JP2013211394A - Semiconductor laser control method, semiconductor laser control device and semiconductor laser device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor laser control method, a semiconductor laser control device and a semiconductor laser device, which can more appropriately reduce an FM noise level of laser beams.SOLUTION: A semiconductor laser control method comprises: a laser drive process of driving a semiconductor laser by a bias current; a frequency discrimination process of converting frequency fluctuation of laser beams output from the semiconductor laser to intensity fluctuation; and a current control process of feedback controlling the bias current on the basis of the converted intensity fluctuation so as to inhibit the intensity fluctuation. In the current control process, the feedback control is performed based on intensity fluctuation in a first frequency band not exceeding a frequency at which a phase of a difference between a phase of current fluctuation of the bias current and a phase of frequency fluctuation of the laser beams caused by the current fluctuation is inverted.

Description

  The present invention relates to a semiconductor laser control method, a semiconductor laser control device, and a semiconductor laser device.

  Conventionally, as a method of controlling the spectral width of a semiconductor laser, a method of taking out optical power in a partial frequency band of laser light output from a semiconductor laser and controlling bias current based on the optical power (Patent Document 1) And a method of reducing the influence on the semiconductor laser by bypassing a high-frequency signal that becomes noise in a capacitor connected in parallel with the semiconductor laser (see Patent Document 2). These methods are based on the theory that there is a correlation between the FM noise characteristics of the laser light output from the semiconductor laser and the spectrum width, and the spectrum width becomes narrower when the FM noise level is reduced. Further, since fluctuation of the bias current of the semiconductor laser is superimposed on the laser light as FM noise, it is important to stabilize the bias current in order to narrow the spectrum width.

Japanese Unexamined Patent Publication No. 64-73687 Japanese Patent Laid-Open No. 7-74420

  By the way, in a method of controlling a bias current using optical power in a specific frequency band as in Patent Document 1, a low frequency band of about 10 kHz is conventionally used for bias current control. However, as a semiconductor laser used as a light source of an optical communication system used for recent large-capacity transmission, the bias current is controlled using optical power in a higher frequency band in order to reduce the FM noise level to a higher frequency. It is preferable.

  However, as will be described later, if the frequency band used for control is inappropriate, the FM noise level may increase.

  The present invention has been made in view of the above, and provides a semiconductor laser control method, a semiconductor laser control device, and a semiconductor laser device capable of appropriately reducing the FM noise level of laser light. Objective.

  In order to solve the above-described problems and achieve the object, a semiconductor laser control method according to the present invention includes a laser driving process for driving a semiconductor laser with a bias current, and a frequency variation of laser light output from the semiconductor laser. In the current control step, a frequency discrimination step of converting the bias current into a fluctuation in intensity and a current control step in which the bias current is feedback-controlled based on the converted fluctuation in intensity so as to suppress the fluctuation in intensity. The feedback control is performed based on an intensity fluctuation in a first frequency band equal to or lower than a phase inversion frequency at which a phase of a difference between a phase of current fluctuation of the bias current and a phase of frequency fluctuation of the laser light due to the current fluctuation is reversed. It is characterized by that.

  The method of controlling a semiconductor laser according to the present invention further includes a blocking step of blocking a fluctuation component having a frequency higher than a second frequency included in the bias current to be feedback-controlled, wherein the second frequency is the first frequency band. It is in the range of.

  The semiconductor laser control device according to the present invention includes a bias current generating unit that generates a bias current for driving the semiconductor laser, a branching unit that branches a part of laser light output from the semiconductor laser, and the branch A frequency discriminating unit that converts the frequency variation of the laser beam into an intensity variation, a detection unit that detects the laser beam in which the frequency variation is converted into an intensity variation, and the intensity variation of the laser beam detected by the detection unit A control unit that feedback-controls the bias current so that the intensity variation is suppressed, and the control unit is configured to control the phase variation of the bias current and the frequency variation of the laser light due to the current variation. The feedback control is performed based on intensity fluctuation in a first frequency band equal to or lower than a phase inversion frequency at which the phase of the difference from the phase is inverted.

  The semiconductor laser control device according to the present invention further includes a low-pass filter provided between the control unit and the semiconductor laser and having a cutoff frequency within the first frequency band.

  The semiconductor laser control device according to the present invention further includes a modulation power generation unit that generates modulation power of a frequency in the vicinity of a frequency at which a gain of frequency variation of the laser light due to current variation of the bias current becomes a minimum value. It is characterized by.

  In the semiconductor laser control device according to the present invention, the frequency of the modulation power is a frequency at which the gain is equal to or less than an allowable value.

  A semiconductor laser device according to the present invention includes a semiconductor laser and a semiconductor laser control device according to the present invention.

  A semiconductor laser device according to the present invention includes a capacitor connected in parallel with the semiconductor laser, and the capacitance of the capacitor is a cutoff of a low-pass CR circuit including the capacitor and the semiconductor laser having a differential resistance. The frequency is set to be within the first frequency band.

  According to the present invention, there is an effect that the FM noise level of laser light can be more appropriately reduced.

FIG. 1 is a schematic configuration diagram of the semiconductor laser device according to the first embodiment. FIG. 2 is a diagram showing the frequency response characteristics of the characteristics of the semiconductor laser. FIG. 3 is a schematic configuration diagram of the semiconductor laser device according to the second embodiment. FIG. 4 is a diagram illustrating an example of a transmission spectrum of the etalon filter. FIG. 5 is a diagram showing frequency response characteristics of gain of each semiconductor laser. FIG. 6 is a diagram showing the frequency response characteristic of the phase of each semiconductor laser. FIG. 7 is a schematic diagram of frequency characteristics of the effective line width of the semiconductor laser according to the second embodiment. FIG. 8 is a schematic configuration diagram of the semiconductor laser device according to the third embodiment. FIG. 9 is a diagram illustrating frequency characteristics of the effective line width of the semiconductor laser according to the third embodiment. FIG. 10 is a schematic configuration diagram of a semiconductor laser device according to a modification of the third embodiment. FIG. 11 is a schematic configuration diagram of a semiconductor laser device according to the fourth embodiment. FIG. 12 is a diagram illustrating an example of a transmission spectrum of the FBG.

  Hereinafter, embodiments of a semiconductor laser control method, a semiconductor laser control device, and a semiconductor laser device according to the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the embodiments. Moreover, in each drawing, the same code | symbol is attached | subjected suitably to the same or corresponding element.

(Embodiment)
FIG. 1 is a schematic configuration diagram of the semiconductor laser device according to the first embodiment. As shown in FIG. 1, the semiconductor laser device 10 according to the first embodiment includes a semiconductor laser 11, an optical branching unit 12, a frequency discriminating unit 13, a light intensity detecting unit 14, and a control unit 15. ing. The optical branching unit 12, the frequency discriminating unit 13, the light intensity detecting unit 14, and the control unit 15 constitute a control device for the semiconductor laser 11.

  The semiconductor laser 11 outputs laser light L11 having a wavelength of, for example, a wavelength of 1520 nm to 1620 nm. The semiconductor laser 11 is, for example, a DFB (Distributed Feedback) laser, a DBR (Distributed Bragg Reflector) laser, an FP (Fabry-Perot) laser, or an external resonator laser. The laser beam L11 includes FM noise.

  The optical branching unit 12 branches a part (for example, 1% to 20%) of the laser light L11 output from the semiconductor laser 11 as the branched laser light L13, and outputs the remaining as the output laser light L12. The optical branching unit 12 is realized using an optical coupler, a half mirror, or the like. The output laser light L12 is used as signal light for optical communication.

  The frequency discriminating unit 13 converts the frequency fluctuation of the branched laser light L13 into an intensity fluctuation, and outputs a converted laser light L14 including the converted intensity fluctuation. The frequency discriminating unit 13 is realized using an FP interferometer, an MZ (Mach-Zehnder) interferometer, a Michelson interferometer, or the like. The frequency discriminating unit 13 may be housed in a package of the semiconductor laser 11 and modularized together, or provided outside the package.

  The light intensity detector 14 receives the converted laser light L14, converts it into an electric signal E11 including an intensity fluctuation, and outputs it. The light intensity detection unit 14 is realized using a Pin-PD (Photo Diode), an APD (Avalanche Photo Diode), or the like.

  The control unit 15 generates a bias current E12 that is feedback-controlled so as to suppress the intensity fluctuation based on the intensity fluctuation included in the electrical signal E11, and outputs the bias current E12 to the semiconductor laser 11. That is, in the first embodiment, the bias current generation unit is included in the control unit 15. The semiconductor laser 11 is feedback driven by the feedback-controlled bias current E12. The control unit 15 is configured using an analog circuit or a digital circuit, but the configuration is not particularly limited.

A control method (driving method) of the semiconductor laser 11 performed by the control unit 15 will be described more specifically. FIG. 2 is a diagram showing frequency response characteristics of typical semiconductor laser characteristics. The horizontal axis shows the frequency of fluctuation (fluctuation) of the bias current. The normalized intensity on the vertical axis is obtained by normalizing the intensity at which the bias current fluctuation is superimposed as the frequency fluctuation of the laser light with a value at a frequency of 10 ⁴ Hz (10 kHz). The phase of the vertical axis indicates the phase difference between the bias current fluctuation phase and the laser light frequency fluctuation phase, that is, the phase delay of the laser light frequency fluctuation with reference to the bias current fluctuation. . Region S1 is a region where thermal factors are dominant, and region S2 is a region where carrier factors are dominant.

  As shown in FIG. 2, the phase delay is about 180 degrees on the low frequency side, but the phase delay decreases as the frequency increases, and becomes smaller than 90 degrees at about 5 MHz.

  When the phase delay is from 180 degrees to 90 degrees, such as on the low frequency side of about 5 MHz, when the bias current varies so as to increase, the intensity of the laser light varies so as to decrease. Therefore, if feedback control is performed so as to suppress an increase in the bias current, the frequency variation of the laser light is also suppressed.

  However, if the phase lag is less than 90 degrees, the phase of the difference between the bias current fluctuation phase and the laser light frequency fluctuation phase is reversed, and the laser light intensity also decreases when the bias current fluctuates to increase. To fluctuate. When feedback control is performed based on such intensity fluctuation in the frequency band after phase inversion, the frequency fluctuation of the laser light is amplified in the frequency band before phase inversion.

  The control unit 15 according to the first embodiment performs the first frequency band equal to or lower than the phase inversion frequency at which the phase of the difference between the current fluctuation phase of the bias current E12 and the phase of the frequency fluctuation of the laser light L11 is inverted. Feedback control is performed based on the intensity fluctuation. Thereby, feedback control is realized such that the frequency fluctuation in the first frequency band is reliably suppressed. As a result, the FM noise level (frequency fluctuation) of the laser light L11 is lowered, and the spectrum width is narrowed. Along with the narrowing of the spectrum width, the coherency of the laser beam L11 is improved and the coherent length is also increased.

  Specifically, the power component in the first frequency band may be extracted from the electrical signal E11 including the intensity fluctuation of the laser beam using a frequency filter, and feedback control may be performed based on this power component.

  The first frequency band is not particularly limited as long as it is equal to or lower than the phase inversion frequency, but is preferably as high as possible, and more preferably the phase inversion frequency. For example, in the case of the semiconductor laser having the frequency response characteristic shown in FIG.

  As described above, the semiconductor laser apparatus 10 according to the first embodiment can reduce the FM noise level of the laser light more appropriately and effectively.

  In the first embodiment, since the phase delay starts from about 180 degrees on the low frequency side, the phase is inverted when the phase delay reaches 90 degrees. However, the start angle of the phase delay on the low frequency side is not limited to about 180 degrees. This starting angle depends on how the phase reference is taken. Therefore, for example, if the start angle of the phase delay on the low frequency side is about 0 degrees, the angle at which the phase relationship is inverted is -90 degrees. In this way, the frequency at which the relationship between the increase / decrease direction of the bias current on the low frequency side and the increase / decrease direction of the laser light frequency is reversed halfway when the frequency is increased is the phase inversion frequency. It is.

(Embodiment 2)
FIG. 3 is a schematic configuration diagram of a semiconductor laser device according to the second embodiment of the present invention. As shown in FIG. 3, the semiconductor laser device 20 according to the second embodiment includes a semiconductor laser 21, an optical coupler 22 as an optical branching unit, an etalon filter 23 as a frequency discriminating unit, and a light intensity detecting unit. And a bias circuit 25 as a bias current generating unit and a control unit. The optical coupler 22, the etalon filter 23, the photodiode 24, and the bias circuit 25 constitute a control device for the semiconductor laser 21.

  FIG. 4 is a diagram illustrating an example of a transmission spectrum of the etalon filter 23. The horizontal axis indicates the frequency of light. In the example of FIG. 4, the FSR (Free Spectral Range) of the etalon filter 23 is 100 GHz, which is a characteristic often used in optical fiber communication. The FSR of the etalon filter 23 is not particularly limited, and may be, for example, 50 GHz.

  As shown in FIG. 4, in the region where the transmission spectrum is inclined with respect to the frequency, when the frequency varies with the width f1, the transmittance varies with the width T1. Therefore, the etalon filter 23 can convert the frequency variation of the input light into an intensity variation.

  The semiconductor laser device 20 operates in the same manner as the semiconductor laser device 10 according to the first embodiment. That is, the semiconductor laser 11 is driven by the bias current E22 and outputs the laser light L21. The optical coupler 22 branches a part of the laser light L21 as a branched laser light L23 and outputs the rest as an output laser light L22. The etalon filter 23 converts the frequency fluctuation of the branched laser light L23 into an intensity fluctuation, and outputs a converted laser light L24 including the converted intensity fluctuation. The photodiode 24 receives the converted laser light L24, converts it into an electric signal E21 including intensity fluctuations, and outputs it. The bias circuit 25 outputs to the semiconductor laser 21 a bias current E22 that is feedback-controlled so as to suppress the intensity fluctuation based on the intensity fluctuation included in the electrical signal E21. The semiconductor laser 21 is feedback driven by the feedback-controlled bias current E22.

  At this time, the bias circuit 25 is based on the intensity fluctuation in the first frequency band equal to or lower than the phase inversion frequency at which the phase of the difference between the current fluctuation phase of the bias current E22 and the phase of the frequency fluctuation of the laser light L21 is inverted. Feedback control. As a result, the FM noise level of the laser beam L21 is reduced and the spectrum width is narrowed.

  Here, the determination method of the 1st frequency band used for feedback control is demonstrated. First, the frequency response characteristic of the semiconductor laser 21 is measured. Here, as an example of the semiconductor laser 21, the frequency response characteristics of four DFB lasers, in which the wavelengths of laser beams output from the respective DFB lasers are set to different wavelengths in the 1.55 μm band, were measured.

FIG. 5 is a diagram showing frequency response characteristics of gain of each semiconductor laser. FIG. 6 is a diagram showing the frequency response characteristic of the phase of each semiconductor laser. Here, the gain on the vertical axis in FIG. 5 means the gain when the bias current fluctuation is superimposed as the frequency fluctuation of the laser beam. Note that when the value corresponding to the frequency fluctuation amount on the vertical axis is multiplied by the conversion coefficient [mW / Hz] of the frequency discriminating means, the intensity fluctuation amount equivalent value is obtained. Moreover, the phase of FIG. 6 shows the delay of the phase of the frequency fluctuation of each laser beam based on the bias current fluctuation. 5 and 6 show the measurement results up to approximately 10 ⁷ Hz. As shown in FIGS. 5 and 6, the frequency response characteristics of the DFB laser did not change much between DFB lasers having different wavelengths.

Since it is confirmed from FIG. 6 that the phase is 90 degrees near 10 ⁶ Hz (1 MHz) and the phase is inverted, in this case, the band where the 3 dB band is 1 MHz can be determined as the first frequency band. .

  The frequency at which the phase is inverted varies depending on the semiconductor laser, for example, in the range of 1 MHz to 5 MHz. However, as described above, the frequency response characteristic of the semiconductor laser used in advance is measured. A frequency band can be determined.

  FIG. 7 is a schematic diagram of the frequency characteristics of the effective line width of the semiconductor laser when the 3 dB band of the first frequency band is set in the vicinity of 1 MHz based on the characteristics shown in FIG. 6 in the semiconductor laser device of the second embodiment. FIG. Here, the effective line width is an index that standardizes PSD (Power Spectral Density) of FM noise. For comparison, the frequency characteristic of the effective line width of a semiconductor laser in which constant current control is performed so that the laser beam intensity in the frequency band of about 10 kHz is stabilized as in the prior art is indicated by a broken line.

  As shown in FIG. 7, when the 3 dB band of the first frequency band is in the vicinity of 1 MHz, the FM noise level of 1 MHz or less is suppressed compared to the conventional example indicated by the broken line.

(Embodiment 3)
FIG. 8 is a schematic configuration diagram of a semiconductor laser device according to Embodiment 3 of the present invention. As shown in FIG. 3, the semiconductor laser device 30 according to the third embodiment includes a semiconductor laser unit 31, an optical coupler 22, an etalon filter 23, a photodiode 24, a bias circuit 25, and a low-pass filter. 36. The optical coupler 22, the etalon filter 23, the photodiode 24, the bias circuit 25, and the low-pass filter 36 constitute a control device for the semiconductor laser unit 31.

  The semiconductor laser unit 31 has a configuration in which the semiconductor laser 21 of the second embodiment and a capacitor are connected in parallel. Since the semiconductor laser has a differential resistance, a low-pass CR circuit for a bias current is configured by connecting the semiconductor laser in parallel with a capacitor. The capacitance of the capacitor is set so that the cut-off frequency of the low-pass transmission CR circuit is a frequency within the first frequency band used for feedback control. In order to set the cutoff frequency to 1 MHz, for example, when the differential resistance value of the semiconductor laser 21 is 5Ω, a capacitor having a capacitance of 30000 pF may be connected in parallel.

  The low-pass filter 36 is provided between the bias circuit 25 and the semiconductor laser unit 31 and has a cutoff frequency in the first frequency band used for feedback control. For example, the low-pass filter 36 can be composed of an RC circuit. In order to set the cut-off frequency to 1 MHz, for example, an RC circuit can be configured with a resistor having a resistance value of 1 KΩ and a capacitor having a capacitance of 1500 pF and used as the low-pass filter 36.

  That is, the semiconductor laser device 30 has a configuration in which a capacitor is connected in parallel to the semiconductor laser 21 of the semiconductor laser device 20 of the second embodiment to form a semiconductor laser unit 31 and a low-pass filter 36 is added.

  In this semiconductor laser device 30, the semiconductor laser unit 31 is driven by a bias current E33 and outputs a laser beam L31. The optical coupler 22 branches a part of the laser light L31 as a branched laser light L33 and outputs the remainder as an output laser light L32. The etalon filter 23 converts the frequency variation of the branched laser beam L33 into an intensity variation, and outputs a converted laser beam L34 including the converted intensity variation. The photodiode 24 receives the converted laser beam L34, converts it into an electric signal E31 including intensity fluctuations, and outputs it. The bias circuit 25 outputs a bias current E32 that is feedback-controlled so as to suppress the intensity fluctuation based on the intensity fluctuation included in the electrical signal E31. The low-pass filter 36 cuts off a fluctuation component having a frequency higher than the cutoff frequency included in the bias current E32, and outputs it to the semiconductor laser unit 31 as the bias current E33. The semiconductor laser unit 31 is feedback driven by the feedback-controlled bias current E33.

  At this time, the bias circuit 25 is based on the intensity fluctuation in the first frequency band equal to or lower than the phase inversion frequency at which the phase of the difference between the current fluctuation phase of the bias current E33 and the phase of the frequency fluctuation of the laser light L31 is inverted. Feedback control. As a result, the FM noise level of the laser beam L31 is reduced and the spectrum width is narrowed.

  In addition, since the low-pass CR circuit and the low-pass filter 36 configured in the semiconductor laser unit 31 block the fluctuation components having frequencies higher than the respective cutoff frequencies, the FM noise level of the laser light L31 is reduced. Also, it decreases on the higher frequency side than the first frequency. As a result, the spectral width of the laser beam L31 is further narrowed.

  FIG. 9 shows that in the semiconductor laser device of the third embodiment, the 3 dB band of the first frequency band used for feedback control is set near 1 MHz, and the cutoff frequency of the low-pass CR circuit and the low-pass filter is any FIG. 6 is a schematic diagram of frequency characteristics of an effective line width of a semiconductor laser when 1 MHz is also used. For comparison, the frequency characteristic of the effective line width of a semiconductor laser in which constant current control is performed so that the laser beam intensity in the frequency band of about 10 kHz is stabilized as in the prior art is indicated by a broken line.

  As shown in FIG. 9, when the 3 dB band of the first frequency band is close to 1 MHz and the cutoff frequencies of the low-pass CR circuit and the low-pass filter are both 1 MHz, compared with the conventional example shown by the broken line. Thus, both FM noise levels below 1 MHz and above 1 MHz are suppressed.

  By the way, in the semiconductor laser device, for example, the control unit may be mounted with a circuit or the like that generates high-frequency modulated power. For example, when a semiconductor laser is driven by controlling the temperature with a Peltier element, the control unit is equipped with a PWM control circuit that performs PWM (Pulse Width Modulation) control on the Peltier element. Generate modulation power at frequency. In addition, when the control unit is configured by a digital circuit, a reference clock generator for generating a reference clock such as a CPU for current control is mounted, and this becomes a generation source of modulation power.

  When such a modulation power generation unit is mounted in a control unit or other semiconductor laser device, the modulation frequency in the modulation power generation unit is set to the gain of the frequency fluctuation of the laser beam due to the current fluctuation of the bias current. Is preferably set to a frequency in the vicinity of a frequency at which becomes a minimum value (that is, low sensitivity). By setting the modulation frequency in this way, even if the modulation power generated by the modulation power generator is superimposed on the current fluctuation of the bias current, the degree of superposition on the frequency fluctuation of the laser light can be minimized. . Here, the frequency in the vicinity of the frequency at which the gain becomes the minimum value means a frequency range in which the gain is not more than the allowable value including the frequency at which the gain becomes the minimum value. The allowable gain is determined based on, for example, the required specification of the FM noise level allowed in the semiconductor laser device.

  The gain of the frequency fluctuation of the laser light due to the current fluctuation of the bias current can be obtained by measuring the frequency response characteristic of the semiconductor laser as shown in FIG. In the example illustrated in FIG. 5, if the frequency of the modulation power generated by the modulation power generation unit is in the vicinity of 1 MHz, superimposition on the laser light can be suppressed to a minimum.

  FIG. 10 is a schematic configuration diagram of a semiconductor laser device according to a modification of the third embodiment of the present invention. A semiconductor laser device 30A shown in FIG. 10 has a configuration in which the bias circuit 25 of the semiconductor laser device 30 according to the third embodiment is replaced with a bias circuit 35. The bias circuit 35 includes a modulation power generation unit 35a in which the modulation frequency is set to a frequency in the vicinity of a frequency at which the gain of the frequency variation of the laser light due to the current variation of the bias current becomes a minimum value. The modulated power generator 35a is, for example, a reference clock generator or a PWM control circuit. The bias circuit 35 outputs a bias current E32A on which the modulation frequency component of the modulation power generator 35a is superimposed, but the degree to which the modulation component is superimposed on the frequency variation of the laser light is small. Further, if the cut-off frequency of the low-pass filter 36 is made lower than the modulation frequency, the modulation component of the bias current E32A is largely cut off by the low-pass filter 36, and is converted from the low-pass filter 36 to the bias current E33A. Since it is output, the degree to which the modulation component is superimposed on the frequency fluctuation of the laser beam becomes extremely small.

(Embodiment 4)
FIG. 11 is a schematic configuration diagram of a semiconductor laser device according to Embodiment 4 of the present invention. As shown in FIG. 11, the semiconductor laser device 30 according to the fourth embodiment includes a semiconductor laser unit 31, an optical circulator 42, an FBG 43, a photodiode 24, a bias circuit 25, a low-pass filter 36, and the like. It has. The optical circulator 42 and the FBG 43 constitute an optical branching unit, and the FBG 43 constitutes a frequency discriminating unit. The optical circulator 42, the FBG 43, the photodiode 24, the bias circuit 25, and the low-pass filter 36 constitute a control device for the semiconductor laser 31.

  FIG. 12 is a diagram illustrating an example of a transmission spectrum of the FBG 43. The horizontal axis indicates the frequency of light. As shown in FIG. 12, in the region where the transmission spectrum is inclined with respect to the frequency, when the frequency varies with the width f2, the transmittance (or reflectance) varies with the width T2. Therefore, the FBG 23 can convert the frequency variation of the input light into an intensity variation.

  In this semiconductor laser device 40, the semiconductor laser unit 31 is driven by a bias current E43 and outputs a laser beam L31. The optical circulator 42 transmits the laser beam L31 to the FBG 43. The FBG 43 converts the frequency fluctuation of the laser light L31 into intensity fluctuation, and a part thereof is branched to the optical circulator 42 side as converted reflected laser light L44 including the converted intensity fluctuation, and the rest is output laser light L32. Output as. The optical circulator 42 outputs the converted reflected laser light L44 to the photodiode 24. The photodiode 24 receives the converted reflected laser light L44, converts it into an electric signal E41 including intensity fluctuations, and outputs it. The bias circuit 25 outputs a bias current E42 that is feedback-controlled so as to suppress the intensity fluctuation based on the intensity fluctuation included in the electrical signal E41. The low-pass filter 36 cuts off a fluctuation component having a frequency higher than the cutoff frequency included in the bias current E42, and outputs it to the semiconductor laser unit 31 as the bias current E43. The semiconductor laser unit 31 is feedback driven by the feedback-controlled bias current E43.

  At this time, the bias circuit 25 performs feedback control based on the intensity fluctuation in the first frequency band equal to or lower than the phase inversion frequency at which the phase relationship between the current fluctuation of the bias current E43 and the frequency fluctuation of the laser light L31 is inverted. I do. As a result, the FM noise level of the laser beam L31 is reduced and the spectrum width is narrowed.

  In addition, since the low-pass CR circuit and the low-pass filter 36 configured in the semiconductor laser unit 31 block the fluctuation components having frequencies higher than the respective cutoff frequencies, the FM noise level of the laser light L31 is reduced. It also decreases on the higher frequency side than the first frequency band. As a result, the spectral width of the laser beam L31 is further narrowed.

  The present invention is not limited to the above embodiment. What comprised each component of each said embodiment combining suitably is also contained in this invention. For example, the semiconductor laser device 20 of the second embodiment may be provided with the low-pass filter 36 of the third embodiment, and the bias circuit 25 includes the modulation power generation unit 35a of the modification of the third embodiment. The bias circuit 35 may be replaced, and the etalon filter 23 may be replaced with an MZ interferometer or a Michelson interferometer. Further effects and modifications can be easily derived by those skilled in the art. Therefore, the broader aspect of the present invention is not limited to the above-described embodiment, and various modifications can be made.

10, 20, 30, 30A, 40 Semiconductor laser device 11, 21 Semiconductor laser 12 Optical branching unit 13 Frequency discriminating unit 14 Light intensity detecting unit 15 Control unit 22 Optical coupler 23 Etalon filter 24 Photodiode 25, 35 Bias circuit 31 Semiconductor laser Unit 35a modulation power generation unit 36 low-pass filter 40 semiconductor laser device 42 optical circulator E11, E21, E31, E41 electrical signal E12, E22, E32, E32A, E33, E33A, E42, E43 bias currents f1, f2, T1, T2 Width L11, L21, L31 Laser light L12, L22, L32 Output laser light L13, L23, L33 Branched laser light L14, L24, L34 Conversion laser light L44 Conversion reflection laser light S1, S2 region

Claims (8)

A laser driving step of driving the semiconductor laser with a bias current;
A frequency discrimination step of converting the frequency fluctuation of the laser beam output from the semiconductor laser into an intensity fluctuation;
A current control step of feedback-controlling the bias current based on the converted intensity fluctuation, so that the intensity fluctuation is suppressed;
In the current control step, the intensity in the first frequency band below the phase inversion frequency at which the phase of the difference between the phase of the current fluctuation of the bias current and the phase of the frequency fluctuation of the laser light due to the current fluctuation is reversed. A method of controlling a semiconductor laser, wherein the feedback control is performed based on fluctuation.   The method further comprises a blocking step of blocking a fluctuation component having a frequency higher than a second frequency included in the bias current to be feedback-controlled, wherein the second frequency is within the range of the first frequency band. Item 2. A method for controlling a semiconductor laser according to Item 1. A bias current generator for generating a bias current for driving the semiconductor laser;
A branching portion for branching a part of the laser light output from the semiconductor laser;
A frequency discriminator for converting frequency fluctuations of the branched laser light into intensity fluctuations;
A detection unit for detecting laser light in which the frequency fluctuation is converted into intensity fluctuation;
A control unit that feedback-controls the bias current based on the intensity variation of the laser light detected by the detection unit so that the intensity variation is suppressed;
The control unit includes an intensity fluctuation in a first frequency band equal to or lower than a phase inversion frequency at which a phase of a difference between a phase of a current fluctuation of the bias current and a phase of a frequency fluctuation of the laser light due to the current fluctuation is reversed. The semiconductor laser control apparatus is characterized in that the feedback control is performed based on the above.   4. The semiconductor laser control device according to claim 3, further comprising a low-pass filter provided between the control unit and the semiconductor laser and having a cutoff frequency within the first frequency band. 5.   5. The modulation power generation unit further generates a modulation power of a frequency in the vicinity of a frequency at which a gain of a frequency variation of the laser light due to the current variation of the bias current becomes a minimum value. Semiconductor laser control device.   6. The semiconductor laser control device according to claim 5, wherein the frequency of the modulation power is a frequency at which the gain is equal to or less than an allowable value.   A semiconductor laser device comprising: a semiconductor laser; and the semiconductor laser control device according to claim 3.   A capacitor connected in parallel with the semiconductor laser, wherein the capacitance of the capacitor is such that a cutoff frequency of a low-pass CR circuit composed of the capacitor and the semiconductor laser having a differential resistance is within the first frequency band. The semiconductor laser device according to claim 7, wherein the semiconductor laser device is set to be

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