JP2022040898A - Optical comb parameter detector and optical comb parameter detection method - Google Patents

Abstract

To provide an optical comb parameter detector and an optical comb parameter detection method, with which it is possible to easily and efficiently detect an offset frequency of an optical comb.SOLUTION: An optical comb parameter detector 1A comprises: an optical comb generator 2 for generating an optical comb having a plurality of frequency modes specified by a repletion frequency frep and an offset frequency fCEO; a dispersion shifted fiber 4 for generating at least one of a Stokes pulse and an anti-Stokes pulse by inputting the optical comb as incident pulse; and a detection unit 11 for detecting the offset frequency fCEO on the basis of a first pulse which is one of the incident pulse and the Stokes pulse and a second pulse which is one of the incident pulse and the anti-Stokes pulse and is different from the first pulse.SELECTED DRAWING: Figure 1

Description

The present disclosure relates to an optical comb parameter detection device and an optical comb parameter detection method.

An optical comb (also referred to as an "optical frequency comb"), which is an ultrashort pulse laser having a plurality of frequency modes (longitudinal modes) arranged at equal intervals, is known (see Non-Patent Document 1). The optical comb is represented by two parameters, the repeat frequency f _rep and the offset frequency f _CEO . Specifically, the nth mode frequency f (n) included in the optical comb is expressed as "f (n) = n × f _rep + f _CEO ".

The optical comb can be used for precise spectroscopic measurement and the like, but in order to perform the measurement using the optical comb, the above two parameters (that is, the repetition frequency _frep and the offset frequency f _CEO ) that define the optical comb are set. It is necessary to detect it in advance. The repetition frequency _frep can be detected relatively easily by injecting an optical comb into a photodetector having a sufficient band. On the other hand, in order to detect the offset frequency f _CEO , it is necessary to perform the following processing as an example.

Non-Patent Document 1 discloses a "self-referencing technique" which is a method for detecting an offset frequency f _CEO . In this method, the first wavelength component (for example, 1000 nm) of the optical comb having a spectral component of one octave or more and the second wavelength component (for example, 2000 nm) which is twice the wavelength component of the first wavelength component are acquired. When the second wavelength component corresponds to the nth mode frequency f (n) of the optical comb, the first wavelength component corresponds to the second mode frequency f (2n) of the optical comb. Here, the 2nth mode frequency f (2n) is expressed as "f (2n) = 2n × f _rep + f _CEO ". Further, the frequency of the second harmonic (also referred to as “second harmonic”) of the nth mode frequency f (n) is expressed as “2f (n) = 2n × f _rep + 2f _CEO ”. Therefore, the offset frequency f _CEO is determined by obtaining the difference frequency between the frequency of the first wavelength component (that is, f (2n)) and the frequency of the second harmonic of the second wavelength component (that is, 2f (n)). Can be detected.

In Non-Patent Document 1, in order to obtain a spectral component of one octave or more required in the above method, the output (incident pulse) of an optical comb is incident on a highly nonlinear fiber, and the light is produced by the nonlinear effect of the highly nonlinear fiber. Techniques for expanding the spectrum of combs are disclosed.

David J. Jones, Scott A. Diddams, Jinendra K. Ranka, Andrew Stentz, Robert S. Windeler, John L. Hall, Steven T. Cundiff, “Carrier-Envelope PhaseControl of Femtosecond Mode-Locked Lasers and Direct Optical FrequencySynthesis”, SCIENCE VOL 288, 28 APRIL 2000 (635-639).

However, such highly non-linear fibers are custom-made products manufactured by some fiber manufacturers, and are relatively expensive and rare, so that they cannot be easily obtained. Further, the spectrum contributing to the detection of the offset frequency component f _CEO (that is, the frequency component required for the detection of the offset frequency component f _CEO ) is a small part of the spectrum obtained by the highly nonlinear fiber, and most of the spectra. It is inefficient because the ingredients are wasted.

Therefore, one aspect of the present disclosure is to provide an optical comb parameter detection device and an optical comb parameter detection method capable of easily and efficiently detecting an optical comb offset frequency.

The optical comb parameter detection device according to one aspect of the present disclosure is to input an optical comb generator that generates an optical comb having a plurality of frequency modes specified by a repetition frequency and an offset frequency, and an optical comb as an incident pulse. Therefore, the pulse generating part that generates at least one of the Stokes pulse and the anti-Stokes pulse, the first pulse which is one of the incident pulse and the Stokes pulse, and the one of the incident pulse and the anti-Stokes pulse and the first pulse are different from each other. A second pulse and a detection unit for detecting an offset frequency based on the second pulse are provided.

According to the optical comb parameter detection device, the offset frequency of the optical comb can be easily and efficiently detected by using at least one of the Stokes pulse and the anti-Stokes pulse generated based on the incident pulse. More specifically, unlike the conventional method (for example, “self-referencing technique”), it is not necessary to expand the spectrum of the optical comb (incident pulse) itself by the non-linear effect of the highly non-linear fiber. Therefore, it is not necessary to prepare a highly nonlinear fiber, which is relatively difficult to obtain. Further, the offset frequency can be efficiently detected based on the appropriately selected first pulse and second pulse.

The pulse generator may include a distributed shift fiber. In this case, at least one of the Stokes pulse and the anti-Stokes pulse can be generated by a relatively simple configuration using the distributed shift fiber.

The optical comb parameter detection device separates the first optical path of the first pulse and the second optical path of the second pulse downstream of the pulse generation unit, and combines the first pulse and the second pulse upstream of the detection unit. An optical system and a delay control unit provided in at least one of the first optical path and the second optical path and causing a time delay in order to match the phases of the first pulse and the second pulse may be further provided. According to the above configuration, the phase shift between the first pulse and the second pulse can be appropriately corrected.

The optical comb parameter detection device may be provided in at least one of the first optical path and the second optical path, and may further include a polarization adjusting unit for adjusting the polarization direction of at least one of the first pulse and the second pulse. The unit may adjust the polarization direction of at least one of the first pulse and the second pulse so that the polarization direction of the first pulse and the polarization direction of the second pulse are substantially the same. According to the above configuration, since the interference intensity between the first pulse and the second pulse to be combined can be increased, the signal intensity of the offset frequency detected by the detection unit can be increased.

The optical comb parameter detection device is provided in at least one of the first optical path and the second optical path, and the first pulse or the second pulse is provided so that the pulse width of the first pulse and the pulse width of the second pulse substantially match. A pulse width adjusting unit for adjusting the pulse width of the above may be further provided. According to the above configuration, since the interference intensity between the first pulse and the second pulse to be combined can be increased, the signal intensity of the offset frequency detected by the detection unit can be increased.

The optical comb parameter detection device may further include a bandpass filter provided upstream of the detection unit and passing components in a predetermined frequency band of the first pulse and the second pulse, and the detection unit may further include a band. The offset frequency may be detected based on the components of the first pulse and the second pulse that have passed through the pass filter. According to the above configuration, since the spectral component of the frequency band (that is, the component that becomes noise) unnecessary for detecting the offset frequency can be removed by the bandpass filter, the signal of the offset frequency detected by the detection unit can be removed. S / N ratio can be increased.

The optical comb parameter detection device is connected to the end on the input side of the detection unit, and by passing the first pulse and the second pulse, the optical wave shape of the first pulse and the beam shape of the second pulse are aligned. Further, a waveguide portion may be provided. According to the above configuration. By aligning the beam shapes of the first pulse and the second pulse that have passed through the optical waveguide portion, the coherence of the first pulse and the second pulse can be improved. As a result, the signal strength of the offset frequency detected by the detection unit can be increased.

The optical comb parameter detection device may be further provided with a dispersion compensating fiber for transmitting the first pulse and the second pulse, which is arranged between the pulse generation unit and the detection unit, and the dispersion compensation fiber is the pulse generation unit. It may have the opposite characteristics to the wavelength dispersion. According to the above configuration, the time delay between the first pulse and the second pulse generated in the dispersion shift fiber (pulse generation unit) can be canceled by the dispersion compensation fiber. This makes it possible to match the phases of the first pulse and the second pulse without using an optical system that separates the optical path between the first pulse and the second pulse as described above.

The optical comb parameter detection device may further include a second harmonic generation unit that generates a second harmonic of the first pulse, and the first pulse may have a wavelength twice that of the second pulse. Often, the detection unit may detect the difference frequency between the second harmonic of the first pulse and the second pulse as an offset frequency. According to the above configuration, the offset frequency can be appropriately detected from the first pulse and the second pulse having a wavelength difference of one octave.

The optical comb parameter detection device further includes a third harmonic generation unit that generates the third harmonic of the first pulse, and a second harmonic generation unit that generates the second harmonic of the second pulse. The first pulse may have a wavelength 1.5 times that of the second pulse, and the detection unit may have a frequency difference between the third harmonic of the first pulse and the second harmonic of the second pulse. May be detected as an offset frequency. According to the above configuration, the offset frequency can be appropriately detected from the first pulse and the second pulse having a wavelength difference of 2/3 octave.

The optical comb parameter detection device may be provided between the optical comb generation unit and the pulse generation unit, and may further include an amplification unit that amplifies the optical comb generated by the optical comb generation unit, and is amplified by the amplification unit. The pulse energy of the optical frequency comb may be 1 nJ or more. According to the above configuration, the wavelength shift amount of the Stokes pulse or the anti-Stokes pulse can be increased by amplifying the pulse energy of the optical comb incident on the pulse generating portion. Thereby, the first pulse and the second pulse having a wavelength difference of a predetermined magnitude (for example, one octave or two-thirds octave) can be appropriately obtained.

The detection unit may detect the repeating frequency together with the offset frequency based on the first pulse and the second pulse. According to the above configuration, two parameters (offset frequency and repetition frequency) for specifying a plurality of frequency modes of the optical comb can be detected at the same time.

The optical comb parameter detection method according to another aspect of the present disclosure comprises a step of generating an optical comb having a plurality of frequency modes specified by a repetition frequency and an offset frequency, and inputting the optical comb as an incident pulse. The step of generating at least one of the Stokes pulse and the anti-Stokes pulse, the first pulse which is one of the incident pulse and the Stokes pulse, and the second pulse which is one of the incident pulse and the anti-Stokes pulse and different from the first pulse. , A step of detecting the offset frequency based on, and the like.

According to the optical comb parameter detection method, the offset frequency of the optical comb can be easily and efficiently detected by using at least one of the Stokes pulse and the anti-Stokes pulse generated based on the incident pulse. More specifically, unlike the conventional method (for example, “self-referencing technique”), it is not necessary to expand the spectrum of the optical comb (incident pulse) itself by the non-linear effect of the highly non-linear fiber. Therefore, it is not necessary to prepare a highly nonlinear fiber, which is relatively difficult to obtain. Further, the offset frequency can be efficiently detected based on the appropriately selected first pulse and second pulse.

According to one aspect of the present disclosure, it is possible to provide an optical comb parameter detection device and an optical comb parameter detection method capable of easily and efficiently detecting an optical comb offset frequency.

FIG. 1 is a diagram showing an overall configuration of an optical comb parameter detection device according to a first embodiment. FIG. 2 is a diagram for explaining an optical frequency comb. FIG. 3 is a diagram showing an example of an incident pulse, a Stokes pulse, and an anti-Stokes pulse. FIG. 4 is a diagram showing an example of the relationship between the fiber length of the distributed shift fiber and the center wavelength of the Stokes pulse. FIG. 5 is a diagram showing an example of the relationship between the pulse energy of the optical comb and the center wavelengths of the Stokes pulse and the anti-Stokes pulse. FIG. 6 is a diagram showing the overall configuration of the optical comb parameter detection device according to the second embodiment. FIG. 7 is a diagram showing the overall configuration of the optical comb parameter detection device according to the third embodiment. FIG. 8 is a diagram showing the overall configuration of the optical comb parameter detection device according to the fourth embodiment.

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In the description of the drawings, the same elements are designated by the same reference numerals, and duplicate description will be omitted.

[First Embodiment]
As shown in FIG. 1, the optical comb parameter detection device 1A according to the first embodiment has a repeat frequency _frep and an offset frequency f _CEO , which are parameters defining an optical comb (also referred to as “optical frequency comb”). It is a device that can be detected at the same time. The optical comb is a frequency-controlled ultrashort pulse laser (mode-synchronized laser). As shown in FIG. 2, the optical comb is represented as an ultrashort pulse train (upper portion of FIG. 2) when viewed on the time axis (time domain). By Fourier transforming the ultra-short pulse trains constituting the optical comb, an optical spectrum (lower portion of FIG. 2) in which frequency modes (longitudinal modes) are arranged at equal intervals can be obtained. That is, the optical comb is represented as a comb-shaped optical spectrum when viewed on the frequency axis (frequency domain).

The optical comb has a plurality of frequency modes specified by a repeat frequency f _rep and an offset frequency f _CEO . More specifically, the nth mode frequency f (n) of the optical comb is expressed by the following equation (1).
f (n) = n × f _rep + f _CEO … (1)

In order to carry out an experiment using an optical comb, it is necessary to detect the above two parameters of the optical comb (that is, the repetition frequency f _rep and the offset frequency f _CEO ) in advance. By detecting the repetition frequency f _rep and the offset frequency f _CEO and performing phase control for a stable microwave oscillator, a frequency-stabilized laser, or the like, the frequency f (n) of each mode of the optical comb can be precisely determined. .. The optical frequency comb in which the above two parameters are controlled can be used in various fields such as spectroscopic measurement.

As shown in FIG. 1, the optical comb parameter detection device 1A includes an optical comb generator 2 (optical comb generator), an optical pulse amplifier 3 (amplification unit), a distributed shift fiber 4 (pulse generator), and the like. It includes a dichroic mirror 5, a second harmonic generation unit 6, a fixed mirror 7, a delay control unit 8, a fixed mirror 9, a beam splitter 10, and a detection unit 11 that constitute an optical system OS. The optical comb parameter detection device 1A is configured to detect the offset frequency f _CEO based on the first pulse and the second pulse having a wavelength difference of one octave. In this embodiment, as an example, the first pulse is a Stokes pulse and the second pulse is an anti-Stokes pulse.

The optical comb generator 2 is a device that generates an optical comb. The optical comb generator 2 is, for example, a fiber laser that outputs a femtosecond pulse. As the gain fiber of the fiber laser, for example, erbium (Er), ytterbium (Yb), thulium (Tm) and the like are used. As an example, the center wavelength of the optical comb generator 2 is the 1550 nm band when erbium (Er) is used as the gain fiber, and the 1000 nm band when ytterbium (Yb) is used as the gain fiber, and the gain. When thulium (Tm) is used as the fiber, it is in the 1900 nm band. The optical comb generator 2 is not limited to the fiber laser. For example, the optical comb generator 2 may be a titanium sapphire laser. In this case, the center wavelength of the optical comb generator 2 is, for example, the 800 nm band. Further, the optical comb generator 2 may be a microcom having a continuous wave laser and a microring resonator. A microcom is a device that generates an optical comb by injecting a continuous wave laser into a microring resonator integrated on a substrate such as silicon nitride. In this case, the central wavelength of the optical comb generator 2 is the wavelength of the continuous wave laser, for example, in the 1550 nm band. Here, as an example, it is assumed that erbium (Er) is used as the gain fiber of the optical comb generator 2, and the central wavelength of the optical comb generator 2 is 1550 nm.

The optical pulse amplifier 3 is provided between the optical comb generator 2 and the distributed shift fiber 4. The optical pulse amplifier 3 amplifies the optical comb (optical pulse) generated by the optical comb generator 2. The optical pulse amplifier 3 may be composed of, for example, an Er fiber, an excited semiconductor laser (LD) having a wavelength of 980 nm, a wavelength division multiplexing (WDM) coupler, or the like. In this case, the wavelength of the excitation LD may be any wavelength suitable for excitation the optical pulse generated by the optical comb generator 2. An example of the parameter of the optical pulse amplified by the optical pulse amplifier 3 is shown below.
・ Half width of spectrum = 100 nm
・ Pulse width = 60 fs
・ Average power = 355mW
・ Repeat frequency f _rep = 30MHz

From the viewpoint of efficiently generating Stokes pulse and anti-Stokes pulse by the distributed shift fiber 4 located after the optical pulse amplifier 3, the optical pulse after being amplified by the optical pulse amplifier 3 is a positively charped pulse. It is preferable to have. The pulse energy is preferably 1 nJ or more, and more preferably 5 nJ or more.

The distributed shift fiber 4 receives a Stokes pulse and an anti-Stokes by inputting an optical comb generated by the optical comb generator 2 (in this embodiment, an optical pulse after being amplified by the optical pulse amplifier 3) as an incident pulse. Generate a pulse. The Stokes pulse is an optical pulse obtained by shifting the incident pulse to the long wavelength side (that is, an optical pulse having a frequency lower than that of the incident pulse). The anti-Stokes pulse is an optical pulse obtained by shifting the incident pulse to the short wavelength side (that is, an optical pulse having a higher frequency than the incident pulse). The dispersion shift fiber 4 is an optical fiber having zero dispersion at the center wavelength of the optical comb generator 2 (in this embodiment, the 1550 nm band).

FIG. 3 is a diagram showing an example of spectra of Stokes pulse and anti-Stokes pulse generated when an incident pulse is input to the dispersion shift fiber 4 having a fiber length of 63.4 m. Note that FIG. 3 also shows the spectrum of the incident pulse. That is, the pulse located in the 1550 nm band in FIG. 3 corresponds to the incident pulse. In the example of FIG. 3, a Stokes pulse having a wavelength shifted to 2142 nm and an anti-Stokes pulse having a wavelength shifted to 1071 nm can be confirmed. Further, in the example of FIG. 3, a plurality of spectral peaks are observed for each of the Stokes pulse and the anti-Stokes pulse. These show higher order Stokes pulses and anti-Stokes pulses. A first-order Stokes pulse and an anti-Stokes pulse may be used for detecting the offset frequency f _CEO , or a higher-order Stokes pulse and an anti-Stokes pulse may be used.

When the fiber length of the distributed shift fiber 4 or the pulse energy of the incident pulse input to the distributed shift fiber 4 increases, the wavelength shift amount (spectral shift amount) of the Stokes pulse and the anti-Stokes pulse increases. Therefore, by adjusting the fiber length of the dispersion shift fiber 4 and the pulse energy of the incident pulse input to the dispersion shift fiber 4, the wavelength difference between the Stokes pulse and the anti-Stokes pulse can be made one octave. In the present embodiment, it is assumed that the distributed shift fiber 4 generates an anti-Stokes pulse having a wavelength of 1071 nm and a Stokes pulse having a wavelength (2142 nm) twice that of the anti-Stokes pulse.

FIG. 4 is a diagram showing the relationship between the fiber length (DSF length) of the distributed shift fiber 4 and the center wavelength of the Stokes pulse. In this example, the pulse energy of the optical pulse after being amplified by the optical pulse amplifier 3 is about 2nJ. As shown in FIG. 4, it was confirmed that the longer the fiber length of the distributed shift fiber 4, the larger the wavelength shift amount of the Stokes pulse. Further, in order to generate a wavelength shift, it is necessary to make the fiber length of the dispersion shift fiber 4 at least 1 m or more. Therefore, the fiber length of the distributed shift fiber 4 can be selected from, for example, a range of 1 m or more and 1000 km or less. It is known that the distributed shift fiber 4 has a propagation loss of about 0.5 mW / m. Therefore, from the viewpoint of suppressing propagation loss, the fiber length of the distributed shift fiber 4 is preferably 1 km or less.

FIG. 5 is a diagram showing an example of the relationship between the pulse energy of the optical frequency comb (the incident pulse input to the dispersion shift fiber 4) and the center wavelengths of the Stokes pulse and the anti-Stokes pulse. In this example, the fiber length of the distributed shift fiber 4 is 63.4 m. As shown in FIG. 5, it was confirmed that the larger the pulse energy of the optical comb (incident pulse), the larger the wavelength difference between the Stokes pulse and the anti-Stokes pulse. In the example of FIG. 5, when the pulse energy of the optical comb is about 10 nJ, the wavelength difference between the Stokes pulse and the anti-Stokes pulse reaches one octave. However, the pulse energy of the optical comb does not necessarily have to be 10 nJ or more. This is because, as described above, the wavelength shift amount can be increased (that is, the wavelength difference between the Stokes pulse and the anti-Stokes pulse can be increased) by increasing the fiber length of the distributed shift fiber 4. .. From the viewpoint of ensuring the wavelength difference between the Stokes pulse and the anti-Stokes pulse while keeping the fiber length of the distributed shift fiber 4 within an appropriate range, the pulse energy of the optical frequency comb is preferably 1 nJ or more, more preferably 5 nJ. That is all.

The Stokes pulse and the anti-Stokes pulse generated by the distributed shift fiber 4 are spatially separated by the optical system OS. The optical system OS separates the first optical path L1 of the Stokes pulse and the second optical path L2 of the anti-Stokes pulse downstream of the distributed shift fiber 4, and combines the Stokes pulse and the anti-Stokes pulse upstream of the detection unit 11. It is a system. The first optical path L1 is a path that passes through the dichroic mirror 5, the second harmonic generation unit 6, the fixed mirror 7, and the beam splitter 10 in this order. The second optical path L2 is a path that passes through the dichroic mirror 5, the delay control unit 8, the fixed mirror 9, and the beam splitter 10 in this order.

As shown in FIG. 1, the Stokes pulse and the anti-Stokes pulse emitted from the tip portion 4a of the distributed shift fiber 4 are separated by the dichroic mirror 5. In the example of FIG. 1, the dichroic mirror 5 is configured to transmit a Stokes pulse and reflect an anti-Stokes pulse.

The Stokes pulse transmitted through the dichroic mirror 5 is incident on the second harmonic generation unit 6 along the first optical path L1. The second harmonic generation unit 6 generates the second harmonic of the Stokes pulse. When the center wavelength of the Stokes pulse is 2142 nm, the center wavelength of the second harmonic of the Stokes pulse is 1071 nm, which is half of that. The second harmonic generation unit 6 may be composed of, for example, a condenser lens, a second harmonic generation crystal, a collimating lens, or the like. The second harmonic generation crystal is, for example, a commercially available non-linear crystal (non-linear optical crystal) used for periodic polarization inversion or angular phase matching. An example of a non-linear crystal is a BBO crystal. The Stokes pulse (second harmonic) that has passed through the second harmonic generation unit 6 is reflected by the fixed mirror 7 and incident on the beam splitter 10.

Here, in the distributed shift fiber 4, the band of the Stokes pulse on the long wavelength side is anomalous dispersion, and the band of the anti-Stokes pulse on the short wavelength side is normally dispersed. Therefore, since the propagation speeds of the Stokes pulse and the anti-Stokes pulse in the distributed shift fiber 4 are different, a time difference occurs between the Stokes pulse and the anti-Stokes pulse. Therefore, a delay control unit 8 is provided on the second optical path L2 of the anti-Stokes pulse to cause a time delay for matching the phases of the Stokes pulse and the anti-Stokes pulse (that is, the generation timing of each pulse). In the present embodiment, as an example, the delay control unit 8 is composed of two movable mirrors 8a and 8b that operate in conjunction with each other. In FIG. 1, when the positions of the movable mirrors 8a and 8b are moved upward (direction toward the dichroic mirror 5), the second optical path L2 becomes shorter, and the positions of the movable mirrors 8a and 8b are moved downward (direction away from the dichroic mirror 5). The second optical path L2 becomes longer when it is moved to. In this way, the delay control unit 8 adjusts the length of the second optical path L2 by adjusting the positions of the movable mirrors 8a and 8b, and adjusts the time delay amount. The form of the delay control unit 8 is not limited to the above. The delay control unit 8 may be configured by, for example, a delay stage, an optical fiber having an appropriate dispersion, or the like. The amount of time delay controlled by the delay control unit 8 is, for example, about several hundred ps. The anti-Stokes pulse that has passed through the delay control unit 8 is reflected by the fixed mirror 9 and incident on the beam splitter 10.

The beam splitter 10 functions as a combiner that temporally and spatially combines a Stokes pulse (second harmonic) and an anti-Stokes pulse given an appropriate time delay by the delay control unit 8. The Stokes pulse (second harmonic) and anti-Stokes pulse combined in the beam splitter 10 are incident on the detection unit 11.

The detection unit 11 detects the offset frequency f _CEO based on the Stokes pulse and the anti-Stokes pulse. The detection unit 11 is, for example, a photodetector or the like. Hereinafter, the method of detecting the offset frequency f _CEO by the detection unit 11 will be described in detail.

As described above, in the present embodiment, the center wavelength of the Stokes pulse (2142 nm) is twice the center wavelength of the anti-Stokes pulse (1071 nm). That is, the wavelength difference between the Stokes pulse and the anti-Stokes pulse is one octave. Here, when the Stokes pulse corresponds to the nth mode frequency f (n) of the optical comb, the anti-Stokes pulse having half the wavelength corresponds to the 2nth mode frequency f (2n) of the optical comb.

From the above equation (1) for obtaining the nth mode frequency f (n), the 2nth mode frequency f (2n) is expressed by the following equation (2). Further, the mode frequency 2f (n) of the second harmonic (second harmonic) of the nth mode frequency f (n) is represented by the following equation (3). Further, the following equation (4) can be obtained by taking the difference between the following equation (3) and the following equation (2).
f (2n) = 2n × f _rep + f _CEO … (2)
2f (n) = 2n × f _rep + 2f _CEO … (3)
2f (n) -f (2n) = f _CEO ... (4)

Therefore, the detection unit 11 can detect the difference frequency between the second harmonic (2f (n)) of the Stokes pulse and the anti-Stokes pulse (f (2n)) as the offset frequency f _CEO .

Further, the repetition frequency _frep can be detected by incidenting the output of the optical comb on a photodetector having a sufficient band. That is, the detection unit 11 can detect the repeat frequency _frep based on the first pulse and the second pulse. In this embodiment, the detection unit 11 can detect the repeat frequency f _rep together with the offset frequency f _CEO based on the second harmonic of the Stokes pulse and the anti-Stokes pulse. According to the above configuration, two parameters (that is, offset frequency f _CEO and repetition frequency f _rep ) for specifying a plurality of frequency modes of the optical comb can be detected at the same time. However, the optical comb parameter detection device 1A may include a detection unit for detecting the repeat frequency f _rep , in addition to the detection unit 11 configured to detect the offset frequency f _CEO . The detection unit for detecting the _repetition frequency frequency is arranged, for example, at the end of a branched route (a route different from the route toward the distributed shift fiber 4) in the route from the optical comb generator 2 to the distributed shift fiber 4. Can be done.

According to the optical comb parameter detection method executed by the optical comb parameter detection device 1A and the optical comb parameter detection device 1A described above, it is easy and easy to use the Stokes pulse and the anti-Stokes pulse generated based on the incident pulse. The offset frequency f _CEO of the optical comb can be efficiently detected. More specifically, unlike the conventional method (for example, “self-referencing technique”), it is not necessary to expand the spectrum of the optical comb (incident pulse) itself by the non-linear effect of the highly non-linear fiber. Therefore, it is not necessary to prepare a highly nonlinear fiber, which is relatively difficult to obtain. Further, the offset frequency f _CEO can be efficiently detected based on the appropriately selected first pulse and second pulse (Stokes pulse and anti-Stokes pulse in this embodiment).

Further, the optical comb parameter detection device 1A includes a dispersion shift fiber 4 as a pulse generating unit for generating a Stokes pulse and an anti-Stokes pulse. In this case, at least one of the Stokes pulse and the anti-Stokes pulse (in this embodiment, both the Stokes pulse and the anti-Stokes pulse) can be generated by a relatively simple configuration using the distributed shift fiber 4.

Further, the optical comb parameter detection device 1A separates the first optical path L1 of the Stokes pulse and the second optical path L2 of the anti-Stokes pulse downstream of the distributed shift fiber, and outputs the Stokes pulse and the anti-Stokes pulse upstream of the detection unit 11. It includes an optical system OS that combines waves, and a delay control unit 8 that is provided in the second optical path and causes a time delay to match the phases of the Stokes pulse and the anti-Stokes pulse. According to the above configuration, the phase shift between the Stokes pulse and the anti-Stokes pulse can be appropriately corrected.

Further, the optical comb parameter detection device 1A includes a second harmonic generation unit 6 that generates a second harmonic of the Stokes pulse. The Stokes pulse has twice the wavelength of the anti-Stokes pulse. The detection unit 11 detects the difference frequency between the second harmonic of the Stokes pulse and the anti-Stokes pulse as the offset frequency f _CEO . According to the above configuration, the offset frequency f _CEO can be appropriately detected from the Stokes pulse and the anti-Stokes pulse having a wavelength difference of one octave.

Further, the optical comb parameter detection device 1A is provided between the optical comb generator 2 and the dispersion shift fiber 4, and includes an optical pulse amplifier 3 for amplifying the optical comb generated by the optical comb generator 2. The pulse energy of the optical comb amplified by the optical pulse amplifier 3 is 1 nJ or more (10 nJ or more in this embodiment). According to the above configuration, the wavelength shift amount of the Stokes pulse or the anti-Stokes pulse can be increased by amplifying the pulse energy of the optical comb incident on the distributed shift fiber 4. Thereby, a Stokes pulse and an anti-Stokes pulse having a wavelength difference of a predetermined magnitude (one octave in this embodiment) can be appropriately obtained.

[Modified example of the first embodiment]
In the first embodiment, a Stokes pulse is used as the first pulse and an anti-Stokes pulse is used as the second pulse, but the combination of the first pulse and the second pulse is not limited to the above. For example, when the wavelength difference between the Stokes pulse and the incident pulse (optical comb incident on the dispersion shift fiber 4) can be set to one octave, the Stokes pulse is used as the first pulse and is incident as the second pulse. Pulses may be utilized. In this case, the dichroic mirror 5 may be configured to transmit the Stokes pulse output from the tip portion 4a of the dispersion shift fiber 4 and reflect the incident pulse output from the tip portion 4a of the dispersion shift fiber 4. .. When the wavelength difference between the incident pulse and the anti-Stokes pulse can be set to one octave, the incident pulse may be used as the first pulse and the anti-Stokes pulse may be used as the second pulse. In this case, if the dichroic mirror 5 is configured to transmit the incident pulse output from the tip portion 4a of the dispersion shift fiber 4 and reflect the anti-Stokes pulse output from the tip portion 4a of the dispersion shift fiber 4. good.

Further, when the pulse energy of the optical comb output from the optical comb generator 2 is sufficiently large, the optical pulse amplifier 3 may be omitted.

Further, the delay control unit 8 may be provided on the first optical path L1 instead of the second optical path L2.

[Second Embodiment]
The optical comb parameter detection device 1B according to the second embodiment will be described with reference to FIG. The optical comb parameter detection device 1B includes a first polarization adjusting unit 12 (polarization adjusting unit), a second polarization adjusting unit 13 (polarization adjusting unit), a pulse width adjusting unit 14, a bandpass filter 15, and an optical waveguide unit. It is mainly different from the optical comb parameter detection device 1A in that 16 is further provided.

The first polarization adjusting unit 12 is provided on the first optical path L1. As an example, the first polarization adjusting unit 12 is provided between the dichroic mirror 5 and the second harmonic generation unit 6. The first polarization adjusting unit 12 adjusts the polarization direction of the first pulse (Stokes pulse or incident pulse) passing through the first optical path L1. The first polarization adjusting unit 12 is, for example, a wave plate or the like.

The second polarization adjusting unit 13 is provided on the second optical path L2. As an example, the second polarization adjusting unit 13 is provided between the delay control unit 8 and the pulse width adjusting unit 14. The second polarization adjusting unit 13 adjusts the polarization direction of the second pulse (the incident pulse or the anti-Stokes pulse, which is different from the first pulse) passing through the second optical path L2. The second polarization adjusting unit 13 is, for example, a wave plate or the like.

The polarization adjusting unit (in this embodiment, the first polarization adjusting unit 12 and the second polarization adjusting unit 13) has the first pulse and the polarization direction of the second pulse so that the polarization direction of the first pulse and the polarization direction of the second pulse are substantially the same. The polarization direction of at least one of the second pulses (in this embodiment, each of the first pulse and the second pulse) is adjusted. According to the above configuration, since the interference strength between the first pulse and the second pulse combined in the beam splitter 10 can be increased, the signal strength of the offset frequency f _CEO detected by the detection unit 11 is increased. Can be done.

The pulse width adjusting unit 14 is provided in the second optical path L2. As an example, the pulse width adjusting unit 14 is provided between the second polarization adjusting unit 13 and the beam splitter 10. The pulse width adjusting unit 14 adjusts the pulse width of the second pulse so that the pulse width of the first pulse and the pulse width of the second pulse substantially match. The pulse width adjusting unit 14 is, for example, a chirp mirror or the like. According to the above configuration, since the interference strength between the first pulse and the second pulse combined in the beam splitter 10 can be increased, the signal strength of the offset frequency f _CEO detected by the detection unit 11 is increased. Can be done. The pulse width adjusting unit 14 may be provided in the first optical path L1 instead of the second optical path L2. In this case, the pulse width adjusting unit 14 adjusts the pulse width of the first pulse so that the pulse width of the first pulse and the pulse width of the second pulse substantially match.

The bandpass filter 15 is provided downstream of the beam splitter 10 (upstream of the detection unit 11). The bandpass filter 15 passes components in a predetermined frequency band of the first pulse and the second pulse. More specifically, the bandpass filter 15 has a frequency of the second harmonic of the first pulse (“2f (n)” represented by the above-mentioned equation (3)) and a frequency of the second pulse (the above-mentioned equation). It may be configured to allow components in the frequency band including "f (2n)") represented by (2) to pass through and not pass through (attenuate) other components. In this case, the detection unit 11 detects the offset frequency f _CEO based on the components of the first pulse and the second pulse that have passed through the bandpass filter 15. According to the above configuration, since the bandpass filter 15 can remove the spectral component (that is, the component that becomes noise) of the frequency band unnecessary for detecting the offset frequency f _CEO , it is detected by the detection unit 11. The S / N ratio of the signal of the offset frequency f _CEO can be increased.

The optical waveguide unit 16 is connected to an end portion on the input side of the detection unit 11. That is, the optical waveguide section 16 is provided between the bandpass filter 15 and the detection section 11. The optical waveguide portion 16 has a function of aligning the beam shape of the first pulse and the beam shape of the second pulse by passing the components of the first pulse and the second pulse that have passed through the bandpass filter 15. The optical waveguide portion 16 is, for example, an optical fiber or the like. According to the above configuration. By aligning the beam shapes of the first pulse and the second pulse that have passed through the optical waveguide portion 16, the coherence of the first pulse and the second pulse can be improved. As a result, the signal strength of the offset frequency f _CEO detected by the detection unit 11 can be increased.

In the optical comb parameter detection device 1B, it is not necessary to provide all of the first polarization adjusting unit 12, the second polarization adjusting unit 13, the pulse width adjusting unit 14, the bandpass filter 15, and the optical waveguide unit 16. Needless to say. That is, in the optical comb parameter detection device 1B, a part of the first polarization adjusting unit 12, the second polarization adjusting unit 13, the pulse width adjusting unit 14, the bandpass filter 15, and the optical waveguide unit 16 may be omitted. For example, the optical comb parameter detection device 1B described above has a first polarization adjustment unit that adjusts the polarization direction of the first pulse as a polarization adjustment unit for substantially matching the polarization direction of the first pulse with the polarization direction of the second pulse. Although both the unit 12 and the second polarization adjusting unit 13 for adjusting the polarization direction of the second pulse are provided, the optical comb parameter detection device 1B is either the first polarization adjusting unit 12 or the second polarization adjusting unit 13. Only one may be provided. In this case, only the polarization direction of one of the first pulse and the second pulse is adjusted so as to substantially coincide with the polarization direction of the other of the first pulse and the second pulse.

[Third Embodiment]
The optical comb parameter detection device 1C according to the third embodiment will be described with reference to FIG. 7. The optical comb parameter detectors 1A and 1B detect the offset frequency f _CEO based on the first pulse and the second pulse having a wavelength difference of one octave, whereas the optical comb parameter detector 1C detects 2/3. It is configured to detect the offset frequency f _CEO based on the first and second pulses having an octave wavelength difference. The optical comb parameter detection device 1C includes a third harmonic generation unit 21 and a detection unit 23 in place of the second harmonic generation unit 6 and the detection unit 11, and further includes a second harmonic generation unit 22. It is mainly different from the comb parameter detection device 1A. Hereinafter, as an example, the center wavelength of the first pulse (Stokes pulse or incident pulse) is 1800 nm, and the center wavelength of the second pulse (incident pulse or anti-Stokes pulse and different from the first pulse) is 1200 nm. As an example, the optical comb parameter detection device 1C will be described.

The third harmonic generation unit 21 is provided on the first optical path L1 in the same manner as the second harmonic generation unit 6 in the optical comb parameter detection devices 1A and 1B. The third harmonic generation unit 21 generates the third harmonic of the first pulse. When the center wavelength of the first pulse is 1800 nm, the center wavelength of the third harmonic of the first pulse is 600 nm, which is 1/3 of that. The third harmonic generation unit 21 may include, for example, a plurality of nonlinear optical crystals such as BBO and LBO. The third harmonic is obtained, for example, by generating a sum frequency of the incident light (first pulse) and the second harmonic. As an example, the third harmonic generation unit 21 first irradiates a first optical crystal such as a BBO with incident light (for example, light having a center wavelength of 1800 nm) to irradiate the second harmonic (for example, light having a center wavelength of 900 nm). ) Is generated. After that, the third harmonic generation unit 21 causes the incident light transmitted through the first optical crystal and the second harmonic to be incident on the second optical crystal such as LBO, thereby causing the sum frequency of the incident light and the second harmonic. Is generated to generate a third harmonic (for example, light having a center wavelength of 600 nm).

The second harmonic generation unit 22 is provided on the second optical path L2. In the present embodiment, as an example, the second harmonic generation unit 22 is provided between the fixed mirror 9 and the beam splitter 10. The second harmonic generation unit 22 generates the second harmonic of the second pulse. When the center wavelength of the second pulse is 1200 nm, the center wavelength of the second harmonic of the second pulse is 600 nm, which is half of that. The second harmonic generation unit 22 is, for example, a member similar to the second harmonic generation unit 6.

The detection unit 23 detects the difference frequency between the third harmonic of the first pulse and the second harmonic of the second pulse as the offset frequency f _CEO . The detection unit 23 is, for example, a photodetector or the like. Hereinafter, the method of detecting the offset frequency f _CEO by the detection unit 23 will be described in detail.

As described above, in the present embodiment, the center wavelength (1800 nm) of the first pulse is 1.5 times the center wavelength (1200 nm) of the second pulse. That is, the wavelength difference between the first pulse and the second pulse is 2/3 octave. Here, when the first pulse corresponds to the 2nth mode frequency f (2n) of the optical comb, the second pulse having a wavelength of 2/3 thereof corresponds to the 3nth mode frequency f (3n) of the optical comb. do.

From the above equation (1) for obtaining the nth mode frequency f (n), the mode frequency 3f (2n) of the third harmonic (third harmonic) of the second mode frequency f (2n) is the following equation. It is represented by (5). Further, the mode frequency 2f (3n) of the second harmonic (second harmonic) of the 3nth mode frequency f (3n) is represented by the following equation (6). Further, the following equation (7) can be obtained by taking the difference between the following equation (5) and the following equation (6).
3f (2n) = 6n x f _rep + 3f _CEO ... (5)
2f (3n) = 6n x f _rep + 2f _CEO ... (6)
3f (2n) -2f (3n) = f _CEO ... (7)

Therefore, the detection unit 23 detects the difference frequency between the third harmonic (3f (2n)) of the first pulse and the second harmonic (2f (3n)) of the second pulse as the offset frequency f _CEO . Can be done.

According to the optical comb parameter detection device 1C, the offset frequency f _CEO can be appropriately detected from the first pulse and the second pulse having a wavelength difference of 2/3 octave.

[Fourth Embodiment]
The optical comb parameter detection device 1D according to the fourth embodiment will be described with reference to FIG. The optical comb parameter detection device 1D is provided with a dispersion compensating fiber 31 that connects the tip portion 4a of the dispersion shift fiber 4 and the end portion on the input side of the detection unit 11 instead of the optical system OS. It is mainly different from the device 1A. The dispersion compensating fiber 31 is arranged between the dispersion shift fiber 4 and the detection unit 11 and transmits the first pulse and the second pulse. In the optical comb parameter detection device 1D, the second harmonic generation unit 6 is incorporated in the dispersion compensating fiber 31. For example, the second harmonic generation unit 6, which is a non-linear crystal, is directly coupled to the dispersion compensating fiber 31.

In the optical comb parameter detection device 1D, the first pulse (Stokes pulse or incident pulse) and the second pulse (incident pulse or anti-Stokes pulse) output from the tip portion 4a of the dispersion shift fiber 4 are different from the first pulse. The pulse) is directly input to the dispersion compensating fiber 31. The dispersion compensating fiber 31 is an optical fiber having characteristics opposite to the wavelength dispersion of the dispersion shift fiber 4. The fiber length of the dispersion compensating fiber 31 is adjusted to an appropriate length so as to cancel the time difference between the first pulse and the second pulse generated in the dispersion shift fiber 4.

According to the optical comb parameter detection device 1D, the time delay between the first pulse and the second pulse generated in the dispersion shift fiber 4 can be canceled by the dispersion compensation fiber 31. As a result, the first pulse and the optical system OS provided in the above-mentioned optical comb parameter detection devices 1A, 1B, 1C are not used to separate the optical path between the first pulse and the second pulse. The phase of the second pulse can be matched. That is, since it is not necessary to spatially separate the first pulse and the second pulse and output them to the free space, it is possible to simplify the device configuration.

Although one embodiment of the present disclosure has been described above, the present disclosure is not limited to the above-described embodiment. Some configurations in one embodiment or variant described above can be optionally applied to the configurations in other embodiments or variants. For example, in the optical comb parameter detection device 1C, the configuration provided in the optical comb parameter detection device 1B (that is, the first polarization adjusting unit 12, the second polarization adjusting unit 13, the pulse width adjusting unit 14, the bandpass filter 15, and the optical waveguide). Part or all of the part 16) may be provided.

Further, in the above embodiment, the distributed shift fiber is exemplified as the pulse generating unit for generating the optical comb, but the pulse generating unit is not limited to the distributed shift fiber. The pulse generating unit may generate at least one of a Stokes pulse that is faster in time with respect to the incident pulse and an anti-Stokes pulse that is slower in time with respect to the incident pulse.

1A, 1B, 1C, 1D ... Optical comb parameter detector, 2 ... Optical comb generator (optical comb generator), 3 ... Optical pulse amplifier (amplification unit), 4 ... Distributed shift fiber (pulse generator), 6 ... 2nd harmonic generation unit, 8 ... Delay control unit, 11,23 ... Detection unit, 12 ... First polarization adjustment unit (polarization adjustment unit), 13 ... Second polarization adjustment unit (polarization adjustment unit), 14 ... Pulse width Adjustment unit, 15 ... band path filter, 16 ... optical waveguide section, L1 ... first optical path, L2 ... second optical path, OS ... optical system.

Claims (13)

An optical comb generator that generates an optical comb having a plurality of frequency modes specified by a repetition frequency and an offset frequency,
A pulse generating unit that generates at least one of a Stokes pulse and an anti-Stokes pulse by inputting the optical comb as an incident pulse.
The offset frequency is detected based on the first pulse, which is one of the incident pulse and the Stokes pulse, and the second pulse, which is one of the incident pulse and the anti-Stokes pulse and is different from the first pulse. Detection unit and
An optical comb parameter detector equipped with. The optical comb parameter detection device according to claim 1, wherein the pulse generating unit includes a distributed shift fiber. An optical system that separates the first optical path of the first pulse and the second optical path of the second pulse downstream of the pulse generation unit, and combines the first pulse and the second pulse upstream of the detection unit. When,
Claim 1 or 2 further includes a delay control unit provided in at least one of the first optical path and the second optical path and causing a time delay in order to match the phases of the first pulse and the second pulse. Optical comb parameter detector according to. Further provided with a polarization adjusting unit provided in at least one of the first optical path and the second optical path and adjusting the polarization direction of at least one of the first pulse and the second pulse.
The polarization adjusting unit adjusts the polarization direction of at least one of the first pulse and the second pulse so that the polarization direction of the first pulse and the polarization direction of the second pulse substantially coincide with each other. The optical comb parameter detection device according to 3. The pulse of the first pulse or the pulse of the second pulse is provided in at least one of the first optical path and the second optical path so that the pulse width of the first pulse and the pulse width of the second pulse substantially match. The optical comb parameter detection device according to claim 3 or 4, further comprising a pulse width adjusting unit for adjusting the width. Further, a bandpass filter provided upstream of the detection unit and passing a component of a predetermined frequency band of the first pulse and the second pulse is provided.
The optical comb parameter according to any one of claims 3 to 5, wherein the detection unit detects the offset frequency based on the components of the first pulse and the second pulse that have passed through the bandpass filter. Detection device. An optical waveguide portion that is connected to the input side end of the detection unit and that aligns the beam shape of the first pulse with the beam shape of the second pulse by passing the first pulse and the second pulse. The optical comb parameter detection device according to any one of claims 3 to 6, further comprising. A dispersion compensating fiber, which is arranged between the pulse generating unit and the detecting unit and transmits the first pulse and the second pulse, is further provided.
The optical comb parameter detection device according to claim 2, wherein the dispersion compensating fiber has characteristics opposite to those of the wavelength dispersion of the pulse generating unit. Further, a second harmonic generation unit for generating the second harmonic of the first pulse is provided.
The first pulse has twice the wavelength of the second pulse and has a wavelength twice that of the second pulse.
The optical comb parameter detection according to any one of claims 1 to 8, wherein the detection unit detects the difference frequency between the second harmonic of the first pulse and the second pulse as the offset frequency. Device. A third harmonic generator that generates the third harmonic of the first pulse,
A second harmonic generation unit that generates the second harmonic of the second pulse is further provided.
The first pulse has a wavelength 1.5 times that of the second pulse.
The detection unit detects the difference frequency between the third harmonic of the first pulse and the second harmonic of the second pulse as the offset frequency, according to any one of claims 1 to 8. The described optical comb parameter detector. An amplification unit provided between the optical comb generation unit and the pulse generation unit to amplify the optical comb generated by the optical comb generation unit is further provided.
The optical comb parameter detection device according to any one of claims 1 to 10, wherein the pulse energy of the optical comb amplified by the amplification unit is 1 nJ or more. The optical comb parameter detection device according to any one of claims 1 to 11, wherein the detection unit detects the repetition frequency together with the offset frequency based on the first pulse and the second pulse. Steps to generate an optical comb with multiple frequency modes specified by repetition frequency and offset frequency,
A step of generating at least one of a Stokes pulse and an anti-Stokes pulse by inputting the optical comb as an incident pulse,
The offset frequency is detected based on the first pulse, which is one of the incident pulse and the Stokes pulse, and the second pulse, which is one of the incident pulse and the anti-Stokes pulse and is different from the first pulse. Steps to do and
Optical comb parameter detection method including.

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