A feedback integrated linear sweep laser based on thin film lithium niobate

Abstract

A lithium niobate-based feedback integrated linear sweep laser, by forming an external cavity laser with a gain chip and a lithium niobate chip, and utilizing the linear electro-optic effect of lithium niobate to realize linear change of cavity length, thereby generating linear sweep laser, after the output end of the lithium niobate chip, a lithium niobate acousto-optic frequency shifter is connected, and a mirror is arranged at the output end of the acousto-optic frequency shifter, so that the optical signal passes through the acousto-optic frequency shifter twice, thereby obtaining a total frequency shift of 2Deltaf. This frequency shifted signal is injected into the laser to form a feedback, effectively improving the sweep linearity and suppressing random noise. By adjusting the relationship between the sweep slope gamma, the frequency shift 2Deltaf and the total delay T experienced by the feedback signal, gamma=2Deltaf / T is satisfied. The laser of the present application has high precision, high stability and excellent linear sweep performance, and is suitable for various application scenarios requiring high precision linear sweep light source.

Description

TECHNICAL FIELD

[0001] The present application relates to an integrated linear sweep laser, in particular to a feedback type integrated linear sweep laser based on thin film lithium niobate. BACKGROUND

[0002] In frequency-modulated continuous wave (FMCW) laser ranging technology, the linearity of the sweep laser output signal of the linear sweep laser as the core device directly determines the resolution of the beat signal after ranging and the ranging accuracy of FMCW ranging, and is the key factor to ensure the measurement accuracy.

[0003] In the prior art, document 1 (Zhang X, Pouls J, Wu M C. Laser frequency sweep linearization by iterative learning pre-distortion for FMCW LiDAR. Optics Express, 2019, 27(7): 9965-9974.) proposes a scheme for linear sweep calibration based on an iterative algorithm, but cannot cope with long-term slow drift and random noise disturbance when the laser works. Document 2 (Zhang J, Liu C, Su L, et al. Wide range linearization calibration method for DFB Laser in FMCW LiDAR. Optics and Lasers in Engineering, 2024, 174: 107961.) proposes a method of using FPGA and phase-locked loop for further real-time feedback correction of linear optical frequency, which can calibrate the frequency in real time, but the integrability is poor. Document 3 (Zhang J, Zheng X, Li S, Xue X. Ultra-Linear FMCW Laser Based on Time-Frequency Self-Injection Locking. Photonics Research. 2024, preprint 10.1364 / PRJ.537952.) uses a time-frequency self-injection locking method, which can greatly improve the sweep linearity, but requires the use of discrete devices such as circulators, erbium-doped fiber amplifiers, etc., which are difficult to integrate, and the integrability is also poor.

[0004] Therefore, while pursuing high sweep linearity in current frequency-modulated continuous wave laser ranging technology, there are problems such as low integration and susceptibility to external interference. Developing a laser source that has both high sweep linearity and high integration has become a key technical problem to be solved in this field. SUMMARY

[0005] In view of the deficiencies of the prior art, the present application provides a feedback type integrated linear sweep laser based on a thin film lithium niobate, which utilizes the electro-optic effect and the acousto-optic effect of lithium niobate to construct a laser with simple structure, high integration and excellent sweep linearity, and realizes the generation of sweep laser with high linearity to meet the needs of high-precision laser ranging, laser radar and other fields.

[0006] The working principle of the present application is as follows:

[0007] The present application forms an external cavity laser with a reflective semiconductor gain chip RSOA and a lithium niobate chip, utilizes the linear electro-optic effect of lithium niobate to realize linear change of the cavity length, and generates linear sweep laser. Meanwhile, a lithium niobate acousto-optic frequency shifter is connected after the output of the lithium niobate chip to form a frequency shift feedback path. Assuming that the acousto-optic frequency shifter shifts the frequency by Δf in a single pass, a mirror is added at the output end of the acousto-optic frequency shifter so that the signal passes through the acousto-optic frequency shifter again, and a total frequency shift of 2Δf is obtained. This frequency-shifted signal is injected into the laser to form feedback, thereby improving the sweep linearity and suppressing random noise. In order to ensure that the frequency of the feedback signal is consistent with the current frequency of the sweep laser, it is necessary to satisfy γ = 2Δf / T, where γ is the sweep slope of the sweep laser (unit: Hz / s), 2Δf is the total frequency shift of the feedback signal, and T is the total delay of the feedback signal path.

[0008] The technical solution of the present application is as follows:

[0009] A feedback type integrated linear sweep laser and a preparation method thereof, comprising a gain chip and a lithium niobate chip, the gain chip is a semiconductor gain chip RSOA with two end faces, one end face of which is coated with a high reflection film, and the other end face is coated with an anti-reflection film; the upper layer of the lithium niobate chip is a lithium niobate thin film, and the lower layer is a lower cladding layer, characterized in that:

[0010] On the lithium niobate thin film, a mode spot converter, an electro-optic phase shifter, a frequency selection filter, a partial mirror, a coupler, an acousto-optic frequency shifter and a total reflection mirror are sequentially prepared along the direction of light signal propagation;

[0011] The mode spot converter is a two-port device, one end of which is at the edge of the chip and is used to be coupled with the output waveguide of the gain chip coated with an anti-reflection film, the mode spot size of which matches the mode spot size of the end face of the gain chip, and the other end is a lithium niobate single-mode waveguide, and the mode spot size is converted by a tapered waveguide between the two ends. In order to suppress the reflection of the chip end face, the waveguide on the chip side of the mode spot converter reaches the chip end face in a way of oblique incidence;

[0012] The electro-optic phase shifter realizes the phase shift of the optical signal, the direction of the electric field generated by the electrode should be parallel to the lithium niobate crystal axis and perpendicular to the optical waveguide;

[0013] The frequency selective filter is used to select the required laser emission frequency, and the bandwidth thereof should cover the required sweep frequency range of the sweep laser;

[0014] The partial reflector is used as an output reflector of the external cavity laser, and the mode spot converter, the electro-optical phase shifter, the frequency selective filter and the partial reflector on the gain chip and the lithium niobate chip jointly constitute an external cavity laser;

[0015] The coupler is a 2x2 directional coupler, and the output of the external cavity laser passes through the coupler, most of the light (e.g. 80%) is directly output from the directional coupler, and a small part of the light (e.g. 20%) is connected to the subsequent acousto-optical frequency shifter and the total reflector to form a feedback branch;

[0016] The acousto-optical frequency shifter can realize frequency shift of the light signal, and is a two-port device, when the light signal is input from one port and output from another port, the frequency shift of Δf is obtained, and due to the reciprocity of the acousto-optical device, no matter which port is input, the output signal will obtain the frequency shift of Δf;

[0017] The total reflector is a one-port device, and can return the input light signal in the original path;

[0018] The acousto-optical frequency shifter and the total reflector constitute a feedback branch, the light signal enters the acousto-optical frequency shifter through the coupler, and then is reflected by the total reflector and passes through the acousto-optical frequency shifter again, and a total frequency shift of 2Δf is obtained. Subsequently, the frequency-shifted signal passes through the coupler and is injected into the laser to form a feedback. When the condition γ = 2Δf / T is met, where γ is the sweep slope (unit: Hz / s) of the sweep laser, 2Δf is the total frequency shift of the feedback signal, and T is the total delay of the path experienced by the feedback signal, the feedback signal will form an injection locking to the laser, which can greatly improve the sweep linearity of the laser and suppress random noise;

[0019] In addition, there are three driving signals generated by the driving module, the first one is the driving of the gain chip to control the output power of the gain chip; the second one is a pre-corrected triangular wave-like driving signal to drive the electro-optical phase shifter, so that the sweep signal output by the external cavity laser has good initial sweep linearity. The other one is the driving signal of the acousto-optical frequency shifter for controlling the frequency shift of the acousto-optical frequency shifter;

[0020] Further, the crystal axis of the lithium niobate thin film is the z-axis, the x-axis is along the normal direction of the thin film, the thin film plane is the yz plane, and the propagation direction of the light signal in the above-mentioned devices is along the y-axis;

[0021] Further, the frequency shift Δf generated by the acousto-optical frequency shifter can be positive or negative.

[0022] Further, the coupling mode of the gain chip and the lithium niobate chip can be end face close coupling, end face coupling through a spatial light device, flip coupling or bonding.

[0023] Preferably, the mode of the optical signal in the electro-optic phase shifter is TE0, and the single-mode operation is achieved.

[0024] Preferably, the frequency selection filter is a Bragg grating or a double-micro-ring or multi-micro-ring structure based on the vernier effect.

[0025] Preferably, the partial mirror and the full mirror are both based on a ring mirror structure.

[0026] Preferably, the coupling ratio of the coupler is selected appropriately so that the power injected into the laser through the feedback branch is 10% to 0.01% of the output power of the laser.

[0027] Preferably, the frequency shift Δf of the acousto-optic frequency shifter is selected in the range of -4GHz to 4GHz.

[0028] Preferably, the coupling mode of the gain chip and the lithium niobate chip is end face close coupling.

[0029] The application also provides a preparation method of the feedback integrated linear sweep frequency laser, which is characterized in that a mode spot converter, an electro-optic phase shifter, a frequency selection filter, a partial mirror, a coupler, an acousto-optic frequency shifter and a full mirror are prepared on an x-cut lithium niobate thin film. In the above devices, an optical waveguide is prepared by etching the lithium niobate thin film, an upper cladding layer is prepared by deposition, and an electrode is prepared by deposition and stripping.

[0030] Compared with the prior art, the application has the following beneficial effects:

[0031] 1) Compared with the pre-distortion correction scheme in document 1, the application retains the ability to generate a linear frequency modulation using a pre-distortion voltage while introducing integrated feedback self-injection locking, which can simultaneously suppress the influence of long-term operation state slow drift and random noise of the laser on the sweep frequency linearity.

[0032] 2) Compared with the scheme for realizing real-time correction based on an FPGA phase-locked loop in document 2, the application has low external electronic equipment requirements, does not require high-frequency detection and complex signal processing, and has high integration.

[0033] 3) Compared with the delay feedback scheme in document 3, the present application does not need a circulator and an erbium-doped fiber amplifier which are difficult to integrate, and realizes on-chip integration. Moreover, the present application fully utilizes the reciprocity of the acousto-optic frequency shifter, and obtains double frequency shift amount by passing the light signal through the frequency shifter twice. Finally, the final output end of the present application is not in the feedback branch, so although the end face reflection of the output end or the end face reflection of the external output path can also generate a feedback signal, the frequency of the feedback signal returning to the laser is different from the current frequency of the laser, and the injection locking cannot be generated, so the operation of the laser is not easily disturbed.

[0034] 4) The present application greatly simplifies the structure of the laser by integrating a plurality of functional devices on a single lithium niobate chip through an integrated design. The high linearity sweep laser generation is realized by utilizing the feedback mechanism and the electro-optic and acousto-optic effects of lithium niobate. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 is a schematic diagram of the feedback type integrated linear sweep laser of the present application based on thin film lithium niobate.

[0036] 1-gain chip, 2-lithium niobate chip, 3-mode spot converter, 4-electro-optic phase shifter, 5-frequency selection filter, 6-partial reflector, 7-coupler, 8-acousto-optic frequency shifter, 9-full reflector, 10-driving module.

[0037] Figure 2 is a schematic diagram of the principle of realizing the sweep linearity improvement of the laser by utilizing the feedback. γ is the sweep slope (unit: Hz / s) of the sweep laser, 2Δf is the total frequency shift amount of the feedback signal, and T is the total delay experienced by the feedback signal. DETAILED DESCRIPTION

[0038] The present application will be further described below in combination with the drawings and examples, but the protection scope of the present application should not be limited thereby. The embodiments of the present application include but are not limited to the following examples.

[0039] The present application utilizes the unique properties of lithium niobate material, i.e. both electro-optic effect and acousto-optic effect, to realize high linearity sweep laser generation. Lithium niobate is an important optoelectronic material, and its electro-optic effect allows the refractive index of the material to be changed by an electric field, thereby realizing the phase modulation of the optical signal; and the acousto-optic effect allows the frequency of the optical signal to be changed by the dynamic grating generated by the acoustic wave in the material.

[0040] Please refer to Figure 1 , Figure 1 is a schematic diagram of the feedback type integrated linear sweep laser of the present application based on thin film lithium niobate. As shown in the figure, a feedback type integrated linear sweep laser based on thin film lithium niobate comprises:

[0041] Gain chip 1, semiconductor gain chip RSOA with two end faces, one end face is coated with high reflection film, and the other end face is coated with anti-reflection film.

[0042] Lithium niobate chip 2: the upper layer is a lithium niobate film, and the lower layer is a lower cladding layer. On the lithium niobate film, a mode spot converter 3, an electro-optic phase shifter 4, a frequency selection filter 5, a partial mirror 6, a coupler 7, an acousto-optic frequency shifter 8 and a total reflection mirror 9 are sequentially prepared. Among them, the mode spot converter realizes the matching of the mode spot size between the gain chip and the lithium niobate chip, and suppresses the reflection of the chip end face. The electro-optic phase shifter realizes the phase shift of the optical signal through the electric field to improve the linearity of the sweep signal. The frequency selection filter selects the required laser emission frequency to ensure that the sweep range covers the demand. The partial mirror serves as the output mirror of the external cavity laser, and together with the gain chip and other devices on the lithium niobate chip forms an external cavity laser. The coupler divides the output of the external cavity laser into two parts, most of which is directly output, and a small part is used for feedback. The acousto-optic frequency shifter realizes the frequency shift of the optical signal and constitutes a feedback branch. The total reflection mirror returns the optical signal to the original route to realize the second frequency shift. Part of the optical signal is introduced into the feedback branch through the coupler, and after passing through the acousto-optic frequency shifter and the total reflection mirror, a frequency shift of 2Δf is obtained. When the condition γ=2Δf / T is met, the feedback signal will form an injection locking to the laser, greatly improving the sweep linearity of the laser and suppressing random noise.

[0043] Drive module 10: generates three-way signals to drive gain chip 1, electro-optic phase shifter 4 and acousto-optic frequency shifter 8 respectively, to ensure the normal operation of the laser and the stable operation of the feedback mechanism.

[0044] The working principle of the embodiment is as follows:

[0045] The application combines the reflective semiconductor gain chip RSOA with the lithium niobate chip to form an external cavity laser. One end face of the RSOA is coated with a high reflection film for reflecting the optical signal back into the cavity, and the other end face is coated with an anti-reflection film to reduce the reflection loss of the optical signal and effectively couple the optical signal to the lithium niobate chip.

[0046] On the lithium niobate chip, the linear electro-optic effect of lithium niobate is utilized to phase shift the optical signal through the electro-optic phase shifter. Due to the electro-optic effect, when the electric field applied to the electro-optic phase shifter changes, the phase of the optical signal will also change accordingly, thereby realizing the linear change of the cavity length and generating a linear sweep laser.

[0047] After the output of the lithium niobate chip, a lithium niobate acousto-optic frequency shifter is connected to form a frequency shift feedback path. The acousto-optic frequency shifter changes the frequency of the optical signal through the dynamic grating generated by the acoustic wave in the lithium niobate material. When the optical signal passes through the acousto-optic frequency shifter, a frequency shift Δf is obtained. By adding a mirror at the output end of the acousto-optic frequency shifter, the signal passes through the acousto-optic frequency shifter again, thereby obtaining a total frequency shift of 2Δf.

[0048] The frequency-shifted signal is injected into the laser through the coupler to form feedback. In order to ensure that the frequency of the feedback signal is consistent with the current frequency of the swept laser, the condition γ = 2Δf / T needs to be met, where γ is the sweep slope of the swept laser, 2Δf is the total frequency shift of the feedback signal, and T is the total delay of the feedback signal. When this condition is met, the feedback signal will form an injection locking to the laser, thereby greatly improving the sweep linearity of the laser and suppressing random noise.

[0049] Figure 2 is a schematic diagram of the principle of using feedback to improve the sweep linearity of the laser. By introducing a feedback branch, the output signal of the laser is processed and then re-injected into the laser, thereby correcting the sweep linearity of the laser. As shown in Figure 2 When the laser is sweeping up (sweep slope γ > 0), the laser frequency increases linearly with time. At this time, when the output signal of the laser is re-injected into the laser from the feedback branch, the frequency of the feedback signal should be 2Δf = γT, where T is the total delay of the feedback path, and 2Δf is the positive frequency shift that the acousto-optic frequency shifter needs to generate, thereby ensuring that the feedback signal can interfere with the original signal of the laser and produce a stable beat signal. Similarly, when the laser is sweeping down, the sweep slope γ < 0, and the laser frequency decreases linearly with time. At this time, when the signal is re-injected into the laser from the feedback branch, the frequency of the feedback signal should be 2Δf = γT, T is the total delay of the feedback path, and 2Δf is the negative frequency shift that the acousto-optic frequency shifter needs to generate 2Δf. At the edge of the sweep transition, i.e. when the laser transitions from up-sweeping to down-sweeping or from down-sweeping to up-sweeping, the corresponding feedback branch injection frequency has already corresponded to the down-sweeping region when the laser frequency in the up-sweeping has already approached the maximum frequency fmax. At this time, there is no need to further change the frequency shift 2Δf, because the feedback delay T (typical value: picoseconds to nanoseconds) is much smaller than the sweep time (typical value: microseconds to milliseconds), accounting for less than one thousandth of the total time, and can be directly deducted from the measured frequency-modulated continuous wave beat signal.

Claims

1. A thin film lithium niobate based feedback integrated linear sweep laser, characterized in that, The application relates to a semiconductor gain chip and a lithium niobate chip. The gain chip is a semiconductor gain chip RSOA with two end faces, one of which is coated with a high reflection film, and the other is coated with an anti-reflection film. The lithium niobate chip has a lithium niobate film as an upper layer and a lower cladding layer as a lower layer, and a mode spot converter, an electro-optic phase shifter, a frequency selection filter, a partial reflector, a coupler, an acousto-optic frequency shifter and a total reflector are sequentially prepared on the lithium niobate film along the light signal propagation direction. The mode spot converter is a two-port device, one end of which is coupled with the output waveguide of the end of the gain chip coated with the anti-reflection film, the mode spot size of which matches the mode spot size of the end face of the gain chip, and the other end is a lithium niobate single-mode waveguide, and the mode spot size is adiabatically converted between the two ends through a tapered waveguide. The electro-optic phase shifter is used for phase-shifting the light signal to linearly change the cavity length and generate a linear sweep frequency laser. The frequency selection filter is used for selecting a required laser emission frequency, and the bandwidth should cover the required sweep frequency range of the sweep frequency laser. The partial reflector is used as an output reflector of an external cavity laser, and the gain chip and the mode spot converter, the electro-optic phase shifter and the frequency selection filter jointly form an external cavity laser. The coupler is a 2x2 directional coupler, which is used for dividing the output of the external cavity laser into two parts, most of which is directly output, and a small part of which forms a feedback branch with the acousto-optic frequency shifter and the total reflector. The acousto-optic frequency shifter is a two-port device, which obtains a frequency shift of Delta f when the light signal is input from one port and output from the other port, and the output signal obtains a frequency shift of Delta f no matter which port is input. The total reflector returns all the light signals after frequency shift through the acousto-optic frequency shifter to form a feedback signal. After the feedback signal passes through the acousto-optic frequency shifter again and obtains a total frequency shift of 2Delta f, the feedback signal is injected into the external cavity laser through the coupler to form a feedback, and when the condition gamma=2Delta f / T is met, the feedback signal forms an injection locking to the laser, wherein gamma is the sweep frequency slope of the sweep frequency laser, unit Hz / s, 2Delta f is the total frequency shift of the feedback signal, and T is the total delay of the feedback signal.

2. The thin film lithium niobate based feedback integrated linear sweep frequency laser of claim 1, wherein, The application further comprises a driving module for generating three signals. The first signal is a driving signal of the gain chip, which is used for controlling the output power of the gain chip. The second signal is a pre-corrected triangular wave-like driving signal, which is used for driving the electro-optic phase shifter so that the sweep frequency signal output by the external cavity laser has good initial sweep frequency linearity. The third signal is a driving signal of the acousto-optic frequency shifter, which is used for controlling the frequency shift of the acousto-optic frequency shifter.

3. Thin film lithium niobate based feedback integrated linear sweep laser according to claim 1 or 2, characterized in that: The crystal axis of the lithium niobate film is the z axis, the x axis is along the normal direction of the film, the film plane is the yz plane, and the propagation direction of the light signal in the lithium niobate film is along the y axis.

4. The thin-film lithium niobate-based feedback integrated linear sweep laser of claim 1 or 2, wherein, The coupling mode of the gain chip and the lithium niobate chip can be end face close coupling, end face coupling through a spatial optical device, or flip chip coupling or bonding.

5. The thin-film lithium niobate-based feedback integrated linear sweep frequency laser of claim 1 or 2, wherein, The mode of the light signal in the electro-optic phase shifter is TE0, and the single-mode operation is realized.

6. The thin-film lithium niobate-based feedback integrated linear sweep frequency laser of claim 1 or 2, wherein, The frequency selection filter is a Bragg grating or a double-micro-ring or multi-micro-ring structure based on the vernier effect.

7. The thin-film lithium niobate-based feedback integrated linear sweep frequency laser of claim 1 or 2, wherein, The partial mirror and the full mirror are both based on a ring mirror structure.

8. The thin-film lithium niobate-based feedback integrated linear sweep laser of claim 1 or 2, wherein, The optional range of the frequency shift Δf of the acousto-optic frequency shifter is -4GHz~4GHz.

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