angular velocity is ω1, the semiconductor laser 1 emits the pulsed light L1, whereby the tolerance to jitter can be improved.

  The wavelength variable light source 10 according to the present embodiment has been described above, but the present invention is not limited to this.

  FIG. 6 is a configuration diagram of a wavelength tunable light source according to a modification. The wavelength tunable light source 10A further includes a mirror 30 that reflects the pulsed light L1 emitted from the first end 1a and collimated by the lens 3, and the pulsed light L1 reflected by the mirror 30 is incident on the diffraction grating 4. The diffraction grating 4 is a point that returns the diffracted light L2 to the first end 1a via the mirror 30, and a point that the rotation drive unit 5 rotates the mirror 30 to rotate the wavelength tunable light source 10 (see FIG. 2). The main difference is that it is the same in other respects.

  In the wavelength tunable light source 10 </ b> A, since the step operation type rotation drive unit 5 is used to rotate the mirror 30, the angular velocity of the mirror 30 periodically changes. The rotation angle α of the mirror 30 by the rotation driving unit 5 is an angle formed by the optical axis A of the pulsed light L1 and the normal line of the reflecting surface. From the geometric relationship, it can be seen that the incident angle θ is correlated with the rotation angle α. Therefore, also in the wavelength tunable light source 10 </ b> A, similarly to the case of the wavelength tunable light source 10, fluctuations in the wavelength change rate can be suppressed.

  Further, in the wavelength tunable light source 10A, since the optical path length of the external resonator E can be increased by the configuration including the mirror 30, the wavelength resolution is improved. Furthermore, from the above geometric relationship, the diffraction angle (that is, the incident angle θ) can be changed twice as much as the change amount of the rotation angle α by changing the rotation angle α of the mirror 30 by a predetermined amount. That is, the amount of rotation of the rotation drive unit 5 necessary for changing the diffraction angle is half that of the wavelength variable light source 10.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to the said embodiment, It deform | transforms in the range which does not change the summary described in each claim, or applied to another thing. There may be.

  In the wavelength tunable light sources 10 and 10A, the laser driver 2 drives the semiconductor laser 1 so as to emit the pulsed light L1 in synchronization with the movement command signal S1 (as a result, in synchronization with the change in angular velocity). The semiconductor laser 1 may be driven other than when the angular velocity is ω1. In other words, the laser drive unit 2 may drive the semiconductor laser 1 every time a predetermined time T (0 ≦ T <t) has elapsed since the rising of the movement command signal S1, as long as it is synchronized with the change in angular velocity. If the laser beam L3 is instantaneous with respect to the change in angular velocity, the angular velocity can be regarded as theoretically constant.

  For example, after about t / 2 has elapsed since the rising of the movement command signal S1, that is, when the angular velocity is ω2 (at the time of the high-speed rotation period and the top of the time waveform of the angular velocity), the laser drive unit 2 It may be driven. As shown in FIG. 3D, when the angular velocity is ω2, the amount of change in angular velocity is small, so that jitter can be suppressed.

  The rotation drive unit 5 may be an ultrasonic motor. The ultrasonic motor is a piezoelectric actuator using a piezoelectric element, and operates by reversing the polarity of the voltage applied to the piezoelectric element at predetermined intervals in accordance with the rising and falling edges of the movement command signal S1. Therefore, in the case of an ultrasonic motor, the cycle of change in angular velocity is ½ of the cycle of the movement command signal S1, and the ultrasonic motor takes two steps of the stepping motor every time one pulse of the movement command signal S1 is input. Rotate minutes as the minimum moving step. The cycle of the movement command signal S1 can be set to, for example, 50 kHz (cycle t is 0.020 msec). The laser drive unit 2 may output the drive current signal S2 in synchronization with the change in angular velocity by using, for example, the rising and falling of the movement command signal S1 as a trigger.

  DESCRIPTION OF SYMBOLS 1 ... Semiconductor laser, 2 ... Laser drive part, 4 ... Diffraction grating, 5 ... Rotation drive part, 10 ... Wavelength variable light source, 30 ... Mirror, E ... External resonator, L1 ... Pulse light, L2 ... Diffracted light, L3 ... Laser light, θ ... incident angle.

Claims (3)

A semiconductor laser that emits pulsed light; and
A laser driving section for driving the semiconductor laser;
A diffraction grating constituting an external resonator together with the semiconductor laser, wherein the pulsed light emitted from the semiconductor laser is incident, and light having a wavelength corresponding to an incident angle is fed back to the semiconductor laser. A diffraction grating,
A step operation type rotational drive unit that rotationally drives the diffraction grating to change the incident angle so that the angular velocity of the diffraction grating periodically changes;
The wavelength tunable light source, wherein the laser driving unit drives the semiconductor laser to emit the pulsed light in synchronization with the change in the angular velocity. The angular velocity is periodically changing between a first angular velocity and a second angular velocity greater than the first angular velocity,
The wavelength tunable light source according to claim 1, wherein the laser driving unit drives the semiconductor laser to emit the pulsed light when the angular velocity is the first angular velocity. A semiconductor laser that emits pulsed light; and
A laser driving section for driving the semiconductor laser;
A mirror that reflects the pulsed light emitted from the semiconductor laser;
A diffraction grating constituting an external resonator together with the semiconductor laser, wherein the pulsed light reflected by the mirror is incident, and light having a wavelength corresponding to an incident angle among the pulsed light is transmitted through the mirror. A diffraction grating to be fed back to the semiconductor laser;
A step operation type rotational drive unit that rotationally drives the mirror to change the incident angle so that the angular velocity of the mirror periodically changes, and
The wavelength tunable light source, wherein the laser driving unit drives the semiconductor laser to emit the pulsed light in synchronization with the change in the angular velocity.

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