Optical parametric oscillator based on Bragg grating dispersion regulation

Abstract

The invention discloses an optical parametric oscillator based on Bragg grating dispersion regulation and control. The device comprises a Bragg grating, a periodically poled waveguide and a directional coupler, the two Bragg gratings serve as reflectors and are used for forming a Fabry-Perot cavity, a pump light source is coupled into the resonant cavity through straight-through ports of the two directional couplers, optical parametric down-conversion is achieved through periodic polarization waveguides in the cavity, and generated signal light and idler frequency light are oscillated and amplified in the cavity. And the signal is transmitted through the cross port of the second directional coupler and is finally output from the straight-through port. Resonance and flexible dispersion regulation and control of signal light and idler frequency light are achieved through design of a Bragg grating reflection spectrum and dispersion characteristics. The optical parametric oscillator provided by the invention can realize high efficiency, low threshold and tunability of signals, and has wide application prospects in the fields of tunable mid-infrared light sources, spectrum detection, optical communication and the like.

Description

Technical Field

[0001] This invention relates to the field of integrated nonlinear optics, and more specifically to an optical parametric oscillator based on Bragg grating dispersion modulation. Background Technology

[0002] Mid-infrared laser sources have broad application prospects in fields such as materials processing, spectral analysis, free-space communication, and biomedical diagnosis.

[0003] Second-order nonlinear optical parametric oscillators (OPOs) can achieve laser wavelength conversion over a wide tuning range, making them an effective method for realizing mid-infrared laser sources. However, traditional mid-infrared OPOs based on bulk crystals suffer from problems such as large footprint, high threshold power, high cost, and poor stability.

[0004] Optical parametric oscillators based on nanophotonics can achieve strong nonlinear interactions and dispersion modulation under subwavelength mode confinement. However, due to the curved waveguide structure of typical toroidal microcavities, the polarization direction of light cannot always remain parallel to the crystal axis, thus limiting the nonlinear conversion efficiency because a strong nonlinear coefficient cannot be achieved at every point. Furthermore, the dispersion characteristics of toroidal microcavities are mainly determined by material dispersion and waveguide geometric dispersion, making it difficult to achieve flexible design and modulation over a wide range. Summary of the Invention

[0005] To overcome the shortcomings of existing on-chip optical parametric oscillator (OPO) technology, this invention proposes an OPO based on Bragg grating dispersion modulation. This invention uses two Bragg gratings directly etched onto a thin-film lithium niobate film as mirrors to form a Fabry-Perot cavity. The pump light undergoes optical parametric down-conversion using a periodically polarized lithium niobate waveguide within the cavity. Through flexible design of the Bragg grating's reflection spectrum and dispersion characteristics, resonance and dispersion modulation of the signal and idler light are achieved. This method utilizes a straight waveguide structure, ensuring a high degree of matching between the optical field polarization direction and the crystal axis. Simultaneously, the Bragg grating provides powerful dispersion modulation capabilities, ultimately enabling a high-efficiency, low-threshold, wide-tuning-range, and flexibly dispersion-modulated OPO.

[0006] The technical solution of the present invention is as follows: I. An Optical Parametric Oscillator Based on Bragg Grating Dispersion Modulation The optical parametric oscillator includes a pump source, a first directional coupler, a second directional coupler, a periodically polarized waveguide, a first Bragg grating, and a second Bragg grating; the pump light emitted from the pump source is transmitted to the second directional coupler after passing through the first directional coupler, and the first directional coupler is connected to the first port of the second directional coupler; The second port of the second directional coupler is connected to the second Bragg grating, the first port of the periodically polarized waveguide is connected to the third port of the second directional coupler, and the fourth port of the second directional coupler is used to connect to external devices; the first and third ports of the second directional coupler are two ports of the same waveguide, and the second and fourth ports of the second directional coupler are two ports of the same waveguide; the periodically polarized waveguide is connected to the first Bragg grating. A periodically polarized waveguide is used to generate signal light and idler light through a second-order nonlinear effect under the action of pump light transmitted by the second directional coupler, and transmits the signal light and idler light to the first Bragg grating; the first Bragg grating, the second Bragg grating, the periodically polarized waveguide, and the second directional coupler form an on-chip Fabry-Perot cavity, in which the signal light and idler light resonate.

[0007] The first and second Bragg gratings have the same structure, both being apodized chirped Bragg gratings; the Bragg wavelength gradually increases from the front end to the back end of the grating.

[0008] The first and second Bragg gratings provide negative group delay dispersion in the signal and idler light bands, and the sum of the group delay dispersion values ​​of the two gratings is equal to the dispersion value of the waveguide, thus canceling each other out.

[0009] The first and second Bragg gratings achieve a reflection bandwidth exceeding a first preset threshold and a reflectivity higher than a second preset threshold.

[0010] The waveguide includes lithium niobate and lithium tantalate.

[0011] II. A laser wavelength conversion device The device includes an optical parametric oscillator based on Bragg grating dispersion modulation.

[0012] The beneficial effects of this invention are as follows: This invention utilizes the flexible and designable dispersion characteristics of on-chip Bragg gratings to accurately compensate for the inherent dispersion of waveguides, achieving near-zero dispersion within the cavity, thereby enabling controllable wavelength tuning.

[0013] The present invention uses an on-chip Bragg grating as a reflector for a Fabry-Perot cavity. By achieving high reflectivity for both signal and idler light, a high-Q resonant cavity is realized, thereby reducing the power threshold of the optical parametric oscillator and improving the conversion efficiency.

[0014] This invention uses a periodically polarized lithium niobate waveguide to achieve wavelength conversion of the pump light. The frequency of the pump light, as well as the signal light and idler light generated by the optical parametric oscillation, are not restricted, resulting in higher degrees of freedom and a larger bandwidth. Attached Figure Description

[0015] Figure 1 A schematic diagram of the on-chip thin-film lithium niobate optical parametric oscillator based on Bragg grating dispersion modulation designed for this invention; Figure 2 A schematic diagram of the apodized chirped Bragg grating designed for this invention; In the figure: 1. C-band pump source, 2. First directional coupler, 3. Second directional coupler, 4. Periodically polarized waveguide, 5. First Bragg grating, 6. Second Bragg grating. Detailed Implementation

[0016] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the scope of the disclosure.

[0017] like Figure 1 As shown, the optical parametric oscillator based on Bragg grating dispersion modulation proposed in this invention includes a C-band pump source 1, a first directional coupler 2, a second directional coupler 3, a periodically polarized waveguide 4, a first Bragg grating 5, and a second Bragg grating 6. The pump light emitted from the C-band pump source 1 is transmitted to the second directional coupler 3 after passing through the first directional coupler 2. Specifically, the first output terminal b of the first directional coupler 2 is connected to the first port d of the second directional coupler 3. The input terminal and the first output terminal b of the first directional coupler 2 are the two ends of the same waveguide. Most of the pump light is emitted from the first output terminal b of the first directional coupler 2, and a small portion of the pump light is emitted from the second output terminal c of the first directional coupler 2. This port is used for the alignment of the optical fiber and the waveguide during testing.

[0018] The second port f of the second directional coupler 3 is connected to the second Bragg grating 6. The first port of the periodically polarized waveguide 4 is connected to the third port e of the second directional coupler 3. The fourth port g of the second directional coupler 3 is used to connect external devices, allowing observation of the generation of signal light and idler light, as well as output of the generated signal light and idler light. Most of the signal light and idler light exits from the third port e of the second directional coupler 3, while a smaller portion exits from the fourth port g. The first port d and the third port e of the second directional coupler 3 are two ports of the same waveguide, as are the second port f and the fourth port g of the second directional coupler 3. The periodically polarized waveguide 4 is connected to the first Bragg grating 5. The periodically polarized waveguide 4 is used to generate signal light and idler light through a second-order nonlinear effect under the action of the pump light transmitted by the second directional coupler 3, and transmits the signal light and idler light to the first Bragg grating 5. The first Bragg grating 5, the second Bragg grating 6, the periodically polarized waveguide 4, and the second directional coupler 3 form an on-chip Fabry-Perot cavity, in which the signal light and idler light resonate. Specifically, the signal light and idler light reflected by the first Bragg grating 5 are transmitted sequentially to the periodically polarized waveguide 4, the second directional coupler 3, and the second Bragg grating 6, and then sequentially transmitted back to the first Bragg grating 5.

[0019] As two directional couplers function as wavelength division multiplexers, their coupling lengths and waveguide spacings need to be designed separately. To reduce intracavity loss of the optical signal, the first directional coupler 2 is designed such that the pump light transmittance at the through port exceeds 97%, with a small portion of the pump light output from the cross port for fiber and waveguide alignment during testing. The second directional coupler 3 is designed such that the pump light transmittance at the through port exceeds 99%, allowing for efficient coupling into the resonant cavity; the signal light and idler light transmittance at the cross port exceeds 98%, with a small portion of the remaining light output from the through port.

[0020] The first Bragg grating 5 and the second Bragg grating 6 have the same structure; both are apodized chirped Bragg gratings. From the front end to the back end of the grating, the Bragg wavelength gradually increases, as... Figure 2 As shown, two identical Bragg gratings serve as cavity mirrors for the resonant cavity, reflecting both signal and idler light. The center wavelength of the gratings is designed to fit within the desired signal and idler light bands by adjusting the grating period.

[0021] Since longer wavelength light travels a longer distance before being reflected, its group delay is greater. The first Bragg grating 5 and the second Bragg grating 6 provide negative group delay dispersion in the signal light and idler light bands, that is, the opposite sign to the waveguide dispersion value. Moreover, the group delay dispersion value is equal in magnitude to the waveguide dispersion value, so that the net dispersion in the resonant cavity is tuned to near zero, thereby achieving stable and controllable wavelength tuning.

[0022] By designing the grating period of the Bragg grating, its center wavelength is positioned within the expected signal and idler light bands. Further optimization of the grating length, period, and chirp ensures that the group delay dispersion is equal to the waveguide dispersion, and that the Bragg grating achieves a reflectivity exceeding 99%. This results in lower intracavity loss, allowing the signal and idler light to resonate efficiently within the resonant cavity. Consequently, the threshold power of the optical parametric oscillator (OPO) is significantly reduced, improving conversion efficiency.

[0023] Specifically, after the pump light enters the periodically polarized lithium niobate waveguide, it generates a signal light and an idler light with frequencies half of the pump light's frequency under nonlinear interaction. During this process, energy and momentum conservation (i.e., phase matching conditions) must be satisfied among the pump light, signal light, and idler light. However, due to the material dispersion of thin-film lithium niobate and the waveguide dispersion, it is difficult to satisfy the phase matching conditions. Therefore, this invention employs a quasi-phase matching technique, introducing an additional wave vector to compensate for phase mismatch by periodically polarizing the waveguide, thus enabling efficient nonlinear optical parametric processes. The generated signal light and idler light are selected and fed back by the resonant cavity, oscillating repeatedly within the cavity to obtain gain, and then output from within the cavity.

[0024] The first Bragg grating 5 and the second Bragg grating 6 proposed in this invention serve as cavity mirrors for the resonant cavity. They are designed to have high reflectivity for both signal light and idler light, and introduce specific group delay dispersion to accurately compensate for the inherent dispersion of the thin-film lithium niobate waveguide at the resonant wavelength. This achieves a resonant cavity with high Q value and near-zero dispersion, thereby reducing the threshold power and enabling controllable wavelength tuning.

[0025] The periodic reverse domains of the periodically polarized waveguide 4 are obtained by applying high voltage pulses to the periodic toothed electrodes on both sides of the waveguide.

[0026] The dispersion characteristics of a thin-film lithium niobate waveguide in the signal and idler bands were calculated using waveguide mode simulations. Simulation results show that, under the most basic single-mode conditions, the waveguide typically exhibits normal dispersion. To compensate for this dispersion, this invention introduces chirp, specifically through its axially gradually varying periodic structure, to provide anomalous dispersion compensation. Its working mechanism is as follows: Figure 2 As shown, the front end of the grating reflects short wavelengths, while the rear end reflects long wavelengths. Since the long wavelength light travels a longer distance before being reflected, its group delay is greater, thus providing negative group delay dispersion in the signal light and idler light bands. This dispersion value is precisely designed to be equal in magnitude but opposite in sign to the dispersion value of the waveguide, so that the net dispersion in the resonant cavity is tuned to near zero, thereby achieving stable and controllable wavelength tuning.

[0027] During device testing, unavoidable process errors occur during waveguide patterning and etching, causing deviations between the actual waveguide dimensions (including width, thickness, and sidewall angles) and the design values, which in turn alters the effective refractive index. This change disrupts the quasi-phase-matching condition, reducing nonlinear conversion efficiency. Therefore, this invention requires the addition of a temperature control device to regulate the temperature of the periodically polarized waveguide in the chip, compensating for phase mismatch by altering the quasi-phase-matching condition, thereby increasing nonlinear conversion efficiency. Simultaneously, phase matching can also be optimized by tuning the pump light wavelength; however, the effective tuning range of the pump light is limited by the bandwidth of the Bragg grating reflection spectrum. This is because the optical parametric oscillation process requires the generated signal and idler wavelengths to be within the high-reflection band of the Bragg grating for the resonant cavity to provide sufficient feedback to maintain oscillation.

[0028] The present invention also proposes a laser wavelength conversion device, which includes an optical parametric oscillator based on Bragg grating dispersion modulation.

[0029] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. An optical parametric oscillator based on Bragg grating dispersion modulation, characterized in that, It includes a pump light source (1), a first directional coupler (2), a second directional coupler (3), a periodically polarized waveguide (4), a first Bragg grating (5), and a second Bragg grating (6); the pump light emitted from the pump light source (1) is transmitted to the second directional coupler (3) after passing through the first directional coupler (2), and the first port of the first directional coupler (2) is connected to the first port of the second directional coupler (3); The second port of the second directional coupler (3) is connected to the second Bragg grating (6), the first port of the periodically polarized waveguide (4) is connected to the third port of the second directional coupler (3), and the fourth port of the second directional coupler (3) is used to connect external devices; the first port and the third port of the second directional coupler (3) are two ports of the same waveguide, and the second port and the fourth port of the second directional coupler (3) are two ports of the same waveguide; the periodically polarized waveguide (4) is connected to the first Bragg grating (5); The periodically polarized waveguide (4) is used to generate signal light and idler light through second-order nonlinear effects under the action of pump light transmitted by the second directional coupler (3), and transmit the signal light and idler light to the first Bragg grating (5); the first Bragg grating (5), the second Bragg grating (6), the periodically polarized waveguide (4) and the second directional coupler (3) form an on-chip Fabry-Perot cavity, and the signal light and idler light resonate in the on-chip Fabry-Perot cavity.

2. The optical parametric oscillator based on Bragg grating dispersion modulation according to claim 1, characterized in that, The first Bragg grating (5) and the second Bragg grating (6) have the same structure, both being apodized chirped Bragg gratings; the Bragg wavelength gradually increases from the front end to the back end of the grating.

3. The optical parametric oscillator based on Bragg grating dispersion modulation according to claim 1, characterized in that, The first Bragg grating (5) and the second Bragg grating (6) provide negative group delay dispersion in the signal light and idler light bands, and the sum of the group delay dispersion values ​​of the two gratings is equal to the dispersion value of the waveguide.

4. The optical parametric oscillator based on Bragg grating dispersion modulation according to claim 1, characterized in that, The first Bragg grating (5) and the second Bragg grating (6) achieve a reflection bandwidth exceeding a first preset threshold and a reflectivity exceeding a second preset threshold.

5. The optical parametric oscillator based on Bragg grating dispersion modulation according to claim 1, characterized in that, The waveguide includes, but is not limited to, nonlinear materials such as lithium niobate and lithium tantalate.

6. A laser wavelength conversion device, characterized in that, The device comprises an optical parametric oscillator based on Bragg grating dispersion modulation as described in claim 1.

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