Frequency stabilization circuit, frequency stabilization method, and optical comb generator

Abstract

Provided is a frequency stabilization circuit comprising: an offset frequency detection unit that detects a carrier envelope offset frequency in an optical comb output from a resonator of a mode-locked fiber laser; a beat frequency detection unit that detects a beat frequency caused by interference between a reference optical spectrum and a wavelength reference laser beam in the optical comb; a first feedback control unit that controls a resonator length in the mode-locked fiber laser on the basis of a first error signal indicating an error of the carrier envelope offset frequency with respect to a reference frequency; a second feedback control unit that controls excitation light power in the mode-locked fiber laser on the basis of a second error signal indicating an error of the carrier envelope offset frequency with respect to the reference frequency; and a third feedback control unit that controls a resonator length in the mode-locked fiber laser on the basis of a third error signal indicating an error of the beat frequency with respect to the reference frequency.

Description

[Technical Field]

[0001] The present invention relates to a frequency stabilization circuit, a frequency stabilization method, and an optical comb generator. [Background technology]

[0002] Patent Document 1 states that "a pulsed laser light source capable of stabilizing the absolute frequencies of all longitudinal oscillation modes is provided." [Prior art document] [Patent documents] [Patent Document 1] JP 2008-251723 [Patent Document 2] JP 2006-179779 A [Patent Document 3] JP 2009-130347 A [Patent Document 4] JP 2018-205546 A [Non-patent literature] [Non-Patent Document 1] JUNGWON Kim et al., Ultralow-noise mode-locked fiber lasers and frequency combs: principles, status, and applications [Non-Patent Document 2] Wolfgang Hansel et al., All Polarization-maintaining fiber laser architecture for robust femtosecond pulse generation General disclosure

[0003] A first aspect of the present invention provides a frequency stabilization circuit comprising: an offset frequency detector for detecting a carrier-envelope offset frequency in an optical comb output from a resonator of a mode-locked fiber laser; a beat frequency detector for detecting a beat frequency generated by interference between a reference optical spectrum in the optical comb and a wavelength reference laser beam; a first feedback controller for controlling a resonator length in the mode-locked fiber laser based on a first error signal indicating an error of the carrier-envelope offset frequency relative to a reference frequency; a second feedback controller for controlling pump light power in the mode-locked fiber laser based on a second error signal indicating an error of the carrier-envelope offset frequency relative to the reference frequency; and a third feedback controller for controlling a resonator length in the mode-locked fiber laser based on a third error signal indicating an error of the beat frequency relative to the reference frequency.

[0004] In the frequency stabilization circuit, the first feedback control unit may include a first phase comparator that compares a signal corresponding to the carrier envelope offset frequency with a signal corresponding to the reference frequency to detect the first error signal, a first loop filter that outputs a first electrical signal corresponding to the first error signal, and a first driver that controls a device that can change the resonator length based on the first electrical signal.

[0005] In any of the frequency stabilization circuits, the first feedback control unit may further include an offset frequency divider that divides the carrier envelope offset frequency and a reference frequency divider that divides the reference frequency, and the first phase comparator may detect the first error signal by comparing a signal output from the offset frequency divider with a signal output from the reference frequency divider.

[0006] In any of the frequency stabilization circuits, the first driver may control, based on the first electrical signal, a piezoelectric element equipped with a reflecting mirror that reflects light emitted from an optical circulator within the resonator and re-enters the optical circulator.

[0007] In any of the frequency stabilization circuits, the first driver may control, based on the first electrical signal, a stepping motor that moves a support member supporting a reflector that reflects light emitted from an optical circulator within the resonator and re-injects it into the optical circulator relative to a housing.

[0008] In any of the frequency stabilization circuits, the first driver may control an optical modulator in the resonator based on the first electrical signal.

[0009] Any of the frequency stabilization circuits may further include a branching unit that branches a signal corresponding to the carrier envelope offset frequency into at least two signals, and supplies one signal to the first feedback control unit and the other signal to the second feedback control unit.

[0010] In any of the frequency stabilization circuits, the second feedback control unit may include: a second phase comparator that compares a signal corresponding to the carrier envelope offset frequency with a signal corresponding to the reference frequency to detect the second error signal; a second loop filter that outputs a second electrical signal corresponding to the second error signal; and a second driver that controls a laser diode that serves as an excitation light source in the resonator based on the second electrical signal.

[0011] In any of the frequency stabilization circuits, the third feedback control unit may include: a third phase comparator that compares a signal corresponding to the beat frequency with a signal corresponding to the reference frequency to detect the third error signal; a third loop filter that outputs a third electrical signal corresponding to the third error signal; and a third driver that controls an optical modulator in the resonator based on the third electrical signal.

[0012] In any of the frequency stabilization circuits, the offset frequency detection unit may include an octave comb generation unit that generates an octave comb by expanding the optical spectrum of the optical comb by more than an octave, and an offset frequency observation unit that observes the carrier envelope offset frequency using the octave comb.

[0013] In any of the frequency stabilization circuits, the beat frequency detection unit may include an optical multiplexing unit that multiplexes the optical comb and the wavelength reference laser light, and a beat frequency observation unit that observes the beat frequency using the multiplexed light.

[0014] In a second aspect of the present invention, there is provided an optical comb generator, comprising the mode-locked fiber laser and any one of the frequency stabilization circuits.

[0015] In the optical comb generator, the mode-locked fiber laser may include, within the resonator, an optical circulator that outputs light incident on a first port from a second port and outputs light incident on the second port from a third port, a reflector that reflects the light output from the second port of the optical circulator and makes the light incident on the second port of the optical circulator enter the second port again, and an actuator that can change the position of the reflector along the optical axis direction.

[0016] In any of the optical comb generators, the actuator may be a piezoelectric element to which the reflecting mirror is attached.

[0017] In any of the optical comb generators described above, the actuator may be a stepping motor that moves a support member that supports the reflecting mirror relative to a housing.

[0018] In any of the optical comb generators, the mode-locked fiber laser may be a figure-of-eight laser in which two input ports and two output ports of a multi-port optical coupler are connected in a figure-of-eight shape by polarization-maintaining fibers.

[0019] In any of the optical comb generators, the mode-locked fiber laser may have, within the resonator, a pumping light source that generates pumping light, a wavelength division multiplexing filter that inputs the pumping light, an optical amplifying fiber that amplifies light by being pumped by the pumping light, an optical modulator that modulates the phase of light propagating within the resonator, and an output optical coupler that outputs the optical comb generated within the resonator.

[0020] A third aspect of the present invention provides a frequency stabilization method comprising: detecting a carrier-envelope offset frequency in an optical frequency comb output from a resonator of a mode-locked fiber laser; detecting a beat frequency generated by interference between a reference optical spectrum and a wavelength reference laser beam in the optical frequency comb; controlling a resonator length in the mode-locked fiber laser based on a first error signal indicating an error of the carrier-envelope offset frequency relative to a reference frequency; controlling pump light power in the mode-locked fiber laser based on a second error signal indicating an error of the carrier-envelope offset frequency relative to the reference frequency; and controlling the resonator length in the mode-locked fiber laser based on a third error signal indicating an error of the beat frequency relative to the reference frequency.

[0021] The above summary of the invention does not list all of the features of the present invention, and subcombinations of these features may also be inventions. [Brief explanation of the drawings]

[0022] [Figure 1] 1 shows an example of the configuration of an optical comb generator 10 including a mode-locked fiber laser 20 and a frequency stabilization circuit 100 according to this embodiment. [Figure 2] 1 shows an example of the resonator configuration of a mode-locked fiber laser 20. [Figure 3] 1 shows an example of the configuration of an offset frequency detection section 300 included in the frequency stabilization circuit 100 according to this embodiment. [Figure 4] 1 shows an example of the configuration of a beat frequency detection section 400 included in the frequency stabilization circuit 100 according to this embodiment. [Figure 5] 1 shows an example of the configuration of a first feedback control section 500 included in the frequency stabilization circuit 100 according to this embodiment. [Figure 6] 1 shows an example of the configuration of a second feedback control section 600 included in the frequency stabilization circuit 100 according to this embodiment. [Figure 7] 1 shows an example of the configuration of a third feedback control section 700 included in the frequency stabilization circuit 100 according to this embodiment. [Figure 8] 1 shows how the carrier envelope offset frequency fce0 is detected by the self-referencing method. [Figure 9] 10 shows how the carrier envelope offset frequency fce0 and the beat frequency fbeat are locked simultaneously. DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention will be described below through embodiments of the invention, but the following embodiments do not limit the scope of the invention as claimed. Furthermore, not all of the combinations of features described in the embodiments are necessarily essential to the solution of the invention.

[0024] 1 shows an example of the configuration of an optical comb generator 10 including a mode-locked fiber laser 20 and a frequency stabilization circuit 100 according to this embodiment. Generally, when a mode-locked fiber laser is used as an optical comb light source, the carrier envelope offset frequency f and the beat frequency f must be simultaneously locked. The frequency stabilization circuit 100 according to this embodiment stabilizes the carrier envelope offset frequency f and the beat frequency f, enabling the simultaneous locking of the carrier envelope offset frequency f and the beat frequency f over a long period of time.

[0025] A frequency stabilization circuit 100 according to the present embodiment is provided. Also provided is an optical comb generator 10 including a mode-locked fiber laser 20 and the frequency stabilization circuit 100 according to the present embodiment.

[0026] The frequency stabilization circuit 100 may include an offset frequency detection section 300, a branching section 350, a beat frequency detection section 400, a first feedback control section 500, a second feedback control section 600, and a third feedback control section 700.

[0027] The offset frequency detection unit 300 detects the carrier envelope offset frequency fceO in the optical comb output from the resonator of the mode-locked fiber laser 20. The offset frequency detection unit 300 will be described in detail later. The offset frequency detection unit 300 supplies a signal corresponding to the detected carrier envelope offset frequency fceO, for example, a pulse signal whose frequency is the carrier envelope offset frequency fceO, to the branching unit 350.

[0028] The branching section 350 branches the signal corresponding to the carrier envelope offset frequency fceо into at least two signals, and supplies one to the first feedback control section 500 and the other to the second feedback control section 600 .

[0029] The beat frequency detection unit 400 detects the beat frequency fbeat generated by interference between a reference optical spectrum and wavelength reference laser light in the optical comb output from the resonator of the mode-locked fiber laser 20. Details of the beat frequency detection unit 400 will be described later. The beat frequency detection unit 400 supplies a signal corresponding to the detected beat frequency fbeat, for example, a pulse signal whose frequency is the beat frequency fbeat, to the third feedback control unit 700.

[0030] The first feedback control unit 500 controls the cavity length of the mode-locked fiber laser 20 based on a first error signal indicating the error of the carrier-envelope offset frequency fce0 relative to the reference frequency. The first feedback control unit 500 will be described in detail later.

[0031] The second feedback control unit 600 controls the pump light power in the mode-locked fiber laser 20 based on a second error signal indicating the error of the carrier envelope offset frequency fce0 relative to the reference frequency. The second feedback control unit 600 will be described in detail later.

[0032] The third feedback control unit 700 controls the cavity length of the mode-locked fiber laser 20 based on a third error signal indicating the error of the beat frequency fbeat relative to the reference frequency. The third feedback control unit 700 will be described in detail later.

[0033] Also provided is a frequency stabilization method according to the present embodiment. The frequency stabilization method may include detecting a carrier-envelope offset frequency f in an optical comb output from a resonator of the mode-locked fiber laser 20, detecting a beat frequency f generated by interference between a reference optical spectrum and a wavelength reference laser beam in the optical comb output from the resonator of the mode-locked fiber laser 20, controlling the resonator length of the mode-locked fiber laser 20 based on a first error signal indicating an error of the carrier-envelope offset frequency f relative to the reference frequency, controlling the pump light power of the mode-locked fiber laser 20 based on a second error signal indicating the error of the carrier-envelope offset frequency f relative to the reference frequency, and controlling the resonator length of the mode-locked fiber laser 20 based on a third error signal indicating the error of the beat frequency f relative to the reference frequency.

[0034] 2 shows an example of the configuration of a resonator of a mode-locked fiber laser 20. In this figure, the mode-locked fiber laser 20 is shown as an example of a figure-eight laser in which two input ports and two output ports of a multi-port optical coupler 200 are connected in a figure-eight shape by polarization-maintaining fiber (PMF). However, the present invention is not limited to this. The mode-locked fiber laser 20 may have a shape other than a figure-eight shape (for example, a figure-nine shape), or each element may be connected by an optical fiber other than a polarization-maintaining fiber (for example, a single-mode fiber (SMF)).

[0035] The mode-locked fiber laser 20 may include, within the resonator, a multi-port optical coupler 200, a first pumping light source 210, a first wavelength division multiplexing filter 215, a first optical amplifying fiber 220, an optical modulator 230, an optical circulator 240, a collimating lens 250, a reflecting mirror 260, a support member 265, an actuator 270, an output optical coupler 280, and an optical splitter 290.

[0036] The multi-port optical coupler 200 has multiple input ports and multiple output ports, and branches and combines light. In this figure, the multi-port optical coupler 200 is shown as a 2×2 port optical coupler having two input ports and two output ports. One of the input ports is connected to an optical modulator 230 via a polarization-maintaining fiber. The other input port is connected to a first port of an optical circulator 240 via a polarization-maintaining fiber. One of the output ports is connected to a first optical amplifying fiber 220 via a polarization-maintaining fiber. The other output port is connected to an output optical coupler 280 via a polarization-maintaining fiber.

[0037] The first pumping light source 210 generates pumping light and provides the amplification medium with the energy required to oscillate the laser. The first pumping light source 210 may be, for example, a pump laser diode with a center wavelength of 980 nm. The first pumping light source 210 outputs a variable optical power depending on the current injected therein. Therefore, the pumping light power can be controlled by controlling the current supplied to the first pumping light source 210. The output of the first pumping light source 210 is connected to the first wavelength division multiplexing filter 215 via a polarization-maintaining fiber.

[0038] The first wavelength division multiplexing filter 215 receives the pumping light. The first wavelength division multiplexing filter 215 may be, for example, a polarization-maintaining WDM (Wavelength Division Multiplexing) filter (also referred to as a "WDM coupler") that combines light of different wavelengths using an optical multilayer filter. One end of the first wavelength division multiplexing filter 215 is connected to the first pumping light source 210 and the output optical coupler 280 via a polarization-maintaining fiber. The other end of the first wavelength division multiplexing filter 215 is connected to the first optical amplifying fiber 220 via a polarization-maintaining fiber.

[0039] The first optical amplifying fiber 220 amplifies light by being pumped with pumping light. The first optical amplifying fiber 220 may be, for example, an erbium (Er) doped fiber. One end of the first optical amplifying fiber 220 is connected to the first wavelength division multiplexing filter 215 via a polarization-maintaining fiber. The other end of the first optical amplifying fiber 220 is connected to the multi-port optical coupler 200 via a polarization-maintaining fiber.

[0040] The optical modulator 230 modulates the phase of light propagating within the resonator. The optical modulator 230 may be, for example, an electro-optical modulator (EOM) in which an element exhibiting an electro-optic effect is used to modulate the phase of light. Such an electro-optic effect may be the Pockels effect, in which the refractive index changes due to a change in polarizability within a material when an external voltage is applied to the material. In such an optical modulator 230, the optical path length changes depending on the applied voltage. Therefore, it is possible to control the resonator length by controlling the voltage applied to the optical modulator 230. One end of the optical modulator 230 is connected to the multi-port optical coupler 200 via a polarization-maintaining fiber. The other end of the optical modulator 230 is connected to the optical circulator 240 via a polarization-maintaining fiber.

[0041] The optical circulator 240 has a first port, a second port, and a third port, and outputs light incident on the first port from the second port and light incident on the second port from the third port. The first port is connected to the multi-port optical coupler 200 via a polarization-maintaining fiber. The second port is connected to the collimating lens 250 via a polarization-maintaining fiber. The third port is connected to the optical modulator 230 via a polarization-maintaining fiber.

[0042] The collimating lens 250 outputs the light from the fiber as collimated light and causes the collimated light to enter the fiber. The collimating lens 250 is connected to the second port of the optical circulator 240 via a polarization-maintaining fiber. The collimating lens 250 is also provided facing the reflecting mirror 260. Therefore, the collimating lens 250 outputs the light from the polarization-maintaining fiber connected to the second port of the optical circulator 240 as collimated light to the reflecting mirror 260 and causes the collimated light reflected from the reflecting mirror 260 to enter the polarization-maintaining fiber.

[0043] The reflecting mirror 260 reflects light. The reflecting mirror 260 may be a total reflecting mirror that reflects all light. The reflecting mirror 260 is provided opposite the collimating lens 250. Therefore, the reflecting mirror 260 reflects the light emitted from the second port of the optical circulator 240 and makes the light enter the second port of the optical circulator 240 again. In other words, the reflecting mirror 260 may function as a folding mirror within the resonator.

[0044] The support member 265 supports the reflecting mirror 260 so as to face the collimating lens 250. The support member 265 is connected to the housing via a stepping motor 274.

[0045] The actuator 270 can change the position of the reflecting mirror 260 along the optical axis direction. The actuator 270 may be at least one of a piezoelectric element 272 and a stepping motor 274.

[0046] Piezo element 272 is a piezoelectric element to which reflecting mirror 260 is attached. Piezo element 272 may be interposed, for example, between reflecting mirror 260 and support member 265. Such piezoelectric element 272 deforms in response to an applied voltage. As a result, the distance between reflecting mirror 260 and support member 265 changes. Therefore, it is possible to control the resonator length by controlling the voltage applied to piezoelectric element 272.

[0047] The stepping motor 274 moves the support member 265 that supports the reflecting mirror 260 relative to the housing. The stepping motor 274 may be interposed, for example, between the support member 265 and the housing. In such a stepping motor 274, the output shaft of the motor rotates in response to a given pulse signal. Therefore, the relative position of the support member 265 with respect to the housing changes. Therefore, it is possible to control the resonator length by controlling the pulse signal given to the stepping motor 274.

[0048] The output optical coupler 280 outputs the optical comb generated in the resonator. One end of the output optical coupler 280 is connected to the first wavelength division multiplexing filter 215 via a polarization-maintaining fiber. The other end of the output optical coupler 280 is connected to the multiport optical coupler 200 and the optical splitter 290 via a polarization-maintaining fiber.

[0049] The optical splitter 290 splits the optical comb generated in the resonator into multiple signals. In this figure, the optical splitter 290 is shown as a three-way splitter that splits the optical comb into three signals. One end of the optical splitter 290 is connected to the output optical coupler 280. The other end of the optical splitter 290 splits into three signals, two of which are connected via polarization-maintaining fiber to the offset frequency detector 300 and the beat frequency detector 400 in the frequency stabilization circuit 100 according to this embodiment, respectively. The remaining branch is connected via polarization-maintaining fiber to the output of the optical comb generator 10.

[0050] When using such a mode-locked fiber laser 20 as an optical comb light source, for example, the frequency stabilization circuit 100 according to this embodiment stabilizes the carrier-envelope offset frequency f and the beat frequency f, thereby enabling simultaneous locking of the carrier-envelope offset frequency f and the beat frequency f over a long period of time. Next, each block of the frequency stabilization circuit 100 according to this embodiment will be described in detail.

[0051] 3 shows an example of the configuration of the offset frequency detection section 300 included in the frequency stabilization circuit 100 according to this embodiment. The offset frequency detection section 300 includes an octave comb generation section 310 and an offset frequency observation section 330.

[0052] The octave comb generator 310 generates an octave comb by expanding the optical spectrum of the optical comb output from the resonator of the mode-locked fiber laser 20 by more than an octave. The octave comb generator 310 may include a second pumping light source 311, a second wavelength division multiplexing filter 313, a second optical amplifying fiber 315, a third pumping light source 317, a third wavelength division multiplexing filter 319, and a highly nonlinear fiber 320.

[0053] The second pumping light source 311 generates pumping light. The second pumping light source 311 may be provided in front of the second optical amplifying fiber 315 and function as a forward pumping light source. The output of the second pumping light source 311 is connected to the second wavelength division multiplexing filter 313 via a polarization maintaining fiber.

[0054] The pump light is input to the second wavelength division multiplexing filter 313. The second wavelength division multiplexing filter 313 may be, for example, a polarization-maintaining WDM filter. One end of the second wavelength division multiplexing filter 313 is connected to the second pump light source 311 and the first branch of the optical branching device 290 via a polarization-maintaining fiber. The other end of the second wavelength division multiplexing filter 313 is connected to the second optical amplifying fiber 315 via a polarization-maintaining fiber.

[0055] The second optical amplifying fiber 315 amplifies light by being pumped with pumping light. The second optical amplifying fiber 315 may be, for example, an erbium-doped fiber. One end of the second optical amplifying fiber is connected to the second wavelength division multiplexing filter 313 via a polarization-maintaining fiber. The other end of the second optical amplifying fiber 315 is connected to the third wavelength division multiplexing filter 319 via a polarization-maintaining fiber.

[0056] The third pumping light source 317 generates pumping light. The third pumping light source 317 is provided after the second optical amplifying fiber 315 and may function as a backward pumping light source. The output of the third pumping light source 317 is connected to the third wavelength division multiplexing filter 319 via a polarization maintaining fiber.

[0057] The pump light is input to the third wavelength division multiplexing filter 319. The third wavelength division multiplexing filter 319 may be, for example, a polarization-maintaining WDM filter. One end of the third wavelength division multiplexing filter 319 is connected to the second optical amplification fiber 315 via a polarization-maintaining fiber. The other end of the third wavelength division multiplexing filter 319 is connected to the third pump light source 317 and the highly nonlinear fiber 320 via a polarization-maintaining fiber.

[0058] In the octave comb generating section 310, the second excitation light source 311, the second wavelength division multiplexing filter 313, the second optical amplifying fiber 315, the third excitation light source 317, and the third wavelength division multiplexing filter 319 function as an optical amplifier section that amplifies the output power of the optical comb.

[0059] The highly nonlinear fiber 320 broadens the spectrum of the optical comb. For example, the highly nonlinear fiber 320 generates an octave comb by broadening the spectrum of the optical comb amplified by the optical amplifier unit to one octave or more. One end of the highly nonlinear fiber 320 is connected to the third wavelength division multiplexing filter 319 via a polarization-maintaining fiber. The other end of the highly nonlinear fiber 320 is connected to the offset frequency observation unit 330.

[0060] The offset frequency observation unit 330 observes the carrier envelope offset frequency fce0 using an octave comb. The offset frequency observation unit 330 may include a lens 331, a PPLN waveguide 333, and a first photodiode 335.

[0061] One end of the lens 331 is connected to the highly nonlinear fiber 320 via a polarization-maintaining fiber. The lens 331 optically couples the PPLN waveguide 333 to the polarization-maintaining fiber.

[0062] The PPLN waveguide 333 is an optical waveguide made of periodically poled lithium niobate (PPLN), and generates a second harmonic wave due to a second-order nonlinear effect.

[0063] The first photodiode 335 observes the carrier-envelope offset frequency fce0 due to interference between the second harmonic 2fn of the nth mode frequency in the optical comb and the 2nth mode frequency f2n in the optical comb. This technique is called the f-2f self-referencing method. Details of the self-referencing method will be described later.

[0064] 4 shows an example of the configuration of the beat frequency detection section 400 included in the frequency stabilization circuit 100 according to this embodiment. The beat frequency detection section 400 includes an optical multiplexing section 410 and a beat frequency observation section 430.

[0065] The optical multiplexing unit 410 multiplexes the optical comb and the wavelength reference laser light. The optical multiplexing unit 410 may include a fourth optical amplifying fiber 411, a fourth pumping light source 413, a fourth wavelength division multiplexing filter 415, an optical bandpass filter 417, a wavelength reference laser 420, and an optical multiplexer 425.

[0066] The fourth optical amplifying fiber 411 amplifies light by being pumped with pumping light. The fourth optical amplifying fiber 411 may be, for example, an erbium-doped fiber. One end of the fourth optical amplifying fiber 411 is connected to the second branch path of the optical splitter 290 via a polarization-maintaining fiber. The other end of the fourth optical amplifying fiber 411 is connected to the fourth wavelength division multiplexing filter 415 via a polarization-maintaining fiber.

[0067] The fourth pumping light source 413 generates pumping light. The fourth pumping light source 413 is provided behind the fourth optical amplifying fiber 411 and may function as a backward pumping light source. The output of the fourth pumping light source 413 is connected to the fourth wavelength division multiplexing filter 415 via a polarization maintaining fiber.

[0068] The pump light is input to the fourth wavelength division multiplexing filter 415. The fourth wavelength division multiplexing filter 415 may be, for example, a polarization-maintaining WDM filter. One end of the fourth wavelength division multiplexing filter 415 is connected to the fourth optical amplifier fiber 411 via a polarization-maintaining fiber. The other end of the fourth wavelength division multiplexing filter 415 is connected to the fourth pump light source 413 and the optical bandpass filter 417 via a polarization-maintaining fiber.

[0069] In the optical multiplexing section 410, the fourth optical amplifying fiber 411, the fourth pumping light source 413, and the fourth wavelength division multiplexing filter 415 function as an optical amplifier section that amplifies the output power of the optical comb.

[0070] The optical bandpass filter 417 extracts a reference optical spectrum. For example, the optical bandpass filter 417 transmits only a specific reference wavelength from the optical comb amplified by the optical amplifier unit and blocks other light. One end of the optical bandpass filter 417 is connected to the fourth wavelength division multiplexing filter 415 via a polarization-maintaining fiber. The other end of the optical bandpass filter 417 is connected to the optical multiplexer 425 via a polarization-maintaining fiber.

[0071] The wavelength reference laser 420 emits wavelength reference laser light. The wavelength reference laser 420 may be, for example, an external cavity laser diode (ECLD). The output of the wavelength reference laser 420 is connected to an optical multiplexer 425 via a polarization-maintaining fiber.

[0072] The optical combiner 425 combines an optical spectrum used as a reference in the optical comb with wavelength reference laser light. The optical combiner 425 combines, for example, the optical spectrum extracted by the optical bandpass filter 417 with wavelength reference laser light emitted from the wavelength reference laser 420. One end of the optical combiner 425 is connected to the optical bandpass filter 417 and the wavelength reference laser 420 via a polarization-maintaining fiber. The other end of the optical combiner 425 is connected to the beat frequency observation unit 430.

[0073] The beat frequency observation section 430 observes the beat frequency using the combined light. The beat frequency observation section 430 may include a second photodiode 431.

[0074] The second photodiode 431 observes the beat frequency fbeat that occurs due to interference between the reference optical spectrum and the wavelength reference laser light in the optical comb.

[0075] 5 shows an example of the configuration of the first feedback control unit 500 included in the frequency stabilization circuit 100 according to this embodiment. The first feedback control unit 500 includes an offset frequency divider 510, a reference frequency divider 520, a first phase comparator 530, a first loop filter 540, and a first driver 550.

[0076] The offset frequency divider 510 divides the carrier envelope offset frequency fceO. The offset frequency divider 510 receives an input of a signal corresponding to the carrier envelope offset frequency fceO detected by the offset frequency detection unit 300, such as a pulse signal whose frequency is the carrier envelope offset frequency fceO and one of the signals branched by the branching unit 350. The offset frequency divider 510 then divides the carrier envelope offset frequency fceO using the input signal. At this time, the offset frequency divider 510 may determine a division ratio according to the operating band frequency of a device to be controlled by the first feedback control unit 500. Such a division ratio may be a value determined in advance for each device, a value optimized using a simulator or the like, or a value learned by machine learning.

[0077] The reference frequency divider 520 divides the reference frequency. A signal corresponding to the reference frequency, for example, a pulse signal whose frequency is a reference frequency generated by a GPS (Global Positioning System) signal or a GNSS (Global Navigation Satellite System) signal, is input to the reference frequency divider 520. The reference frequency divider 520 then divides the reference frequency using the input signal. In this case, similar to the offset frequency divider 510, the reference frequency divider 520 may determine a division ratio according to the operating band frequency of a device to be controlled by the first feedback control unit 500. Such a division ratio may be a value determined in advance for each device, a value optimized using a simulator or the like, or a value learned by machine learning.

[0078] The first phase comparator 530 detects a first error signal by comparing a signal corresponding to the carrier envelope offset frequency fceO with a signal corresponding to the reference frequency. More specifically, two signals, a signal output from the offset frequency divider 510 and a signal output from the reference frequency divider 520, may be input to the first phase comparator 530. The first phase comparator 530 may then compare the signal output from the offset frequency divider 510 with the signal output from the reference frequency divider 520 to detect the first error signal.

[0079] The first loop filter 540 outputs a first electrical signal corresponding to the first error signal. For example, the first loop filter 540 converts the first error signal detected by the first phase comparator 530 into a DC voltage to output the first electrical signal.

[0080] The first driver 550 controls the device capable of changing the resonator length based on the first electrical signal. The first driver 550 feedback-controls the device capable of changing the resonator length in accordance with the first electrical signal output by the first loop filter 540, for example.

[0081] As described above, in the mode-locked fiber laser 20, the cavity length can be controlled by controlling the voltage applied to the piezoelectric element 272. Therefore, the first driver 550 may be a piezoelectric driver that controls, based on a first electrical signal, the piezoelectric element 272 to which the reflecting mirror 260 is attached, which reflects the light emitted from the optical circulator 240 in the cavity and makes the light enter the optical circulator 240 again.

[0082] Furthermore, as described above, in the mode-locked fiber laser 20, the resonator length can be controlled by controlling the pulse signal given to the stepping motor 274. Therefore, the first driver 550 may be a motor driver that controls the stepping motor 274, which moves, based on the first electric signal, the support member 265 that supports the reflecting mirror 260 that reflects the light emitted from the optical circulator 240 in the resonator and makes it incident again on the optical circulator 240, relative to the housing.

[0083] Furthermore, as described above, in the mode-locked fiber laser, the cavity length can be controlled by controlling the voltage applied to the optical modulator 230. Therefore, the first driver 550 may be a modulator driver that controls the optical modulator 230 in the cavity based on the first electrical signal.

[0084] In this way, the first driver 550 may be one of a piezo driver, a motor driver, and a modulator driver, or a combination of these drivers. When the first driver 550 is made up of a combination of multiple drivers, the offset frequency divider 510 and the reference frequency divider 520 may each be provided with multiple dividers that divide the frequency by different division ratios for multiple devices controlled by the multiple drivers.

[0085] 6 shows an example of the configuration of the second feedback control section 600 included in the frequency stabilization circuit 100 according to this embodiment. The second feedback control section 600 includes a second phase comparator 630, a second loop filter 640, and a second driver 650.

[0086] The second phase comparator 630 compares the signal corresponding to the carrier envelope offset frequency fceO with the signal corresponding to the reference frequency to detect a second error signal. Two signals may be input to the second phase comparator 630: a signal corresponding to the carrier envelope offset frequency fceO detected by the offset frequency detection unit 300 (e.g., a pulse signal whose frequency is the carrier envelope offset frequency fceO and which is the other of the signals branched by the branching unit 350); and a signal corresponding to the reference frequency (e.g., a pulse signal whose frequency is a reference frequency generated by a GPS signal or a GNSS signal). The second phase comparator 630 may then compare these two signals to detect a second error signal.

[0087] The second loop filter 640 outputs a second electrical signal corresponding to the second error signal. For example, the second loop filter 640 converts the second error signal detected by the second phase comparator 630 into a DC voltage to output the second electrical signal.

[0088] The second driver 650 controls the laser diode serving as the pumping light source in the resonator based on the second electrical signal. The second driver 650 feedback-controls the laser diode serving as the pumping light source in the resonator in accordance with the second electrical signal output by the second loop filter 640, for example.

[0089] As described above, in the mode-locked fiber laser 20, the pump light power can be controlled by controlling the current supplied to the first pump light source 210, which is a pump laser diode. Therefore, the second driver 650 may be a laser diode driver that controls the first pump light source 210, which generates the pump light to be input into the resonator, based on the second electrical signal.

[0090] 7 shows an example of the configuration of the third feedback control section 700 included in the frequency stabilization circuit 100 according to this embodiment. The third feedback control section 700 includes a third phase comparator 730, a third loop filter 740, and a third driver 750.

[0091] The third phase comparator 730 compares a signal corresponding to the beat frequency fbeat with a signal corresponding to a reference frequency to detect a third error signal. Two signals may be input to the third phase comparator 730: a signal corresponding to the beat frequency fbeat detected by the beat frequency detection section 400, for example, a pulse signal whose frequency is the beat frequency fbeat, and a signal corresponding to the reference frequency, for example, a pulse signal whose frequency is a reference frequency generated by a GPS signal or a GNSS signal. The third phase comparator 730 may then compare these two signals to detect a third error signal.

[0092] The third loop filter 740 outputs a third electrical signal corresponding to the third error signal. For example, the third loop filter 740 converts the third error signal detected by the third phase comparator 730 into a DC voltage to output the third electrical signal.

[0093] The third driver 750 controls the optical modulator in the resonator based on the third electrical signal. The third driver 750 feedback-controls the optical modulator in the resonator in response to the third electrical signal output by the third loop filter 740, for example.

[0094] As described above, in the mode-locked fiber laser 20, the cavity length can be controlled by controlling the voltage applied to the optical modulator 230. Therefore, the third driver 750 may be a modulator driver that controls the optical modulator 230 in the cavity based on the third electrical signal.

[0095] Figure 8 shows how the carrier envelope offset frequency fceо is detected using the self-referencing method. As shown in this figure, when the output of an optical comb is Fourier transformed and viewed in the frequency domain, it has a comb-shaped spectrum consisting of many equally spaced modes. The nth mode frequency fn in an optical comb is expressed as fn = fceо + n × frep, where frep is the repetition frequency. Assuming that the spectrum of the optical comb extends to 0 Hz, fceо indicates the frequency of the mode closest to 0 Hz, and is called the carrier envelope offset frequency. n is the mode number, an integer of around 1 million.

[0096] Here, the second harmonic 2fn of the nth mode frequency fn in the optical comb is 2fn = 2fceo + 2n × frep. Similarly, the 2nth mode frequency f2n in the optical comb is f2n = fceo + 2n × frep. Therefore, the carrier envelope offset frequency fceo can be detected by subtracting the 2nth mode frequency f2n from the second harmonic 2fn of the nth mode frequency fn. The offset frequency detection unit 300 can detect the carrier envelope offset frequency fceo using, for example, this f-2f type self-referencing method.

[0097] When the mode-locked fiber laser 20 that outputs such an optical comb is used as an optical comb light source, the carrier-envelope offset frequency fce0 and the beat frequency fbeat must be locked simultaneously.

[0098] Figure 9 shows how the carrier-envelope offset frequency fceо and the beat frequency fbeat are simultaneously locked. As shown in this figure, by simultaneously locking the carrier-envelope offset frequency fceо and the beat frequency fbeat, the frequency of each mode is stabilized, allowing the mode-locked fiber laser 20 to be used as an optical comb light source. However, simultaneously locking the carrier-envelope offset frequency fceо and the beat frequency fbeat has the following problems.

[0099] Typically, to lock the carrier envelope offset frequency fceо, a phase comparator detects the phase difference between a signal corresponding to the carrier envelope offset frequency fceо and a signal corresponding to a reference frequency, and the current value of the pump laser diode is adjusted to achieve phase synchronization. Specifically, the carrier envelope offset frequency fceо is adjusted by utilizing changes in the refractive index of the optical fiber due to changes in the pump light power, thereby achieving phase synchronization so that the reference frequency and the carrier envelope offset frequency fceо coincide. To achieve phase synchronization, the S / N ratio of the carrier envelope offset frequency fceо must be sufficiently large. However, if the current value of the pump laser diode is changed to maintain phase synchronization, the S / N ratio deteriorates due to changes in the oscillation state of the mode-locked laser, such as the optical pulse output power and optical pulse width, making it difficult to maintain phase synchronization. Furthermore, because the variable range of the carrier envelope offset frequency fceо (the control range of the pump laser diode) is narrow, significant changes in the cavity length due to environmental temperature fluctuations, etc., can exceed the control range and degrade the S / N ratio.

[0100] Furthermore, when simultaneously locking the carrier-envelope offset frequency f and the beat frequency f, the voltage applied to the optical modulator is changed to stabilize the beat frequency f, which changes the cavity length and, accordingly, causes fluctuations in the carrier-envelope offset frequency f and the beat frequency f. Therefore, in order to stabilize the carrier-envelope offset frequency f, it is necessary to significantly change the current value of the pump laser diode, which also deteriorates the S / N ratio.

[0101] Conventionally, to stabilize the carrier-envelope offset frequency fceO so that it does not exceed the control range of the pump laser diode, the temperature of the laser housing is controlled using a Peltier element or the like, and the cavity length is controlled to stabilize it. However, with the conventional method, because the heat capacity of the case containing the laser is large, it is difficult to perform high-speed temperature control in response to sudden environmental temperature fluctuations so as to prevent the phase synchronization of the carrier-envelope offset frequency fceO from being lost.

[0102] In contrast, the frequency stabilization circuit 100 according to this embodiment has a first feedback control unit 500, a second feedback control unit 600, and a third feedback control unit 700, and controls the cavity length in the mode-locked fiber laser 20 based on a first error signal indicating the error of the carrier-envelope offset frequency f with respect to the reference frequency, controls the pump light power in the mode-locked fiber laser 20 based on a second error signal indicating the error of the carrier-envelope offset frequency f with respect to the reference frequency, and controls the cavity length in the mode-locked fiber laser 20 based on a third error signal indicating the error of the beat frequency f with respect to the reference frequency.

[0103] As described above, the frequency stabilization circuit 100 according to this embodiment has two feedback loops, one for controlling the pump light power and the other for controlling the resonator length, in order to phase-lock the carrier envelope offset frequency fceо and the reference frequency, and also has a feedback loop for controlling the resonator length in order to phase-lock the beat frequency fbeat and the reference frequency. Therefore, it is possible to provide a frequency stabilization circuit and a frequency stabilization method that can constantly control the control current of the pump laser diode to be constant and suppress deterioration of the S / N ratio of the carrier envelope offset frequency fceо due to changes in the pump light, thereby enabling sufficient tracking and stabilization even in the event of sudden fluctuations in environmental temperature.

[0104] More specifically, the frequency stabilization circuit 100 according to this embodiment has an independent path (first feedback control unit 500) for feedback control of the resonator length, in parallel with a path (second feedback control unit 600) for feedback control of the pump light power, as a locking mechanism for the carrier envelope offset frequency fceO. Thus, by providing separate parallel paths, the frequency stabilization circuit 100 according to this embodiment can accommodate cases where the control frequency for the device whose pump light power can be changed and the control frequency for the device whose resonator length can be changed are different. More specifically, the frequency stabilization circuit 100 according to this embodiment can adjust the control frequency for the device whose resonator length can be changed to the operating band frequency of the device, independently of the control frequency for the device whose pump light power can be changed, by including, for example, an offset frequency divider 510 and a reference frequency divider 520 in the first feedback control unit 500.

[0105] Furthermore, the frequency stabilization circuit 100 according to this embodiment may have a loop that feedback controls the piezoelectric element 272 as a locking mechanism for the carrier envelope offset frequency fce0. Here, for example, if the change in the cavity length caused by applying a voltage to the piezoelectric element 272 is ΔL, the change Δfrep_pzt in the repetition frequency frep of the mode-locked fiber laser 20 is expressed by the following equation: where c is the speed of light, n is the refractive index of the fiber with respect to the optical carrier frequency, and L is the cavity length.

number

[0106] Furthermore, the carrier envelope offset frequency fceо and the repetition frequency frep have the following relationship: where ΔΦ indicates the phase shift for each pulse.

number

[0107] Here, assuming that the phase is locked, ΔΦ is constant. In this state, if the repetition frequency frep changes by Δfrep_env due to a sudden temperature change, in order to maintain the locked state of the carrier envelope offset frequency fceO, it is necessary to change the carrier envelope offset frequency fceO and the repetition frequency frep as shown in the following equation: where Δfrep and ΔfceO respectively represent changes in the carrier envelope offset frequency fceO and the repetition frequency frep due to control of the pump laser diode.

number

[0108] In the case of a sudden temperature change, the S / N ratio of the carrier envelope offset frequency fceo deteriorates when compensation is performed using Δfceo and Δfrep. Therefore, the frequency stabilization circuit 100 according to this embodiment compensates for Δfrep_env caused by a sudden temperature change using Δfrep_pzt in equation 1. As a result, the frequency stabilization circuit 100 according to this embodiment can reduce Δfceo and Δfrep for compensation, thereby making it possible to suppress the required control current value of the pump laser.

[0109] In the above explanation, the case where the resonator length is changed by controlling the piezoelectric element 272 is shown as an example, but the same can be said for the case where the resonator length is changed by controlling the stepping motor 274 or the optical modulator 230.

[0110] Although the present invention has been described above using embodiments, the technical scope of the present invention is not limited to the scope described in the above embodiments. It will be apparent to those skilled in the art that various modifications and improvements can be made to the above embodiments. It is clear from the claims that such modifications and improvements can also be included within the technical scope of the present invention.

[0111] It should be noted that the order of execution of each process, such as operations, procedures, steps, and stages, in the devices, systems, programs, and methods shown in the claims, specifications, and drawings is not specifically stated as "before," "prior to," etc., and that the processes can be performed in any order unless the output of a previous process is used in a subsequent process. Even if the operational flow in the claims, specifications, and drawings is described using "first," "next," etc. for convenience, this does not mean that the processes must be performed in this order. [Explanation of symbols]

[0112] 10 Optical comb generator 20 Mode-locked fiber laser 100 Frequency stabilization circuit 200 Multiport Optical Coupler 210 First excitation light source 215 First Wavelength Division Multiplexing Filter 220 First optical amplifying fiber 230 Optical Modulator 240 Optical Circulator 250 Collimating Lens 260 Reflector 265 Support member 270 Actuator 272 Piezoelectric element 274 Stepping Motor 280 Output Optical Coupler 290 Optical Splitter 300 Offset frequency detection unit 310 Octave comb generator 311 Second excitation light source 313 Second Wavelength Division Multiplexing Filter 315 Second optical amplifier fiber 317 Third excitation light source 319 Third Wavelength Division Multiplexing Filter 320 Highly Nonlinear Fiber 330 Offset frequency observation unit 331 Lens 333 PPLN waveguide 335 First photodiode 400 Beat frequency detector 410 Optical multiplexing section 411 4th optical amplifier fiber 413 4th excitation light source 415 Fourth Wavelength Division Multiplexing Filter 417 Optical Bandpass Filter 420 wavelength reference laser 425 Optical multiplexer 430 Beat Frequency Observation Unit 431 Second photodiode 500 First feedback control section 510 Offset Frequency Divider 520 Reference Frequency Divider 530 First Phase Comparator 540 First Loop Filter 550 First Driver 600 Second feedback control section 630 Second Phase Comparator 640 Second Loop Filter 650 Second Driver 700 Third feedback control section 730 Third Phase Comparator 740 Third Loop Filter 750 Third Driver

Claims

1. an offset frequency detection unit that detects a carrier envelope offset frequency in an optical comb output from a resonator of the mode-locked fiber laser; a beat frequency detector that detects a beat frequency generated by interference between a reference optical spectrum and a wavelength reference laser beam in the optical comb; a branching unit that branches the signal according to the carrier envelope offset frequency into at least two signals, and supplies one signal to a first feedback control unit and the other signal to a second feedback control unit; the first feedback control unit controlling a cavity length of the mode-locked fiber laser based on a first error signal indicating an error of the carrier-envelope offset frequency with respect to a reference frequency; the second feedback control unit controlling the pumping light power in the mode-locked fiber laser based on a second error signal indicating an error of the carrier envelope offset frequency with respect to the reference frequency; a third feedback control unit that controls a cavity length of the mode-locked fiber laser based on a third error signal that indicates an error of the beat frequency with respect to the reference frequency; A frequency stabilization circuit having:

2. The first feedback control unit a first phase comparator that compares a signal corresponding to the carrier envelope offset frequency with a signal corresponding to the reference frequency to detect the first error signal; a first loop filter that outputs a first electrical signal according to the first error signal; 2. The frequency stabilization circuit according to claim 1, further comprising: a first driver that controls the device capable of changing the resonator length based on the first electrical signal.

3. The first feedback control unit an offset frequency divider that divides the carrier envelope offset frequency; a reference frequency divider that divides the reference frequency; 3. The frequency stabilization circuit according to claim 2, wherein said first phase comparator detects said first error signal by comparing the signal output from said offset frequency divider with the signal output from said reference frequency divider.

4. 3. The frequency stabilization circuit according to claim 2, wherein the first driver controls a piezoelectric element equipped with a reflecting mirror that reflects light emitted from an optical circulator in the resonator and re-enters the optical circulator based on the first electrical signal.

5. 3. The frequency stabilization circuit according to claim 2, wherein the first driver controls a stepping motor that moves a support member supporting a reflector that reflects light emitted from an optical circulator in the resonator and re-injects it into the optical circulator relative to a housing based on the first electrical signal.

6. 3. The frequency stabilization circuit according to claim 2, wherein the first driver controls an optical modulator in the resonator based on the first electrical signal.

7. The second feedback control unit a second phase comparator that compares a signal corresponding to the carrier envelope offset frequency with a signal corresponding to the reference frequency to detect the second error signal; a second loop filter that outputs a second electrical signal according to the second error signal; 2. The frequency stabilization circuit according to claim 1, further comprising: a second driver that controls a laser diode serving as an excitation light source within said resonator based on said second electrical signal.

8. The third feedback control unit a third phase comparator that compares the signal corresponding to the beat frequency with the signal corresponding to the reference frequency to detect the third error signal; a third loop filter that outputs a third electrical signal according to the third error signal; 2. The frequency stabilization circuit according to claim 1, further comprising: a third driver that controls an optical modulator in said resonator based on said third electrical signal.

9. The offset frequency detection unit an octave comb generator that generates an octave comb by expanding the optical spectrum of the optical comb by more than an octave; 2. The frequency stabilization circuit according to claim 1, further comprising: an offset frequency observation unit that observes the carrier envelope offset frequency using the octave comb.

10. The beat frequency detection unit an optical multiplexing unit that multiplexes the optical comb and the wavelength reference laser light; 2. The frequency stabilization circuit according to claim 1, further comprising: a beat frequency observation unit that observes the beat frequency using the combined light.

11. the mode-locked fiber laser; A frequency stabilization circuit according to any one of claims 1 to 10; An optical comb generator comprising:

12. The mode-locked fiber laser includes: an optical circulator that outputs light incident on a first port from a second port and outputs light incident on the second port from a third port; a reflecting mirror that reflects the light emitted from the second port of the optical circulator and makes the light enter the second port of the optical circulator again; an actuator capable of changing the position of the reflecting mirror along the optical axis direction; 12. The optical comb generator of claim 11, comprising:

13. 13. The optical comb generator of claim 12, wherein the actuator is a piezoelectric element to which the reflecting mirror is attached.

14. 13. The optical comb generator according to claim 12, wherein the actuator is a stepping motor that moves a support member that supports the reflecting mirror relative to a housing.

15. 12. The optical comb generator of claim 11, wherein the mode-locked fiber laser is a figure-of-eight laser in which two input ports and two output ports of a multi-port optical coupler are connected in a figure-of-eight shape by polarization-maintaining fibers.

16. The mode-locked fiber laser includes: an excitation light source that generates excitation light; a wavelength division multiplexing filter into which the excitation light is input; a light-amplifying fiber that amplifies light by being excited by the excitation light; an optical modulator that modulates the phase of light propagating within the resonator; an output optical coupler that outputs the optical comb generated in the resonator.

17. Detecting a carrier envelope offset frequency in an optical comb output from a resonator of the mode-locked fiber laser; detecting a beat frequency generated by interference between a reference optical spectrum and a wavelength reference laser beam in the optical comb; branching the signal according to the carrier envelope offset frequency into at least two; receiving one of the branched signals corresponding to the carrier envelope offset frequency, and controlling a cavity length of the mode-locked fiber laser based on a first error signal indicating an error of the carrier envelope offset frequency relative to a reference frequency; receiving the other of the branched signals according to the carrier envelope offset frequency, and controlling the pumping light power in the mode-locked fiber laser based on a second error signal indicating an error of the carrier envelope offset frequency with respect to the reference frequency; controlling a cavity length of the mode-locked fiber laser based on a third error signal indicating an error of the beat frequency with respect to the reference frequency; A frequency stabilization method comprising:

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