CN111613959B

CN111613959B - Narrow linewidth continuous wave frequency modulation laser based on silicon-based external cavity self-injection

Info

Publication number: CN111613959B
Application number: CN202010279703.8A
Authority: CN
Inventors: 陈明华; 汤黎伟; 李佳琛
Original assignee: Tsinghua University
Current assignee: Shenzhen Lingdong Core Light Technology Co ltd
Priority date: 2020-04-10
Filing date: 2020-04-10
Publication date: 2021-12-03
Anticipated expiration: 2040-04-10
Also published as: CN111613959A

Abstract

The present invention provides a continuously tunable narrow linewidth laser, comprising a silicon-based external cavity (111), wherein the silicon-based external cavity (111) includes a substrate layer (101), and light above the substrate layer (101) A waveguide layer (104); the optical waveguide layer (104) constitutes a self-injection microring resonator to obtain a frequency-tunable continuous wave laser, and a distributed feedback laser (103), the distributed feedback laser (103) and the silicon The base external cavity (111) is end-face coupled, and the laser is coupled into the micro-ring resonator (110), and the frequency-modulated continuous wave laser is output through the silicon base external cavity (111). The narrow linewidth laser of the invention can realize continuous tunable frequency modulation and meet the application of frequency modulation light source, and can also achieve the optimization of design size and coupling with the laser.

Description

Narrow linewidth continuous wave frequency modulation laser based on silicon-based external cavity self-injection

Technical Field

The invention belongs to the field of frequency modulation lasers, and particularly relates to a narrow linewidth continuous wave frequency modulation laser based on silicon-based external cavity self-injection.

Background

Narrow line width, i.e., low noise, low Relative Intensity Noise (RIN), and large range continuous wave frequency modulated wave lasers have important applications in a number of application scenarios, such as coherent optical communications, laser radar (LiDAR), and biosensing. Meanwhile, the use of an amplifier is avoided due to the high output optical power, so that the high-power laser has great demand on improving the system performance.

A Frequency Modulated Continuous Wave (FMCW) laser radar transmits a frequency modulated continuous wave signal and receives an echo reflected by an object, and detects the distance and the movement speed of the object by demodulating a coherent signal of the transmitted wave and the received reflected wave. Wherein the maximum detection distance is limited by the coherence length of the laser in order to ensure a good signal-to-noise ratio of the received signalGenerally, the coherence length is required to be more than twice as long as the detection distance, so that the frequency-modulated continuous wave laser light source is required to have a lower spectral line width. Modulation bandwidth B of light source and object distance resolution S_rCorrespond to, i.e. S_rC is not less than c/2B, c is the speed of light, and the large-range tuning range becomes an important index of the frequency modulation light source.

Some laser applications require a narrow laser linewidth, i.e., a narrow spectrum. The narrow linewidth laser refers to a single-frequency laser, namely the laser has a resonant cavity mode, the phase noise is low, and therefore the spectral purity is high. Typically, such lasers are very noisy in intensity.

In order to achieve a narrow radiation bandwidth (linewidth) of the laser, the following factors need to be considered in laser design: first, a single frequency operation needs to be achieved. The goal should be long-term stable single-frequency operation without mode-hopping. Second, it is desirable to minimize the effects of ambient noise. Electrically pumped lasers require low noise current or voltage sources, while optically pumped lasers require pumping sources with low intensity noise. This is easily achieved when the noise frequency is high, but when the noise frequency is low, the line width source is mainly low frequency phase noise (spontaneous radiation). Third, there is a need to optimize the laser design to minimize laser phase noise. High intracavity power and long cavities are preferred, although stable single frequency operation is more difficult to achieve in this case. The importance of different noise sources needs to be known for system optimization, since different measurement methods are used according to different requirements of the dominant noise source.

One important application of narrow linewidth lasers is in the field of sensing, such as pressure or temperature fiber optic sensors, various types of interferometer sensing, the detection of trace gases using different absorption radars, and the measurement of wind velocity using doppler lidar. Some fiber optic sensors require laser linewidths of several kHz, whereas in radar measurements a 100kHz linewidth is sufficient. Optical frequency measurement requires a very narrow line width of the light source and requires a stabilization technique to be implemented. Fiber optic communication systems have relatively relaxed line width requirements and are used primarily for transmitters or for detection or measurement.

The continuously tunable narrow linewidth laser has a wide application range, such as coherent optical communication and laser radar. The broadband swept-frequency laser is usually affected by mode hopping, and a Fabry-perot FP laser coupled with an external cavity can realize lasing at different wavelengths by adjusting the position of a resonant wavelength through a Vernier effect (Vernier effect) according to the difference of FSRs of the external cavity and the FP cavity. However, due to the vernier effect, the longitudinal modes cannot be tuned continuously, and coherence between each longitudinal mode is lost, so that the application of the sweep light source is limited. More importantly, for continuous frequency modulated lidar applications, the laser must achieve continuously tunable frequency modulation. The us OEwaves company proposed a backscatter self-injection narrow linewidth tunable laser based on a Whispering Gallery Mode Resonator (WGMR) coupled to a semiconductor laser. This approach has its limitations due to limitations in WGMR volume and laser coupling issues.

There is a need for a new continuously tunable narrow linewidth laser that can achieve continuously tunable frequency modulation and meet the application of frequency modulated light sources, yet can achieve the optimization of design size and coupling with the laser.

Disclosure of Invention

The invention aims to provide a continuously tunable narrow linewidth laser, which can realize continuously tunable frequency modulation, meet the application of a frequency modulation light source and optimize the design size and the coupling with the laser.

The invention relates to a method for constructing a high-quality factor (high Q value) micro-ring self-injection resonant cavity based on a low-loss silicon nitride platform, which can narrow the line width of a distributed feedback laser (DFB) to 5KHz magnitude, can realize a 10GHz static tuning range by tuning the resonant wavelength of the micro-ring resonant cavity and a thermally tuned phase shifter, and can realize a 1GHz tuning rate to 20kHz under the drive of lead zirconate titanate piezoelectric ceramics (PZT) by introducing an elasto-optic effect. One aspect of the present invention provides a narrow linewidth laser, characterized by comprising: the silicon-based external cavity (111), the silicon-based external cavity (111) comprises a substrate layer (101), and an optical waveguide layer (104) is arranged above the substrate layer (101); the optical waveguide layer (104) forms a self-injection micro-ring resonant cavity to obtain tunable continuous wave laser, and the self-injection micro-ring resonant cavity comprises a first straight waveguide, a micro-ring resonator (110), a second straight waveguide (113), a third straight waveguide (114) and a fourth straight waveguide (116); and

a distributed feedback laser (103), the distributed feedback laser (103) being end-coupled to one end of a first straight waveguide of the optical waveguide layer (104); and is

The first straight waveguide separates laser light from the first straight waveguide to a second straight waveguide (113) and a third straight waveguide (114) through an optical beam splitter (109); and coupling the laser light in the second straight waveguide (113) and the third straight waveguide (114) into the micro-ring resonator (110), and outputting the frequency-modulated continuous wave laser light through the fourth straight waveguide (116).

The narrow linewidth laser of another aspect of the invention is characterized in that a first thermal phase shifter (106) is arranged above the first straight waveguide, the first thermal phase shifter (106) is made of metal, the middle part of the first thermal phase shifter is a first heating resistor (106a), and the first heating resistor (106a) is heated to generate waveguide phase shift; the micro-ring resonator (110) is an annular micro-ring waveguide (115), a second thermal phase shifter (107) for adjusting the resonance wavelength of the micro-ring is arranged above the annular micro-ring waveguide (115), the second thermal phase shifter (107) is made of metal, the middle part of the second thermal phase shifter is a second heating resistor (107a), the second heating resistor (107a) is heated, and the wavelength and the resonance frequency of the micro-ring resonator are tuned; the first thermal phase shifter (106) and the second thermal phase shifter (107) form a tuning unit for wavelength tuning of the narrow linewidth laser.

A narrow linewidth laser of still another aspect of the present invention, wherein the microring resonator (110) functions as both a filter and a mirror reflecting a selected wavelength, reflects part of light near a resonance wavelength back to the distributed feedback laser (103) and simultaneously filters and outputs light near the resonance wavelength.

In a narrow linewidth laser according to still another aspect of the present invention, after the narrow linewidth laser is powered on, the device is in an unlocked state, the thermal tuning electrode (107) is adjusted, and the resonant frequency of the micro-ring resonator (110) is changed to align the obtained resonant frequency with the output frequency intrinsic to the laser output; and changing the fine tuning injection phase of the first thermal phase shifter (106) to enable the laser to jump from a free state to a locked state, so that the laser generates single-mode output with narrow line width.

In a narrow linewidth laser according to a further aspect of the invention, the continuous static tuning is performed by adjusting the center frequency of the micro-ring resonator (110) and its spectral linewidth, and since the heating power on the micro-ring resonator (110) is linear with the detuned resonance frequency, a linear frequency sweep of the laser is achieved by applying a voltage on said second thermal phase shifter (107).

In a narrow linewidth laser according to still another aspect of the present invention, the quality factor of the self-injection micro-ring resonator is 2 × 10⁵The linewidth is 3kHz, the static tuning is 9.1GHz at 1547 nm, the dynamic modulation bandwidth is 1.69GHz at 12kHz, and the 50kHz is 0.91 GHz.

A narrow linewidth laser of still another aspect of the present invention, wherein the formula (1) gives the effect of self-injection effect on noise suppression for reducing the near-phase noise of the laser.

Δν、Δν₀Line widths of the laser before and after self-injection, Q, Q respectively_LDThe quality factors of the micro-ring and the laser are respectively, the quality factor line width narrowing effect of the micro-ring resonant cavity is in direct proportion, and the quality factor of the micro-ring resonant cavity is Q_LD＝2×10⁵Mixing P with_rThe value of/P is 0.3, α is 2.5, and η is approximately equal to 1000, and the line width is reduced by at least three orders of magnitude.

A narrow linewidth laser of a further aspect of the present invention, wherein a voltage across the microring resonator (110) is adjusted, the laser maintaining a free state in the first free region (401); during an injection locking (402) phase, the laser transitions from a free state to an injection locked state; in the injection locking state, the micro-ring tuning frequency and the output frequency are approximately in a linear relation; with the increasing of the voltage of the micro-ring, the laser gradually enters a second free area (403), at the moment, the laser jumps to the next free area when exceeding a locking range, and the laser works in a self-injection locking state (402) of a central linear tuning area to realize a frequency modulation function based on thermal tuning and a static linear tuning result, wherein when the line width is 3.7kHz, the static tuning range is 73 pm.

A narrow linewidth laser according to still another aspect of the present invention, wherein the distributed feedback laser (103) is end-coupled to the first straight waveguide (112) through a silicon photonic spot converter portion (112), the silicon photonic spot converter portion (112) being tapered and being a part of the optical waveguide layer (104).

A narrow linewidth laser of yet another aspect of the invention, wherein the silicon photonic spot converter (112) has a smaller spot size near one end of the distributed feedback laser (103) matching the spot size of the nano-silicon photonic waveguide and a larger spot size at the other end matching the spot size of the standard optical fiber; the silicon photon spot converter (112) realizes that the spot shapes of the distributed feedback laser (103) and the optical waveguide layer (104) are matched as much as possible, and has larger overlapped integral values.

A narrow linewidth laser of yet another aspect of the present invention, wherein the optical beam splitter (109) is a silicon-based multimode interference structure (MMI), the MMI having five parameters: 1. coupling ratio: representing the amount of power coupled by the input channel (i) to the specified output channel (j); 2. additional loss: represents the total loss due to the coupler; 3. channel insertion loss: representing the magnitude of power from the input channel to the output channel; 4. the isolation ratio is as follows: indicating the degree of isolation between the same side ports in the transmissive coupler; 5. return loss: indicating the amount of power returned by the input channel.

A narrow linewidth laser of yet another aspect of the present invention, wherein the optical beam splitter (109) is a 3db optical beam splitter, the 3db optical beam splitter being a 50: a 50 coupling ratio coupler.

The narrow linewidth laser in the further aspect of the invention is characterized in that a piezoelectric ceramic (108) is arranged on the second thermal phase shifter (107) above the micro-ring resonator (110), the piezoelectric ceramic is adhered to the substrate layer (101) through epoxy resin glue, and is driven by a high-frequency voltage signal to be stressed downwards by the piezoelectric ceramic (108), the stress refractive index of the optical waveguide layer (104) is changed to change the resonance wavelength, and the self-injection state is changed accordingly, so that the frequency modulation of the laser is realized.

In a narrow linewidth laser according to still another aspect of the present invention, the piezoelectric ceramic (108) is swept by an elasto-optic effect, and the frequency range is 1.69GHz at a modulation rate of 12kHz and 0.91GHz at a modulation rate of 20 kHz.

The hybrid integrated external cavity self-injection laser provided by the invention can realize that the frequency-adjustable continuous wave laser light source has lower spectral line width, and the frequency-adjustable continuous wave laser light source has a large tuning range. The silicon-based silicon nitride integrated external cavity realizes a high-Q value micro-ring external cavity due to lower transmission loss, provides narrow-band optical feedback, greatly prolongs the photon service life and compresses the spectral line width. The refractive index of the waveguide can be changed by heating a heating resistor deposited above the waveguide, so that the resonance wavelength of the micro-ring can be tuned, and the output frequency of the laser can be tuned along with the change of the resonance wavelength of the micro-ring according to the L-K self-injection principle. Meanwhile, in order to improve the tuning rate and the tuning bandwidth, the PZT is adhered above the waveguide, the stress generated by the inverse piezoelectric effect acts on the waveguide, the refractive index of the waveguide is changed due to the elasto-optical effect to tune the resonance wavelength of the micro-ring, and the tuning bandwidth and the tuning rate are improved.

Drawings

In order to more clearly illustrate the technical solution in the embodiments of the present invention, the drawings required to be used in the embodiments will be briefly described below. It is obvious that the drawings in the following description are only examples of the invention, and that for a person skilled in the art, other drawings can be derived from them without making an inventive step.

Fig. 1(a) is a block diagram of a narrow linewidth laser in one embodiment of the present invention.

Fig. 1(b) is a structural diagram of a narrow linewidth laser in another embodiment of the present invention.

Fig. 2(a) is a perspective view of a narrow linewidth laser in one embodiment of the present invention.

Fig. 2(b) is a perspective view of a narrow linewidth laser in another embodiment of the present invention.

Fig. 3(a) is a schematic diagram of a narrow linewidth laser in accordance with an embodiment of the present invention.

Fig. 3(b) is a schematic diagram of a narrow linewidth laser in another embodiment of the present invention.

Fig. 4 is a graph of the variation of the output frequency of a narrow linewidth laser of the present invention with the tuning frequency of a filter.

Fig. 5(a) is a static tuning diagram of a narrow linewidth laser of the present invention.

Fig. 5(b) is a plot of tuning frequency versus thermal tuning power for a narrow linewidth laser in accordance with the present invention.

Fig. 5(c) is a static measured beat power spectrum of a narrow linewidth laser of the present invention.

Fig. 5(d) is a plot of bias frequency versus phase noise for a narrow linewidth laser of the present invention.

Fig. 6 is a schematic diagram of a frequency of a continuous frequency sweep laser generated by tuning a micro-ring resonant wavelength of a narrow linewidth laser according to the present invention.

Fig. 7(a) and 7(b) are schematic diagrams of PZT driven continuous sweep wavelength variations for narrow linewidth lasers of the present invention.

FIGS. 8(a) and 8(b) are typical performance charts of the piezoelectric ceramic of the present invention.

Detailed Description

Specific embodiments of the present invention will now be described with reference to the accompanying drawings. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. These embodiments are provided only for the purpose of exhaustive and comprehensive description of the invention so that those skilled in the art can fully describe the scope of the invention. The terminology used in the detailed description of the embodiments illustrated in the accompanying drawings is not intended to be limiting of the invention.

Fig. 1(a) is a structural diagram of a narrow linewidth laser of the present invention.Wherein the substrate layer 101 is a silicon substrate, the optical waveguide layer 104 is arranged above the substrate layer 101, the optical waveguide layer 104 is a silicon nitride waveguide, and SiO is arranged around the SiN waveguide₂Oxide layer wrapping to form core-cladding structure of waveguide, SiO₂Not labeled in the figure. Wherein the optical waveguide layer 104 is generally a straight waveguide, and in one embodiment, has a rectangular cross-section of about a few microns by 200nm, wherein the dimension on the nanometer scale is the thickness of the waveguide on the substrate layer. In one embodiment, the substrate layer ranges in a rectangular shape of a few millimeters by a few millimeters. Silicon nitride (SiN) is used as a core layer material of the optical waveguide, compared with an organic polymer material, the SiN has the advantages of large refractive index difference of the core layer, small device size, high integration level, high performance stability and the like, and compared with Silicon On Insulator (SOI), the SiN waveguide has the advantages of simplicity in preparation, lower process cost and the like. On one side of the substrate layer 101 is a distributed feedback laser ("DFB laser") 103 located on a base 102, which may be a commercial DFB laser chip, that is packaged with the substrate layer 101 in a hybrid manner to form an external cavity laser of the narrow linewidth laser of the present invention.

The DFB laser 103 is end-coupled to an optical waveguide 104 on the substrate layer 101. Preferably, the DFB laser 103 is end-coupled to the optical waveguide 104 on the substrate layer 101 through a silicon photonic spot converter section 112. The optional silicon photonic spot converter 112 is part of the optical waveguide 104 and is a critical part of the silicon photonic integrated chip to interface with an external laser, avoiding or reducing coupling losses. The silicon photonic spot size converter 112 is substantially tapered and as part of the optical waveguide layer 104, the portions of the optical waveguide layer 104 that comprise the silicon photonic spot size converter have the same thickness, with the end of the silicon photonic spot size converter 112 near the DFB laser having a smaller spot size that matches the spot size of the nano-silicon photonic waveguide and the other end having a larger spot size that matches the spot size of a standard optical fiber. The silicon photonic spot converter 112 functions to match the spot of the DFB laser 103 with the spot of the optical waveguide layer 104, i.e., SiN, wherein the spot of the DFB laser is slightly larger and the spot of the optical waveguide layer 104, i.e., SiN, is slightly smaller, but more importantly, the spot shapes of the optical waveguide layer 104 and SiN are different, both of which are elliptical spots, and functions to realize the spot shapes of the optical waveguide layer and the optical waveguide layer as matched as possible, thereby realizing high coupling efficiency, i.e., a large overlap integral value.

The fabrication process of the optical waveguide 104 employs a CMOS compatible process, in which a silicon dioxide layer is first obtained on a silicon substrate by thermal oxidation, and a 200nm silicon nitride layer is deposited on the silicon dioxide layer by LPCVD. After etching the required structure and pattern on the silicon nitride, silicon dioxide is deposited on the waveguide by PECVD to obtain the optical waveguide structure.

The specific layout of the optical waveguide layer 104 above the substrate layer 101 can be referred to fig. 2(a) a perspective view of the narrow-linewidth laser of the present invention, and fig. 3(a) a schematic view of the narrow-linewidth laser of the present invention. A first thermal phase shifter 106 and a second thermal phase shifter 107 based on the thermo-optic effect are provided on the upper layer of the optical waveguide layer 104. The first thermal phase shifter 106 and the second thermal phase shifter 107 are made of metal, and the first heating resistor 106(a) and the second heating resistor 107(a) are respectively located in the middle portions of the first thermal phase shifter 106 and the second thermal phase shifter 107 and respectively located above different positions of the optical waveguide layer 104. A silicon dioxide layer (not shown) is interposed between the optical waveguide layer 104 and the intermediate layer.

Also included in optical waveguide layer 104 is an optical splitter 109 (not shown in fig. 1), which may be a 3db optical splitter, which is a device that splits and combines light in input and output fibers, and which splits light from one fiber bundle into several fiber bundles at the same wavelength, and which may be implemented as a silica-based multimode interference (MMI), which has five main parameters: 1. coupling ratio: representing the amount of power coupled by the input channel (i) to the specified output channel (j); 2. additional loss: represents the total loss due to the coupler; 3. channel insertion loss: representing the magnitude of power from the input channel to the output channel; 4. the isolation ratio is as follows: indicating the degree of isolation between the same side ports in the transmissive coupler; 5. return loss: indicating the amount of power returned by the input channel. One of ordinary skill in the art can design the above parameters of the optical beam splitter 109 based on the disclosed narrow linewidth laser design. In one embodiment, the optical splitter 109, i.e., a 3dB optical splitter or 3dB coupler, is a 50: a 50 coupling ratio coupler.

After entering the silicon-based external cavity, the laser is divided into two paths at the optical beam splitter 109 through the first thermal phase shifter 106, and the light is coupled into the micro-ring resonator 110, i.e., the ring-shaped micro-waveguide 115, in the two paths of waveguides. In static operation, the microring resonator functions as both a filter and a mirror that reflect a selected wavelength. It reflects a portion of the light near the resonant wavelength back to the DFB laser 103 and simultaneously filters out the light near the resonant wavelength. The efficiency of self-injection depends on the ratio of the Q-value of the laser and the external cavity. In one embodiment, the Q-factor (i.e., quality factor) of the microring resonator of the present invention is 2 × 10⁵The linewidth was 3 kHz. As described earlier, the resonance wavelength is adjusted by the voltages applied to the electrodes of the first thermal phase shifter 106 and the second thermal phase shifter 107, which are used to heat the waveguide. Based on the thermo-optic ("TO") effect, the static tuning range is up TO 9.1GHz at 1547.1 nm, and the modulation bandwidth is 0.9GHz at 1GHz, enabling FMCW-based LIDAR.

According to the feedback self-injection theory, the reflection external cavity with high quality factor (i.e. high Q value) has the function of narrowing the laser linewidth and suppressing the phase noise as a whole, and also comprises a phase tuning unit, i.e. the first thermally tuned phase shifter 106. As described above, both the first thermal phase shifter 106 and the second thermal phase shifter 107 include the heating resistors 106(a) and 107 (a). The first heating resistor 106(a) functions to heat and produce a waveguide phase shift. The second heating resistor 107(a) functions to heat and tune the resonance wavelength of the micro-ring, and the two together constitute a tuning unit for wavelength tuning.

Specifically, as shown in fig. 3(a), the silicon-based external cavity is designed as a self-injection micro-ring resonator, and includes a first straight waveguide, a ring-shaped micro-ring waveguide 115, a second straight waveguide 113, a third straight waveguide 114, and a fourth straight waveguide 116. The two ends of the first straight waveguide are respectively provided with an Add port and a Drop port, so that the external cavity has the functions of a filter and a feedback (reflecting mirror). A first heating resistor 106(a) and a second heating resistor 107(a) are respectively disposed above the first straight waveguide and the annular micro-ring waveguide 115 to achieve modulation functions of adjusting the locked state and in the locked state. Specifically, after the laser is powered on, the device is in an unlocked state, and the output spectrum transmitted through the micro-ring external cavity is generally multimode and unstable in linewidth. The applied voltage on the annular micro-ring waveguide 115 is adjusted, the resonance frequency of the micro-ring filter is changed to be aligned with the output frequency intrinsic to the output of the laser, and then the injection phase is finely adjusted by changing the voltage above the first straight waveguide, so that the laser jumps from a free state to a locked state, and the laser shows single-mode output and has the effect of line width narrowing. The specific physical mechanism can be explained by Lang-kobayashi rate equation L-K equation:

the rate equations with external feedback self-injection filter are given by (2) - (4), and the physical meanings and simulation parameters of the parameters are shown in the following table 1:

TABLE 1 values of parameters used in the simulation

Substituting the output electric field E (t) and the feedback electric field F (t) to obtain the relation between the tuning parameter and the output light:

Δω_sτ＝-C_effsin{Δω_sτ+ω₀τ+arctan(α)-arctan[(Δω_s-ω_f)/Λ]} (5)

wherein

As shown in equations (5) and (6). Making the center frequency omega of the filter_fAnd the output frequency omega_sThe relationship is shown in fig. 4, where fig. 4 is a graph of output frequency as a function of filter tuning frequency. Adjusting the voltage on the microring resonator 110, and maintaining the free state of the laser in the first free region 401; during the injection locking 402 phase, the laser jumps from a free state to an injection locking state; in the injection locking state, the micro-ring tuning frequency and the output frequency are approximately in a linear relation; as the voltage of the micro-ring increases, the laser gradually enters the second free region 403, at which time the laser jumps to the next free region beyond the locked range. A laser operating in the self-injection locked state 402 of the central linear tuning region can achieve both a hot-tone based frequency modulation function and static linear tuning results. The static tuning range is up to 73pm at a linewidth of 3.7 kHz. The TO frequency sweep is a sweep range of 0.964Ghz and the sweep rate is 1 Khz. PZT drive is 1.69G range rate 12Khz, and 0.91G range rate 20 Khz.

In summary, as shown in fig. 1, the portions outside the DFB laser 103 together form the silicon-based external cavity 111. The working process of the silicon-based external cavity 111 of the invention is divided into the following parts:

self-injection locking: due to self-injection locking, the laser achieves continuous tuning and frequency modulation without mode hopping. The laser spectral linewidth is limited by quantum noise, the commercial semiconductor DFB laser linewidth is usually several megahertz, and formula (1) gives the effect of self-injection effect on noise suppression for reducing the near-phase noise of the laser:

wherein, the value of delta v and the value of delta v₀Line widths of the laser before and after self-injection, Q, Q respectively_LDQuality factors of the microring and the laser, respectively. The higher the quality factor of the micro-ring resonant cavity is, the more obvious the line width narrowing effect isTherefore, the outer cavity is required to be constructed on a low-loss waveguide platform, the outer cavity is based on the silicon-based silicon nitride waveguide, and the quality factor of the micro-ring resonant cavity can reach Q_LD＝2×10⁵Mixing P with_rThe value of/P is 0.3, α is 2.5, and η is approximately 1000, the line width can be reduced by more than three orders of magnitude.

By varying the voltages on the two pairs of electrodes and the operating temperature of the laser, the laser will meet the self-injection equilibrium condition and enter self-injection locked mode.

Continuous static tuning: the principle of laser tuning, i.e. the external cavity tuning principle, can be explained by the Lang-Kobayashi rate equation (Lang-Kobayashi theory) of Filtered Optical Feedback (FOF), which describes the laser dynamics by two tuning parameters, i.e. the center frequency of the micro-ring resonator and its spectral linewidth. This theory leads to the conclusion that the detuning frequency of a laser is linear with the tuning frequency of the laser. The frequency modulation process depends on the TO effect of the waveguide, where the heating power on the microring is linear with the detuned resonance frequency, so that a linear frequency sweep can be achieved by applying an appropriate voltage on the electrodes. The results of the static tuning and the dynamic frequency modulation are experimentally demonstrated and will be described in detail below. The theory of equating the external cavity to a frequency filter with feedback self-injection shows that the lasing frequency of the laser will follow the change of the resonant frequency of the external cavity to achieve the tuning function.

Fig. 5(a) - (d) show the self-injection locked state with narrow linewidth and static frequency tuning. By changing the voltage applied to the microring from 8.7 to 9.7V, 10GHz frequency tuning was obtained. The linewidth was about 4kHz over the entire tuning range with 10km delay self-heterodyne interferometry (DSHI).

Fig. 5(a) is a static tuning diagram of a narrow linewidth laser of the present invention. In the figure, the tuning wavelength of the laser is blue-shifted along with the change of the resonant frequency of the micro-ring resonant cavity, and the lasing wavelength in the figure respectively corresponds to the lasing wavelength when the voltage of the micro-ring thermal tuning electrode is 9.7, 9.5, 9.3, 9.1, 8.9 and 8.7V. The tuning range is about 10G and the side mode suppression ratio is greater than 40 dB.

Fig. 5(b) is a plot of tuning frequency versus thermal tuning power for a narrow linewidth laser in accordance with the present invention. The tuning frequency of the laser has a linear relation with the thermal tuning power, and a straight line in the figure is a linear fitting straight line. The linear relationship of detuned frequency and thermally tuned power fits well in fig. 5(b), which makes a linear frequency sweep possible.

Fig. 5(c) is a static measured beat power spectrum of a narrow linewidth laser of the present invention. A beat frequency power spectrum at a carrier frequency is obtained by using a 10km time delay self-heterodyne measurement method, and the linewidth of a single laser is estimated to be about 3.7kHz by Lorentz line type fitting.

FIG. 5(d) is a plot of the bias frequency versus phase noise for a narrow linewidth laser of the present invention, plotting the measured laser noise spectrum and the detected noise floor, with noise analysis using an asymmetric Mach-Zehnder interferometer (MZI) with a path imbalance (delay line) of 100m and phase noise of-65 dBc/Hz at 1 kHz.

In summary, the laser frequency tracks the micro-ring resonant frequency, which is tuned by the thermo-optic effect (TO). The laser has the advantages of direct-current frequency tuning range of 9.1GHz, line width of 3.7kHz and low noise. The DFB laser is coupled to an external microring resonator using a 14-pin standard butterfly package for practical applications. A thermoelectric cooler (TEC) packaged together is used to maintain a constant temperature. Two electrodes are deposited on the chip for tuning the external round trip time and the resonance frequency based on the thermo-optic effect. The two-dimensional voltage tuning assembly allows the laser to enter a self-injection locking state after passing through a chaotic state and a mode locking state.

Fig. 6 is a schematic diagram of a frequency of a continuously swept laser generated by a tuned micro-ring resonant wavelength of a narrow linewidth laser of the present invention, illustrating frequency modulation. Where the curves in fig. 6 are instantaneous frequencies. Continuous static frequency tuning has reached 10GHz, allowing FMCW bandwidths of up to 10G. Based on the fact that thermal power is proportional to the tuning frequency, the square of the voltage applied to the micro-loop is linear with time, and a chirp is desired. The time-frequency relation of the beat frequency signals of the sweep frequency light source and a narrow-linewidth stable wavelength laser is shown in the figure, the 1kHz sweep frequency rate and the 0.96G sweep frequency bandwidth are realized, and due to the fact that the thermo-optic effect is not high in transient responsivity, the non-linear phenomenon exists in the sweep frequency process. Fig. 6 also shows that the beat tones of the FMCW laser and the reference laser are analyzed by a time-frequency algorithm to obtain a time-frequency diagram. Thermo-optic based frequency modulation has a bandwidth of 0.964GHz/ms and introduces linear distortion.

Constructing a continuous frequency modulation light source: based on a continuous tunable principle, the current can be directly modulated, the modulation frequency is related to the carrier transfer rate and is often up to dozens of GHz, but the tuning bandwidth is limited due to the self-injection locking effect; and secondly, the micro-ring electrode is connected with a linear scanning voltage, and the output light frequency is scanned along with the linear scanning voltage.

In another embodiment of the present invention, the frequency tuning function of the present invention can also be achieved by a strain force generated by a piezoelectric ceramic. Piezoelectric ceramic 108 is optional. Specifically, referring to fig. 1(b), a structural diagram, a perspective view and a schematic diagram of another embodiment in fig. 2(b) and fig. 3(b), the device structure repeated between fig. 1(b), fig. 2(b) and fig. 3(b) and fig. 1(a), fig. 2(a) and fig. 3(a) is not repeated herein. In fig. 1(b), piezoelectric ceramic 108 is bonded to substrate layer 101 by epoxy glue over optical waveguide layer 104. The piezoelectric ceramic 108 may be, for example, PA2AB type, and has a size of 0.9. mu. m.times.0.9. mu. m.times.0.8. mu.m. The PA2AB piezoelectric chip is composed of stacked piezoelectric ceramic layers mechanically connected in series, sandwiched between interdigitated electrodes connected in parallel. It provides a maximum displacement of 0.7 μm. + -. 15%. In the piezoelectric ceramic 108, a black spot is located beside an electrode receiving a positive bias, the other electrodes are grounded, and the electrode is exposed. The piezoelectric ceramic is manufactured by utilizing the piezoelectric effect that the material causes the relative displacement of the centers of positive and negative charges in the material under the action of mechanical stress to generate polarization, so that bound charges with opposite signs appear on the surfaces of two ends of the material, and the piezoelectric ceramic has sensitive characteristics. Table 2 shows the properties of PA2AB at 25 ℃ at room temperature:

TABLE 2

The epoxy glue may be any epoxy that cures at a temperature below 80 ℃, such as Thorlabs items under the number 353NDPK or TS10, or lotai Hysol 9340 may also be used. Since the edges do not translate, the load can only be added to the central area of the maximum facet. Adding a load to a smaller face may lead to mechanical failure.

Under the driving of high-frequency voltage signals, the piezoelectric ceramic 108 generates stress downwards due to the inverse piezoelectric effect, the resonance wavelength of the optical waveguide layer 104 is changed due to the change of the stress refractive index, and the self-injection state is changed accordingly, so that the frequency modulation is realized.

Mechanical stress changes the waveguide refractive index: piezoelectric ceramics (PZT) are adhered above the chip micro-ring, and the refractive index of the waveguide is changed by utilizing mechanical stress generated by inverse piezoelectric effect, so that the resonance frequency of the micro-ring is changed, and the frequency sweeping effect is generated. As shown in fig. 7(a) and 7 (b). Fig. 7(a) and 7(b) are schematic diagrams of PZT driven continuous sweep wavelength variations for narrow linewidth lasers of the present invention. FIGS. 7(a) and 7(b) show the time-frequency relationship between the swept-frequency light source and the beat signal of a narrow-linewidth stable wavelength laser, which achieves a sweep rate of 12kHz and a sweep range of 1.69 GHz; and 20kHz sweep rate, 0.91GHz sweep range.

FIGS. 8(a) and 8(b) are typical performance charts of the piezoelectric ceramic of the present invention. The temperature rise and displacement of these PA2AB was a sine wave drive voltage from 0 to 75V applied for 10 minutes at the specified frequency.

Reference herein to "one embodiment," "an embodiment," or "one or more embodiments" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Moreover, it is noted that instances of the word "in one embodiment" are not necessarily all referring to the same embodiment.

The above description is only for the purpose of illustrating the present invention, and any person skilled in the art can modify and change the above embodiments without departing from the spirit and scope of the present invention. Therefore, the scope of the claims should be accorded the full scope of the claims. The invention has been explained above with reference to examples. However, other embodiments than the above described are equally possible within the scope of this disclosure. The different features and steps of the invention may be combined in other ways than those described. The scope of the invention is limited only by the appended claims. More generally, those of ordinary skill in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are exemplary and that actual parameters, dimensions, materials, and/or configurations will depend upon the particular application or applications for which the teachings of the present invention is/are used.

Claims

1. A Frequency Modulated Continuous Wave (FMCW) narrow linewidth laser, comprising:

the silicon-based external cavity (111), the silicon-based external cavity (111) comprises a substrate layer (101), and an optical waveguide layer (104) is arranged above the substrate layer (101); the optical waveguide layer (104) forms a self-injection micro-ring resonant cavity to obtain tunable continuous wave laser, and the self-injection micro-ring resonant cavity comprises a first straight waveguide, a micro-ring resonator (110), a second straight waveguide (113), a third straight waveguide (114) and a fourth straight waveguide (116); and

The first straight waveguide separates laser light from the first straight waveguide to a second straight waveguide (113) and a third straight waveguide (114) through an optical beam splitter (109); coupling the laser light in the second straight waveguide (113) and the third straight waveguide (114) into the micro-ring resonator (110), and outputting frequency-modulated continuous wave laser light through the fourth straight waveguide (116);

after the narrow linewidth laser is electrified, the device is in an unlocked state, the output spectrum of the silicon-based external cavity (111) is multimode, and the linewidth is unstable; -varying the resonance frequency of the microring resonator (110) such that the obtained resonance frequency is aligned with the output frequency intrinsic to the laser output; changing the fine tuning injection phase of the micro-ring resonator (110) to enable the laser to jump from a free state to a locked state, and enabling the laser to generate single-mode output with narrow line width;

the center frequency and the spectral line width of the micro-ring resonator (110) are adjusted to perform continuous static tuning, as the heating power on the micro-ring resonator (110) is in a linear relationship with the detuned resonance frequency, the linear frequency sweep of the laser is realized by applying a voltage on the micro-ring resonator (110), and the self-injection effect has the effect of noise suppression and is used for reducing the near-phase noise of the laser as shown in formula (1):

wherein, the value of delta v and the value of delta v₀Line widths of the laser before and after self-injection, Q, Q respectively_LDThe quality factors of the micro-ring and the laser are respectively, the quality factor line width narrowing effect of the micro-ring resonant cavity is in direct proportion, and the quality factor of the micro-ring resonant cavity is Q_LD＝2×10⁵A 1 is to P_rThe value of/P is 0.3, α is 2.5, and η is approximately 1000, the line width is reduced by three orders of magnitude.

2. The narrow linewidth laser of claim 1,

a first thermal phase shifter (106) is arranged above the first straight waveguide, the first thermal phase shifter (106) is made of metal, the middle part of the first thermal phase shifter is a first heating resistor (106a), and waveguide phase shift is generated by heating the first heating resistor (106 a);

the micro-ring resonator (110) is an annular micro-ring waveguide (115), a second thermal phase shifter (107) for adjusting the resonance wavelength of the micro-ring is arranged above the annular micro-ring waveguide (115), the second thermal phase shifter (107) is made of metal, the middle part of the second thermal phase shifter is a second heating resistor (107a), the second heating resistor (107a) is heated, and the wavelength and the resonance frequency of the micro-ring resonator are tuned;

the first thermal phase shifter (106) and the second thermal phase shifter (107) form a tuning unit for wavelength tuning of the narrow linewidth laser.

3. The narrow linewidth laser of claim 1,

the micro-ring resonator (110) is an annular micro-ring waveguide (115), piezoelectric ceramics (108) are arranged above the annular micro-ring waveguide (115), and under the driving of a high-frequency voltage signal, the stress refractive index of the optical waveguide layer (104) is changed to change the resonance wavelength under the downward stress of the piezoelectric ceramics (108), and the self-injection state is changed along with the change of the resonance wavelength, so that the frequency modulation of the laser is realized.

4. The narrow linewidth laser of claim 3,

the piezoelectric ceramics (108) are adhered to the substrate layer (101) through epoxy resin glue.

5. The narrow linewidth laser of any of claims 3-4, wherein the piezoelectric ceramic (108) is swept by the elasto-optic effect to a frequency range of 1.69GHz at a modulation rate of 12kHz and 0.91GHz at a modulation rate of 20 Khz.

6. A narrow line width laser according to any of claims 1-3, wherein the microring resonator (110) functions as both a wavelength selective filter and a mirror to reflect a portion of light near the resonant wavelength back to the distributed feedback laser (103) and to filter out light near the resonant wavelength.

7. The narrow linewidth laser of any of claims 1-3, wherein the self-injection microring resonator has a quality factor of 2 x 10⁵The linewidth is 3kHz, the static tuning is 9.1GHz at 1547 nm, the dynamic modulation bandwidth is 1.69GHz at 12kHz, and the 50kHz is 0.91 GHz.

8. A narrow linewidth laser as claimed in any one of claims 1 to 3 wherein the voltage across the microring resonator (110) is adjusted such that, in the first free region (401), the laser maintains a free state; during an injection locking (402) phase, the laser transitions from a free state to an injection locked state; in the injection locking state, the micro-ring tuning frequency and the output frequency are approximately in a linear relation; with the increasing of the voltage of the micro-ring, the laser gradually enters a second free area (403), at the moment, the laser jumps to the next free area when exceeding a locking range, and the laser works in a self-injection locking state (402) of a central linear tuning area to realize a frequency modulation function based on thermal tuning and a static linear tuning result, wherein when the line width is 3.7kHz, the static tuning range is 73 pm.

9. A narrow line width laser according to any of claims 1 to 3, wherein the distributed feedback laser (103) is end-coupled to the first straight waveguide by a silicon photonic spot converter (112), the silicon photonic spot converter (112) being tapered and part of the optical waveguide layer (104).

10. The narrow linewidth laser of claim 9, wherein

One end of the silicon photon spot converter (112) close to the distributed feedback laser (103) has a smaller spot size matched with that of the nano silicon photon waveguide, and the other end has a larger spot size matched with that of the standard optical fiber; the silicon photon spot converter (112) realizes that the spot shapes of the distributed feedback laser (103) and the optical waveguide layer (104) are matched as much as possible, and has larger overlapped integral values.

11. The narrow linewidth laser of any of claims 1-3, wherein the optical beam splitter (109) is a silicon-based multimode interference structure (MMI) having five parameters: 1. coupling ratio: representing the amount of power coupled by the input channel (i) to the specified output channel (j); 2. additional loss: represents the total loss due to the coupler; 3. channel insertion loss: representing the magnitude of power from the input channel to the output channel; 4. the isolation ratio is as follows: indicating the degree of isolation between the same side ports in the transmissive coupler; 5. return loss: indicating the amount of power returned by the input channel.

12. The narrow linewidth laser of claim 11, wherein

The optical splitter (109) is a 3db optical splitter, and the 3db optical splitter is a 50: a 50 coupling ratio coupler.