Wide-spectrum narrow-linewidth semiconductor mode-locked laser

Abstract

The invention belongs to the technical field of photoelectronics, and particularly relates to a wide-spectrum narrow-linewidth semiconductor mode-locked laser, and the collaboration of a saturable absorption region SA and a resonator is reflected in time domain-frequency domain dual optimization. And the resonator optimizes the frequency domain characteristics through nonlinear broadening, phase synchronization and mode purification. The SA adjusts a mode locking threshold value in real time through dynamic bias voltage, the pulse energy stability is ensured, dispersion management of the resonator balances the SPM and group velocity dispersion, spectrum splitting is avoided, and the dissipative soliton form is maintained. Besides, due to heterogeneous integration of the InP-based SA and the SiN-based resonator, wafer bonding of the InP-based SA and the SiN-based resonator is realized, high-efficiency energy transfer is realized through the low-loss conical waveguide, a high-performance frequency comb with a wide spectrum and a narrow pulse width is output, and the high-performance frequency comb can be widely applied to the fields of ultrafast optics, coherent light communication and precision spectroscopy.

Description

Technical Field

[0001] The present application belongs to the field of optoelectronic technology, and specifically relates to a wide spectrum narrow linewidth semiconductor mode-locked laser. Background Art

[0002] Optical resonator is a basic photonic structure, and devices based on this structure are widely used in communications, sensing and other fields. Small-scale microresonators can confine light in a very small volume, achieving a wider free spectral range and more flexible regulation and detection capabilities, and have outstanding performance in high-speed optical communications, spectroscopy and other fields.

[0003] In recent years, resonators of various structures have been developed, including ring, knot, and scroll resonator structures. Among them, micro-ring resonators (MRR) are widely used in many fields. MRR can accurately control the driving current, modulation signal, etc. of semiconductor mode-locked lasers, thereby achieving fine adjustment of the output characteristics of the laser, such as pulse width, repetition frequency, wavelength, etc., and improving the performance and stability of the laser. Traditional semiconductor mode-locked lasers are difficult to implement, and the spectral linewidth, spectral width, and signal-to-noise ratio produced are low, which is limited in applications with high pulse power requirements. Summary of the invention

[0004] Based on the above problems, this application introduces a resonator into the mode-locked laser to optimize the light field distribution in the cavity, enhance the nonlinear effect, improve the mode-locked stability, and enhance the pulse characteristics. SA plus reverse bias voltage achieves the effect of saturable absorption, and finally obtains mode-locked laser output. Its technical solution is:

[0005] A wide spectrum narrow line width semiconductor mode-locked laser comprises a saturable absorption region SA and a resonator arranged in a laser resonant cavity; the saturable absorption region SA and the resonator cooperate as follows:

[0006] The saturable absorption region SA is responsible for pulse time domain shaping and initial noise suppression, and the resonator performs nonlinear broadening;

[0007] The saturable absorption region SA adjusts the mode-locking threshold in real time through dynamic bias to ensure pulse energy stability. The dispersion management of the resonator balances the self-phase modulation SPM and group velocity dispersion to avoid spectral splitting and maintain the dissipative soliton morphology.

[0008] Preferably, the resonator is a microring resonator MRR or a microknot resonator MKR: the time-frequency characteristics of the pulse are optimized through a wavelength selective feedback mechanism: that is, when a wavelength in the pulse spectral component meets the resonance condition of the microring, the light wave of this wavelength will undergo multiple cycles of interference in the ring waveguide, forming a resonance enhancement effect, while the detuned wavelength is suppressed due to destructive interference.

[0009] Preferably, the InP-based saturable absorption region SA is wafer-bonded to the SiN-based resonator, and energy transfer is achieved through a tapered waveguide, outputting a frequency comb with a wide spectrum and a narrow pulse width.

[0010] Preferably, the saturable absorption region SA is introduced into the laser resonator, and its nonlinear absorption characteristics exhibit a dynamic response to changes in light intensity: when low-intensity background light or noise passes through, the absorption region is in an unsaturated state, strongly absorbing light energy and suppressing continuous radiation and spontaneous emission noise; while when the transient light intensity exceeds the saturation threshold, the carrier concentration in the absorption region enters a transparent state due to rapid depletion, allowing high-intensity pulses to pass through without loss.

[0011] Preferably, the nonlinear broadening of the resonator is calculated as follows:

[0012]

[0013] where c is the speed of light, N is the effective refractive index of the medium, R is the radius of the microring, and FSR refers to the frequency interval between adjacent longitudinal modes of the resonator;

[0014] When f p = FSR, it means that the frequency interval between adjacent longitudinal modes is equal to the operating frequency of the main mode of the laser; under this condition, the phases of all longitudinal modes are locked, forming a stable ultrashort pulse output;

[0015] When FSR is consistent with f p and mode locking occurs, the tooth interval of the frequency comb is determined by the integer multiples of f p and FSR:

[0016] f n = f p + k·FSR (k = 0, ±1, ±2...);

[0017] At this time, an optical frequency comb will be formed;

[0018] Under this condition, the phase accumulation of the optical field during multiple reflections in the cavity satisfies the resonance condition, i.e., 2nL = mλ, which will cause spectral broadening; n is the refractive index, m = 0, 1, 2..; λ is the wavelength.

[0019] Preferably, the resonator changes the refractive index with the light intensity through its third-order nonlinear effect, generating self-phase modulation SPM and four-wave mixing FWM in the resonator; self-phase modulation SPM causes symmetric spectral broadening through the phase change dependent on the light intensity, and at the same time balances with the anomalous dispersion in the cavity to form dissipative solitons, enhancing the temporal stability of the mode-locked pulses; FWM generates equally spaced new frequency components through energy transfer, expanding the spectral range; the third-order nonlinear effect causes the refractive index to change with the light intensity as shown in the following formula:

[0020] n(I) = n0 + n2I;

[0021]

[0022] where I is the light intensity, A eff is the effective mode field area, n2 is the nonlinear refractive index, and n0 is the linear refractive index;

[0023] The nonlinear phase change of the optical field after propagating a distance z is:

[0024]

[0025] where ω0 is the central angular frequency of the light, I(z′) is the light intensity at the propagation distance z′ of the optical field, z is the propagation distance of the optical field, P = |A| 2 is the instantaneous power, and γ is the nonlinear coefficient;

[0026] Four-wave mixing coupling equation: Let the pump light frequency be ω p , the signal and idler lights be ω s and ω i , satisfying the energy conservation ω s + ω i = 2ω p , the optical field evolution equation is:

[0027]

[0028] where, A s is the complex amplitude of the signal light, A i is the complex amplitude of the idler light, A p is the complex amplitude of the pump light, γ is the nonlinear coefficient, are the loss terms respectively; z is the propagation distance of the optical field, the wave vector mismatch Δk = 2k p - k s - k i , k p , k s , k i are the wave vectors of the pump light, signal light, and idler light respectively, and the phase matching condition is Δk = 0;

[0029] The wave vector mismatch can be expanded as:

[0030]

[0031] where γ is the nonlinear coefficient, P p is the pump light power, β2 is the second-order dispersion coefficient, Δω = ω s - ω p , ω s , ω pare the angular frequencies of the signal light and the pump light, respectively; β4 is the fourth-order dispersion. In the anomalous dispersion region, Δk may be zero, supporting broadband FWM;

[0032] When Δk = 0, the power gain coefficient of FWM is:

[0033] G = sinh 2 (γP p L eff );

[0034]

[0035] where γ is the nonlinear coefficient, P p is the pump light power, L eff is the effective length, α is the attenuation coefficient, L is the actual propagation length of light in the medium, and the gain bandwidth is determined by the phase matching condition:

[0036]

[0037] The Q value of the MRR is defined as:

[0038]

[0039] where ω0 is the central angular frequency of light, n g is the group refractive index, α rt is the single-round loss, L’ is the cavity length, and the photon lifetime τ photon > 10 ns corresponds to Q > 10 6 .

[0040] Preferably, the preparation method of the saturable absorption region SA:

[0041] First, a multi-layer structure is epitaxially grown on an InP substrate by metalorganic chemical vapor deposition, which are an n-type InP buffer layer, an InGaAs multiple quantum well gain layer, an InGaAsP saturable absorption layer, and a p-type InP top layer in sequence; among them, the InGaAs multiple quantum well is used for laser gain, and the narrow bandgap characteristic of InGaAsP can meet the saturable absorption requirements of the 1550 nm communication band;

[0042] Subsequently, a SiN thin film is deposited on the InP surface by plasma-enhanced chemical vapor deposition. An electron beam lithography special glue PMMA is spin-coated on the SiN layer to achieve a uniform coating of 1.2 μm at a set rotation speed. After drying and curing, the rectangular waveguide and resonator patterns of the saturable absorption region SA are defined by electron beam lithography in zones; after exposure, it is developed with a mixed solution of methyl isobutyl ketone and isopropyl alcohol, and post-baked at 110 °C to enhance the etching resistance;

[0043] Finally, the waveguide etching in the SA region adopts an inductively coupled plasma process with a Cl2 / Ar mixed gas. The characteristic spectrum line of InGaAs is monitored in real time through optical emission spectroscopy (OES) to ensure an accurate stop at the p-InP layer interface, and the verticality of the etched sidewalls reaches more than 80°.

[0044] Preferably, the preparation method of the microring resonator (MRR) is as follows:

[0045] Bond a prefabricated SiN-on-insulator wafer to an InP substrate;

[0046] After defining the ring waveguide by electron beam lithography (EBL), etch the SiN with an SF6 / C4F8 mixed gas inductively coupled plasma (ICP) until the remaining thickness is 150 nm, and the verticality of the sidewalls is >85°. Subsequently, optimize the sidewall roughness by dynamically soaking in diluted HF;

[0047] After removing the hard mask with a Cr wet etching solution, perform rapid thermal annealing in a nitrogen atmosphere at 400 °C to repair the etching damage;

[0048] Evaporate Ti / Pt / Au as the p-type electrode on the surface of the SA region by electron beam evaporation, and the n-type electrode is realized by backside evaporation of Ge / Au;

[0049] After selectively ultrasonically stripping the excess metal with acetone, perform rapid annealing in nitrogen at 280 °C to form ohmic contacts;

[0050] By applying a reverse bias voltage, the depletion region covers the InGaAsP absorption layer to achieve electro-optically controlled saturable absorption characteristics. Finally, the device is encapsulated by depositing a 2-μm-thick SiO2 cladding layer by plasma-enhanced chemical vapor deposition (PECVD) and chemical mechanical polishing;

[0051] The preparation method of the MKR is as follows:

[0052] Adopt the plasma-activated bonding technology to bond the prefabricated SiN-on-insulator wafer to the InP substrate after oxygen plasma pretreatment, and apply pressure to complete room-temperature bonding, with the interface loss controlled within <0.2 dB / cm;

[0053] Define the ring waveguide and coupled waveguide patterns of the MKR by electron beam lithography, set the ring diameter, waveguide width, coupling gap, and exposure parameters, and develop using a TMAH solution;

[0054] After using the ICP etching process, soak in a diluted HF solution for seconds, and then perform high-temperature annealing in an N2 atmosphere to eliminate the etching damage and improve the SiN lattice quality. Deposit a 1-μm-thick SiO2 upper cladding layer by PECVD, and then achieve surface planarization through chemical mechanical polishing. Finally, the surface roughness is <0.5 nm, and the Q value of the MKR is >1×10 5 , and the deviation of the free spectral range (FSR) from the theoretical value is <0.1 nm.

[0055] Preferably, in a narrow linewidth and broad spectrum mode-locked laser, the ring resonator MRR and the electro-optically modulated saturable absorber SA achieve the efficient generation of a broad spectrum, narrow linewidth, and high coherence frequency comb through multi-level synergistic effects. The steps are as follows:

[0056] First, the absorption saturation intensity is adjusted by a dynamic reverse bias. Using its non-linear absorption effect, high-intensity pulses are quickly selected and spontaneous emission noise is suppressed, forcing the formation of initial mode-locked pulses in the cavity. During this process, the short recovery time of the SA significantly compresses the pulse temporal width by accelerating carrier sweep-out. At the same time, its electro-tuning ability dynamically suppresses mode competition and reduces the time jitter on a short time scale.

[0057] Subsequently, under the combined action of injecting pulses into the high-Q resonator of the ring resonator MRR, the non-linear phase shift dominated by self-phase modulation SPM causes the spectral symmetric broadening, while four-wave mixing FWM generates equally spaced frequency components through energy transfer. At the same time, the high-Q resonance characteristics of the ring resonator MRR selectively filter the longitudinal mode phase noise. Combining with the active frequency stabilization technology, the single longitudinal mode linewidth is compressed to sub-kilohertz, and long-range phase locking is established through the resonance-enhanced Kerr non-linear effect to suppress the long-term time jitter between pulses.

[0058] Considering thermal noise and shot noise, the corrected linewidth is:

[0059]

[0060] where v0 is the initial frequency, is the phase noise spectral density function, and the active frequency stabilization technology can suppress the phase noise to 10 -14 rad 2 / Hz, achieving a sub-hertz linewidth.

[0061] Preferably, in a narrow linewidth and broad spectrum mode-locked laser, the micro-ring resonator MKR and the electro-modulated saturable absorption region achieve efficient generation of a broad-spectrum and narrow-linewidth frequency comb through multi-dimensional coupling of material properties and dynamic regulation: The electro-modulated saturable absorption region SA, based on the strong non-linear absorption characteristics of InP-based multiple quantum wells, quickly filters high-intensity pulses and suppresses spontaneous emission noise through the carrier sweep-out effect under reverse bias, triggering the formation of initial mode-locked pulses. Its dynamically adjustable absorption saturation intensity and the high-Q value resonator of MKR jointly compress the pulse time-domain width to the sub-picosecond level, while simultaneously suppressing mode competition in real-time through electro-tuning to reduce time jitter; After the pulse is injected into the SiN-based MKR, the self-phase modulation SPM induced by its third-order non-linear effect forms dissipative solitons by balancing with group velocity dispersion in the anomalous dispersion region, symmetrically broadening the spectrum to cover the C+L band, that is, 1530nm - 1625nm; The four-wave mixing FWM generates equally spaced frequency components through energy transfer, and selectively filters the longitudinal mode phase noise in combination with the high photon lifetime of MKR, compressing the single longitudinal mode linewidth to sub-kilohertz; MKR establishes long-range phase locking through a non-linear spectral expansion and phase synchronization mechanism.

[0062] Advantageous Effects

[0063] The synergy between SA and the resonator is reflected in the dual optimization of the time domain - frequency domain: SA completes the pulse time-domain shaping and initial noise suppression, while MRR optimizes the frequency domain characteristics through non-linear broadening, phase synchronization, and mode purification. SA adjusts the mode-locking threshold in real-time through dynamic biasing to ensure the stability of pulse energy, while the dispersion management of the resonator balances SPM and group velocity dispersion, avoiding spectral splitting and maintaining the dissipative soliton form. In addition, the heterogeneous integration of the two, the wafer bonding of InP-based SA and SiN-based resonator in the present invention, realizes efficient energy transfer through a low-loss tapered waveguide, outputting a high-performance frequency comb with a broad spectrum and narrow pulse width, which can be widely applied in the fields of ultrafast optics, coherent optical communication, and precision spectroscopy. Description of the Drawings

[0064] Figure 1 This is the schematic diagram of the present application (the resonator is MRR);

[0065] Figure 2 This is the schematic diagram of the present application (the resonator is MKR). Detailed Embodiments

[0066] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the embodiments of the present invention and the accompanying drawings of the specification. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0067] A wide-spectrum narrow-linewidth semiconductor mode-locked laser, comprising a saturable absorption region SA and a resonator disposed in a laser resonator cavity. The InP-based saturable absorption region SA is wafer-bonded to the SiN-based resonator, and energy transfer is achieved through a tapered waveguide, outputting a frequency comb with a wide spectrum and narrow pulse width. The saturable absorption region SA and the resonator cooperate as follows:

[0068] The saturable absorption region SA is responsible for pulse time-domain shaping and initial noise suppression, and the resonator performs non-linear broadening to optimize the frequency-domain characteristics.

[0069] The saturable absorption region SA adjusts the mode-locking threshold in real time through a dynamic bias voltage to ensure the stability of the pulse energy. The dispersion management of the resonator balances self-phase modulation (SPM) and group velocity dispersion to avoid spectral splitting and maintain the dissipative soliton form.

[0070] The resonator is a microring resonator (MRR) or a microjunction resonator (MKR). Through a wavelength-selective feedback mechanism, the time-frequency characteristics of the pulse are optimized. That is, when a certain wavelength in the pulse spectral components satisfies the resonance condition of the microring, the light wave of this wavelength will undergo multiple cyclic interferences in the ring waveguide, forming a resonance enhancement effect, while the detuned wavelengths are suppressed due to destructive interference.

[0071] In the present invention, the most representative electrically modulated saturable absorption region is selected as SA to realize semiconductor mode-locked laser. One advantage of the electrically modulated saturable absorption region is that it can use the same process as the gain region, and then apply a reverse bias voltage to achieve the saturable absorption effect, and finally obtain mode-locked laser output.

[0072] Preparation method of the saturable absorption region SA:

[0073] First, a multi-layer structure is epitaxially grown on an InP substrate by metalorganic chemical vapor deposition (MOCVD), which are an n-type InP buffer layer, an InGaAs multiple quantum well gain layer, an InGaAsP saturable absorption layer (SA layer), and a p-type InP top layer in sequence. Among them, the InGaAs multiple quantum wells are used for laser gain, and the narrow bandgap characteristics of InGaAsP can meet the saturable absorption requirements of the 1550 nm communication band.

[0074] Subsequently, a 400-nm-thick SiN film was deposited on the InP surface using plasma-enhanced chemical vapor deposition (PECVD). The deposition temperature was controlled below 300 °C to avoid thermal damage to the substrate. Electron beam lithography special glue PMMA was spin-coated on the SiN layer, and a uniform coating of 1.2 μm was achieved at a rotation speed of 3000 rpm. After curing by pre-baking at 90 °C for 60 seconds, electron beam lithography (EBL) was used to define the rectangular waveguide and micro-ring resonator (MRR) patterns of the SA region in partitions. After exposure, it was developed using a mixed solution of methyl isobutyl ketone (MIBK) and isopropyl alcohol (IPA), and post-baked at 110 °C to enhance the etching resistance.

[0075] For the etching of the SA waveguide, an inductively coupled plasma (ICP) process with a Cl2 / Ar mixed gas was used. The radio frequency power was set at 100 W and the chamber pressure was 5 mTorr. The characteristic spectrum line of InGaAs (such as the In spectrum at 451 nm) was monitored in real time through optical emission spectroscopy (OES) to ensure precise stopping at the p-InP layer interface, and the etching sidewall perpendicularity reached more than 80°.

[0076] The preparation method of the MRR is as follows:

[0077] The MRR was prepared by bonding a prefabricated SiN-on-insulator wafer to the InP substrate. Before bonding, the surface was activated by oxygen plasma, and the interface loss was <0.1 dB under the conditions of 2 MPa pressure and room temperature. After defining the ring waveguide by EBL, the SiN was etched to a remaining thickness of 150 nm using an SF6 / C4F8 mixed gas ICP, and the sidewall perpendicularity >85°. Subsequently, it was dynamically soaked in diluted HF (1:50) for 5 seconds to optimize the sidewall roughness (RMS <2 nm). After removing the hard mask using a Cr wet etching solution, rapid thermal annealing (RTA) was performed in a nitrogen atmosphere at 400 °C for 10 seconds to repair the etching damage. This temperature could avoid thermal decomposition of the InP substrate while maintaining low optical loss of SiN (<1 dB / cm).

[0078] Finally, Ti / Pt / Au (20 / 50 / 150 nm) was electron beam evaporated on the SA surface as the p-type electrode, and the n-type electrode was achieved by back evaporation of Ge / Au. After selectively ultrasonic stripping of the excess metal using acetone, rapid annealing was performed in nitrogen at 280 °C for 20 seconds to form an ohmic contact (specific contact resistance <1×10 -5 Ω·cm 2 ). By applying a reverse bias voltage, the depletion region covered the InGaAsP absorption layer to achieve electrically controlled saturated absorption characteristics. Finally, the device was encapsulated by depositing a 2-μm-thick SiO2 cladding layer by PECVD and chemical mechanical polishing (CMP).

[0079] The preparation method of the MKR is as follows:

[0080] First, the plasma-activated bonding technology is adopted. After the prefabricated SiN-on-insulator wafer (SiN layer thickness 400 nm, buried oxide layer 2 μm) and the InP substrate (including n-type InP buffer layer, multiple quantum well gain layer and p-type InP top layer) are pretreated in oxygen plasma (power 50 W, time 60 s), a pressure of 10 MPa is applied to complete the room-temperature bonding, and the interface loss is controlled within <0.2 dB / cm. Subsequently, the annular waveguide and coupled waveguide patterns of the MKR are defined by electron beam lithography (EBL), with the annular diameter set at 50 μm, the waveguide width at 1.5 μm, and the coupling gap at 200 nm. The exposure parameters are an acceleration voltage of 100 kV and a dose of 350 μC / cm 2 . The development is carried out by treating with a TMAH (2.38%) solution for 60 seconds. Then, the ICP etching process is used to etch the SiN layer to a remaining thickness of 150 nm at a gas ratio of CF4 / CHF3 / Ar = 30 / 20 / 10 sccm, a radio frequency power of 150 W, and a chamber pressure of 15 mTorr. The etching rate is 80 nm / min, the sidewall perpendicularity is >85°, and the surface roughness is <1 nm (verified by AFM); after the etching is completed, the sidewall morphology is optimized by soaking in a diluted HF solution (HF:H2O = 1:50) for 10 seconds, and then high-temperature annealing is carried out at 1000 °C for 30 minutes in an N2 atmosphere to eliminate the etching damage and improve the SiN lattice quality (the full width at half maximum of the Raman spectrum is reduced to 5 cm- 1 ). To reduce the optical transmission loss, a 1-μm-thick SiO2 upper cladding is deposited by PECVD, and then chemical mechanical polishing (CMP) is carried out to achieve surface planarization (polishing pressure 3 psi, rate 50 nm / min). Finally, the surface roughness is <0.5 nm. This process ensures that the Q value of the MKR is >1×105, and the deviation of the free spectral range (FSR) from the theoretical value is <0.1 nm, providing a high-precision resonance basis for wide-spectrum tuning.

[0081] The Electrically Modulated Saturable Absorber (SA) is one of the core components of semiconductor mode-locked lasers. Its main function is to achieve the startup and maintenance of mode locking by electrically controlling the absorption characteristics, thereby generating ultrashort optical pulses. The two major characteristics of SA are the saturable absorption effect and self-starting mode locking. At low optical intensities, SA has a high absorption rate for light, suppressing low-intensity noise and spontaneous emission light in the cavity; at high optical intensities, the concentration of photo-generated carriers increases rapidly, the absorption saturates, allowing high-intensity pulses to pass through, and finally screening out high-intensity pulses through nonlinear absorption to initiate the mode-locking process. SA automatically screens out high-intensity pulses through the saturable absorption effect and can achieve mode locking without an external modulation signal. Under a reverse bias voltage, the carriers in SA are quickly swept out by the electric field, and the recovery time can be as short as the picosecond level, and the absorption recovery speed is much faster than the carrier lifetime of the gain medium. The absorption recovery speed is synchronized with the pulse evolution, further compressing the pulse width to the femtosecond level. Compared with ordinary SA, the electrically modulated saturable absorber can adjust the absorption depth by changing the magnitude of the reverse bias voltage, thereby controlling the absorption saturation intensity.

[0082] However, relying solely on SA cannot achieve the wide-spectrum and narrow-pulse-width pulses we want. The directly output spectral width is generally on the order of several nanometers, and the output signal-to-noise ratio is also relatively low. The inherent gain bandwidth of semiconductor gain media is usually narrow, which limits the width of the directly output spectrum. In the initial mode-locking stage, only some longitudinal modes satisfy the phase-locking condition, and other wavelengths are suppressed due to insufficient gain. Moreover, the peak power of the initial mode-locked pulses is low, and the nonlinear effect is too weak to effectively broaden the spectrum. Although the high absorption in the SA region suppresses multi-longitudinal-mode oscillation and only allows the phase-locked longitudinal modes to pass through, reducing the noise caused by mode competition, when the mode locking is not completely stable, the spontaneous emission noise is not fully suppressed, resulting in background noise between pulses. Moreover, if the debugging depth is low and the recovery time does not match, it may deteriorate the signal-to-noise ratio. To achieve the goal of wide spectrum and narrow pulse width, we introduce the cooperation of a resonator and SA.

[0083] The Micro-Ring Resonator (MRR) is a high-performance integrated optical resonator. Its core role is to significantly improve the performance of photonic devices by minimizing losses and maximizing the localization of the optical field. The Q value of MRR can reach 10 6 or more, while the Q value of traditional micro-rings is about 10 4 to 10 5 . The high Q value ensures an extended photon lifetime.

[0084] The Free Spectral Range (FSR) is an important parameter of the MRR. The FSR refers to the frequency interval between adjacent longitudinal modes of the resonator cavity, which is determined by the radius R and the effective refractive index n of the medium. The expression is as follows,

[0085]

[0086] where c is the speed of light, N is the effective refractive index of the medium, and R is the radius of the micro-ring.

[0087] f p is the main mode frequency output by the laser, which is usually determined by the gain spectrum of the gain medium and the cavity mode matching. When f p = FSR, it means that the frequency interval between adjacent longitudinal modes is equal to the operating frequency of the main mode of the laser. Under this condition, the phases of all longitudinal modes are locked to form a stable ultrashort pulse output.

[0088] When FSR is consistent with f p and mode locking occurs, the tooth interval of the frequency comb is determined by f p and integer multiples of FSR:

[0089] f n = f p + k·FSR (k = 0, ±1, ±2……)

[0090] At this time, an optical frequency comb will be formed. Moreover, under this condition, the phase accumulation of the light field reflected multiple times in the cavity satisfies the resonance condition, that is, 2nL = mλ, which will cause spectral broadening.

[0091] The MRR (micro-ring resonator) makes the refractive index change with the light intensity through its third-order nonlinear effect (such as the optical Kerr effect), generating self-phase modulation (SPM) and four-wave mixing (FWM) in the resonator cavity. SPM causes symmetric spectral broadening through the phase change dependent on the light intensity, and at the same time forms dissipative solitons by balancing with the anomalous dispersion in the cavity, enhancing the temporal stability of the mode-locked pulse; FWM generates equally spaced new frequency components through energy transfer, significantly expanding the spectral range.

[0092] The third-order nonlinear effect causes the refractive index to change with the light intensity as shown in the following formula,

[0093]

[0094] where I is the light intensity, A eff is the effective mode field area, n0 is the linear refractive index, and n2 is the nonlinear refractive index coefficient.

[0095] The nonlinear phase change of the light field after propagating a distance z is:

[0096]

[0097] where ω0 is the central angular frequency of light, c is the speed of light, n2 is the nonlinear refractive index, I(z′) is the light intensity at the re-propagation distance z′ of the light field, z is the propagation distance of the light field, and P = |A| 2 is the instantaneous power, and γ is the nonlinear coefficient.

[0098] Four-wave mixing coupling equation: Let the pump light frequency be ω p , the signal and idler lights be ω s and ω i , satisfying the energy conservation ω s +ω i = 2ω p . The light field evolution equation is

[0099]

[0100] where, A s is the complex amplitude of the signal light, A i is the complex amplitude of the idler light, A p is the complex amplitude of the pump light, γ is the nonlinear coefficient, are the loss terms respectively; z is the propagation distance of the light field, the wave vector mismatch Δk = 2k p -k s -k i , k p , k s , k i are the wave vectors of the pump light, signal light, and idler light respectively, and the phase matching condition is Δk = 0.

[0101] The wave vector mismatch can be expanded as:

[0102]

[0103] where γ is the nonlinear coefficient, P p is the pump light power, β2 is the second-order dispersion coefficient, Δω = ωs - ωp, and β4 is the fourth-order dispersion. In the anomalous dispersion region, Δk may be zero, supporting broadband FWM.

[0104] When Δk = 0, the power gain coefficient of FWM is:

[0105]

[0106] where, γ is the nonlinear coefficient, P p is the pump light power, L eff is the effective length, α is the attenuation coefficient, and L is the actual propagation length of light in the medium. The gain bandwidth is determined by the phase matching condition:

[0107]

[0108] The high-Q characteristic of the MRR (photon lifetime > 10 ns) compresses the single longitudinal mode linewidth to the sub-kilohertz level. Combining with the active frequency stabilization technology to suppress phase noise, thereby improving the coherence of the frequency comb. The mode-locking mechanism drives the longitudinal mode phase synchronization, while SPM and FWM further optimize the pulse waveform and spectral flatness, and finally achieve the output of an optical frequency comb with a wide spectrum, narrow linewidth and stable frequency interval.

[0109] The Q value of the MRR is defined as:

[0110]

[0111] where ω0 is the central angular frequency of the light, n g is the group refractive index, α rt is the single-round loss, L’ is the cavity length, and the photon lifetime τ photon > 10 ns corresponds to Q > 10 6 .

[0112] Figure 1 In a narrow-linewidth and wide-spectrum mode-locked laser, the micro-ring resonator and the electro-optically modulated saturable absorption region achieve the efficient generation of a wide-spectrum, narrow-linewidth, and high-coherence frequency comb through multi-level synergistic effects. The electro-optically modulated saturable absorption region, as the triggering core of mode locking, first adjusts the absorption saturation intensity through a dynamic reverse bias, and uses its nonlinear absorption effect to quickly select high-intensity pulses and suppress spontaneous emission noise, forcing the formation of initial mode-locked pulses in the cavity. In this process, the short recovery time of the SA region significantly compresses the pulse temporal width by accelerating the carrier sweep-out, and at the same time its electro-tuning ability dynamically suppresses mode competition and reduces the time jitter on a short time scale. Subsequently, under the combined action of injecting the pulse into the high-Q resonator of the MRR, the self-phase modulation (SPM)-dominated nonlinear phase shift leads to symmetric spectral broadening, while four-wave mixing (FWM) generates equally spaced frequency components through energy transfer. At the same time, the high-Q resonance characteristic of the MRR selectively filters the longitudinal mode phase noise. Combining with the active frequency stabilization technology, the single longitudinal mode linewidth is compressed to sub-kilohertz, and long-range phase locking is established through the resonance-enhanced Kerr nonlinear effect to suppress the long-term time jitter between pulses.

[0113] Considering thermal noise and shot noise, the corrected linewidth is:

[0114]

[0115] where, v0 is the initial frequency, is the phase noise spectral density function. The active frequency stabilization technology can suppress the phase noise to 10 -14 rad 2 / Hz, achieving a sub-hertz linewidth.

[0116] Figure 2 In a narrow-linewidth and wide-spectrum mode-locked laser, the micro-ring resonator (MKR) and the electro-modulated saturable absorption region achieve the efficient generation of a wide-spectrum and narrow-linewidth frequency comb through multi-dimensional coupling of material properties and dynamic regulation: The electro-modulated saturable absorption region (SA) is based on the strong non-linear absorption characteristics of InP-based multiple quantum wells. Under reverse bias (-3 to -5 V), high-intensity pulses are rapidly screened and spontaneous emission noise is suppressed through the carrier sweep-out effect (recovery time < 0.5 ns), triggering the formation of the initial mode-locked pulse. Its dynamically adjustable absorption saturation intensity (10 to 50 cm- 1 ) and the high-Q resonator of the MKR (Q > 1×105) jointly compress the pulse temporal width to the sub-picosecond level. At the same time, mode competition is suppressed in real time through electro-tuning to reduce time jitter; After the pulse is injected into the SiN-based MKR, the self-phase modulation (SPM) induced by its third-order non-linear effect forms dissipative solitons in the anomalous dispersion region in balance with group velocity dispersion, broadening the spectrum symmetrically to cover the C+L band, that is, 1530 nm - 1625 nm. Four-wave mixing (FWM) generates equally spaced frequency components through energy transfer (the interval strictly corresponds to the FSR). Combining with the high photon lifetime of the MKR (> 10 ns), the longitudinal mode phase noise is selectively filtered, and the single longitudinal mode linewidth is compressed to sub-kilohertz (Δν < 500 Hz); The synergy between the SA and the MKR is reflected in the dual optimization in the time-frequency domain - the SA adjusts the mode-locking threshold through dynamic bias to stabilize the pulse energy (fluctuation < 0.5 dB), while the MKR establishes long-range phase locking through the non-linear spectral expansion and phase synchronization mechanism (active frequency stabilization technology compensates for thermal drift, frequency stability < 10 MHz / h). At the same time, the heterogeneous integration process ensures the efficient transmission of the lower optical field and the enhancement of non-linear interaction, and finally outputs an optical frequency comb with both narrow linewidth, high flatness and frequency interval stability, providing a breakthrough solution for applications such as coherent optical communication and precision spectroscopy measurement.

[0117] In a laser system, the semiconductor gain medium achieves population inversion through current injection, stimulates stimulated emission, and generates an initial laser output. At this time, the laser usually appears as a continuous wave or a weakly modulated pulse with a wide spectrum. To convert the broadband laser into a narrow linewidth ultrashort pulse with high stability, a semiconductor saturable absorber region is introduced into the laser resonator. Its nonlinear absorption characteristics exhibit a dynamic response to changes in light intensity: when low-intensity background light or noise passes through, the absorber region is in an unsaturated state, strongly absorbing the light energy and suppressing continuous radiation and spontaneous emission noise; while when the transient light intensity exceeds the saturation threshold, the carrier concentration in the absorber region enters a transparent state due to rapid depletion, allowing high-intensity pulses to pass through without loss. This light intensity-dependent "gating" effect forces the energy in the cavity to concentrate on the instantaneous high-peak pulses, prompting the initial wide pulses to be compressed into ultrashort optical pulses in the picosecond to femtosecond range. At the same time, the microresonator integrated in the laser cavity optimizes the time-frequency characteristics of the pulses through a wavelength-selective feedback mechanism, that is, when a certain wavelength in the pulse spectral components satisfies the resonance condition of the microresonator, the light wave of this wavelength will undergo multiple cyclic interferences in the ring waveguide, forming a resonance enhancement effect, while the detuned wavelengths are suppressed due to destructive interference. This frequency-domain filtering effect not only significantly compresses the laser linewidth but also promotes the symmetric broadening of the pulse spectrum through the third-order nonlinear effect in the resonator and generates an equally spaced frequency comb structure.

Claims

1. A wide spectrum narrow linewidth semiconductor mode-locked laser, characterized in that: The invention comprises a saturable absorption region SA and a resonator arranged in a laser resonant cavity; the saturable absorption region SA and the resonator cooperate as follows: The saturable absorption region SA is responsible for pulse time domain shaping and initial noise suppression, and the resonator performs nonlinear broadening; The saturable absorption region SA adjusts the mode-locking threshold in real time through dynamic bias to ensure pulse energy stability. The dispersion management of the resonator balances the self-phase modulation SPM and group velocity dispersion to avoid spectral splitting and maintain the dissipative soliton morphology.

2. The wide spectrum narrow linewidth semiconductor mode-locked laser according to claim 1, characterized in that: The resonator is a microring resonator MRR or a microknot resonator MKR: the time-frequency characteristics of the pulse are optimized through a wavelength selective feedback mechanism: that is, when a wavelength in the pulse spectrum component meets the resonance condition of the microring, the light wave of this wavelength will undergo multiple cyclic interference in the ring waveguide, forming a resonance enhancement effect, while the detuned wavelength is suppressed due to destructive interference.

3. The wide spectrum narrow linewidth semiconductor mode-locked laser according to claim 2, characterized in that: The InP-based saturable absorption region SA is bonded to the wafer of the SiN-based resonator, and energy transfer is achieved through a conical waveguide to output a frequency comb with a wide spectrum and narrow pulse width.

4. The wide spectrum narrow linewidth semiconductor mode-locked laser according to claim 1, characterized in that: The saturable absorption region SA is introduced into the laser resonant cavity, and its nonlinear absorption characteristics show a dynamic response to changes in light intensity: when low-intensity background light or noise passes through, the absorption region is in a non-saturated state, and has a strong absorption effect on light energy, suppressing continuous radiation and spontaneous radiation noise; when the transient light intensity exceeds the saturation threshold, the carrier concentration in the absorption region enters a transparent state due to rapid depletion, allowing high-intensity pulses to pass through without loss.

5. The wide spectrum narrow linewidth semiconductor mode-locked laser according to claim 1, characterized in that: The nonlinear broadening of the resonator is calculated as follows: Among them, c is the speed of light, N is the effective refractive index of the medium, R is the radius of the microring, and FSR refers to the frequency interval between adjacent longitudinal modes of the resonant cavity; When f p =FSR, which means that the frequency interval of adjacent longitudinal modes is equal to the operating frequency of the main mode of the laser; under this condition, all longitudinal modes are phase locked to form a stable ultrashort pulse output; When FSR and f p When the mode is locked, the tooth spacing of the frequency comb is f p And the integer multiples of FSR are determined as: f n =f p +k·FSR(k=0,±1,±2……); At this point, an optical frequency comb is formed; Under this condition, the phase accumulation of multiple reflections of the light field in the cavity meets the resonance condition, that is, 2nL=mλ, which will cause spectral broadening; n is the refractive index, m=0.1.2...; λ is the wavelength.

6. The wide spectrum narrow linewidth semiconductor mode-locked laser according to claim 1, characterized in that: The resonator makes the refractive index change with the light intensity through its third-order nonlinear effect, generating self-phase modulation SPM and four-wave mixing FWM in the resonant cavity; self-phase modulation SPM causes symmetrical broadening of the spectrum through the phase change dependent on the light intensity, and at the same time forms dissipative solitons with the anomalous dispersion balance in the cavity, enhancing the time domain stability of the mode-locked pulse; FWM generates new frequency components with equal intervals through energy transfer, expanding the spectral range; The third-order nonlinear effect causes the refractive index to vary with light intensity as shown below: n(I)=n0+n2I; Where I is the light intensity, A eff is the effective mode field area, n2 is the nonlinear refractive index, and n0 is the linear refractive index; The nonlinear phase change of the light field after propagation distance z is: Where ω0 is the central angular frequency of the light, I(z′) is the light intensity at the light field re-propagation distance z′, z is the light field propagation distance, P = |A| 2 is the instantaneous power, γ is the nonlinear coefficient; Four-wave mixing coupling equation: Assume the pump light frequency is ω p , the signal and idler light are ω s and ω i , satisfying the energy conservation law ω s +ω i =2ω p , the light field evolution equation is: Among them, A s is the signal light complex amplitude, A i is the idle light recovery amplitude, A p The pump light complex amplitude, γ is the nonlinear coefficient, are loss terms respectively; z is the propagation distance of the light field, and the wave vector mismatch Δk=2k p -k s -k i , k p , k s , k i are the pump light wave vector, signal light wave vector, and idle light wave vector, respectively, and the phase matching condition is Δk=0; The wave vector mismatch can be expanded as: Where γ is the nonlinear coefficient, P p is the pump light power, β2 is the second-order dispersion coefficient, Δω=ω s -ω p ,ω s ,ω p are the signal light angular frequency and the pump light angular frequency respectively; β4 is the fourth-order dispersion. In the anomalous dispersion region, Δk may be zero, supporting broadband FWM; When Δk = 0, the power gain coefficient of FWM is: G=birth 2 (γP p L eff ); Among them, γ is the nonlinear coefficient, P p is the pump light power, L eff is the effective length, α is the attenuation coefficient, L is the actual propagation length of light in the medium, and the gain bandwidth is determined by the phase matching condition: The Q value of MRR is defined as: Where ω0 is the central angular frequency of light, n g is the group refractive index, α rt is the single-turn loss, L' is the cavity length, and the photon lifetime τ photon >10ns corresponds to Q>10 6 .

7. A wide spectrum narrow linewidth semiconductor mode-locked laser, characterized in that: Preparation method of saturable absorption region SA: First, a multilayer structure is epitaxially grown on an InP substrate by metal organic chemical vapor deposition, which includes an n-type InP buffer layer, an InGaAs multi-quantum well gain layer, an InGaAsP saturable absorption layer, and a p-type InP top layer. Among them, the InGaAs multi-quantum well is used for laser gain, and the narrow bandgap characteristics of InGaAsP can adapt to the saturation absorption requirements of the 1550nm communication band. Then, SiN film was deposited on the InP surface using plasma enhanced chemical vapor deposition, and PMMA, a special adhesive for electron beam lithography, was spin-coated on the SiN layer to achieve a uniform coating of 1.2 μm at a set rotation speed. After drying and curing, the rectangular waveguide and resonant cavity patterns of the saturable absorption region SA were defined by electron beam lithography. After exposure, a mixture of methyl isobutyl ketone and isopropyl alcohol was used for development, and the etching resistance was enhanced by post-baking at 110°C. Finally, the SA region waveguide etching adopts the inductively coupled plasma process of Cl2 / Ar mixed gas, and the InGaAs characteristic spectrum line is monitored in real time by optical emission spectroscopy (OES) to ensure precise stopping at the p-InP layer interface and the verticality of the etching side wall reaches more than 80°.

8. A wide spectrum narrow linewidth semiconductor mode-locked laser, characterized in that: Ring resonator MRR preparation method: Use prefabricated SiN-on-insulator wafers to bond with InP substrates; After the ring waveguide is defined by EBL, SiN is etched by ICP with SF6 / C4F8 mixed gas to a remaining thickness of 150nm with a sidewall verticality of >85°, followed by dynamic immersion with diluted HF to optimize the sidewall roughness; After removing the hard mask using Cr wet etching solution, the etching damage was repaired by rapid thermal annealing in a nitrogen atmosphere at 400°C; Ti / Pt / Au was electron-beam evaporated on the surface of the SA region as the p-type electrode, and the n-type electrode was realized by back-evaporation of Ge / Au; After selective ultrasonic stripping of excess metal using acetone, ohmic contacts were formed by rapid annealing in nitrogen at 280°C; By applying reverse bias, the depletion region covers the InGaAsP absorption layer to achieve electrically controlled saturated absorption characteristics. Finally, the device is packaged by depositing a 2μm thick SiO2 cladding layer by PECVD and chemical mechanical polishing. The MKR preparation method is as follows: Using plasma activated bonding technology, the prefabricated SiN-on-insulator wafer and the InP substrate are pretreated with oxygen plasma, and then pressure is applied to complete room temperature bonding, and the interface loss is controlled at <0.2dB / cm; The annular waveguide and coupled waveguide patterns of MKR are defined by electron beam lithography, and the annular diameter, waveguide width, coupling gap, and exposure parameters are set. The development is processed by TMAH solution. After using the ICP etching process, the surface was immersed in a dilute HF solution for 10 seconds, followed by high-temperature annealing in a N2 atmosphere to eliminate etching damage and improve the SiN lattice quality. A 1μm thick SiO2 upper cladding layer was deposited by PECVD, and then the surface was flattened by chemical mechanical polishing. The final surface roughness was <0.5nm, and the Q value of MKR was >1×10 5 , the free spectral range FSR deviates from the theoretical value by <0.1nm.

9. A wide spectrum narrow linewidth semiconductor mode-locked laser, characterized in that: In the narrow-linewidth and wide-spectrum mode-locked laser, the ring resonator MRR and the electrically modulated saturable absorption region SA achieve efficient generation of wide-spectrum, narrow-linewidth, and highly coherent frequency combs through multi-level synergy. The steps are as follows: First, the absorption saturation intensity is adjusted by dynamic reverse bias, and its nonlinear absorption effect is used to quickly screen high-intensity pulses and suppress spontaneous radiation noise, forcing the formation of initial mode-locked pulses in the cavity. In this process, the short recovery time of SA significantly compresses the pulse time domain width by accelerating carrier sweeping out, while its electrical tuning ability dynamically suppresses mode competition and reduces short-time scale time jitter. Subsequently, under the combined effect of the pulse injection into the high-Q resonant cavity of the ring resonator MRR, the nonlinear phase shift dominated by the self-phase modulation SPM leads to symmetrical broadening of the spectrum, while the four-wave mixing FWM generates equally spaced frequency components through energy transfer; At the same time, the high-Q resonance characteristics of the ring resonator MRR selectively filter the longitudinal mode phase noise, and combined with active frequency stabilization technology, the single longitudinal mode linewidth is compressed to sub-kilohertz, and long-range phase locking is established through the resonance-enhanced Kerr nonlinear effect to suppress the long-term time jitter between pulses; Taking into account thermal noise and shot noise, the corrected linewidth is: Among them, v0 is the initial frequency, The active frequency stabilization technology can reduce the phase noise to Suppress to 10 -14 rad 2 / Hz, achieving sub-Hertz linewidth.

10. A wide spectrum narrow linewidth semiconductor mode-locked laser, characterized in that: In the narrow-linewidth and wide-spectrum mode-locked laser, the microring resonator MKR and the electrically modulated saturable absorption region are coupled in multiple dimensions through material properties and dynamic regulation to achieve efficient generation of wide-spectrum and narrow-linewidth frequency combs: the electrically modulated saturable absorption region SA is based on the strong nonlinear absorption characteristics of InP-based multiple quantum wells. Under reverse bias, it quickly screens high-intensity pulses and suppresses spontaneous radiation noise through the carrier sweep-out effect, triggering the formation of initial mode-locked pulses. Its dynamically adjustable absorption saturation intensity and the high-Q value resonant cavity of MKR jointly compress the pulse time domain width to the sub-picosecond level. At the same time, through electrical tuning Mode competition is suppressed in real time to reduce time jitter; after pulse injection into SiN-based MKR, the self-phase modulation SPM induced by its third-order nonlinear effect is balanced with the group velocity dispersion in the anomalous dispersion region to form dissipative solitons, so that the spectrum is symmetrically broadened to cover the C+L band, i.e. 1530nm-1625nm; while four-wave mixing FWM generates equally spaced frequency components through energy transfer, and combines the high photon lifetime of MKR to selectively filter the longitudinal mode phase noise, compressing the single longitudinal mode linewidth to sub-kilohertz; MKR establishes long-range phase locking through nonlinear spectral expansion and phase synchronization mechanism.

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