CN115706390A

CN115706390A - Frequency modulation external cavity laser device

Info

Publication number: CN115706390A
Application number: CN202110931331.7A
Authority: CN
Inventors: 梁伟; 刘云凤
Original assignee: Suzhou Institute of Nano Tech and Nano Bionics of CAS
Current assignee: Suzhou Institute of Nano Tech and Nano Bionics of CAS
Priority date: 2021-08-13
Filing date: 2021-08-13
Publication date: 2023-02-17

Abstract

The embodiment of the invention discloses a frequency modulation external cavity laser device, which comprises a seed light source, a feedback loop external cavity, a light source frequency adjusting module, an FP (Fabry-Perot) cavity frequency adjusting module and an external cavity frequency adjusting module, wherein the seed light source is connected with the feedback loop external cavity through a frequency modulation filter; the feedback loop outer cavity comprises an FP cavity; the light source frequency adjusting module is used for adjusting the eigenfrequency of the seed light beam; the FP cavity frequency adjusting module is used for adjusting the resonant frequency of the FP cavity; the external cavity frequency adjusting module is used for adjusting the resonant frequency of the external cavity of the feedback loop; the light source frequency adjusting module, the FP cavity frequency adjusting module and the outer cavity frequency adjusting module are cooperatively modulated, so that the eigen frequency of the seed light beam, the resonance frequency of the FP cavity and the resonance frequency of the outer cavity of the feedback loop meet the outer cavity self-injection locking condition, and the frequency-locked laser is formed. The embodiment of the invention solves the problem of small frequency modulation range of the existing narrow linewidth laser, can greatly improve the frequency modulation range, constructs the external cavity narrow linewidth laser with rapid large-range continuous frequency modulation, and expands the application field of the laser device.

Description

Frequency modulation external cavity laser device

Technical Field

The embodiment of the invention relates to a laser technology, in particular to a frequency modulation external cavity laser device.

Background

The linewidth of the laser is directly related to the quality factor of the laser cavity, which is limited by the material properties and processes typically low. The frequency-adjustable single-frequency narrow-linewidth laser has important application in the fields of laser radar, optical frequency domain reflectometer, quantum and the like. The mode of reducing the laser linewidth of the current frequency-adjustable laser device usually uses an external cavity feedback method, wherein one scheme is to use a grating to selectively feed back light with specific wavelength back to the laser, and the other scheme is to use an external reflector, and simultaneously, to avoid the multi-longitudinal mode effect of a long optical cavity, an interference filter is used to select a single longitudinal mode. The two types of external cavity lasers can realize frequency modulation through rotating a grating or an interference filter, however, due to the fact that the quality factor of the external cavity is low, the line width of the obtained laser is generally in the order of tens of kilohertz to hundreds of kilohertz, and a narrower line width cannot be obtained.

Currently, various external cavity laser schemes using high Q external cavities have their fast frequency tuning range limited to hundreds of mhz to gigahertz levels, although ultra-narrow linewidths can be achieved. In the fields of frequency modulated continuous wave laser radars, optical frequency domain reflectometers, and the like, the distance measurement accuracy is inversely proportional to the frequency modulation range, and in many quantum application fields, multiple absorption spectra of atoms are used, and also ultra-narrow linewidth laser with wide-range frequency modulation is required. The current laser device can not meet the application requirements of the fields.

Disclosure of Invention

The invention provides a frequency modulation external cavity laser device, which realizes an external cavity laser scheme of large-range high-speed continuous frequency modulation through a high-Q-value FP optical cavity, and obtains a larger quick continuous frequency modulation range while obtaining an ultra-narrow line width.

The embodiment of the invention provides a frequency modulation external cavity laser device, which comprises a seed light source and a feedback loop external cavity, wherein the feedback loop external cavity comprises an FP (Fabry-Perot) cavity, and the cavity length of the FP cavity is less than or equal to 10cm;

the seed light source is used for outputting a seed light beam;

the FP cavity is used for filtering the seed beam to form a transmission beam;

the feedback loop outer cavity is used for feeding the transmitted light beam back to the seed light source to form a feedback light path;

the frequency modulation external cavity laser device also comprises a light source frequency adjusting module, an FP cavity frequency adjusting module and an external cavity frequency adjusting module;

the light source frequency adjusting module is used for adjusting the eigenfrequency f1 of the seed light beam;

the FP cavity frequency adjusting module is used for adjusting the resonant frequency f2 of the FP cavity;

the external cavity frequency adjusting module is used for adjusting the resonant frequency f3 of the external cavity of the feedback loop;

the light source frequency adjusting module, the FP cavity frequency adjusting module and the external cavity frequency adjusting module are cooperatively modulated, so that the eigen frequency f1 of the seed light beam, the resonant frequency f2 of the FP cavity and the resonant frequency f3 of the external cavity of the feedback loop meet the external cavity self-injection locking condition, and frequency-locked laser is formed.

Optionally, the outer chamber self-injection locking condition comprises:

the difference between the eigenfrequency f1 of the seed beam and the resonance frequency f2 of the FP cavity is smaller than the external cavity self-injection locking range of the external cavity of the feedback loop;

the difference between the resonance frequency f2 of the FP cavity and the resonance frequency f3 of the feedback loop external cavity is less than or equal to one half of the free spectral range of the feedback loop external cavity.

Optionally, the FP cavity frequency adjustment module and the external cavity frequency adjustment module are electrically controlled displacement modules; the electric control displacement module is respectively assembled on at least one optical component of the feedback loop external cavity and the FP cavity;

the electronic control displacement module is used for changing the cavity length of the feedback loop external cavity or the FP cavity, or the electronic control displacement module is used for changing the optical path of the light beam in the optical component so as to adjust the resonant frequency of the feedback loop external cavity or the FP cavity.

Optionally, the feedback loop external cavity and the FP cavity respectively comprise at least one reflection unit, and the electronic control displacement module is mounted on the reflection unit;

the electronic control displacement module is used for changing the cavity length of the feedback loop outer cavity or the FP cavity according to a formula of delta f/f = delta L/L so as to adjust the resonant frequency of the feedback loop outer cavity or the FP cavity;

wherein f is the current resonant frequency of the feedback loop external cavity or the FP cavity, Δ f is the variation of the resonant frequency of the feedback loop external cavity or the FP cavity, L is the current cavity length of the feedback loop external cavity or the FP cavity, and Δ L is the cavity length variation of the feedback loop external cavity or the FP cavity.

Optionally, the feedback loop outer cavity and the FP cavity each comprise at least one prism unit on which the electronically controlled displacement module is mounted;

the electronic control displacement module is used for changing the optical path length of the light beam in the optical component according to the formula of delta f/f = n1 × delta L/(n 2 × L) so as to adjust the resonant frequency of the feedback loop external cavity or the FP cavity;

wherein f is the current resonant frequency of the feedback loop external cavity or the FP cavity, Δ f is the variation of the resonant frequency of the feedback loop external cavity or the FP cavity, n2 × L is the total optical path of the feedback loop external cavity or the FP cavity, and n1 × Δ L is the variation of the optical path of the feedback loop external cavity or the FP cavity.

Optionally, the FP cavity frequency adjustment module and the external cavity frequency adjustment module are an electrically controlled refractive index module or a thermally controlled refractive index module, respectively; the electric control refractive index module or the thermal control refractive index module is respectively positioned in the feedback loop outer cavity or the FP cavity;

the electric control refractive index module is used for changing the refractive index through an electro-optic effect, and the thermal control refractive index module is used for changing the refractive index through a thermo-optic effect so as to adjust the optical path of the light beam in the electric control refractive index module or the thermal control refractive index module, thereby adjusting the resonant frequency of the feedback loop external cavity or the FP cavity.

Optionally, the seed light source comprises a first end and a second end; the seed light beam is output from a first end of the seed light source, and the transmitted light beam is input from the first end of the seed light source;

the feedback loop outer cavity further comprises a first collimating unit, a one-way transmission unit and a reflection unit;

the first collimation unit is used for collimating the seed light beam;

the unidirectional transmission unit is used for transmitting the seed light beam to the FP cavity and blocking a reflected light beam of the FP cavity from being incident to the seed light source;

the reflecting unit is used for reflecting the transmission beam of the FP cavity back to the seed light source to form a feedback light path.

Optionally, the first collimating unit includes a first lens, the unidirectional transmission unit includes a polarization beam splitter, a first quarter-wave plate, a second quarter-wave plate, and a second lens in sequence, and the reflection unit includes a first mirror;

the FP cavity is located between the first quarter wave plate and the second quarter wave plate; the second lens is located between the FP cavity and the second quarter wave plate or between the second quarter wave plate and the first mirror.

Optionally, the seed light source comprises a first end and a second end; the seed light beam is output from a first end of the seed light source, and the transmitted light beam is input from the first end of the seed light source; alternatively, the seed light beam is output from a first end of the seed light source and the transmitted light beam is input from a second end of the seed light source;

the feedback loop outer cavity also comprises a one-way transmission unit and a light steering unit;

the one-way transmission unit is used for transmitting the seed light beam to the FP cavity and blocking a reflected light beam of the FP cavity from being incident to the seed light source;

the light steering unit is used for changing the transmission direction of the transmitted light beam of the FP cavity so that the transmitted light beam is fed back to the seed light source to form a feedback light path.

Optionally, the unidirectional transmission unit comprises a circulator, and the light turning unit comprises a second mirror, a third mirror and a fourth mirror;

the light beam transmission path in the frequency modulation external cavity laser device is as follows:

the seed light beam is output from the first end of the seed light source, input from the first end of the circulator, output from the second end of the circulator, and incident to the FP cavity for transmission to form the transmitted light beam; the transmitted light beam is reflected by the second reflector, the third reflector and the fourth reflector in sequence, then enters the third end of the circulator and is output from the first end of the circulator and fed back to the seed light source.

Optionally, the unidirectional transmission unit includes an isolator, and the light turning unit includes a fifth mirror, a sixth mirror, a seventh mirror, and an eighth mirror;

the seed light beam is output from the first end of the seed light source, input from the first end of the isolator, output from the second end of the isolator, and incident to the FP cavity to be transmitted to form the transmitted light beam; the transmitted light beam is reflected by the fifth reflector, the sixth reflector, the seventh reflector and the eighth reflector in sequence and then enters the second end of the seed light source.

Optionally, the unidirectional transmission unit further comprises at least one isolator, and the at least one isolator is located between the seed light source and the FP cavity.

Optionally, the FP cavity comprises a ninth mirror and a tenth mirror parallel to each other, and the seed beam is incident from the ninth mirror and exits from the tenth mirror;

or, the FP cavity includes an eleventh mirror, a tenth mirror, and a thirteenth mirror, and the seed beam is incident from the eleventh mirror, and is reflected by the tenth mirror and the thirteenth mirror in sequence, and then exits from the eleventh mirror.

Optionally, the seed light source comprises a semiconductor laser; or the seed light source comprises a combination of a gain chip and an optical filter, and the optical filter is positioned at any position of the external cavity of the feedback loop.

In the embodiment of the invention, the seed light source and the feedback loop outer cavity are arranged, the short FP cavity with the cavity length less than or equal to two centimeters is arranged in the feedback loop outer cavity, the seed light beam is emitted by using the seed light source, the seed light beam is filtered by using the FP cavity with a high Q value to form a transmission light beam, and then the transmission light beam is fed back to the seed light source by using the feedback loop outer cavity to form a feedback light path, so that the ultra-narrow line width frequency-locked laser is realized. In addition, in this embodiment, three frequency adjustment modules are further provided to perform cooperative modulation on the frequencies of the three optical cavities, so that the frequencies of the three optical cavities meet the external cavity self-injection locking condition, a frequency-locked laser is formed, and a very-large-range high-speed continuous frequency modulation is realized. The embodiment of the invention solves the problem that the frequency modulation range of the existing narrow-linewidth laser is smaller, utilizes the FP short cavity with a high Q value to participate in the frequency locking of the feedback external cavity, simultaneously carries out the frequency cooperative modulation on three optical cavities in the frequency modulation external cavity laser device, can improve the frequency modulation range to dozens to hundreds of GHZ, and constructs the fast large-range continuous frequency modulation external cavity narrow-linewidth laser, so that the frequency modulation external cavity laser device can improve the distance measurement precision in the fields of frequency modulation continuous wave laser radar, optical frequency domain reflectometer and the like, and meanwhile, for the quantum application field, the frequency modulation external cavity laser can meet the requirement of using a plurality of atomic absorption spectrums, thereby expanding the application field of the laser device.

Drawings

Fig. 1 and fig. 2 are schematic diagrams of simplified structures of two frequency-modulated external cavity laser devices provided by an embodiment of the present invention;

FIGS. 3 and 4 are spectral diagrams of the optical cavity of FIGS. 1 and 2, respectively;

fig. 5 is a schematic structural diagram of another frequency-modulated external cavity laser device according to an embodiment of the present invention;

fig. 6 is a schematic structural diagram of another frequency-modulated external cavity laser device according to an embodiment of the present invention;

fig. 7 and fig. 8 are schematic structural diagrams of two further frequency-modulated external cavity laser devices provided by the embodiment of the present invention;

fig. 9 is a schematic structural diagram of another frequency-modulated external cavity laser device according to an embodiment of the present invention;

fig. 10 is a schematic structural diagram of another frequency-modulated external cavity laser device according to an embodiment of the present invention.

Detailed Description

The terminology used in the embodiments of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. It should be noted that the terms "upper", "lower", "left", "right", and the like used in the description of the embodiments of the present invention are used in the angle shown in the drawings, and should not be construed as limiting the embodiments of the present invention. In addition, in this context, it is also to be understood that when an element is referred to as being "on" or "under" another element, it can be directly formed on "or" under "the other element or be indirectly formed on" or "under" the other element through an intermediate element. The terms "first," "second," and the like, are used for descriptive purposes only and not for purposes of limitation, and do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The specific meanings of the above terms in the present invention can be understood by those skilled in the art according to specific situations.

As described in the background section, various external cavity schemes using high-Q optical cavities have been developed in recent years, and ultra-narrow linewidth lasers with a linewidth below kilohertz and kilohertz can be obtained, for example, a high-quality-factor echo wall optical cavity is used as a filtering to construct a composite optical cavity, and laser with a linewidth of hundreds of hertz or even hertz can be realized, but the rapid tuning range of piezoelectric ceramics is only hundreds of megahertz; or the annular cavity on the plate is used, the static thermal frequency modulation range can reach 9GHz, but the piezoelectric ceramic rapid frequency modulation only has modulation frequency of 1GHz @ 12kHz; there are also laser schemes that use fiber optic cavities or high Q FP cavities, however, their fast tuning range is also small.

In view of this, the embodiment of the present invention provides a frequency-modulated external cavity laser device. Fig. 1 and fig. 2 are schematic diagrams of simplified structures of two frequency-modulated external cavity laser devices provided by an embodiment of the present invention, and referring to fig. 1 and fig. 2, the frequency-modulated external cavity laser device includes a seed light source 10 and a feedback loop external cavity 30, the feedback loop external cavity 30 includes an FP cavity 20, and a cavity length of the FP cavity 20 is less than or equal to 10cm. The seed light source 10 is used for outputting a seed light beam; the FP cavity 20 is used for filtering the seed beam to form a transmission beam; the feedback loop external cavity 30 is used for feeding back the transmitted light beam to the seed light source 10 to form a feedback light path.

The frequency modulation external cavity laser device also comprises a light source frequency adjusting module, an external cavity frequency adjusting module and an FP cavity frequency adjusting module (not shown in the figure); the light source frequency adjusting module is used for adjusting the eigenfrequency f1 of the seed light beam; the external cavity frequency adjusting module is used for adjusting the resonant frequency f3 of the external cavity of the feedback loop; the FP cavity frequency adjusting module is used for adjusting the resonant frequency f2 of the FP cavity; the light source frequency adjusting module, the FP cavity frequency adjusting module and the outer cavity frequency adjusting module are cooperatively modulated, so that the eigen frequency f1 of the seed light beam, the resonant frequency f2 of the FP cavity and the resonant frequency f3 of the outer cavity of the feedback loop meet the outer cavity self-injection locking condition, and the frequency-locked laser is formed.

The seed light source 10 is an optical cavity with gain, and may be a semiconductor laser, which can generate a seed beam with a wider line width, or a combination of a gain chip and an optical filter, where the gain chip has higher gain for a specific band (e.g., C band), and the optical filter can select a laser wavelength within a gain spectral line range (tens of nm) of the gain chip. The feedback loop external cavity 30 is used for enabling the seed beam to form external cavity self-injection locking, and the feedback loop external cavity 30 is a low-Q-value optical cavity in essence and can reduce the laser line width to a certain extent. The external feedback loop cavity 30 may be an optical cavity that returns coaxially along the original path, or may be a feedback loop formed by a turning component, such as an annular feedback loop. In the embodiment of the invention, the feedback loop external cavity 30 is provided with the FP cavity 20, the FP cavity 20 is a high-Q-value optical cavity in nature, and the laser after filtering feedback through the FP cavity 20 can form external cavity self-injection locking and can narrow the original line width of the laser at the same time.

It should be noted that, in the present invention, the cavity length of the FP cavity 20 is set to be less than or equal to 10cm, and the essence is that a short FP cavity is adopted, and on the premise of realizing ultra-narrow line width laser by using a high Q value, the FP cavity is used to realize rapid and large-range frequency modulation of frequency. Compared with other solid optical cavities, the short FP cavity can enlarge the modulation range, and particularly can improve the modulation range of several orders of magnitude. For example, when a FP short cavity with a cavity length of 2mm is used, the reflectivity of the cavity mirror reaches 99.9More than 92 percent, and the Q value is more than 10 ⁸ When the cavity length is modulated to be 2 mu m, one thousandth of the FP cavity length can be changed, the frequency of the optical cavity can be changed to be about 200GHz near 1550nm (corresponding to the optical frequency of 200 THz), and compared with high-Q external cavity lasers such as optical fiber lasers, echo wall optical cavities and on-chip micro-ring cavities, the frequency adjusting range of the optical cavity of the embodiment can be improved by 1-2 orders of magnitude. For another example, when the cavity length of the FP cavity is 2cm, by changing the cavity length of 2 μm, that is, by changing one ten-thousandth of the cavity length, a wide-range frequency modulation of 20ghz @1550nm can be realized for frequency adjustment, and the frequency modulation range can be greatly increased as compared with the conventional high-Q external cavity laser.

The embodiment of the invention essentially adopts three optical cavities to form the composite external cavity laser, and the necessary condition for forming the effective external cavity self-injection locking is that the frequencies of the three optical cavities are aligned as much as possible, so that the external cavity laser device of the embodiment can be understood to need to carry out the cooperative modulation on the three optical cavities in the frequency modulation process, and ensure that the frequencies of the three optical cavities always meet the external cavity self-injection locking condition, thereby dynamically forming the frequency-locked laser. Specifically, the embodiment of the invention is provided with the light source frequency adjusting module, the FP cavity frequency adjusting module and the external cavity frequency adjusting module, which are used for correspondingly adjusting the frequencies of the three optical cavities synchronously or in equal proportion, so as to ensure the alignment of the frequencies of the three optical cavities, thereby still meeting the external cavity self-injection locking condition in the dynamic frequency modulation process and forming the frequency-locked laser. FIGS. 3 and 4 are spectral diagrams of the optical cavity of FIGS. 1 and 2, respectively, where the solid vertical black line represents the eigenfrequency f1 of the seed light source, as shown in FIGS. 3 and 4; a narrower periodic dotted line represents the transmission spectrum of the FP cavity, and the transmission peak frequency close to the eigenfrequency f1 of the seed light source is the resonant frequency f2 of the FP cavity; the wider lorentz-shaped solid line represents the transmission spectrum of the external cavity of the feedback loop, and the transmission peak frequency close to the eigenfrequency f1 of the seed light source is the resonant frequency f3 of the external cavity of the feedback loop. In order to ensure that the three optical cavities form effective self-injection locking, the eigenfrequency f1 of the seed light source, the transmission peak frequency f2 of the FP cavity and the transmission peak frequency f3 of the external cavity of the feedback loop need to be kept aligned on the frequency spectrum as much as possible. When the frequency modulation external cavity laser device performs frequency modulation, for example, when the resonant frequency of the FP cavity changes, that is, the narrow periodic dotted line moves left and right, in order to maintain effective self-injection locking, the transmission peak frequency f3 of the feedback loop external cavity and the eigen frequency f1 of the seed light source both have to keep moving synchronously with the transmission peak frequency f2 of the FP cavity, that is, it is necessary to ensure the cooperative modulation of the three optical cavities, thereby avoiding the loss of self-injection locking and the occurrence of mode hopping.

In addition, it should be noted that the three frequency adjustment modules may specifically adopt the same control device to perform frequency modulation control, so as to implement cooperative modulation of the three optical cavities, and those skilled in the art may design the frequency adjustment modules according to actual situations, which is not limited herein. In addition, the three frequency adjustment modules may be additionally provided frequency adjustment structures, for example, for the FP cavity, the corresponding frequency adjustment module may be a piezoelectric ceramic PZT, and the piezoelectric ceramic PZT is used to adjust the resonant frequency in a manner of changing the cavity length of the FP cavity, or may be a frequency adjustment component possessed by the seed light source 10, the feedback loop external cavity 30 and the FP cavity 20, taking the seed light source 10 as a DFB distributed feedback laser as an example, the seed light source itself is provided with a frequency adjustment component bragg grating, and the frequency of the light beam emitted by the distributed feedback laser may be adjusted by using the bragg grating by changing the current injected by the DFB.

The embodiment of the invention provides a detailed scheme aiming at the specific conditions of the frequency cooperative modulation of the three optical cavities. With continued reference to fig. 1-4, further optional external cavity self-injection locking conditions include: the difference between the eigenfrequency f1 of the seed beam and the resonance frequency f2 of the FP cavity is smaller than the external cavity self-injection locking range of the external cavity of the feedback loop; the difference between the resonance frequency f2 of the FP cavity and the resonance frequency f3 of the feedback loop external cavity is less than or equal to one-half of the free spectral range of the feedback loop external cavity.

Wherein, for the resonance frequency f2 of the FP cavity and the resonance frequency f3 of the feedback loop external cavity, it can be seen from the spectrum that the difference should not exceed half of the free spectral range FSR3 of the feedback loop external cavity. At this time, the resonant frequency f2 is located in the free spectral range FSR3 of the external cavity of the feedback loop, indicating that the resonant frequency f2 of the FP cavity and the resonant frequency f3 of the external cavity of the feedback loop can be substantially aligned. For the eigenfrequency f1 of the seed beam and the resonant frequency f2 of the FP cavity, taking a conventional distributed feedback laser chip as an example, the external cavity self-injection locking range is several hundred mhz to several ghz under appropriate feedback conditions, and in order to ensure self-injection locking, the difference between the eigenfrequency f1 of the seed beam and the resonant frequency f2 of the FP cavity should be smaller than the external cavity self-injection locking range. At this time, the eigenfrequency f1 of the seed beam is spectrally located close to the resonance frequency f2. Based on the fact that the frequencies f1, f2 and f3 can be close to each other or in a mutually aligned range on the frequency spectrum in the dynamic frequency modulation process, the external cavity self-injection locking condition can be met, and frequency-locked laser can be formed.

On the basis, the embodiment of the invention also provides a plurality of modes for adjusting the frequency of the optical cavity, and particularly optionally, the FP cavity frequency adjusting module and the external cavity frequency adjusting module are electric control displacement modules; the electric control displacement module is respectively assembled on at least one optical component of the feedback loop external cavity and the FP cavity; the electronic control displacement module is used for changing the cavity length of the feedback loop external cavity or the FP cavity, or the electronic control displacement module is used for changing the optical path of the light beam in the optical component so as to adjust the resonant frequency of the feedback loop external cavity or the FP cavity.

In addition, the FP cavity frequency adjusting module and the outer cavity frequency adjusting module can be selected to be an electric control refractive index module or a thermal control refractive index module respectively; the electric control refractive index module or the thermal control refractive index module is respectively positioned in the feedback loop outer cavity or the FP cavity; the electric control refractive index module is used for changing the refractive index through an electro-optic effect, and the thermal control refractive index module is used for changing the refractive index through a thermo-optic effect so as to adjust the optical path of the light beam in the electric control refractive index module or the thermal control refractive index module, thereby adjusting the resonant frequency of the feedback loop external cavity or the FP cavity.

First, a detailed example of the scheme for adjusting the frequency of the optical cavity by the electrically controlled displacement module is described. Fig. 5 is a schematic structural diagram of another frequency-modulated external cavity laser device according to an embodiment of the present invention, and referring to fig. 5, the feedback loop external cavity 30 and the FP cavity 20 respectively include at least one reflection unit, and the electrically controlled displacement module 40 is mounted on the reflection unit; the electronic control displacement module 40 is configured to change the cavity length of the feedback loop outer cavity 30 or the FP cavity 20 according to the formula Δ f/f = Δ L/L, so as to adjust the resonant frequency of the feedback loop outer cavity 30 or the FP cavity 20; wherein f is the current resonant frequency of the feedback loop external cavity 30 or the FP cavity 20, Δ f is the variation of the resonant frequency of the feedback loop external cavity 30 or the FP cavity 20, L is the current cavity length of the feedback loop external cavity 30 or the FP cavity 20, and Δ L is the cavity length variation of the feedback loop external cavity 30 or the FP cavity 20.

It should be noted that the scheme of adjusting the optical cavity frequency by the electronic control displacement module can be applied to different composite external cavity laser structures, taking the composite external cavity laser structure shown in fig. 5 as an example, in this embodiment, the seed light source 10 includes a first end 1 and a second end 2; the seed light beam is output from the first end 1 of the seed light source 10, and the transmission light beam is input from the first end 1 of the seed light source 10; the feedback loop external cavity 30 further comprises a first collimating unit 31, a one-way transmission unit 32 and a reflection unit 33; the first collimating unit 31 is used for collimating the seed beam; the one-way transmission unit 32 is used for transmitting the seed light beam to the FP cavity 20 and blocking the reflected light beam of the FP cavity 20 from being incident to the seed light source 10; the reflection unit 33 is used to reflect the transmitted beam of the FP cavity 20 back to the seed light source 10 to form a feedback light path.

More specifically, the first collimating unit 31 may be arranged to include a first lens 311, the unidirectional transmission unit 32 may include a polarization beam splitter 321, a first quarter-wave plate 322, a second quarter-wave plate 323, and a second lens 324 in this order, and the reflection unit 33 may include a first mirror 331; the FP cavity 20 is located between the first quarter wave plate 322 and the second quarter wave plate 323, and the second lens 324 is located between the FP cavity 20 and the second quarter wave plate 323.

The FP cavity may be a hollow FP cavity or a solid FP cavity. The hollow FP cavity can be a parallel cavity, a plano-concave cavity, or a concave-concave cavity. Specifically, the FP cavity 20 may be configured to include a ninth mirror 21 and a tenth mirror 22 parallel to each other, and the seed light beam is incident from the ninth mirror 21 and exits from the tenth mirror 22.

In the frequency modulation external cavity laser device, a feedback light path specifically comprises: the seed light source 10 emits a seed light beam, which is collimated by the first lens 311 and then passes through the polarization beam splitter 321, and the polarization beam splitter 321 transmits the P-parallel polarization component of the seed light beam and reflects the S-vertical polarization component. The P-parallel polarization component is converted into circular polarization light by the first quarter-wave plate 322, coupled out by the FP cavity and collimated by the second lens 324, and then converted into first line polarization light by the second quarter-wave plate 323, wherein the first line polarization light is perpendicular to the polarization direction of the P-parallel polarization component; the first linearly polarized light is reflected by the first reflector 331, then transformed into circularly polarized light by the second quarter wave plate 323, passes through the FP cavity, and then transformed into a second linearly polarized light by the first quarter wave plate 322, where the polarization direction of the second linearly polarized light is the same as that of the P-parallel polarized component, and at this time, the second linearly polarized light may pass through the polarization beam splitter 321, and finally, may be fed back to the seed light source 10 through the first lens 311.

It should be noted that, in the embodiment, the polarization beam splitter 321, the first quarter-wave plate 322, the second quarter-wave plate 323, and the second lens 324 substantially constitute the unidirectional transmission unit 32, which can be used to block the light beam reflected by the FP cavity, and ensure that the light beam in the external cavity of the feedback loop returns coaxially. Specifically, it can be understood that the circularly polarized light transformed by the first quarter-wave plate 322 forms a reflected light toward the seed light source 10 on the reflective structure in the FP cavity 20 when passing through the FP cavity 20, and the reflected light directly entering the seed light source 10 affects the self-injection locking of the entire external cavity. In the structure of this embodiment, by arranging the first quarter-wave plate 322 and the polarization beam splitter 321, the reflected light forms third-line polarized light after passing through the first quarter-wave plate 322, and the third-line polarized light is perpendicular to the polarization direction of the P-parallel polarization component, and at this time, the third-line polarized light is reflected when passing through the polarization beam splitter 321 and cannot be transmitted through the polarization beam splitter 321, so that the reflected light formed by the FP cavity can be effectively prevented from being fed back to the seed light source.

In this embodiment, the electrically controlled displacement module 40 corresponding to the external cavity 30 of the feedback loop is a first electrically controlled displacement module 41, which can be mounted on the back surface of the reflection unit 33, i.e. the first reflecting mirror 331, as shown in the figure, but also can be mounted on the side surface or the front surface of the first reflecting mirror 331. The electrically controlled displacement module 40 may be a piezoelectric ceramic PZT or a voice coil motor. It will be understood by those skilled in the art that under the control of the electrical signal, the piezo-ceramic PZT or the voice coil motor can precisely move the position of the reflection unit 33, so as to adjust the cavity length of the external cavity 30 of the feedback loop, in other words, the resonant frequency f3 of the external cavity 30 of the feedback loop can be adjusted by changing the cavity length through the first electrically controlled displacement module 41. Similarly, for the FP cavity 20, the corresponding electrically controlled displacement module 40 is a second electrically controlled displacement module 42, which may specifically be a piezoelectric ceramic PZT or a voice coil motor. As shown, the second electrically controlled displacement module 42 can be disposed on the back of the tenth mirror 22, or can be mounted on the side or front of the tenth mirror 22, and the resonant frequency f2 of the FP cavity 20 can be adjusted by precisely moving the position of the tenth mirror 22 through the piezoelectric ceramics PZT or the voice coil motor.

In this embodiment, the electrically controlled displacement module 40 is mainly responsible for moving the position of a component of the optical cavity, so as to adjust the cavity length of the optical cavity and change the frequency of the optical cavity by using the cavity length. Therefore, when adjusting the cavity length of the optical cavity, it is necessary to ensure the frequency cooperative modulation of the three optical cavities, and specifically, the cavity length of the feedback loop external cavity 30 or the FP cavity 20 may be changed according to that the ratio of the cavity length variation to the current cavity length is equal to the ratio of the frequency variation to the current frequency, so as to guide the adjustment of the resonant frequency of the feedback loop external cavity 30 or the FP cavity 20.

Fig. 6 is a schematic structural diagram of another frequency-modulated external cavity laser device provided by an embodiment of the present invention, and comparing fig. 5 and fig. 6, the second lens 324 in the optional unidirectional transmission unit 32 is located between the second quarter-wave plate 323 and the first mirror 331. The frequency-modulated external cavity laser device shown in fig. 6 is substantially a modification of fig. 5, and in the structure of the laser device shown in fig. 6, the transmission light of the FP cavity 20 is converged onto the first reflecting mirror 331 through the second lens 324, so that the accuracy and difficulty of the angular alignment of the first reflecting mirror 331 can be reduced.

The embodiment of the invention also provides other composite external cavity laser structures aiming at the scheme that the electronic control displacement module adjusts the frequency of the optical cavity. Fig. 7 and 8 are schematic structural diagrams of two further frequency-modulated external cavity laser devices provided by an embodiment of the present invention, and referring to fig. 7 and 8, in the two embodiments, the seed light source 10 includes a first end 1 and a second end 2; the seed light beam is output from the first end 1 of the seed light source 10, and the transmission light beam is input from the first end 1 of the seed light source 10; alternatively, the seed light beam is output from the first end 1 of the seed light source 10, and the transmitted light beam is input from the second end 2 of the seed light source 10; the feedback loop outer cavity 30 further comprises a one-way transmission unit 32 and a light steering unit 35; the one-way transmission unit 32 is used for transmitting the seed light beam to the FP cavity 20 and blocking the reflected light beam of the FP cavity 20 from being incident to the seed light source 10; the light steering unit 35 is used for changing the transmission direction of the transmitted light beam of the FP cavity 20 so that the transmitted light beam is fed back to the seed light source 10 to form a feedback light path.

Specifically, referring to fig. 7, in this embodiment, the seed light beam is output from the first end 1 of the seed light source 10, and the transmitted light beam is input from the first end 1 of the seed light source 10. Unidirectional transmission unit 32 includes circulator 325, and light-diverting unit 35 includes second mirror 351, third mirror 352, and fourth mirror 353. The light beam transmission path in the frequency modulation external cavity laser device is as follows: the seed light beam is output from the first end 1 of the seed light source 10, input from the first end 1 of the circulator 325, output from the second end 2 of the circulator 325, and incident to the FP cavity 20 for transmission to form a transmission light beam; the transmitted light beam is reflected by the second reflector 351, the third reflector 352 and the fourth reflector 353 in sequence, then enters the third end 3 of the circulator 325, and is output from the first end 1 of the circulator 325 and fed back to the seed light source 10.

Referring to fig. 8, in this embodiment, the seed light beam is output from the first end 1 of the seed light source 10 and the transmitted light beam is input from the second end 2 of the seed light source 10. The unidirectional transmission unit 32 includes an isolator 326, and the light ray diverting unit 35 includes a fifth mirror 354, a sixth mirror 355, a seventh mirror 356, and an eighth mirror 357. The light beam transmission path in the frequency modulation external cavity laser device is as follows: the seed beam is output from the first end 1 of the seed light source 10, input from the first end 1 of the isolator 326, output from the second end 2 of the isolator, and incident to the FP cavity for transmission to form a transmitted beam; the transmitted beam is reflected by a fifth mirror 354, a sixth mirror 355, a seventh mirror 356 and an eighth mirror 357 in sequence and then enters the second end 2 of the seed light source 10.

In both embodiments shown in fig. 7 and 8, the first electronically controlled displacement module 41 corresponding to the external cavity 30 of the feedback loop can be mounted on the back of any one of the mirrors in the light redirecting unit 35, and by adjusting the position of the mirror, the cavity length of the external cavity 30 of the feedback loop can be changed, thereby adjusting the frequency of the optical cavity. For the FP cavity 20, the second electronically controlled displacement module 42 may be disposed on the back of the tenth mirror 22, and the resonant frequency f2 of the FP cavity 20 may be adjusted by precisely moving the position of the tenth mirror 22 through piezoelectric ceramic PZT or a voice coil motor.

In addition, for the composite external cavity laser structure provided in the four embodiments of fig. 5-8, a person skilled in the art may optionally add at least one isolator in the unidirectional transmission unit, where the isolator is located between the seed light source and the FP cavity. At the moment, the isolator can further obstruct the reflected light of the FP cavity, and the influence of the direct reflected light of the FP cavity on the frequency locking of the whole outer cavity is effectively reduced.

Fig. 9 is a schematic structural diagram of another frequency-modulated external cavity laser device according to an embodiment of the present invention, and referring to fig. 9, the embodiment provides another implementation manner for the structure of the FP cavity. As shown in fig. 9, the FP cavity 20 may include an eleventh mirror 23, a tenth mirror 24, and a thirteenth mirror 25, wherein the seed beam is incident from the eleventh mirror 23, and is sequentially reflected by the tenth mirror 24 and the thirteenth mirror 25, and then exits from the eleventh mirror 23.

In this embodiment, the FP cavity 20 is substantially a ring FP cavity composed of three-sided high-reflection mirrors, and the cavity length modulation of the ring FP cavity is realized by piezoelectric ceramic PZT on a one-sided mirror. The seed beam enters the ring-shaped FP cavity 20 through reflective coupling of a reflector arranged in the outer cavity of the feedback loop, and after passing through the eleventh reflector 23, the seed beam is reflected by the tenth reflector 24, the eleventh reflector 23, the thirteenth reflector 25, the eleventh reflector 23 and the tenth reflector 24 in sequence, and then is emitted by the eleventh reflector 23 to form a transmitted beam, and the transmitted beam returns to the seed light source 10 along the original path. The cavity length of the external cavity of the feedback loop is realized by arranging piezoelectric ceramic PZT on the reflector, and the details are not repeated here.

In the frequency-modulated external cavity laser device according to the above embodiments, besides the deformation of the feedback loop external cavity and the FP cavity, the seed light source may also adopt different implementation manners. Taking the embodiment shown in fig. 5-9 as an example, the optional seed light source 10 is a semiconductor laser 11; alternatively, the seed light source 10 may be configured to include a combination of the gain chip 12 and the filter 13, and the filter 13 may be disposed at any position of the external cavity 30 of the feedback loop.

In addition, as can be understood by those skilled in the art, the frequency-locked laser of each frequency-modulated external cavity laser device has different light emitting modes, as shown in fig. 5 and 6, the frequency-locked laser may be reflected upward by the polarization beam splitter 321 to exit, as shown in the laser devices in fig. 9 and 10, and the frequency-locked laser may be output by the ring-shaped FP cavity 20. In the laser device shown in fig. 7, a beam splitter may be optionally disposed on the light path between the third mirror 352 and the fourth mirror 353, and the frequency-locked laser light is output by the beam splitter. Similarly, for the laser device shown in fig. 8, a beam splitter may be optionally disposed on the optical path between the sixth mirror 355 and the seventh mirror 356, and the frequency-locked laser light is output by using the beam splitter.

In other embodiments of the present invention, the electrically controlled displacement module may be used to implement frequency adjustment of the optical cavity based on different principles. Fig. 10 is a schematic structural diagram of another frequency-modulated external cavity laser device provided in an embodiment of the present invention, and referring to fig. 10, in this embodiment, the feedback loop external cavity 30 and/or the FP cavity 20 respectively include at least one prism unit 50, and the electronically controlled displacement module 40 is assembled on the prism unit 50; the electronically controlled displacement module 40 is configured to change the optical path length of the light beam in the optical assembly according to the formula Δ f/f = n1 × Δ L/(n 2 × L) to adjust the resonant frequency of the feedback loop external cavity 30 or the FP cavity 20. At this time, f is the current resonant frequency of the feedback loop external cavity or the FP cavity, Δ f is the variation of the resonant frequency of the feedback loop external cavity or the FP cavity, n2 × L is the total optical path of the feedback loop external cavity or the FP cavity, and n1 × Δ L is the variation of the optical path of the feedback loop external cavity or the FP cavity.

The prism unit 50 is disposed in the feedback loop external cavity 30 and/or the FP cavity 20, so that the seed beam has a part of optical path in the prism unit 50, and since the prism unit 50 has a certain refractive index n1, the position of the prism unit 50 can be adjusted by the electrically controlled displacement module 40, such as a piezoelectric ceramic PZT or a voice coil motor, or the like, conversely, the relative position of the light beam in the prism unit 50 can be adjusted, so that the optical path of the light beam in the prism unit 50 can be changed. Here, the optical path length in the prism unit 50 may be denoted by n1 × L, and the optical path length L and the optical path length n1 × L of the light in the prism unit 50 may be changed when the prism unit 50 moves in a direction perpendicular to the light beam. In this embodiment, the cavity length of the feedback loop external cavity 30 or the FP cavity 20 can be changed to guide the adjustment of the resonant frequency of the feedback loop external cavity 30 or the FP cavity 20 according to the fact that the ratio of the optical path variation to the current optical path is equal to the ratio of the frequency variation to the current frequency. The electrically controlled displacement module 40 may be installed at the side of the prism unit 50 as shown in the figure, and may be set at any other position according to the actual design by those skilled in the art, which is not limited herein.

It should be noted that the structure of the FP cavity and the feedback loop outer cavity shown in fig. 10 is only one implementation manner, and those skilled in the art may refer to the composite outer cavity structures shown in fig. 5 to fig. 9 to reasonably deform the FP cavity and the feedback loop outer cavity, and details are not repeated here. It should be noted that, since the optical path of the optical cavity has a substantially linear relationship with the phase (θ =2 π L/λ), the optical path is used as an adjusting factor in the above embodiment as a mere expression, and those skilled in the art can understand that the phase of the optical cavity can be adjusted by the prism unit and the electrically controlled displacement module. Therefore, those skilled in the art can design the frequency of the optical cavity to be adjusted by changing the phase based on the above embodiments, which is substantially the same as the frequency of the optical cavity to be adjusted by changing the optical length in the embodiments of the present invention, and therefore, the invention does not depart from the scope of the present invention.

Based on the above-mentioned solutions of the prism unit and the electrically controlled displacement module, a person skilled in the art can understand that the optical path change of the optical cavity depends not only on the stroke L of the light beam in the optical component, but also on the refractive index n of the optical component. Based on this, in the embodiment of the present invention, an electro-optical effect or a thermo-optical effect may be utilized, and the corresponding electrically controlled refractive index module and the thermally controlled refractive index module are respectively disposed in the feedback loop external cavity or the FP cavity, and the electrically controlled refractive index module or the thermally controlled refractive index module is utilized to change the optical path by changing the refractive index, so as to adjust the resonant frequency of the feedback loop external cavity and the FP cavity. It is understood that the composite external cavity structure provided in the above embodiments can also be applied to the scheme of adjusting the frequency of the optical cavity by using the electrically controlled refractive index module or the thermally controlled refractive index module, and those skilled in the art can design the structure according to practical situations, and the structure is not illustrated here.

It is to be noted that the foregoing is only illustrative of the preferred embodiments of the present invention and the technical principles employed. It will be understood by those skilled in the art that the present invention is not limited to the particular embodiments described herein, but is capable of various obvious modifications, rearrangements, combinations and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, although the present invention has been described in some detail by the above embodiments, the invention is not limited to the above embodiments, and may include other equivalent embodiments without departing from the spirit of the invention, and the scope of the invention is determined by the scope of the appended claims.

Claims

1. A frequency modulation external cavity laser device is characterized by comprising a seed light source and a feedback loop external cavity, wherein the feedback loop external cavity comprises an FP (Fabry-Perot) cavity, and the cavity length of the FP cavity is less than or equal to 10cm;

the seed light source is used for outputting a seed light beam;

the FP cavity is used for filtering the seed beam to form a transmission beam;

the light source frequency adjusting module, the FP cavity frequency adjusting module and the external cavity frequency adjusting module are cooperatively modulated, so that the eigenfrequency f1 of the seed light beam, the resonant frequency f2 of the FP cavity and the resonant frequency f3 of the external cavity of the feedback loop meet the external cavity self-injection locking condition, and frequency-locked laser is formed.

2. A frequency-modulated external cavity laser device according to claim 1, wherein said external cavity self-injection locking condition comprises:

3. A frequency-modulated external cavity laser device according to claim 1,

the FP cavity frequency adjusting module and the outer cavity frequency adjusting module are electric control displacement modules; the electric control displacement module is respectively assembled on at least one optical component of the feedback loop external cavity and the FP cavity;

4. A frequency-modulated external cavity laser device according to claim 3,

the feedback loop outer cavity and the FP cavity respectively comprise at least one reflecting unit, and the electronic control displacement module is assembled on the reflecting unit;

5. A frequency-modulated external cavity laser device according to claim 3,

the feedback loop outer cavity and the FP cavity respectively comprise at least one prism unit, and the electronic control displacement module is assembled on the prism unit;

6. A frequency-modulated external cavity laser device according to claim 1,

the FP cavity frequency adjusting module and the outer cavity frequency adjusting module are respectively an electric control refractive index module or a thermal control refractive index module; the electric control refractive index module or the thermal control refractive index module is respectively positioned in the feedback loop outer cavity or the FP cavity;

the electric control refractive index module is used for changing the refractive index through an electro-optic effect, and the thermal control refractive index module is used for changing the refractive index through a thermo-optic effect so as to adjust the optical path of the light beam in the electric control refractive index module or the thermal control refractive index module, and thus the resonant frequency of the feedback loop external cavity or the FP cavity is adjusted.

7. A frequency-modulated external cavity laser device according to claim 1,

the seed light source comprises a first end and a second end; the seed light beam is output from a first end of the seed light source, and the transmitted light beam is input from the first end of the seed light source;

the first collimation unit is used for collimating the seed light beam;

8. A frequency-modulated external cavity laser device according to claim 7,

the first collimation unit comprises a first lens, the unidirectional transmission unit sequentially comprises a polarization beam splitter, a first quarter-wave plate, a second quarter-wave plate and a second lens, and the reflection unit comprises a first reflector;

9. A frequency-modulated external cavity laser device according to claim 1,

the seed light source comprises a first end and a second end; the seed light beam is output from a first end of the seed light source, and the transmitted light beam is input from the first end of the seed light source; alternatively, the seed light beam is output from a first end of the seed light source and the transmitted light beam is input from a second end of the seed light source;

10. A frequency-modulated external cavity laser device according to claim 9,

the one-way transmission unit comprises a circulator, and the light steering unit comprises a second reflector, a third reflector and a fourth reflector;

11. A frequency-modulated external cavity laser device according to claim 9,

the one-way transmission unit comprises an isolator, and the light steering unit comprises a fifth reflector, a sixth reflector, a seventh reflector and an eighth reflector;

12. A frequency-modulated external cavity laser device according to any one of claims 7 to 11,

the unidirectional transmission unit further comprises at least one isolator, and the at least one isolator is located between the seed light source and the FP cavity.

13. A frequency-modulated external cavity laser device according to claim 1,

the FP cavity comprises a ninth reflector and a tenth reflector which are parallel to each other, and the seed light beam is incident from the ninth reflector and emergent from the tenth reflector;

14. A frequency-modulated external cavity laser device according to claim 1,

the seed light source comprises a semiconductor laser; or the seed light source comprises a combination of a gain chip and an optical filter, and the optical filter is positioned at any position of the external cavity of the feedback loop.