An E-band Fourier domain mode locked fiber laser

Abstract

The application discloses an E-band Fourier domain mode-locked fiber laser, comprising a sweep filter, a first optical amplifier, a first optical isolator, a first optical splitter, a first polarization controller and a fiber delay line; each device is connected to form a ring laser cavity; the sweep filter is used for selectively filtering the wavelength of the optical signal in the laser cavity; the first optical amplifier is used for amplifying the optical signal passing through the sweep filter; the first optical isolator is used for controlling the amplified optical signal to transmit in a single direction; the first optical splitter is used for outputting part of the amplified optical signal to outside the laser cavity, and the rest part is re-injected into the laser cavity; the first polarization controller is used for controlling the polarization state of the optical signal; and the fiber delay line is used for making the wavelength dispersion in the E-band to be near-zero dispersion, and prolonging the cavity length of the laser cavity, so that the cavity length of the laser cavity matches the filtering period of the sweep filter. The application realizes the E-band Fourier domain mode-locked fiber laser.

Description

Technical Field

[0001] This invention relates to the field of Fourier domain mode-locked fiber laser technology, and more particularly to an E-band Fourier domain mode-locked fiber laser. Background Technology

[0002] The E-band refers to the wavelength range of laser light between 1360-1460 nm. Due to the low level of early optical fiber manufacturing technology, optical fibers in this band have high losses. This loss originates from the vibrational absorption of OH ions in water. The specific amount of loss is related to the water molecule content in the optical fiber. Therefore, this absorption peak is called the "water peak".

[0003] Currently realized Fourier Domain Mode Locking (FDML) fiber lasers are limited to the C-band (1530-1565nm), O-band (1260-1360nm), L-band (1565-1625nm), and 1060nm bands. Because the E-band is located near the water peak, there are no corresponding dispersion management devices available for realizing E-band FDML fiber lasers, thus limiting the development of E-band FDML fiber lasers.

[0004] Therefore, existing technologies still need to be improved and developed. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the purpose of this invention is to provide an E-band Fourier domain mode-locked fiber laser to solve the problem that there is no corresponding wavelength dispersion management device for realizing an E-band FDML fiber laser because the E-band is located near the water peak.

[0006] The technical solution of the present invention is as follows:

[0007] An E-band Fourier domain mode-locked fiber laser includes: a swept-frequency filter, a first optical amplifier, a first optical isolator, a first beam splitter, a first polarization controller, and an optical fiber delay line.

[0008] The swept-frequency filter, the first optical amplifier, the first optical isolator, the first beam splitter, the first polarization controller, and the fiber delay line are interconnected to form a ring laser cavity; wherein...

[0009] The sweep frequency filter is used to periodically select and filter the wavelength of the optical signal in the laser cavity;

[0010] The first optical amplifier is used to amplify the optical signal that has passed through the swept frequency filter;

[0011] The first optical isolator is disposed on one side of the first optical amplifier and is used to control the optical signal amplified by the first optical amplifier to be transmitted in one direction.

[0012] The first beam splitter is used to output part of the amplified optical signal outside the laser cavity, while the rest is re-injected into the laser cavity;

[0013] The first polarization controller is used to control the polarization state of the optical signal within the laser cavity;

[0014] The fiber delay line is used to make the wavelength dispersion in the E-band near zero while extending the cavity length of the laser cavity, so that the cavity length of the laser cavity matches the filtering period of the sweep filter; wherein, the equivalent zero dispersion point of the fiber delay line is located in the E-band.

[0015] In a further embodiment of the present invention, the optical fiber delay line comprises: a non-zero dispersion-shifted optical fiber and a single-mode optical fiber; wherein,

[0016] One end of the non-zero dispersion-shifted fiber is connected to the single-mode fiber, and the other end of the non-zero dispersion-shifted fiber is connected to the sweep frequency filter.

[0017] The other end of the single-mode optical fiber is connected to the first polarization controller;

[0018] The total length of the non-zero dispersion-shifted fiber and the single-mode fiber is controlled according to the frequency of the swept-frequency filter; the length ratio of the non-zero dispersion-shifted fiber to the single-mode fiber is controlled according to the near-zero dispersion point.

[0019] In a further embodiment of the present invention, one end of the first optical isolator is connected to the first polarization controller, and the other end of the first optical isolator is connected to one end of the first optical amplifier;

[0020] The other end of the first optical amplifier is connected to the first beam splitter;

[0021] The first beam splitter is also connected to the sweep frequency filter.

[0022] In a further embodiment of the present invention, the E-band Fourier domain mode-locked fiber laser further includes: a second polarization controller; the second polarization controller is disposed between the first optical amplifier and the first beam splitter.

[0023] In a further embodiment of the present invention, the fiber delay line comprises: a non-zero dispersion-shifted fiber and a single-mode fiber; the E-band Fourier domain mode-locked fiber laser further comprises: a fiber circulator and a Faraday rotator; wherein,

[0024] The non-zero dispersion-shifted fiber is connected to the fiber circulator, and the other end of the non-zero dispersion-shifted fiber is connected to one end of the single-mode fiber.

[0025] The fiber optic circulator is also connected to the first polarization controller;

[0026] The other end of the single-mode optical fiber is connected to the Faraday rotator mirror.

[0027] The total length of the non-zero dispersion-shifted fiber and the single-mode fiber is determined by the frequency of the swept-frequency filter; the length ratio of the non-zero dispersion-shifted fiber to the single-mode fiber is determined by the near-zero dispersion point.

[0028] In a further embodiment of the present invention, one end of the first optical amplifier is connected to the sweep frequency filter, and the other end of the first optical amplifier is connected to one end of the first optical isolator.

[0029] The other end of the first optical isolator is connected to the first polarization controller;

[0030] The first optical splitter is connected between the optical fiber circulator and the swept frequency filter.

[0031] In a further embodiment of the present invention, the E-band Fourier domain mode-locked fiber laser further includes: a second polarization controller; the second polarization controller is disposed between the first optical amplifier and the first beam splitter.

[0032] In a further embodiment of the present invention, the E-band Fourier domain mode-locked fiber laser further includes: a second optical amplifier, a second optical isolator, a second beam splitter, and a third beam splitter; wherein,

[0033] The second optical amplifier and the second optical isolator are connected in series, and then connected in parallel with the first optical amplifier and the first optical isolator;

[0034] The second beam splitter is connected to the first polarization controller, the first optical isolator, and the second optical isolator, respectively.

[0035] The third beam splitter is connected to the first optical amplifier, the second optical amplifier, and the first beam splitter, respectively.

[0036] In a further embodiment of the present invention, the first optical amplifier is an E-band optical amplifier; the second optical amplifier is an O-band or C-band amplifier.

[0037] In a further embodiment of the present invention, the E-band Fourier domain mode-locked fiber laser further includes: an arbitrary waveform generator, which is connected to the sweep frequency filter. The arbitrary waveform generator is used to generate a periodic signal to control the sweep frequency filter to periodically selectively filter the wavelength of the optical signal in the laser cavity.

[0038] This invention provides an E-band Fourier domain mode-locked fiber laser, comprising: a swept-frequency filter, a first optical amplifier, a first optical isolator, a first beam splitter, a first polarization controller, and a fiber delay line; the swept-frequency filter, the first optical amplifier, the first optical isolator, the first beam splitter, the first polarization controller, and the fiber delay line are interconnected to form a ring laser cavity; wherein, the swept-frequency filter is used to periodically selectively filter the wavelength of the optical signal within the laser cavity; the first optical amplifier is used to amplify the optical signal after passing through the swept-frequency filter; the first optical isolator is disposed on one side of the first optical amplifier and is used to control the amplified optical signal to be transmitted in a single direction; the first beam splitter is used to output part of the amplified optical signal outside the laser cavity, and the remaining part is re-injected into the laser cavity; the first polarization controller is used to control the polarization state of the optical signal within the laser cavity; the fiber delay line is used to make the wavelength dispersion in the E-band near zero dispersion, and at the same time extend the cavity length of the laser cavity so that the cavity length of the laser cavity matches the filtering period of the swept-frequency filter; wherein, the equivalent zero-dispersion point of the fiber delay line is located within the E-band. This invention achieves near-zero wavelength dispersion in the E-band by using an optical fiber delay line. This ensures that the laser wavelength matches the transmission window of the sweep filter each time the laser passes through it. After multiple transmissions, the time domain position of each wavelength will not deviate due to dispersion, thus realizing a Fourier domain mode-locked fiber laser in the E-band. Attached Figure Description

[0039] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0040] Figure 1 This is a schematic diagram of the structure of the E-band Fourier domain mode-locked fiber laser in this invention. Figure 1 .

[0041] Figure 2 This is a schematic diagram illustrating the principle of the equivalent zero-dispersion point transfer technology for hybrid optical fibers achieved by combining non-zero dispersion-shifted optical fibers and single-mode optical fibers in this invention.

[0042] Figure 3 This is a schematic diagram of the structure of the E-band Fourier domain mode-locked fiber laser in this invention. Figure 2 .

[0043] Figure 4 This is a schematic diagram of the structure of the E-band Fourier domain mode-locked fiber laser in this invention. Figure 3 .

[0044] The labels in the attached diagram are as follows: 1. Sweep filter; 2. First optical amplifier; 3. First optical isolator; 4. First beam splitter; 5. First polarization controller; 6. Fiber delay line; 61. Non-zero dispersion shifted fiber; 62. Single-mode fiber; 7. Second polarization controller; 8. Fiber circulator; 9. Faraday rotator; 10. Second optical amplifier; 11. Second optical isolator; 12. Second beam splitter; 13. Third beam splitter; 14. Arbitrary waveform generator. Detailed Implementation

[0045] This invention provides an E-band Fourier domain mode-locked fiber laser. To make the objectives, technical solutions, and effects of this invention clearer and more explicit, the invention is further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0046] In the implementation methods and claims, unless otherwise specified in the text, the terms "a," "an," "the," and "the" may also include plural forms. If the embodiments of the present invention involve descriptions of "first," "second," etc., such descriptions are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined with "first" or "second" may explicitly or implicitly include at least one of those features.

[0047] It should be further understood that the term "comprising" as used in this specification means the presence of the stated features, integers, steps, operations, elements, and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. It should be understood that when we say an element is "connected" or "coupled" to another element, it can be directly connected or coupled to the other element, or there may be intermediate elements. Furthermore, "connected" or "coupled" as used herein can include wireless connections or wireless coupling. The term "and / or" as used herein includes all or any unit and all combinations of one or more associated listed items.

[0048] It will be understood by those skilled in the art that, unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. It should also be understood that terms such as those defined in general dictionaries should be understood to have the same meaning as in the context of the prior art, and should not be interpreted in an idealized or overly formal sense unless specifically defined as herein.

[0049] Furthermore, the technical solutions of the various embodiments can be combined with each other, but only if they are feasible for those skilled in the art. If the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by this invention.

[0050] Fourier domain mode-locked fiber lasers (FDMLs) represent a completely new operating state for lasers. Unlike traditional amplitude modulation or phase modulation mode-locking mechanisms, FDMLs employ frequency domain modulation. FDML fiber lasers output a continuously scanning laser frequency, with all scanned frequency information stored in the laser resonant cavity (typically a ring cavity) in laser form. FDMLs are also a type of wavelength-scanning laser (SLS), achieving high-speed wavelength scanning within a specific wavelength range and instantaneously outputting lasers with narrow linewidths.

[0051] FDML technology possesses the characteristics required for OCT (Optical Coherence Tomography) light sources. FDML lasers are widely used in OCT imaging of biological tissues to improve their imaging performance, such as 3D imaging of the retina, 3D imaging of bird embryonic heart tissue, and imaging of biological tissues such as fingers or pig esophagus. In addition, FDML has wide applications in fiber optic sensing, such as dynamic pressure sensing, strain sensing, long-distance sensing, and linear demodulation of fiber optic grating sensing systems. Furthermore, FDML also has coherent applications in ultrafast measurements, such as high-speed measurements of fiber dispersion and analytical absorption spectra.

[0052] The E-band refers to the laser wavelength range of 1360-1460nm. Due to the lower manufacturing technology of early optical fibers, this band exhibited higher losses. With advancements in fiber manufacturing processes, the absorption loss in the water-peak band has been significantly reduced, now only about 0.5dB / km higher than the traditional communication band (C-band). Although this absorption loss is still higher than in the traditional communication band, the laser cavity in FDML typically uses high-gain media, such as semiconductor optical amplifiers (SOAs). Therefore, this relatively high absorption loss does not affect laser generation.

[0053] The function of dispersion management devices is to ensure that the net dispersion within the laser is zero. However, strictly speaking, FDML does not require zero net dispersion for all wavelengths within the operating band; near-zero dispersion is sufficient. O-band FDML utilizes the fact that the zero-dispersion point of ordinary single-mode fiber is in the O-band, and the dispersion of wavelengths near this point is near-zero. This ensures that each time the laser passes through the filter, the laser wavelength matches the filter's current transmission window, and the time-domain positions of each wavelength will not deviate due to dispersion after multiple transmissions. Otherwise, the laser will be unable to pass through the filter, resulting in loss and preventing laser generation.

[0054] Currently realized Fourier-domain mode-locked fiber lasers are limited to the C-band (1530-1565nm), O-band (1260-1360nm), L-band (1565-1625nm), and 1060nm bands. Because the E-band is located near the water peak, there are no corresponding wavelength dispersion management devices available for realizing E-band FDML fiber lasers, thus limiting the development of E-band FDML fiber lasers.

[0055] A fiber Bragg grating (FBG) is a passive device that uses techniques such as photolithography and holographic interference to modulate the refractive index of its core along its axis periodically. This variation in the refractive index period along the axial direction (gradually increasing or decreasing) is typically called a chirped fiber Bragg grating (CFBG). Different positions along the CFBG's axis can reflect different wavelengths of light; therefore, by designing the refractive index period, the CFBG can achieve dispersion management or broadening. In other words, by adding a CFBG to a laser cavity, the net dispersion within the laser cavity can be made near-zero, meeting the operating requirements of FDML fiber lasers. However, the bandwidth of a CFBG is typically only tens of nanometers, therefore the bandwidth of the FDML output laser is also only tens of nanometers, which is detrimental to the practical application of FDML.

[0056] To address the aforementioned technical problems, this invention provides an E-band Fourier domain mode-locked fiber laser, comprising: a swept-frequency filter, a first optical amplifier, a first optical isolator, a first beam splitter, a first polarization controller, and an optical fiber delay line. The swept-frequency filter, the first optical amplifier, the first optical isolator, the first beam splitter, the first polarization controller, and the fiber delay line are interconnected to form a ring laser cavity. The swept-frequency filter is used to periodically selectively filter the wavelength of the optical signal within the laser cavity. The first optical amplifier is used to amplify the optical signal after passing through the swept-frequency filter. The first optical isolator is located on one side of the first optical amplifier and is used to control the amplified optical signal to transmit in a single direction. The first beam splitter outputs a portion of the amplified optical signal outside the laser cavity, while the remaining portion is re-injected into the laser cavity. The first polarization controller is used to control the polarization state of the optical signal within the laser cavity. The fiber delay line is used to achieve near-zero wavelength dispersion in the E-band and simultaneously extend the cavity length of the laser cavity so that the cavity length matches the filtering period of the swept-frequency filter. The equivalent zero-dispersion point of the fiber delay line is located within the E-band. This invention achieves near-zero wavelength dispersion in the E-band by using an optical fiber delay line. This ensures that the laser wavelength matches the transmission window of the sweep filter each time the laser passes through it, and the time-domain positions of each wavelength do not deviate due to dispersion after multiple transmissions. This realizes a Fourier domain mode-locked fiber laser in the E-band, and further expands the current FDML band range through an E-band FDML laser. This provides a new band option for applications that require FDML, such as swept-source optical coherence tomography (SS-OCT), fiber optic communication, and fiber optic sensing.

[0057] Please also refer to Figures 1 to 4 The present invention provides a preferred embodiment of an E-band Fourier domain mode-locked fiber laser.

[0058] like Figure 1 As shown, the present invention provides an E-band Fourier domain mode-locked fiber laser, which includes: a swept frequency filter 1, a first optical amplifier 2, a first optical isolator 3, a first beam splitter 4, a first polarization controller 5, and an optical fiber delay line 6. The swept-frequency filter 1, the first optical amplifier 2, the first optical isolator 3, the first beam splitter 4, the first polarization controller 5, and the fiber delay line 6 are interconnected to form a ring laser cavity. The swept-frequency filter 1 is used to periodically selectively filter the wavelength of the optical signal within the laser cavity. The first optical amplifier 2 is used to amplify the optical signal after passing through the swept-frequency filter 1. The first optical isolator 3 is located on one side of the first optical amplifier 2 and is used to control the amplified optical signal to transmit in a single direction. The first beam splitter 4 is used to output part of the amplified optical signal outside the laser cavity, while the remaining part is re-injected into the laser cavity 6. The first polarization controller 5 is used to control the polarization state of the optical signal within the laser cavity. The fiber delay line 6 is used to make the wavelength dispersion in the E-band near zero dispersion, while simultaneously extending the cavity length of the laser cavity so that the cavity length matches the filtering period of the swept-frequency filter 1. The equivalent zero-dispersion point of the fiber delay line 6 is located within the E-band.

[0059] Specifically, the Fourier mode-locked laser is a ring laser cavity formed by interconnecting the sweep filter 1, the first optical amplifier 2, the first optical isolator 3, the first beam splitter 4, the first polarization controller 5, and the fiber delay line 6, wherein the connection order of each laser device can be different.

[0060] In some embodiments, the first optical amplifier 2 provides gain to the laser and may be, but is not limited to, a semiconductor optical amplifier, a doped fiber amplifier, a Raman amplifier, an optical parametric amplifier, etc. The first optical isolator 3 ensures that the optical signal propagates in a single direction, isolating backscattered light from interfering with the system; it may be, but is not limited to, a fiber optic isolator or a free-space isolator. The first polarization controller 5 adjusts the polarization state within the cavity to ensure that polarization-sensitive devices such as optical modulators and semiconductor optical amplifiers operate in optimal conditions; it may be, but is not limited to, a three-ring type, a squeeze type, or other different types of fiber-based online polarization controller, or a free-space polarization controller composed of glass plates. The swept-frequency filter 1 may be, but is not limited to, a mechanical or integrated swept-frequency filter 1 based on a scanning lens and grating, controlled by piezoelectric ceramics; the swept-frequency filter 1 performs narrowband filtering of the optical signal by periodically controlling the transmission wavelength of the filter. The first beam splitter 4 may be an all-fiber fiber coupler, or a solid-state output mirror or polarization beam splitter with semi-transmissive and semi-reflective properties; the beam splitting ratio is not limited.

[0061] This invention incorporates the fiber delay line 6, whose equivalent zero-dispersion point is located in the E-band, into the optical resonant cavity. The swept-frequency filter 1 (narrowband optical filter) tunes to the next position after one wavelength of light completes one round trip within the resonant cavity, allowing the next wavelength of light to be filtered out again, thus forming a quasi-steady-state operating mode. When the transmission window (the wavelength of light that can pass through) of the swept-frequency filter 1 is tuned to match the wavelength of the light that returns to the swept-frequency filter 1 after one round trip, this light is coupled back to the gain medium (first optical amplifier), eliminating the need for the laser to accumulate from spontaneous emission. In other words, light of all frequencies is stored in the fiber delay line 6 of the laser cavity. The swept-frequency filter 1 consumes almost no energy (under ideal operating conditions) because the wavelength of light entering the swept-frequency filter 1 at each moment perfectly matches its transmission window. This eliminates the limitation of the laser's settling time on the sweep-frequency light source's sweep speed, thereby increasing the sweep speed.

[0062] Therefore, this invention achieves a Fourier-domain mode-locked fiber laser in the E-band by using the fiber delay line 6 to make the wavelength dispersion in the E-band near zero. This ensures that the laser wavelength matches the transmission window of the sweep filter 1 each time the laser passes through it, and the time-domain position of each wavelength will not deviate due to dispersion after multiple transmissions. This allows the E-band Fourier-domain mode-locked fiber laser to be used in sensing systems, fiber optic communication systems, and SS-OCT systems, providing more channels for sensing systems and fiber optic communication systems, and providing a more suitable light source band for SS-OCT systems in certain samples.

[0063] Please see Figure 1 In a further embodiment of one example, the E-band Fourier domain mode-locked fiber laser further includes: an arbitrary waveform generator 14, which is connected to the sweep frequency filter 1. The arbitrary waveform generator 14 is used to generate periodic signals to control the sweep frequency filter 1 to periodically selectively filter the wavelength of the optical signal in the laser cavity.

[0064] Specifically, the arbitrary waveform generator 14 can generate a periodic driving signal to drive the sweep frequency filter 1 to match the cavity length of the laser, wherein the frequency of the driving signal is matched with the cavity fundamental frequency. In other words, the transmission wavelength of the sweep frequency filter 1 can be controlled by the periodic voltage signal generated by the arbitrary waveform generator 14.

[0065] Please see Figure 1 In a further embodiment of one example, the optical fiber delay line 6 includes a non-zero dispersion-shifted fiber 61 and a single-mode fiber 62. One end of the non-zero dispersion-shifted fiber 61 is connected to the single-mode fiber 62, and the other end of the non-zero dispersion-shifted fiber 61 is connected to the swept-frequency filter 1; the other end of the single-mode fiber 62 is connected to the first polarization controller 5; the total length of the non-zero dispersion-shifted fiber 61 and the single-mode fiber 62 is controlled according to the frequency of the swept-frequency filter 1; the length ratio of the non-zero dispersion-shifted fiber 61 to the single-mode fiber 62 is controlled according to the near-zero dispersion point.

[0066] Specifically, the technical principle of realizing the transfer of the zero-dispersion point in hybrid optical fibers is based on the combination of non-zero dispersion-shifted fiber 61 and ordinary single-mode fiber 62, as follows: Figure 2 As shown, the zero-dispersion point of single-mode fiber 62 is generally around 1310 nm, while that of non-zero dispersion-shifted fiber 61 is around 1550 nm. Therefore, by mixing non-zero dispersion-shifted fiber 61 and ordinary single-mode fiber 62 in different proportions, the equivalent zero-dispersion point of the final hybrid fiber can be shifted between the zero-dispersion points of the two fibers. The center wavelength of the E-band is precisely between the zero-dispersion wavelengths of these two fibers. Therefore, by combining ordinary single-mode fiber 62 and non-zero dispersion-shifted fiber 61 in different proportions, a hybrid fiber with an equivalent zero-dispersion point of 1410 nm can be obtained. By adding the fiber delay line 6, whose equivalent zero-dispersion point is located in the E-band, to the optical resonant cavity, it can simultaneously achieve dispersion management and cavity length extension, making the cavity length of the laser match the frequency of the sweep filter 1, and simultaneously making the dispersion of the laser cavity near zero.

[0067] Please see Figure 1In some embodiments, one end of the first optical isolator 3 is connected to the first polarization controller 5, and the other end of the first optical isolator 3 is connected to one end of the first optical amplifier 2; the other end of the first optical amplifier 2 is connected to the first beam splitter 4; and the first beam splitter 4 is also connected to the sweep frequency filter 1.

[0068] Specifically, the optical signal inside the laser cavity undergoes wavelength selective filtering after passing through the sweep frequency filter 1. The optical signal after passing through the sweep frequency filter 1 is then transmitted to the first optical amplifier 2 for amplification, and then passes through the first optical isolator 3 to ensure unidirectional transmission of the optical signal. Finally, a portion of the signal is output to both the inside and outside of the cavity via the first beam splitter 4. The signal returning into the cavity enters the fiber delay line 6, which consists of a non-zero dispersion-shifted fiber 61 and a single-mode fiber 62. After the optical signal is output from the fiber delay line 6, its polarization state is adjusted by the first polarization controller 5. Simultaneously, the arbitrary waveform generator 14 generates a periodic sweep frequency signal, which is then loaded onto the sweep frequency filter 1. The periodic sweep frequency signal causes the sweep frequency filter 1 to periodically selectively filter the wavelengths in the laser cavity.

[0069] It should be noted that the length of the fiber delay line 6 needs to be specially calculated to ensure that the propagation time of the optical signal within the entire fiber ring cavity containing the fiber delay line 6 is an integer multiple of the modulation period of the sweep filter 1. Simultaneously, the length ratio of the two fibers needs to be calculated to ensure that the equivalent zero-dispersion point of the hybrid fiber is located in the E-band (1360-1460 nm), resulting in near-zero wavelength dispersion. Specifically, the laser cavity length matching the sweep frequency of the sweep filter 1 is calculated, and the ratio of the two fibers that moves the equivalent zero-dispersion point of the hybrid fiber (fiber delay line) to the E-band is calculated while satisfying the cavity length requirement.

[0070] Please see Figure 1 Furthermore, the E-band Fourier domain mode-locked fiber laser also includes a second polarization controller 7, which is disposed between the first optical amplifier 2 and the first beam splitter 4. By adding the second polarization controller 7, the polarization state of the optical signal can be better adjusted.

[0071] Please see Figure 3In some embodiments, the fiber delay line 6 includes a non-zero dispersion-shifted fiber 61 and a single-mode fiber 62; the E-band Fourier domain mode-locked fiber laser further includes a fiber circulator 8 and a Faraday rotator 9. The non-zero dispersion-shifted fiber 61 is connected to the fiber circulator 8, and one end of the non-zero dispersion-shifted fiber 61 is connected to one end of the single-mode fiber 62; the fiber circulator 8 is also connected to the first polarization controller 5; the other end of the single-mode fiber 62 is connected to the Faraday rotator 9; the total length of the non-zero dispersion-shifted fiber 61 and the single-mode fiber 62 is determined according to the frequency of the swept-frequency filter 1; the length ratio of the non-zero dispersion-shifted fiber 61 to the single-mode fiber 62 is determined according to the near-zero dispersion point.

[0072] Furthermore, one end of the first optical amplifier 2 is connected to the sweep frequency filter 1, and the other end of the first optical amplifier 2 is connected to one end of the first optical isolator 3; the other end of the first optical isolator 3 is connected to the first polarization controller 5; and the first beam splitter 4 is connected between the fiber optic circulator 8 and the sweep frequency filter 1.

[0073] Specifically, the optical signal inside the laser cavity undergoes wavelength selective filtering after passing through the sweep frequency filter 1. The optical signal after passing through the sweep frequency filter 1 is then transmitted to the first optical amplifier 2 for amplification, and then passes through the first optical isolator 3 to ensure unidirectional transmission of the optical signal. Finally, a portion of the signal is output to both the inside and outside of the cavity via the first beam splitter 4. The signal returning into the cavity enters the fiber delay line 6, which consists of a non-zero dispersion-shifted fiber 61 and a single-mode fiber 62. After the optical signal is output from the fiber delay line 6, its polarization state is adjusted by the first polarization controller 5. Simultaneously, the arbitrary waveform generator 14 generates a periodic sweep frequency signal, which is then loaded onto the sweep frequency filter 1. The periodic sweep frequency signal causes the sweep frequency filter 1 to periodically selectively filter the wavelengths in the laser cavity.

[0074] The use of the fiber circulator 8 and Faraday rotator 9 allows the laser to pass back and forth twice through the non-zero dispersion shift fiber 61 and the single-mode fiber 62 within the laser cavity, thus achieving the effect of keeping the physical length of the fiber constant while doubling the cavity length.

[0075] Please see Figure 3 Furthermore, the E-band Fourier domain mode-locked fiber laser also includes a second polarization controller 7, which is disposed between the first optical amplifier 2 and the first beam splitter 4 to better adjust the polarization state of the optical signal.

[0076] Please see Figure 4In some embodiments, the E-band Fourier domain mode-locked fiber laser further includes: a second optical amplifier 10, a second optical isolator 11, a second beam splitter 12, and a third beam splitter 13. Specifically, the second optical amplifier 10 and the second optical isolator 11 are connected in series and then in parallel with the first optical amplifier 2 and the first optical isolator 3; the second beam splitter 12 is connected to the first polarization controller 5, the first optical isolator 3, and the second optical isolator 11, respectively; and the third beam splitter 13 is connected to the first optical amplifier 2, the second optical amplifier 10, and the first beam splitter 4, respectively.

[0077] Specifically, the first optical amplifier 2 is an E-band optical amplifier, and the second optical amplifier 10 is an O-band or C-band amplifier. The first optical amplifier 2 and the first optical isolator 3, as well as the second optical amplifier 10 and the second optical isolator 11, serve as two arms with different gain ranges. Two parallel ring laser cavities are constructed through the second beam splitter 12 and the third beam splitter 13. The first optical amplifier 2 is E-band, and the second optical amplifier 10 can be an O-band or C-band OA, which is close to the E-band. This method allows the final output FDML laser to have a wider spectrum, increasing the bandwidth of the output laser.

[0078] In summary, the E-band Fourier domain mode-locked fiber laser provided by this invention has the following beneficial effects:

[0079] By using fiber delay lines to achieve near-zero wavelength dispersion in the E-band, the laser wavelength is matched to the transmission window of the sweep filter each time the laser passes through it. After multiple transmissions, the time domain positions of each wavelength will not deviate due to dispersion, thus realizing a Fourier domain mode-locked fiber laser in the E-band. Furthermore, the current FDML band range can be extended through E-band FDML lasers, providing a new band option for applications that require FDML, such as swept-frequency optical coherence tomography, fiber optic communication, and fiber optic sensing.

[0080] It should be understood that the application of the present invention is not limited to the examples above. Those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims.

Claims

1. An E-band Fourier domain mode-locked fiber laser, characterized in that, include: The components include a swept-frequency filter, a first optical amplifier, a first optical isolator, a first beam splitter, a first polarization controller, and an optical fiber delay line. The swept-frequency filter, the first optical amplifier, the first optical isolator, the first beam splitter, the first polarization controller, and the fiber delay line are interconnected to form a ring laser cavity; wherein... The sweep frequency filter is used to periodically select and filter the wavelength of the optical signal in the laser cavity; The first optical amplifier is used to amplify the optical signal that has passed through the swept frequency filter; The first optical isolator is disposed on one side of the first optical amplifier and is used to control the optical signal amplified by the first optical amplifier to be transmitted in one direction. The first beam splitter is used to output part of the amplified optical signal outside the laser cavity, while the rest is re-injected into the laser cavity; The first polarization controller is used to control the polarization state of the optical signal within the laser cavity; The fiber delay line is used to make the wavelength dispersion in the E-band near zero, while extending the cavity length of the laser cavity so that the cavity length of the laser cavity matches the filtering period of the sweep filter; wherein, the equivalent zero dispersion point of the fiber delay line is located in the E-band. The optical fiber delay line comprises: non-zero dispersion-shifted fiber and single-mode fiber; wherein... One end of the non-zero dispersion-shifted fiber is connected to the single-mode fiber, and the other end of the non-zero dispersion-shifted fiber is connected to the sweep frequency filter. The other end of the single-mode optical fiber is connected to the first polarization controller; The total length of the non-zero dispersion-shifted fiber and the single-mode fiber is determined by the frequency of the swept-frequency filter; the length ratio of the non-zero dispersion-shifted fiber to the single-mode fiber is determined by the near-zero dispersion point. The non-zero dispersion shifted fiber and the single-mode fiber are mixed so that the equivalent zero dispersion point of the resulting hybrid fiber moves between the zero dispersion points of the two fibers, and the center wavelength of the E-band lies between the zero dispersion wavelengths of the two fibers.

2. The E-band Fourier domain mode-locked fiber laser according to claim 1, characterized in that, One end of the first optical isolator is connected to the first polarization controller, and the other end of the first optical isolator is connected to one end of the first optical amplifier. The other end of the first optical amplifier is connected to the first beam splitter; The first beam splitter is also connected to the sweep frequency filter.

3. The E-band Fourier domain mode-locked fiber laser according to claim 1, characterized in that, Also includes: A second polarization controller is disposed between the first optical amplifier and the first beam splitter.

4. The E-band Fourier domain mode-locked fiber laser according to claim 1, characterized in that, The fiber delay line comprises: a non-zero dispersion-shifted fiber and a single-mode fiber; the E-band Fourier domain mode-locked fiber laser further comprises: a fiber circulator and a Faraday rotator; wherein... The non-zero dispersion-shifted fiber is connected to the fiber circulator, and the other end of the non-zero dispersion-shifted fiber is connected to one end of the single-mode fiber. The fiber optic circulator is also connected to the first polarization controller; The other end of the single-mode optical fiber is connected to the Faraday rotator mirror. The total length of the non-zero dispersion-shifted fiber and the single-mode fiber is determined by the frequency of the swept-frequency filter; the length ratio of the non-zero dispersion-shifted fiber to the single-mode fiber is determined by the near-zero dispersion point.

5. The E-band Fourier domain mode-locked fiber laser according to claim 4, characterized in that, One end of the first optical amplifier is connected to the sweep frequency filter, and the other end of the first optical amplifier is connected to one end of the first optical isolator. The other end of the first optical isolator is connected to the first polarization controller; The first optical splitter is connected between the optical fiber circulator and the swept frequency filter.

6. The E-band Fourier domain mode-locked fiber laser according to claim 5, characterized in that, Also includes: A second polarization controller is disposed between the first optical amplifier and the first beam splitter.

7. The E-band Fourier domain mode-locked fiber laser according to claim 2, characterized in that, Also includes: The system comprises a second optical amplifier, a second optical isolator, a second beam splitter, and a third beam splitter; among which... The second optical amplifier and the second optical isolator are connected in series, and then connected in parallel with the first optical amplifier and the first optical isolator; The second beam splitter is connected to the first polarization controller, the first optical isolator, and the second optical isolator, respectively. The third beam splitter is connected to the first optical amplifier, the second optical amplifier, and the first beam splitter, respectively.

8. The E-band Fourier domain mode-locked fiber laser according to claim 7, characterized in that, The first optical amplifier is an E-band optical amplifier; the second optical amplifier is an O-band or C-band amplifier.

9. The E-band Fourier domain mode-locked fiber laser according to claim 1, characterized in that, Also includes: An arbitrary waveform generator is connected to the sweep frequency filter. The arbitrary waveform generator is used to generate a periodic signal to control the sweep frequency filter to periodically select and filter the wavelength of the optical signal in the laser cavity.

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