使用一系列的绝热tapers56，在激光活性区域的INP/Si杂种模式之间将光学模式转移到超过4.8μm的垂直距离上 ，转移到Ultrahigh-Q谐振器的ULL SIN波导层。该模式首先从INP/SI混合波导转移到Si波导中
，然后从Si波导转移到SIN RDL中 。由于这些INP，SI和SIN层是在接触中（如果是INP和SI）或近距离（对于SI和SIN RDL的情况下）进行制造 ，则将光学模式在它们之间迅速转移，并且它们的锥度长度总计小于300μm
。特别是
，INP到Si Rib波导的转变可能非常短（约25μm），因为INP和SI具有相似的折射率45。随后将具有231 nm蚀刻深度的Si Rib波导逐渐变细至200 nm宽度 ，以将模式转移到厚度为269 nm的薄Si波导中 。薄的Si锥度从约3μm到150 nm
，以匹配RDL SIN波导的有效指数，以有效地进行Si -Sin功率传递。为了跨越分离sin rdl的垂直距离和高Q谐振器所在的Ull sin层 ，光学模式逐渐从上罪演变为下部罪行层
。RDL SIN和ULL SIN波导具有100 nm的相同核心厚度
，因此它们的有效指标很容易匹配。因此，RDL SIN波导宽度从2,800 nm逐渐缩小到200 nm ，同时将ULL SIN波导宽度从200 nm扩大到2,800 nm
，距离长度接近1 cm 。该方案可实现有效的电力传输（<1-dB insertion loss) from the RDL SiN waveguide to the ULL SiN waveguide. 　　In weakly confined ULL SiN waveguides, the optical mode extends significantly into the silicon dioxide (SiO2) cladding. Previous work31, in which a ULL SiN waveguide was heterogeneously integrated in close vertical proximity with an InP/Si hybrid waveguide, resulted in a relatively high propagation loss of 0.43 dB cm−1. In this work, a 3D layer transition enables the ULL SiN waveguide to be buried deeper within the SiO2 cladding, such that impurities originating from the back-end heterogeneous integration process do not influence the ULL waveguide performance. 　　To motivate the addition of a SiN RDL in such a 3D layer transition, we compare the performance of a Si-to-SiN waveguide transition with a SiN-to-SiN waveguide transition. The optimal length of an adiabatic coupler transition is given by , in which κ represents the coupling coefficient between the waveguides in the coupling region, and ϵ = 0.01 represents the tolerance of power transferred to the undesired, anti-symmetric system mode (that is, loss)56. This optimal (minimum) length is calculated as a function of vertical separation in Extended Data Fig. 1a, which demonstrates that a SiN-to-SiN layer results in a more efficient (shorter) transition for vertical separation exceeding 2 μm. 　　Thus, the inclusion of a SiN RDL beneath the Si waveguide provides improved vertical coupling efficiency, enabling the ULL SiN waveguide to be buried deeper below. The SiN RDL is further motivated by additional performance and fabrication concerns. Efficient power transfer between Si and SiN waveguides requires a very narrow Si width to match the propagation constants of the respective waveguides, as shown in Supplementary Fig. 1b. Such narrow Si waveguides feature significant sidewall-roughness-induced scattering loss, limiting the length of such structures. Furthermore, the combination of narrow width and long length of a Si-to-SiN transition capable of spanning several-micrometre distance would yield a fragile structure that is susceptible to damage during the fabrication process. As such, the close proximity of the SiN RDL to the Si waveguide enables a short Si-to-SiN transition, improving process yield. In this work, the SiN-to-SiN transition length was chosen to be excessively long (approaching 1 cm) to enable flexibility in the choice of vertical separation while retaining transition efficiency. 　　To experimentally evaluate the achievable transition efficiency from the RDL SiN to the ULL SiN, two such layer transition structures were placed within a racetrack resonator. In contrast to a cut-back approach, in which multiple identical structures are cascaded in series to extract an aggregate insertion loss, a resonator-based measurement technique enables insertion loss of a structure to be measured independently of fibre-to-chip coupling losses, resulting in a more accurate measurement. Previous work has demonstrated this approach to accurately measure insertion losses well below 0.1 dB (ref. 57). The transmission spectrum of the resonator was measured and fitted to extract the internal round-trip loss, as shown in Supplementary Fig. 2. From this measurement, the insertion loss was inferred to be below 0.03 dB per transition. However, the resonator-based test structure was fabricated on a separate wafer that did not undergo any heterogeneous integration process and featured a narrower spacer thickness of roughly 3.5 μm. Thus we conservatively expect the insertion loss of the RDL-to-ULL transition within the heterogeneous laser to be well below 1 dB. 　　Fabrication of the SiN waveguides was performed at Tower Semiconductor, a commercial CMOS foundry, on a 200-mm-diameter Si wafer with 15-μm-thick thermal SiO2. Low-pressure chemical vapour deposition SiN with 100-nm thickness was deposited and patterned using deep ultraviolet (DUV) stepper lithography and reactive ion etching to form the ULL waveguide layer. Tetraethyl orthosilicate-based oxide was deposited on the ULL layer, annealed at 1,150 °C, and underwent chemical mechanical polishing to form an approximately 4-μm-thick spacer layer35. To form the RDL waveguide transition, another 100-nm-thick low-pressure chemical vapour deposition SiN was deposited and patterned by the same process. The adiabatic RDL taper was defined and etched on this layer. Additional tetraethyl orthosilicate-based oxide was deposited, annealed and underwent chemical mechanical polishing to leave around 500-nm-thick SiO2 on top of the RDL. The processed 200-mm wafer was then transferred out of the foundry for subsequent processing. The wafer was cored into 100-mm wafers to be compatible with an ASML 248-nm DUV stepper. Diced silicon-on-insulator pieces with 500-nm-thick Si device layer were bonded on the polished SiO2 surface using plasma-activated direct bonding. The Si substrate was removed by mechanical polishing plus deep Si Bosch etching. The buried SiO2 layer was removed by buffered hydrofluoric acid. The fabricated Si/SiN RDL/SiN ULL wafer was then ready for the heterogeneous InP process on Si similar to our previous studies31. In general, Si waveguides and tapers were patterned with a DUV stepper whereas the grating was patterned with electron beam lithography with a period of 240 nm. The Si layer underwent several patterned etches with different etch depths. The first etch had a 231-nm etch depth to form Si shallow etched rib waveguides in the InP/Si and phase-tuner sections. Then the Si gratings and thin Si tapers were formed respectively with 269-nm etch depth. Si outgassing channels were patterned later and etched with an etch depth of 500 nm in the area that had no Si waveguides. The Si etch was reactive-ion-etched with a mixed etching gas of C4F8/SF6 and the etch depth was controlled by an etch monitor Intellemetrics LEP400. After Si processing, InP dies with the layer stack shown in Fig. 1c were bonded on the fabricated Si circuits, with the InP substrate removed by mechanical polishing and 3:1 hydrochloric acid:deionized water. A thin layer of p-type contact-metal Pd/Ge/Pd/Au was formed using a lift-off process. The p-InP mesa was etched using CH4/H2/Ar, with a SiO2 hard mask. The dry etch was monitored using an etch monitor and stopped at the AlInGaAs  quantum well (QW) layer. After another round of QW layer lithography, the QW layer was etched using a mixed solution of H2O/H2O2/H3PO4 15/5/1. An n-type InP mesa etch followed the QW etch to complete the mesa definition with the same etching gas CH4/H2/Ar. The excess Si on top of the SiN devices was removed using a XeF2 isotropic gas etch. The entire chip was passivated using low-temperature deuterated SiO2 (ref. 58) followed by the contact-metal window opening through CF4-based inductively coupled plasma etching. The n-type contact-metal Pd/Ge/Pd/Au and another layer of Ti/Au on top of the p-type contact metal were deposited and formed. Proton implantation was performed to define the current channels. Ti/Pt was deposited as heaters for the phase tuner on Si and resonance tuner on SiN. The chip went through another round of SiO2 deposition and contact via opening. The Ti/Au probe metal was deposited to finish the wafer fabrication. The fabricated 100-mm-diameter 3D PIC wafer was diced and polished to expose the SiN edge couplers for fibre-coupled device characterization. The detailed process flow charts can be found in Extended Data Fig. 7. 　　To analyse the impurity distribution along the depth direction (depth profiling), a secondary ion mass spectrometry system (also known as ion microprobes, CAMECA IMS 7f) was used to analyse the devices. In the measurement, a raster area of 50 μm × 50 μm was swept with the primary beam (for ionization and sputtering) and secondary ions generated only in the centre area of 20 μm × 20 μm were collected by the instrument filtering aperture to prevent impacts from other layers at the edge of the hole drilled. To obtain conductivity required for secondary ion mass spectrometry, 20 nm of gold was deposited onto device surfaces. Reference devices NIST SRM 610 and 612 (ref. 59) (National Institute of Standards and Technology Standard Reference Materials (NIST SRM)) were used for the calibrations of elementary concentrations. The measurement was implemented in a vacuum level of 3 × 10−9 torr. For the positive-ion measurements, O− ions were the primary beam. For the negative-ion measurements, Cs+ ions were the primary beam, and the electron beam was also engaged to neutralize the sample to avoid charging effects. The results are plotted in Extended Data Fig. 3. 　　The sample area measured here is a pure waveguide region without the top laser structure but experienced the full back-end-of-line process. The appearance of boron atoms indicates the boundary of the lower thermal oxide cladding layer because the substrate Si wafers are of p-type (resistivity of about 100 Ω cm) to accelerate thick thermal oxidation. The appearances of both Si–N clusters and C–N clusters indicate the thin SiN waveguide layer because nitrogen atoms begin to appear in large amounts. The coincidence of B, Si–N and C–N traces cross-verify each other and gives the SiN waveguide depth position of 5.9 μm. 　　The lasers are characterized on a temperature-controlled copper stage with a precision temperature controller (Vescent SLICE-QTC) for device characterization at 20 °C. We screened the lasers before the self-injection-locking characterization, phase-noise measurement and so on. The laser light-current measurement results are shown in Extended Data Fig. 4a, which exhibit an approximately 74-mA laser threshold, influenced by the DFB grating strength. Compared with typical laser light-current behaviours, one difference of such a laser-resonator device is that with the increase of laser gain current, the recorded power would see several dips in the light-current curve when the laser output power is filtered by the ring resonator. The spacing of the resonance dips is determined by the ring resonator FSR (30 GHz in this work) when the laser wavelength is swept across multiple resonances during the gain-current increase. It has to be noted that in this light-current sweep, the laser gain current is stepped at 1 mA so not every resonance can be matched and recorded. 　　We can thus lock the laser to different resonances by tuning the laser gain current. Besides, the thermal tuning of ring resonances allows the continuous tuning of the SIL laser wavelengths across the DFB laser wavelength. This capability is critical in microwave generation as microwave frequency can be synthesized precisely based on the laser gain and ring resonance controls. We lock two SIL lasers at two resonances with over 3-nm-wavelength space and the laser spectra are shown in Extended Data Fig. 4b. This wavelength separation promises >如果有快速的PD，则为375-GHz毫米波生成
。更重要的是 ，相位噪声将与低载体频率相同
，与激光相噪声确定。 　　通过将激光波长调整为从环的共振，从环将激光波长锁定到谐振的后散射光 ，前提是到达激光器的反向散射光的相位是正向激光输出相的2π的整数值 。换句话说，激光的波长和共振在频域中匹配
，而激光和反向散射光的相位则在时域中匹配，如图2a所示
。匹配波长是通过调谐激光增益电流或环加热器电流执行的 ，而与相位匹配的相位则通过调谐相敲击电流来完成。激光增益电流和相位调节器电流均由低噪声激光电流源（ILX Lightwave LDX-3620）驱动
，以确保稳定且低噪声操作 。自我注射锁定状态的检测不仅可以观察到激光波长触发共振时的输出功率的下降，而且还可以观察到自我注射锁定锁定的线路的降低 ，也可以降低自我形态锁定的线宽
。自组织干涉仪的设置基本上由具有偏振控制器的马赫德干涉仪（由两个3 dB耦合器制成）
，其一个手臂中的一个和短延迟线以及在另一个手臂中的纤维耦合声音调节器（Gooch＆Housego 27 MHz）中的另一个手臂，如图2b所示。在将其发送到电谱分析仪（ESA）（Rohde＆Schwarz FSWP）之前 ，使用PD（Newport 1811）检测到Self Hertrodyne干涉仪的BEAT频率（Newport 1811） 。
。在自我注射锁定过程中
，大致调整了相位调节器电流，以允许锁定发生并随后进行精细调整 ，以确保稳定的自我注射锁定。值得一提的是
，自我注射锁定状态可以持续数小时，而无需包装激光芯片 。这可以归因于同一芯片上激光和谐振器的整合 ，从而降低了激光器和从环的反向散射光之间的相位波动。 　　通过在三个2π注射锁定周期内扫描其应用的电力（Keithley 2604b），同时记录SIL SIL Laser检测到的自求odydrogy beat的ESA频谱（图2C
，图2C，顶部） ，研究了相敲打对自注射锁定的动力学
。注射锁定周期的光谱图（如图2C所示）证明了稳定的SIL周期（深蓝色区域）
，其次是混乱的区域（浅蓝色），然后是解锁区域。在仅一个时期内 ，在相位调节期间
，在示波器（Tektronix MSO64B）上还检测到激光功率，这清楚地显示了上述行为（图2C ，底部） 。另一个重要的参数是自我注射锁定持续存在的频率范围
。可以通过将激光频率在环共振上扫描或在激光器上扫环。我们通过使用应用于电流源的三角信号（Keithley 2604b）扫描环加热器的电流来选择第二个方案 。为了检测扫描过程中激光宽度的变化
，使用SIL激光器和窄线宽光纤激光器之间的3 dB耦合器进行节拍
。用ERBIUM掺杂的纤维放大器（Amonics AEDFA-IL-18-B-FA）光学放大，并发送到连接到ESA的快速PD（Finisar HPDV2120R） ，如图2B的下分支所示。共振扫描期间记录的频谱图如图2D所示 。激光频率噪声和最终的基本线宽取自商业激光相分析仪（OEWAVES OE4000）
，该分析仪内部在测得的相位噪声上进行平均
。对于几个测量运行，我们尚未观察到1 kHz和1 MHz之间的噪声光谱有显着差异 ，这非常稳定，并且相信该范围内的噪声谱是通过与仿真结果进行比较的谐振器的热效率噪声所支配的（图2E）。作为比较
， 延迟的自组织设置使用两个PD来接收杂差BEAT60，该杂种BEAT60先前用于超高噪声激光线宽特征36 ，并允许对统计测量误差进行更详细的分析 。 　　图3b示意性地描述了用于分析激光反馈灵敏度的实验构型
。耦合激光发射被发送到90/10光纤束弹奏者
，此后，耦合功率的90％将用于外部光学反馈。反馈回路由8米长的单模光纤 ，一个三端口光学循环器
，偏振控制器和可变光学衰减器（VOA）组成，该衰减器（VOA）允许衰减范围为0至40 dB（EXFO MOA-3800） 。应当指出的是 ，激光反馈灵敏度还取决于反射场的极化
，必须调整这些磁场以在进行分析之前最大程度地提高反馈影响。其余10％的激光输出用于反馈灵敏度表征
。通过光学隔离器后，将其转移到相位噪声分析仪（OEWAVES OE4000）中进行频率 - 噪声表征或用于电谱的激光相干检查的延迟自生态设置。 　　在这项研究中 ，反馈强度由反射能力（Prefl）和输出功率（pout）确定：通过以下关系： 　　应考虑来自反馈回路的所有损失以计算反射能力
，从而计算反馈强度 。优化设置后，纤维芯片耦合损耗为-3 dB（往返耦合损耗为-6 dB） ，来自90％Beamsplitter的总损耗，光学循环器
，VOA的插入损失，偏光控制器和纤维为-4.05 db
。因此 ，可以将VOA衰减的反馈强度从-10.05 dB调整为-50.05 dB。 　　为了进一步减少设置的损失
，从而最大程度地提高了反馈强度，我们使用100％的光纤反向反射器（BKR ，Thorlabs）来替换90％BeamSplitter端口之后的配置 。然后将反馈回路的损失降低至-0.9 dB
，最大反馈强度为-6.9 dB
。 　　两个激光器是SIL到两个30 GHz-FSR环的谐振器，它们的频率在不调谐环共振的情况下以10 GHz的速度移动。尽管每个环谐振器可以通过在环上的加热器上施加约0.5 W的电源来调整30 GHz ，但我们仅使用一个环加热器进行调整 。第一个FSR的调整范围为-10 GHz至20 GHz
，而下一个完整的FSR调整通过将第二激光锁定到下一个共振，从而覆盖完整的50 GHz ，从而导致20 GHz至50 GHz调谐
。如图4B所示
，两个激光器的输出是通过纤维V型槽阵列从芯片中收集的，并发送到3-DB纤维耦合器 ，然后将ERBIUM掺杂的纤维放大器收集到与ESA连接的Fast PD（Finisar HPDV2120R）之前，然后击败了ESA。如果激光器在预期的共振上
，则将一小部分激光输出（1％）发送到光谱分析仪（Yokogawa AQ6370C） 。尽管我们的芯片可用于在50 GHz上生成任何任意微波频率，但此处用于在整个50 GHz上以1 GHz的步骤生成微波频率进行演示（图4C）
。通过将一种激光器的相位锁定到另一个激光器的相位 ，使用偏移相锁伺服电路（Vescent D2-135）来提高最高10 GHz的生成的微波信号的长期稳定性。伺服控制框中的反馈信号被发送到激光器的一个环加热器之一
，以将其阶段锁定到第二激光器的阶段 。因此，稳定的微波信号具有SIL激光器的低相位特征 ，并在扩展数据中绘制了VOIGT拟合图6。
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