Intracavity frequency multiplication optical frequency comb based on fiber coupler

Abstract

The application discloses an intra-cavity frequency multiplication optical frequency comb based on a fiber coupler, relates to the technical field of frequency comb measurement, and comprises a gain medium, a pump source, a saturable absorber, an intra-cavity and an output test port, which are connected through optical fibers or spatial optical paths and jointly form an external cavity of the optical frequency comb. The intra-cavity is formed by two fiber couplers which are connected with each other. The intra-cavity is nested in the external cavity, the lengths of the intra-cavity and the external cavity are accurately controlled, the free spectral ranges of the intra-cavity and the external cavity strictly satisfy an integer multiple relationship, and the multiplication of the repetition frequency of the original external cavity optical frequency comb is realized. The application has the advantages of stable structure, easy coupling and low cost. The optical frequency comb with high repetition frequency can be applied to the fields of gas absorption spectrum detection, precise optical frequency comb ranging and astronomical spectrometer calibration.

Description

Technical Field

[0001] This invention relates to the field of optical frequency comb technology, specifically an intracavity frequency multiplication optical frequency comb based on an optical fiber coupler. Background Technology

[0002] Optical frequency combs based on ultrafast lasers have garnered significant attention in recent years as a bridge between light waves and microwaves. In the frequency domain, an optical frequency comb appears as a series of equally spaced optical frequency teeth, with the tooth spacing equal to the repetition frequency of the laser. Due to the extremely high accuracy and traceability of the comb tooth frequency, it has been applied to direct precision spectral measurements, time-frequency transfer, and the measurement of fundamental physical constants. Currently, the main techniques for generating optical frequency combs include electro-optic modulation (EOM), passive mode-locking, and nonlinear Kerr microcavities. Each of these methods has its own advantages and disadvantages. Frequency combs based on EOM technology can have their repetition frequency flexibly tuned over a wide range, but they suffer from relatively high comb noise, lagging behind passively mode-locked lasers. Passive mode-locking typically utilizes optical resonators with saturable absorbers, offering advantages such as low noise and high output energy. However, the repetition frequency is limited by the physical size of the optical resonator, often falling below several hundred MHz. Nonlinear Kerr microcavities have extremely small physical dimensions and can be partially integrated into chips, but the comb spacing of the optical frequency combs they generate is often limited by the microcavity size, reaching tens of GHz. This is detrimental to high-resolution spectral measurements, and the output energy and electro-optical conversion efficiency are relatively low, making them unsuitable for applications requiring high energy and high efficiency. In summary, existing mainstream methods generally struggle to achieve low-noise, high-performance frequency combs with repetition frequencies around 1 GHz.

[0003] Repetition rate is a critical parameter in many optical frequency comb applications. Applications such as high-precision ranging, astronomical observation, and lidar all require frequency combs with repetition rates in the GHz range. In high-precision ranging, compared to lower repetition rate optical frequency comb systems, GHz frequency combs can increase the spacing between adjacent comb teeth, preventing spectral aliasing and improving the ranging accuracy of femtosecond optical combs. In astronomical applications, high-precision measurement of Doppler velocity drift is crucial for studying the expansion of the universe and searching for exoplanets; optical frequency combs with repetition rates in the GHz range are excellent calibrators for astronomical spectrometers. Furthermore, frequency combs can further enhance existing lidar technology, enabling simultaneous detection of multi-component spectra. However, lidar typically uses nanosecond-width pulse modulation; to ensure the overlap of the frequency comb pulses and the lidar modulation pulses in the time domain, optical frequency combs with repetition rates above GHz must be used.

[0004] Currently, optical frequency comb systems based on mode-locked fiber lasers are the most mature method for generating optical frequency combs, and also the earliest and most widely used commercial optical frequency systems. However, the repetition frequency of such optical frequency combs is usually within several hundred MHz, which greatly limits their application scope and power capabilities. To further improve the repetition rate of the optical frequency comb of mode-locked fiber lasers, it is generally necessary to minimize the physical size of the fiber resonator. For example, mode-locked fiber lasers can be fabricated using ultrashort cavities and encapsulated saturable absorbers (SESAMs) (see D. Song, K. Yin, R. Miao, C. Zhang, Z. Xu, and T. Jiang, Theoretical and experimental investigations of dispersion-managed, polarization-maintaining 1-GHz mode-locked fiber lasers, Opt. Express 31, 1916 (2023)), or NPR mode-locked fiber lasers with small-sized modular optical packages (see R. Yang, M. Zhao, X. Jin, Q. Li, Z. Chen, A. Wang, and Z. Zhang, Attosecond timing jitter from high repetition rate femtosecond “solid-state fiber lasers”, Optica 9, 874 (2022)). The former has the limited lifespan of SESAM, while the latter, based on NPR mode-locked fiber lasers, employs a large number of spatial structures, resulting in some loss of long-term stability. Summary of the Invention

[0005] To address the limitations in lifetime and long-term stability resulting from increasing the repetition rate of optical frequency combs based on passively mode-locked fiber lasers, and to overcome the physical size constraints of the resonant cavity imposed on repetition rate increases in fiber lasers, this invention provides an intracavity frequency multiplication optical frequency comb based on an fiber coupler. The aim is to achieve simple, low-cost, and stable repetition rate multiplication of optical frequency combs directly generated from various fiber lasers.

[0006] The solution of this invention is as follows: an intracavity frequency multiplication optical frequency comb based on an optical fiber coupler, characterized in that: the external resonant cavity laser uses rare-earth-doped ion-fiber as the gain medium and can operate with independent mode-locking. (Optical fiber laser external resonant cavity) With internal resonant cavity The relationship is satisfied as follows: ,in For frequency multiplication factor, It is a positive integer. To ensure the optical frequency comb has a single mode and suppress the generation of supermodes, the free spectral range of the internal resonant cavity satisfies the following relationship: ,in , Let A:B be the loss factor of the inner resonant cavity, and C:D be the splitting ratios of the two fiber couplers, respectively. By nesting the inner resonant cavity within the outer resonant cavity and adjusting appropriate pump source parameters, the repetition frequency of the output optical frequency comb of the original outer cavity fiber laser can be multiplied.

[0007] Specifically, when the optical frequency comb is in the external resonant cavity of the fiber laser... =82.5 MHz, the required frequency is 907.5 MHz, then the internal resonant cavity needs to be controlled. =907.5 MHz, amplification factor The ratio is 11. The splitting ratio of both fiber couplers is set to 85:15.

[0008] The gain fiber doped ions include , , , , , , and One or more combinations thereof.

[0009] The independent mode-locking mechanism of the external resonant cavity can be an artificial saturable absorber (e.g., nonlinear polarization rotation, nonlinear amplification ring mirror) or a material-based saturable absorber (e.g., graphene, SESAM, topological insulator).

[0010] With the removal of the inner resonant cavity and compensation of an equal length of optical fiber, and adjustment of the pump source power, the frequency comb can operate at a low repetition frequency without doubling.

[0011] Specifically, optical fibers and optical fiber devices are all polarization-maintaining.

[0012] The optimization also includes an optical frequency comb reference source and CEO frequency acquisition module, as well as microwave circuit processing and PID locking loop, which enable the optical frequency comb to be locked to a specific reference frequency source, achieving a stable and traceable optical frequency comb output.

[0013] Compared with the prior art, the technical advantages of the present invention are:

[0014] 1) By utilizing the spectral filtering effect of the inner resonant cavity formed by the interconnection of two fiber couplers, and through the matching design of the parameters of the inner and outer resonant cavities, the physical size limitation of the optical resonant cavity of the original fiber laser is broken, and the repetition frequency of the output optical frequency comb of the laser is multiplied.

[0015] 2) Compared with the traditional approach of reducing the size of the fiber laser resonator cavity to increase the repetition frequency, this invention has the advantages of simple structure, low cost, reliability and stability, and is suitable for increasing the repetition frequency of various optical frequency combs based on mode-locked lasers. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of an intracavity frequency multiplication optical frequency comb frame based on an optical fiber coupler according to the present invention.

[0017] Figure 2 This is a schematic diagram of a resonant cavity constructed from an optical fiber coupler based on an intracavity frequency multiplication optical frequency comb according to the present invention.

[0018] Figure 3 This is a schematic diagram of the framework of the present invention, which uses a nonlinear amplifying ring mirror laser to achieve repetitive frequency multiplication of an intracavity frequency multiplication optical frequency comb based on an optical fiber coupler.

[0019] Figure 4 This is an FSR test diagram of a resonant cavity constructed from an optical fiber coupler based on an intracavity frequency multiplication optical frequency comb according to the present invention.

[0020] Figure 5 This invention relates to an external cavity laser output pulse sequence based on an intracavity frequency multiplication optical frequency comb with an optical fiber coupler, and an output pulse sequence after nesting an inner cavity.

[0021] Figure 6 The CEO frequency of the output laser of an intracavity frequency multiplication optical frequency comb based on an optical fiber coupler, as described in this invention, is measured by an f-2f module. Detailed Implementation

[0022] The present invention will be further described below with reference to an example and accompanying drawings, but this should not be construed as limiting the scope of protection of the present invention.

[0023] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0024] It should be noted that the terminology used herein is for the purpose of describing specific implementations only and is not intended to be limiting.

[0025] It should be understood that when a unit is referred to as "connected," "linked," or "coupled" to another unit in this document, it can be directly connected or coupled to another unit, or an intermediate unit may exist. Conversely, when a unit is referred to as "directly connected" or "directly coupled" to another unit in this document, it indicates that no intermediate unit exists. Furthermore, other words used to describe relationships between units should be interpreted in a similar manner (e.g., "between" versus "directly between," "adjacent" versus "directly adjacent," etc.).

[0026] The intracavity frequency multiplication optical frequency comb based on an optical fiber coupler provided in this embodiment, such as Figure 3 As shown, the optical system includes: fiber optic mirror 1 for optical path folding; third fiber coupler 2 for coupling the beam and providing an output test port; wavelength division multiplexer 3 for coupling pump light and signal light; pump source 4 for providing appropriate pump 976 nm laser; gain fiber 5 is a rare-earth-doped fiber; dispersion compensation fiber 6 is used to control the net dispersion of the resonant cavity; first fiber coupler 7 and second fiber coupler 8 together form the fiber cavity; and non-reciprocal phase shifter 9 provides... The phase difference; the wavelength division multiplexer 3 includes a common terminal, a signal optical terminal and a pump terminal, and the port of the fiber optic mirror 1 and the third fiber optic coupler 2. Connection, port of the third fiber coupler 2 The signal optical end of wavelength division multiplexer 3 is connected to the signal optical end, pump source 4 is connected to the pump end of wavelength division multiplexer 3, the common end of wavelength division multiplexer 3 is connected to gain fiber 5, gain fiber 5 is connected to dispersion compensation fiber 6, and dispersion compensation fiber 6 is connected to the port of first fiber coupler 7. Connected to the port of the first fiber optic coupler 7 Port of the second fiber optic coupler 8 Connected to the port of the second fiber optic coupler 8 Connected to a non-reciprocal phase shifter, the port of non-reciprocal phase shifter 9 is connected to the port of the third fiber coupler 2. The connected loop forms the external resonant cavity of the fiber laser. When the first fiber coupler 7 and the second fiber coupler 8 are removed and replaced with a section of equal-length fiber, it becomes a nonlinear amplifying ring mirror fiber laser with a fully polarization-maintaining fiber.

[0027] The third fiber coupler is a 1550 nm polarization-maintaining fiber coupler with a splitting ratio of 7:3.

[0028] The first fiber wavelength division multiplexer uses a 976 nm / 1550 nm polarization-maintaining fiber wavelength division multiplexer.

[0029] The first and second fiber couplers are 1550 nm polarization-maintaining fiber couplers with a splitting ratio of 85:15.

[0030] Gain fiber is doped Ion polarization-maintaining fiber.

[0031] Mode-locked pulse output can be achieved independently by controlling the length of the pigtails of each device and adjusting the pump source power to 200 mW. The measured output repetition frequency is 82.5 MHz and the center wavelength is 1550 nm.

[0032] like Figure 2 The fiber lengths of the first and second fiber couplers are precisely controlled and connected together to form the resonant cavity inside the fiber coupler. A stable mode-locked laser is then used to transmit power from the first fiber coupler. Port input, from the second fiber coupler Port measurement output pulse interval, see [link / reference] Figure 4 The FSR of this resonant cavity can be calculated to be approximately 907.5 MHz. The resonant cavity within the fiber coupler is nested within the original fiber nonlinear amplification ring mirror cavity, as shown below. Figure 1 Strict control of the first fiber optic coupler port is required here. To the second fiber coupler The fiber length at the port (including the pigtail) is consistent with the fiber length cut from the original external resonant cavity.

[0033] A stable mode-locked pulse sequence output can be observed when the pump power is adjusted to 900 mW, and the repetition frequency of this pulse sequence corresponds to the FSR of the cavity. Figure 4 The original mode-locked output pulse sequence of the fully polarization-maintaining fiber nonlinear amplification ring mirror and the mode-locked output pulse sequence after nesting inner cavity are measured by a photodetector and an oscilloscope through the output test port.

[0034] It should be noted that when it is difficult to precisely match and control the fiber length so that the internal and external cavity FSRs are integer multiples, the fiber reflector can be replaced with a spatial optical path whose length can be adjusted and coupled back and forth with the fiber.

[0035] It should be noted that all three test fiber output ports after nesting can measure pulse sequences with the same repetition frequency.

[0036] It should be noted that when the free spectral range of the inner resonant cavity is strictly smaller than the FSR of the outer cavity, the output CEO frequency of the nested optical frequency comb laser can be obtained as a single frequency through the f-2f module or other measurement methods. (See [reference needed]). Figure 5 By utilizing components such as a microwave conversion link and a PID controller, this signal can be locked to a stable reference frequency to obtain a stable optical frequency comb.

[0037] Thus, by nesting a matched fiber coupler ring within the original fiber frequency comb laser, the physical size limitations of the original laser can be overcome, significantly increasing the frequency spacing of the output optical frequency comb while maintaining a single CEO frequency. This is a solution that offers flexible wavelengths, a simple and stable application structure, and the ability to multiply the repetition frequency of various fiber frequency combs.

Claims

1. An intracavity frequency multiplication optical frequency comb based on an optical fiber coupler, comprising a pump source (V), a gain medium (II), a saturable absorber (III), and an output test port (IV) connected via an optical fiber or spatial optical path, wherein the gain medium, the saturable absorber, and the output test port are connected to form an external resonant cavity, and the frequency comb can operate at a low repetition frequency without multiplication under suitable pump source parameters; characterized in that, It also includes two port and An internal resonant cavity (Ⅰ) is formed by two fiber optic couplers (110, 120) connected in pairs at their ports. The port is connected to an external resonant cavity; By nesting the inner resonant cavity within the outer resonant cavity and precisely controlling the lengths of the inner and outer resonant cavities, their free spectral ranges can be made to strictly satisfy an integer multiple relationship. The inner resonant cavity is placed at any position within the outer resonant cavity, so that the inner and outer resonant cavities share a portion of the optical path. Strictly control the two fiber optic couplers The fiber length between ports is consistent with the fiber length removed from the original external resonant cavity.

2. The intracavity frequency multiplication optical frequency comb based on an optical fiber coupler according to claim 1, characterized in that, The free spectral range (FSR) of the external resonant cavity. Free spectral range of the internal resonant cavity The relationship is satisfied as follows: ,in It is a positive integer, representing the multiple by which the frequency increases.

3. The intracavity frequency multiplication optical frequency comb based on an optical fiber coupler according to claim 1, characterized in that, The gain medium (II) is a rare-earth ion-doped optical fiber; the saturable absorber (III) is an artificial or material-based saturable absorber; and the output test port (IV) is a spatial light output or an output via optical fiber.

4. The intracavity frequency multiplication optical frequency comb based on an optical fiber coupler according to claim 1, characterized in that, The output test port (Ⅳ) generates an ultrafast laser with a pulse width in the range of hundreds of picoseconds to tens of femtoseconds and a comb tooth spacing in the range of hundreds of MHz to tens of GHz.

Images