Feedback type integrated linear frequency sweep laser based on thin film lithium niobate

Abstract

A feedback type integrated linear frequency sweep laser based on lithium niobate is characterized in that an external cavity laser is formed by a gain chip and a lithium niobate chip, the linear change of the cavity length is realized by using the linear electro-optical effect of lithium niobate so as to generate linear frequency sweep laser, and a lithium niobate acousto-optic frequency shifter is connected behind the output end of the lithium niobate chip so as to generate the linear frequency sweep laser. And a reflecting 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, and the total frequency shifting amount 2deltaf is obtained. The signal after frequency shift is injected into the laser to form feedback, so that the sweep frequency linearity is effectively improved, and random noise is suppressed. By adjusting the relationship among the sweep frequency slope gamma, the frequency shift amount 2 [delta] f and the total path delay T experienced by the feedback signal, gamma = 2 [delta] f / T is satisfied. The laser provided by the invention has high precision, high stability and excellent linear sweep frequency performance, and is suitable for various application scenes requiring a high-precision linear sweep frequency light source.

Description

Technical Field

[0001] The invention relates to an integrated linear frequency-sweeping laser, in particular to a feedback-type integrated linear frequency-sweeping laser based on thin-film lithium niobate. Background Art

[0002] In frequency modulated continuous wave (FMCW) laser ranging technology, the linear swept laser is the core device. The linearity of its swept laser output signal directly determines the resolution of the ranging signal after the beat frequency and the ranging accuracy of the FMCW ranging, and is a key factor in ensuring measurement accuracy.

[0003] In the existing technology, 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 solution for linear frequency sweep calibration based on an iterative algorithm, but it cannot cope with the long-term slow drift and random noise disturbances during the operation of the laser. 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 for further real-time feedback correction of linear optical frequency using FPGA and phase-locked loop, which can calibrate the frequency in real time, but the integration is poor. Reference 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.) adopts a time-frequency self-injection locking method, which can greatly improve the frequency sweep linearity, but requires the use of discrete devices that are difficult to integrate, such as circulators, erbium-doped fiber amplifiers, etc., and the degree of integration is also poor.

[0004] Therefore, the current frequency modulated continuous wave laser ranging technology is facing bottleneck problems such as low integration and susceptibility to external interference while pursuing high frequency sweep linearity. Developing a laser light source that has both high frequency sweep linearity and high integration has become a key technical problem that needs to be solved in this field. Summary of the invention

[0005] In view of the above-mentioned deficiencies in the prior art, the present invention provides a feedback integrated linear swept-frequency laser based on thin-film lithium niobate. By utilizing the electro-optic effect and acousto-optic effect of lithium niobate, a laser with simple structure, high integration and excellent swept-frequency linearity is constructed, so as to achieve high-linearity swept-frequency laser generation to meet the needs of high-precision laser ranging, laser radar and other fields.

[0006] Working principle of the present invention:

[0007] The present invention forms an external cavity laser with a reflective semiconductor gain chip RSOA and a lithium niobate chip, and utilizes the linear electro-optic effect of lithium niobate to achieve a linear change in the cavity length and generate a linear frequency-sweeping laser. At the same time, a lithium niobate acousto-optic frequency shifter is connected to the output of the lithium niobate chip to form a frequency-shifting feedback path. Assuming that the frequency shift of the acousto-optic frequency shifter is Δf in a single pass, a reflector is added to 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 frequency-sweeping linearity and suppressing random noise. In order to ensure that the frequency of the feedback signal is consistent with the current frequency of the frequency-sweeping laser, it is necessary to satisfy γ=2Δf / T, where γ is the frequency-sweeping slope of the frequency-sweeping laser (unit Hz / s), 2Δf is the total frequency shift of the feedback signal, and T is the total path delay experienced by the feedback signal.

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

[0009] A feedback integrated linear frequency sweeping laser and a preparation method thereof, comprising a gain chip and a lithium niobate chip, wherein the gain chip is a semiconductor gain chip RSOA having two end faces, one end face of which is plated with a high reflection film, and the other end face is plated with an anti-reflection film; the upper layer of the lithium niobate chip is a lithium niobate film, and the lower layer is a lower cladding layer, and the characteristics are:

[0010] On the niobate film, along the propagation direction of the optical signal, a spot converter, an electro-optic phase shifter, a frequency-selective filter, a partial reflector, a coupler, an acousto-optic frequency shifter and a total reflector are sequentially prepared;

[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 couple with the output waveguide at one end of the gain chip coated with an anti-reflection film. Its mode spot size matches the mode spot size of the gain chip end face, and the other end is a lithium niobate single-mode waveguide. The two ends are connected by a tapered waveguide to achieve adiabatic conversion of the mode spot size. In order to suppress the reflection of the chip end face, the waveguide of the mode spot converter on one side of the chip reaches the chip end face in an oblique incidence manner;

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

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

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

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

[0016] The acousto-optic frequency shifter can achieve frequency shift of optical signals. It is a two-port device. When the optical signal is input from one port and output from the other port, a frequency shift of Δf will be obtained. And due to the mutual difference of the acousto-optic device, no matter which port the input is from, the output signal will obtain a frequency shift of Δf;

[0017] The total reflection mirror is a one-port device that can return all input optical signals along the original path;

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

[0019] In addition, there is a driving module that generates three signals. The first signal is the drive signal of the gain chip, which controls the output power of the gain chip. The second signal is a pre-corrected triangle wave drive signal, which drives the electro-optic phase shifter, so that the frequency sweep signal output by the external cavity laser has better initial frequency sweep linearity. The other signal is the drive signal of the acousto-optic frequency shifter, which is used to control the frequency shift amount of the acousto-optic frequency shifter.

[0020] Furthermore, 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 optical signal in each of the above devices is along the y-axis;

[0021] Furthermore, the frequency shift amount Δf generated by the acousto-optic frequency shifter may be positive or negative.

[0022] Furthermore, the coupling method between 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.

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

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

[0025] Preferably, the partial reflector and the total reflector are both based on a toroidal mirror structure.

[0026] Preferably, the coupling ratio of the coupler should be appropriately selected 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 optional range of the frequency shift amount Δf of the acousto-optic frequency shifter is -4 GHz to 4 GHz.

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

[0029] The present invention also provides a method for preparing a feedback integrated linear frequency sweeping laser, which is characterized in that a 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 prepared on an x-cut lithium niobate film. In the above device, an optical waveguide is prepared by etching the lithium niobate 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 present invention has the following beneficial effects:

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

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

[0033] 3) Compared with the delayed feedback scheme in Document 3, the present invention does not require a circulator and an erbium-doped fiber amplifier that are difficult to integrate, and achieves on-chip integration. In addition, the present invention makes full use of the reciprocity of the acousto-optic frequency shifter, and obtains a double frequency shift by passing the optical signal through the frequency shifter twice. Finally, the final output end of the present invention 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 may also generate a feedback signal, the frequency of the feedback signal returned to the laser is different from the current frequency of the laser, and injection locking cannot be generated, so the operation of the laser is not easily disturbed.

[0034] 4) The present invention integrates multiple functional devices on a single lithium niobate chip through an integrated design, greatly simplifying the structure of the laser. The feedback mechanism and the electro-optical and acousto-optical effects of lithium niobate are used to achieve high-linearity swept-frequency laser generation. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a schematic diagram of an embodiment of a feedback-type integrated linear frequency-sweeping laser based on thin-film lithium niobate according to the present invention.

[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- total reflector, 10- driving module.

[0037] Figure 2 This is a schematic diagram of the principle of using feedback to improve the linearity of laser frequency sweeping. γ is the frequency sweep slope of the frequency sweeping laser (unit: Hz / s), 2Δf is the total frequency shift of the feedback signal, and T is the total path delay experienced by the feedback signal. DETAILED DESCRIPTION

[0038] The present invention will be further described below in conjunction with the accompanying drawings and examples, but the protection scope of the present invention shall not be limited thereto. The embodiments of the present invention include but are not limited to the following examples.

[0039] The present invention utilizes the unique properties of lithium niobate materials, namely, the electro-optic effect and the acousto-optic effect, to achieve high-linearity swept-frequency laser generation. Lithium niobate is an important optoelectronic material. Its electro-optic effect allows the refractive index of the material to be changed by an electric field, thereby achieving phase modulation of the optical signal; while the acousto-optic effect allows the frequency of the optical signal to be changed by a dynamic grating generated in the material by sound waves.

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

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

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

[0043] Driving module 10: generates three signals to drive the gain chip 1, the electro-optic phase shifter 4 and the 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 this embodiment is as follows:

[0045] The present invention combines a reflective semiconductor gain chip RSOA with a 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 used to shift the phase of the optical signal through an 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 achieving a linear change in the cavity length and generating a linear swept 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 sound 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 reflector 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 a 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 path delay experienced by the feedback signal. When this condition is met, the feedback signal will form an injection lock on the laser, thereby greatly improving the sweep linearity of the laser and suppressing random noise.

[0049] Figure 2 This is a schematic diagram of the principle of using feedback to improve the linearity of laser frequency sweeping. By introducing a feedback branch, the output signal of the laser is processed and then re-injected into the laser, thereby correcting the linearity of the laser frequency sweeping. Figure 2 As shown, when the laser performs an upward sweep (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 on the feedback path, and the 2Δf acousto-optic frequency shifter needs to produce a positive frequency shift to ensure that the feedback signal can interfere with the original signal of the laser and produce a stable beat signal. Similarly, when the laser performs a downward sweep, 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, where T is the total delay on the feedback path, and 2Δf is the negative frequency shift 2Δf that the acousto-optic frequency shifter needs to produce. At the edge of the frequency sweep transition, that is, when the laser transitions from up-sweep to down-sweep or from down-sweep to up-sweep, if the laser frequency in the up-sweep is already very close to the maximum frequency fmax, the frequency injected by the corresponding feedback branch has already corresponded to the down-sweep region. At this time, there is no need to further change the frequency shift amount 2Δf, because the feedback delay T (typical value is picoseconds to nanoseconds) is much smaller than the frequency sweep time (typical value is microseconds to milliseconds), accounting for less than one thousandth of the total time. This period of time can be directly deducted from the measured frequency modulated continuous wave beat signal.

Claims

1. A feedback integrated linear swept laser based on thin film lithium niobate, characterized in that: include: 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; A lithium niobate chip, wherein the upper layer is a lithium niobate film, and the lower layer is a lower cladding layer, and a pattern spot converter, an electro-optical phase shifter, a frequency selective 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 propagation direction of the optical signal; wherein, The mode spot converter is a two-port device, one end of which is coupled to the output waveguide of the end of the gain chip coated with an anti-reflection film, and its mode spot size 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 adiabatic conversion of the mode spot size is achieved between the two ends through a tapered waveguide; The electro-optical phase shifter is used to shift the phase of the optical signal to achieve a linear change in the cavity length and generate a linear frequency-sweeping laser; The frequency-selective filter is used to select the required laser emission frequency, and its bandwidth should cover the frequency sweep range required by the frequency-sweep laser; The partial reflector, as an output reflector of the external cavity laser, together with the gain chip and the mode spot converter, the electro-optic phase shifter and the frequency selective filter, constitutes an external cavity laser; The coupler is a 2x2 directional coupler, which is used to divide the output of the external cavity laser into two parts, wherein most of the light is directly output, and a small part of the light forms a feedback branch with the acousto-optic frequency shifter and the total reflection mirror; The acousto-optic frequency shifter is a two-port device. When an optical signal is input from one port and output from the other port, a frequency shift of Δf is obtained. Regardless of which port the optical signal is input from, the output signal will obtain a frequency shift of Δf. A total reflection mirror returns all the optical signals after the frequency shift by the acousto-optic frequency shifter to the original path to form a feedback signal; The feedback signal passes through the acousto-optic frequency shifter again to obtain a total frequency shift of 2Δf, and is then injected into the external cavity laser through a coupler to form feedback; when the condition γ=2Δf / T is met, the feedback signal will form injection locking on the laser, where γ is the sweep slope of the swept laser (unit Hz / s), 2Δf is the total frequency shift of the feedback signal, and T is the total path delay experienced by the feedback signal.

2. The feedback integrated linear swept laser based on thin film lithium niobate according to claim 1, characterized in that: It also includes a driver module for generating three signals: The first signal is a driving signal of the gain chip, which is used to control the output power of the gain chip; The second signal is a pre-corrected triangular wave-like driving signal, which is used to drive the electro-optical phase shifter so that the frequency sweeping signal output by the external cavity laser has better initial frequency sweeping linearity; The third signal is a driving signal of the acousto-optic frequency shifter, and is used to control the frequency shifting amount of the acousto-optic frequency shifter.

3. The feedback integrated linear frequency-sweeping laser based on thin-film lithium niobate 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 optical signal in each of the above devices is along the y-axis.

4. The feedback integrated linear frequency-sweeping laser based on thin-film lithium niobate according to claim 1 or 2, characterized in that: 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 feedback integrated linear frequency-sweeping laser based on thin-film lithium niobate according to claim 1 or 2, characterized in that: The mode of the optical signal in the electro-optical phase shifter is TE0, which is single-mode operation.

6. The feedback integrated linear frequency-sweeping laser based on thin-film lithium niobate according to claim 1 or 2, characterized in that: The frequency selective filter is a Bragg grating or a double micro-ring or multi-micro-ring structure based on a vernier caliper effect.

7. The feedback integrated linear frequency-sweeping laser based on thin-film lithium niobate according to claim 1 or 2, characterized in that: Both the partial reflector and the total reflector are based on a toroidal mirror structure.

8. The feedback integrated linear frequency-sweeping laser based on thin-film lithium niobate according to claim 1 or 2, characterized in that: The optional range of the frequency shift amount Δf of the acousto-optic frequency shifter is -4 GHz to 4 GHz.

Images