A non-linear interferometer

Abstract

In the field of optical interference measurement of the present invention, a non-linear interferometer is disclosed, comprising: a femtosecond laser, an ultra-high continuous spectrum generating device, a polarization state regulating device, an optical frequency doubling device, a time delay adjusting device, and a beat signal detecting device, which are connected in sequence in an optical path. By connecting the femtosecond laser, the ultra-high continuous spectrum generating device, the polarization state regulating device, the optical frequency doubling device, the time delay adjusting device, and the beat signal detecting device in sequence in an optical path, all the light rays in the interferometer are transmitted in one optical path, the optical path structure is simple, the common mode suppression effect is fully utilized to suppress the influence of environmental noise, and the ultra-high continuous spectrum is precisely adjusted in time delay by the time delay adjusting device to realize continuous adjustment of the time delay between the fundamental frequency light and the frequency-doubled light, which can improve the detection stability of the carrier envelope offset frequency while realizing high signal-to-noise ratio measurement of the carrier envelope offset frequency.

Description

Technical Field

[0001] The present invention relates to the field of optical interferometric measurement, and more particularly, to a non-linear interferometer. Background Art

[0002] The carrier-envelope offset frequency of a femtosecond laser frequency comb is the change of the optical pulse carrier relative to the femtosecond pulse envelope, and cannot be directly detected by detection instruments such as photodetectors. Currently, a non-linear process is usually introduced, and an interferometric method is used to detect the carrier-envelope offset frequency.

[0003] The optical path structures of existing interferometers for detecting the carrier-envelope offset frequency mainly include two types: non-collinear and collinear interferometric optical paths. Among them, in the non-collinear interferometer, the second harmonic 2f in the supercontinuum spectrum n and the fundamental frequency f 2n are transmitted separately, so that the time-domain delay between the two can be flexibly adjusted. However, the non-collinear interferometer has the disadvantages of a complex optical path structure, and the equivalent optical path difference of the two interference arms is easily affected by environmental disturbances, and the stability of the obtained carrier-envelope offset frequency f0 is relatively low. The optical path structure of the collinear interferometer is relatively simple, the overall influence of environmental disturbances is small, and there is no non-common mode noise influence. However, the collinear interferometer needs to preset that the time-domain delay between the high-frequency and low-frequency components in the output spectrum is close to zero and cannot be continuously adjusted, and the signal-to-noise ratio of the obtained carrier-envelope offset frequency f0 is relatively low. Summary of the Invention

[0004] The present invention provides a non-linear interferometer to overcome the defects of low stability and low signal-to-noise ratio of the carrier-envelope offset frequency obtained by the prior art.

[0005] To solve the above technical problems, the technical solution of the present invention is as follows:

[0006] In a first aspect, the present invention provides a non-linear interferometer, comprising:

[0007] A femtosecond laser, a supercontinuum spectrum generating device, a polarization state regulating device, an optical frequency doubling device, a time delay regulating device, and a beat signal detecting device that are optically connected in sequence.

[0008] The femtosecond laser outputs femtosecond optical pulses to the supercontinuum spectrum generating device, and the supercontinuum spectrum generating device performs spectral broadening on the femtosecond optical pulses to obtain a supercontinuum spectrum including fundamental frequency light; the supercontinuum spectrum sequentially passes through the polarization state regulating device and the optical frequency doubling device for polarization state regulation and frequency doubling processing of the low-frequency light in the supercontinuum spectrum to obtain second harmonic light; the fundamental frequency light and the second harmonic light are transmitted to the beat signal detecting device after time delay adjustment by the time delay regulating device, and the beat signal detecting device performs beat processing on the fundamental frequency light and the second harmonic light to obtain the carrier-envelope offset frequency.

[0009] In this technical solution, the femtosecond laser outputs femtosecond optical pulses; the femtosecond optical pulses are input into the supercontinuum spectrum generation device, and the supercontinuum spectrum generation device broadens the spectrum of the femtosecond optical pulses, broadening the spectrum of the femtosecond optical pulses to a supercontinuum spectrum with a coverage range of more than one octave, so that the frequency of the high-frequency light in the supercontinuum spectrum reaches twice the frequency of the low-frequency light, thereby obtaining the fundamental frequency light f required for beat frequency. 2n ; The supercontinuum spectrum is input into the polarization state control device to change the polarization direction and power distribution of the supercontinuum spectrum, so that the supercontinuum spectrum reaches the best matching state with the optical frequency doubling device; then the supercontinuum spectrum output by the polarization state control device is input into the optical frequency doubling device for frequency doubling processing, so that the low-frequency components in the supercontinuum spectrum are frequency doubled, thereby obtaining the frequency-doubled light required for beat frequency; the fundamental frequency light and the frequency-doubled light output by the optical frequency doubling device are input into the time delay adjustment device, and the time delay adjustment device performs precise time delay adjustment on the input fundamental frequency light and frequency-doubled light, so as to realize continuous adjustment of the time delay between the fundamental frequency light and the frequency-doubled light; the beam output by the time delay adjustment device is input into the beat frequency signal detection device, and the beat frequency signal detection device performs beat frequency on the fundamental frequency light and the frequency-doubled light to obtain a beat frequency signal, and the beat frequency signal is the carrier-envelope offset frequency.

[0010] Compared with the prior art, the beneficial effect of the technical solution of the present invention is that: by connecting the femtosecond laser, the supercontinuum spectrum generation device, the polarization state control device, the optical frequency doubling device, the time delay adjustment device and the beat frequency signal detection device in sequence on an optical path, all the light rays in the interferometer are transmitted on one optical path, the optical path structure is simple, the common-mode rejection effect is fully utilized to suppress the influence of environmental noise, and the supercontinuum spectrum is precisely adjusted in time delay by the time delay adjustment device to realize continuous adjustment of the time delay between the fundamental frequency light and the frequency-doubled light, which can improve the detection stability of the carrier-envelope offset frequency while realizing high signal-to-noise ratio measurement of the carrier-envelope offset frequency. Description of the Drawings

[0011] Figure 1 It is a schematic structural diagram of a non-linear interferometer of the present invention.

[0012] Figure 2 It is another schematic structural diagram of a non-linear interferometer of the present invention.

[0013] Figure 3 It is a schematic diagram of off-axis parabolic mirror collimating and focusing a beam.

[0014] Figure 4 It is yet another schematic structural diagram of a non-linear interferometer of the present invention.

[0015] Figure 5 Schematic diagram for adjusting the time delay between two light beams by a Wollaston prism.

[0016] Figure 6 Schematic diagram of the optical path of a 2F-2F imaging system.

[0017] Wherein, 1 - femtosecond laser, 2 - supercontinuum spectrum generation device, 201 - half-wave plate, 202 - first fiber collimator, 203 - photonic crystal fiber, 3 - polarization state modulation device, 301 - second fiber collimator, 302 - first linear polarizer, 303 - first convex lens, 304 - off-axis parabolic mirror, 4 - optical frequency doubling device, 5 - time delay adjustment device, 501 - narrowband filter, 502 - second convex lens, 503 - plane mirror, 504 - Wollaston prism, 505 - concave mirror, 6 - beat signal detection device, 601 - second linear polarizer, 602 - third convex lens, 603 - photodetector. Specific embodiments

[0018] The accompanying drawings are only for illustrative purposes and should not be construed as a limitation of this patent;

[0019] The technical solutions of the present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0020] For the convenience of describing the embodiments of the present invention, the basic concepts involved in the embodiments of the present invention are introduced as follows:

[0021] The femtosecond laser outputs a series of femtosecond optical pulses with equal intervals in the time domain. The time interval is generally the time (T) for the optical pulse to travel back and forth in the laser resonator once. Corresponding to the frequency domain, it is manifested as a series of frequency teeth distributed in a comb shape, that is, a series of longitudinal mode frequencies in the femtosecond laser resonator. The interval between adjacent comb teeth is the reciprocal of the time (T) for the optical pulse to circulate in the laser cavity once, that is, the number of pulses output by the laser per unit time, which is generally called the pulse repetition frequency f of the femtosecond laser r , which is mainly affected by the equivalent cavity length (L) of the laser. Due to the existence of dispersion in the laser resonator, the group velocity and phase velocity of the output optical pulse are inconsistent, so there will be a phase difference between the envelope and the carrier of the optical pulse during the oscillation in the cavity, that is, carrier-envelope offset (CEO), and its change frequency is called the carrier-envelope offset frequency (f0). f0 is manifested as the overall offset of the frequency comb teeth in the frequency domain, which is equivalent to the offset between the starting scale of the ruler and the physical end point of the ruler; while the repetition frequency f r is equivalent to the size of the unit scale of the comb teeth. When the femtosecond laser operates freely, affected by various interference factors, its f0 and f rIt will change, which is equivalent to the position of each scale on the ruler being not very certain. When the repetition frequency and the carrier-envelope offset frequency of the laser optical pulse are accurately locked through some technical means, the frequency of each comb tooth can be accurately determined, and the frequency of the nth frequency tooth is expressed as f n = f0 + nf r . This precisely controlled optical frequency comb has revolutionary application value in the fields of precision spectroscopy and precision metrology, etc.

[0022] The repetition frequency f of the femtosecond laser output pulse r Generally ranges from 10 7 Hz to 10 9 Hz, which can be measured by a high-speed photodetector and accurately locked at a fixed frequency value referenced to the international frequency standard by precisely controlling the equivalent cavity length of the laser through a phase-locked loop system. Since the carrier-envelope offset frequency is the change of the optical pulse carrier relative to the femtosecond pulse envelope and cannot be directly detected by a photodetector, etc., a nonlinear process is usually introduced and detected by an interferometric measurement method. The most commonly used one is based on the f-2f self-referencing interferometry technique. The f-2f self-referencing interferometry method requires the output spectrum of the femtosecond laser to be extended to a supercontinuum spectrum with an octave through a strong nonlinear process, that is, the spectrum contains f n = f0 + nf r and f 2n = f0 + 2nf r two frequency components, and the value of f 2n is almost twice that of f n . Then, the low-frequency f n is frequency-doubled to 2f n = 2f0 + 2nf r so as to obtain an interference signal of two beams of light with frequencies close to f 2n and 2f n in the interferometer, and the beat frequency (frequency difference) between the two is detected by a photodetector, f 2n - 2f n = f0, that is, the carrier-envelope offset frequency f0 of the femtosecond laser pulse train is obtained.

[0023] Embodiment 1

[0024] Please refer to Figure 1 , this embodiment proposes a nonlinear interferometer for detecting the carrier-envelope offset frequency, including:

[0025] A femtosecond laser 1, a supercontinuum spectrum generation device 2, a polarization state regulation device 3, an optical frequency doubling device 4, a time delay adjustment device 5, and a beat frequency signal detection device 6 that are sequentially connected in an optical path.

[0026] In the specific implementation process, the femtosecond laser 1 outputs femtosecond optical pulses; the femtosecond optical pulses are input into the supercontinuum spectrum generating device 2, and the supercontinuum spectrum generating device 2 broadens the spectrum of the femtosecond optical pulses to a supercontinuum spectrum with a coverage range of more than one octave, so that the frequency of the high-frequency light in the supercontinuum spectrum reaches twice the frequency of the low-frequency light, thereby obtaining the fundamental frequency light required for beat frequency; the supercontinuum spectrum is input into the polarization state control device 3 to change the polarization direction and power distribution of the supercontinuum spectrum, so that the supercontinuum spectrum reaches the best matching state with the optical frequency doubling device 4; then the supercontinuum spectrum output by the polarization state control device 3 is input into the optical frequency doubling device 4 for frequency doubling processing, so that the low-frequency components in the supercontinuum spectrum are frequency doubled, thereby obtaining the frequency-doubled light required for beat frequency; the fundamental frequency light and the frequency-doubled light output by the optical frequency doubling device 4 are input into the time delay adjustment device 5, and the time delay adjustment device 5 performs precise time delay adjustment on the input fundamental frequency light and frequency-doubled light, thereby realizing continuous adjustment of the time delay between the fundamental frequency light and the frequency-doubled light; the beam output by the time delay adjustment device 5 is input into the beat frequency signal detection device 6, and the beat frequency signal detection device 6 performs beat frequency on the fundamental frequency light and the frequency-doubled light to obtain a beat frequency signal, and the beat frequency signal is the carrier-envelope offset frequency.

[0027] By optically connecting the femtosecond laser 1, the supercontinuum spectrum generating device 2, the polarization state control device 3, the optical frequency doubling device 4, the time delay adjustment device 5, and the beat frequency signal detection device 6 in sequence on an optical path, the fundamental frequency light and the frequency-doubled light in the supercontinuum spectrum are transmitted on the same optical path, and the optical path structure is simple, which can improve the detection stability of the carrier-envelope offset frequency; through the precise time delay adjustment of the supercontinuum spectrum by the time delay adjustment device 5, continuous adjustment of the time delay between the fundamental frequency light and the frequency-doubled light is realized, and by adjusting the time delay between the fundamental frequency light and the frequency-doubled light, high signal-to-noise ratio measurement of the beat frequency signal - the carrier-envelope offset frequency can be achieved.

[0028] Embodiment 2

[0029] This embodiment makes improvements on the basis of the nonlinear interferometer proposed in Embodiment 1.

[0030] In this embodiment, the supercontinuum spectrum generating device 2 includes a half-wave plate 201, a first fiber collimator 202, and a photonic crystal fiber 203 that are optically connected in sequence; the emission end of the femtosecond laser 1 is connected to the receiving end of the half-wave plate 201; the output end of the photonic crystal fiber 203 is connected to the input end of the polarization state control device 3.

[0031] In a specific implementation process, a femtosecond optical pulse is output by a femtosecond fiber laser with a central wavelength of 1030 nm. After passing through the half-wave plate 201, the femtosecond optical pulse is coupled into the photonic crystal fiber 203 through the first fiber collimator 202. The first fiber collimator 202 and the photonic crystal fiber 203 are processed as a whole, or the first fiber collimator 202 and the photonic crystal fiber 203 are connected together by an FC / PC fiber connector. When the output femtosecond optical pulse is coupled into the photonic crystal fiber 203, the spectrum of the femtosecond optical pulse will be broadened to an ultra-wideband spectrum with a coverage range of more than one octave, so that the frequency of the high-frequency light in the ultra-wideband spectrum reaches twice the frequency of the low-frequency light, thereby obtaining the fundamental frequency light f required for beat frequency 2n .

[0032] In this embodiment, the polarization state of the incident light is changed by adjusting the half-wave plate 201, and the output spectrum can be optimized to achieve the desired effect.

[0033] Embodiment 3

[0034] This embodiment makes improvements on the basis of the nonlinear interferometer proposed in Embodiment 2, as Figure 1 shown Figure 1 is a schematic structural diagram of an interferometer of the present invention.

[0035] In this embodiment, the polarization state control device 3 includes a second fiber collimator 301, a first linear polarizer 302, and a first convex lens 303 that are connected in optical path in sequence; the output end of the ultra-wideband spectrum generating device 2 is connected to the input end of the second fiber collimator 301; the emitting end of the first convex lens 303 is connected to the input end of the optical frequency doubling device 4.

[0036] In this embodiment, the output end of the photonic crystal fiber 203 is connected to the input end of the second fiber collimator 301.

[0037] In this embodiment, the second fiber collimator 301 includes an integrated off-axis parabolic reflector collimator with an FC / PC connector. Compared with collimating the light beam with a lens, the off-axis parabolic reflector collimator avoids the chromatic aberration effect of the lens and has better collimation effect. In the traditional design scheme, an achromatic lens is usually used, and the dispersion effect is still very obvious when facing an ultra-wideband spectrum with a spectral width greater than 500 nm, and the collimation effect is poor. At the same time, the reflector usually only has a loss of less than 3%, while the loss of a general near-infrared achromatic microscope objective is greater than 10%. The integrated off-axis parabolic reflector collimator with an FC / PC interface can directly and simply and quickly assemble the output end of the photonic crystal fiber 203 with an FC / PC connector, and is not easily affected by environmental disturbances.

[0038] In the specific implementation process, the output end of the supercontinuum spectrum after spectral broadening is output to the second fiber collimator 301 through the photonic crystal fiber 203, and the second fiber collimator 301 collimates the supercontinuum spectrum output by the photonic crystal fiber 203, so that the supercontinuum spectrum reaches the best matching state with the optical frequency doubling device 4.

[0039] When doubling the frequency of the supercontinuum spectrum, by adjusting the angles of the first half-wave plate 201 in front of the photonic crystal fiber 203 and the first linear polarizer 302 in front of the nonlinear frequency doubling crystal, the polarization direction of the light beam and the power distribution of the supercontinuum spectrum can be changed, so that the supercontinuum spectrum reaches the best matching state with the optical frequency doubling device 4 to meet the phase matching requirements, thereby improving the laser frequency doubling efficiency in order to obtain the optimal frequency doubling and subsequent beat frequency effects.

[0040] In this embodiment, the output end of the photonic crystal fiber 203 is subjected to a collapsing process and an FC / PC connector is installed. The output end of the photonic crystal fiber 203 is directly installed on the second fiber collimator 301. Since the supercontinuum spectrum output by the photonic crystal fiber 203 is in a divergent state with a large divergence angle, the numerical aperture NA of the light beam is generally greater than 0.2. When the focus of the second fiber collimator 301 is on the end face of the photonic crystal fiber 203 and the direction is appropriate, the reflected supercontinuum light beam by the second fiber collimator 301 is in a collimated state, and the light beam emitted by the second fiber collimator 301 is a parallel light, which is focused on the optical frequency doubling device 4 by the first convex lens 303.

[0041] Further, as Figure 2 shown, Figure 2 is another structural schematic diagram of the interferometer of the present invention. In this embodiment, an off-axis parabolic mirror 304 can also be used to focus the supercontinuum spectrum beam collimated by the second fiber collimator 301 so that it is focused on the optical frequency doubling device 4, as Figure 3 shown, Figure 3 is a schematic diagram of the off-axis parabolic mirror 304 collimating and focusing the light beam, that is, the polarization state control device 3 includes a second fiber collimator 301, a first linear polarizer 302 and an off-axis parabolic mirror 304 that are connected in optical path in sequence; the output end of the supercontinuum spectrum generating device 2 is connected to the input end of the second fiber collimator 301; the emitting end of the off-axis parabolic mirror 304 is connected to the input end of the optical frequency doubling device 4. The off-axis reflective parabolic mirror has the advantages of no chromatic aberration and spherical aberration, and can make the supercontinuum spectrum beam better focused on the optical frequency doubling device 4.

[0042] Further, in special cases, if it is necessary to further simplify the optical path, the relative position of the output end face of the photonic crystal fiber 203 and the second fiber collimator 301 can be changed so that the light beam emitted from the second fiber collimator 301 is directly focused on the optical frequency doubling device 4, without adding another first convex lens 303 or off-axis parabolic mirror 304, as Figure 4 shown, Figure 4 is another structural schematic diagram of the interferometer of the present invention. The polarization state adjustment device 3 includes a second fiber collimator 301 and a first linear polarizer 302 that are optically connected in sequence; the output end of the supercontinuum spectrum generating device 2 is connected to the input end of the second fiber collimator 301. The emitting end of the first linear polarizer 302 is connected to the input end of the optical frequency doubling device 4. When the output end of the photonic crystal fiber 203 is slightly away from the mirror focus, the reflected light beam is no longer collimated but in a focused state, and the light beam can be directly focused on the subsequent optical frequency doubling device 4 by the second fiber collimator 301.

[0043] In this embodiment, the optical frequency doubling device 4 includes a type-I phase-matched nonlinear frequency doubling crystal. Common such frequency doubling crystals include BBO, KTP, LBO, etc.

[0044] In the specific implementation process, the collimated light beam is focused into the nonlinear frequency doubling crystal through the first convex lens 303 to achieve the frequency doubling of the low-frequency component f n = f0 + nf r in the supercontinuum spectrum, so as to obtain the frequency components required for beat frequency. The frequency-doubled light 2f n = 2f0 + 2nf r .

[0045] The supercontinuum spectrum collimated by the second fiber collimator 301 is focused into the nonlinear frequency doubling crystal through an achromatic lens such as a double-glued first convex lens 303, or the second fiber collimator 301 directly focuses the supercontinuum spectrum output by the photonic crystal fiber 203 onto the optical nonlinear frequency doubling crystal. The low-frequency component f n = f0 + nf r in the supercontinuum spectrum is frequency-doubled after passing through the nonlinear frequency doubling crystal to obtain the frequency-doubled light 2f n = 2f0 + 2nf r required for subsequent beat frequency. The high-frequency fundamental light f 2n = f0 + 2nf r directly passes through the nonlinear frequency doubling crystal because it does not meet the phase matching condition. The polarization state of the newly generated frequency-doubled light with a frequency of 2f n is perpendicular to the polarization state of the fundamental light f 2n in the supercontinuum spectrum.

[0046] In this embodiment, the nonlinear frequency doubling crystal is a BBO crystal with type-I phase matching, that is, the o-ray is incident on the nonlinear frequency doubling crystal, and the frequency-doubled light is the e-ray, whose polarization state is perpendicular to that of the high-frequency component in the supercontinuum spectrum.

[0047] Embodiment 4

[0048] This embodiment is an improvement based on the nonlinear interferometer proposed in Embodiment 3.

[0049] In this embodiment, the time delay adjusting device 5 includes a narrowband filter 501, a second convex lens 502, a plane mirror 503, a Wollaston prism 504, and a concave mirror 505 that are connected in optical path in sequence; the output end of the optical frequency doubling device 4 is connected to the receiving end of the narrowband filter 501; the transmitting end of the plane mirror 503 is connected to the receiving end of the beat signal detecting device 6.

[0050] In this embodiment, a narrowband filter 501 is placed behind the nonlinear frequency doubling crystal to filter out redundant spectral components. This narrowband filter 501 only transmits the frequency-doubled light 2f n and the fundamental frequency light f 2n . Thus, it is convenient to control the time domain delay between the frequency-doubled light 2f n and the fundamental frequency light f 2n subsequently, and to improve the signal-to-noise ratio.

[0051] In this embodiment, the second convex lens 502 is an achromatic doublet convex lens, and specifically, a double-glued lens can be used. The second convex lens 502 can focus the light beam passing through the narrowband filter 501 on the glued surface inside the Wollaston prism 504.

[0052] In this embodiment, a Wollaston prism 504 is used to split the frequency-doubled light 2f n and the fundamental frequency light f 2n into two paths. Since the polarization states of these two spectral components are perpendicular to each other, although they have been collinearly transmitted before, when the light beam passes through the glued surface of the Wollaston prism 504, the light beams with different polarization states will be separated and transmitted at the designed separation angle. Since the light with different polarization states has different group velocities in the Wollaston prism 504, therefore, by changing the position of the light beam on the glued surface of the Wollaston prism 504, the time delay between the frequency-doubled light 2f n and the fundamental frequency light f 2n can be precisely adjusted.

[0053] In this embodiment, a concave mirror 505 with a relatively large aperture is used to reflect the separated frequency-doubled light 2f n and the fundamental frequency light f 2nReflect and re - intersect on the gluing surface of the Wollaston prism 504. According to the principle of reversibility of light path, after the reflected transmission passes through the prism gluing surface again, the second - harmonic light 2f n and the fundamental - frequency light f 2n are spatially combined again. In addition, by adjusting the height of the beam reflected by the concave mirror 505, the beam reflected through the Wollaston prism 504 and combined can have a different height compared to the incident beam, which is beneficial for separating the reflected beam from the incident beam.

[0054] In the specific implementation process, for the second - harmonic light 2f n and the fundamental - frequency light f 2n in the beam of the narrow - band filter 501, the second - harmonic light 2f n and the fundamental - frequency light f 2n are focused on the gluing interface of the Wollaston prism 504 through the second convex lens 502. Since the Wollaston prism 504 is formed by gluing right - angle prism pairs, and the optical axes of the two prism pairs are perpendicular to each other, when the 2f n and the fundamental - frequency light f 2n with perpendicular polarization states are incident on the end face of the Wollaston prism 504, they travel in the same direction at different speeds in the first prism. When entering the second prism, the optical axis of the prism turns by 90°. The o - light and e - light are converted. Since the refractive index of calcite of the prism base for o - light is greater than that for e - light, the two second - harmonic lights 2f n and the fundamental - frequency light f 2n with perpendicular polarization states deviate from the normal of the prism inclined plane by different angles. Therefore, the two propagate separately at a certain separation angle in the second prism and exit from the rear surface of the Wollaston prism 504. By moving the Wollaston prism 504 to focus the second - harmonic light 2f n and the fundamental - frequency light f 2n at different positions on the gluing inclined plane of the Wollaston prism 504, the second - harmonic light 2f n and the fundamental - frequency light f 2n can travel different optical paths, thereby achieving the purpose of adjusting the time - domain delay between the two.

[0055] To direct the combined beam to the beat - frequency signal detection device 6, adjust the pitch of the concave mirror 505 so that the returned beam is slightly lower than the original optical axis and is incident on the plane mirror 503 below the original optical axis to realize the beam steering and transmit the beam to the beat - frequency signal detection device 6.

[0056] The present invention adopts the splitting light scheme of the Wollaston prism 504. Compared with the traditional non - collinear interferometer, all the light rays in the interferometer can pass through the same optical components, which not only greatly simplifies the optical path of the interferometer and makes the volume more compact, but also enables the two spectral components for interference, namely the second - harmonic light 2f n and the fundamental - frequency light f2n All optical components are shared, and most of the optical paths are collinear, making full use of the common-mode rejection effect to suppress the influence of environmental noise, and a detection signal with a high signal-to-noise ratio can be obtained.

[0057] In addition, a Wollaston prism 504 is used for precise time delay adjustment of femtosecond optical pulses, and the adjustment direction is close to perpendicular to the direction where the time delay occurs. The time delay generated has a small sensitivity coefficient to the displacement of the Wollaston in the adjustment direction, as Figure 5 shown. Figure 5 As shown in the figure, it is a schematic diagram of the time delay adjustment between two beams by the Wollaston prism 504, that is, a small time delay adjustment is generated by adjusting a longer distance. In this way, the system has high stability, and the beat frequency signal is insensitive to the position vibration and drift of the Wollaston prism 504.

[0058] The light beam in the present invention is focused on the splitting and gluing surface of the Wollaston prism 504. The spot size on the gluing surface is very small, which fully suppresses the transverse dispersion inconsistency effect caused by the too large transverse distribution of the light beam on the gluing surface, and is beneficial to improving the detection signal-to-noise ratio of the interferometer.

[0059] The present invention uses a concave mirror 505 to simultaneously reflect the frequency-doubled light 2f n and the fundamental frequency light f 2n back to the splitting surface of the Wollaston prism 504 and form a combined beam. Since the light spot on the splitting surface is located at the center of the sphere of the concave mirror, that is, at twice the focal length 2F, the light spot on the splitting surface of the prism transmitted forward and the light spot reflected back to the splitting surface of the prism form a 2F-2F system, that is, the object distance is equal to the image distance, as Figure 6 shown. Figure 6 It is a schematic diagram of the optical path of the 2F-2F imaging system. The reflected light beam can reproduce the incident light beam on the splitting surface of the Wollaston prism 504. In this way, the combined beam effect of the two light beams is good, and the anti-interference ability of the spatial overlap effect is strong.

[0060] Embodiment 5

[0061] This embodiment makes improvements on the basis of the non-linear interferometer proposed in Embodiment 4.

[0062] Further, the beat frequency signal detection device 6 includes a second linear polarizer 601, a third convex lens 602, and a photodetector 603 that are optically connected in sequence; the receiving end of the second linear polarizer 601 is connected to the emitting end of the plane mirror 503.

[0063] In this embodiment, by placing the second linear polarizer 601 in front of the photodetector 603, the frequency-doubled light 2f n and the fundamental frequency light f 2nAfter passing through the second linear polarizer 601, they have the same polarization state, so that an interference effect can be generated.

[0064] In this embodiment, the photodetector 603 uses an avalanche photodetector 603 to detect the second harmonic light 2f with similar frequencies n and the fundamental frequency light f 2n of the interference beat frequency, and obtains the carrier-envelope offset frequency f0 of the femtosecond laser pulse, providing a signal for applications such as the precise locking of the femtosecond laser frequency comb.

[0065] In the specific implementation process, the light beam emitted from the plane mirror 503 is focused onto the avalanche photodetector 603 by the third convex lens 602 after passing through the second linear polarizer 601. The combined beam with originally perpendicular polarization states selects the same polarization state after passing through the second linear polarizer 601, so as to realize the beat frequency of the second harmonic light 2f n and the fundamental frequency light f 2n Finally, the avalanche photodetector 603 measures its beat frequency signal, that is, the carrier-envelope offset frequency f0.

[0066] The terms describing the positional relationship in the drawings are only for illustrative purposes and should not be construed as a limitation of this patent;

[0067] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, rather than limitations on the implementation manners of the present invention. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to enumerate all the implementation manners here. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included in the protection scope of the claims of the present invention.

Claims

1. A non-linear interferometer for detecting the carrier-envelope offset frequency, characterized in that, Comprising: A femtosecond laser (1), an ultra - continuum generation device (2), a polarization state modulation device (3), an optical frequency doubling device (4), a time - delay adjustment device (5), and a beat - frequency signal detection device (6) that are optically connected in sequence; The femtosecond laser (1) outputs femtosecond optical pulses to the ultra - continuum generation device (2), and the ultra - continuum generation device (2) broadens the spectrum of the femtosecond optical pulses to obtain an ultra - continuum spectrum including fundamental frequency light; The ultra - continuum spectrum successively passes through the polarization state modulation device (3) and the optical frequency doubling device (4) for polarization state modulation and frequency doubling of the low - frequency light in the ultra - continuum spectrum to obtain frequency - doubled light; The time - delay adjustment device (5) includes a narrow - band filter (501), a second convex lens (502), a plane mirror (503), a Wollaston prism (504), and a concave mirror (505) that are optically connected in sequence. The output end of the optical frequency doubling device (4) is connected to the receiving end of the narrow - band filter (501), and the transmitting end of the plane mirror (503) is connected to the receiving end of the beat - frequency signal detection device (6); The fundamental frequency light and the frequency - doubled light successively pass through the narrow - band filter (501), the second convex lens (502), the plane mirror (503), the Wollaston prism (504), reach the concave mirror (505), are reflected by the concave mirror (505) and then reach the Wollaston prism (504), the plane mirror (503), and are then transmitted to the beat - frequency signal detection device (6) after being turned by the plane mirror (503); The beat - frequency signal detection device (6) performs beat - frequency processing on the fundamental frequency light and the frequency - doubled light to obtain the carrier - envelope offset frequency.

2. The non-linear interferometer according to claim 1, wherein The ultra - continuum generation device (2) includes a half - wave plate (201), a first fiber collimator (202), and a photonic crystal fiber (203) that are optically connected in sequence; The emitting end of the femtosecond laser (1) is connected to the receiving end of the half - wave plate (201); The output end of the photonic crystal fiber (203) is connected to the input end of the polarization state modulation device (3).

3. The non-linear interferometer according to claim 1, wherein, The polarization state modulation device (3) includes a second fiber collimator (301), a first linear polarizer (302), and a first convex lens (303) that are optically connected in sequence; The output end of the ultra - continuum generation device (2) is connected to the input end of the second fiber collimator (301); The emitting end of the first convex lens (303) is connected to the input end of the optical frequency doubling device (4).

4. The non-linear interferometer according to claim 1, characterized in that, The polarization state modulation device (3) includes a second fiber collimator (301), a first linear polarizer (302), and an off - axis parabolic mirror (304) that are optically connected in sequence; The output end of the ultra - continuum generation device (2) is connected to the input end of the second fiber collimator (301); The emitting end of the off - axis parabolic mirror (304) is connected to the input end of the optical frequency doubling device (4).

5. The non-linear interferometer according to claim 1, wherein The polarization state modulation device (3) includes a second fiber collimator (301) and a first linear polarizer (302) that are optically connected in sequence; The output end of the supercontinuum spectrum generating device (2) is connected to the input end of the second fiber collimator (301); The emitting end of the first linear polarizer (302) is connected to the input end of the optical frequency doubling device (4).

6. The non-linear interferometer according to claim 5, characterized in that, The beat signal detecting device (6) includes a second linear polarizer (601), a third convex lens (602), and a photodetector (603) that are optically connected in sequence; The receiving end of the second linear polarizer (601) is connected to the emitting end of the plane mirror (503).

7. The nonlinear interferometer according to any one of claims 1 to 6, characterized in that, The optical frequency doubling device (4) includes a nonlinear frequency doubling crystal.

8. The non-linear interferometer according to claim 7, characterized in that, The nonlinear frequency doubling crystal is a type-I phase-matched frequency doubling crystal.

9. The non-linear interferometer according to any one of claims 3 to 5, characterized in that, The second fiber collimator (301) includes an integrated off-axis parabolic mirror reflective collimator with an FC / PC connector.

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