A wideband,high-resolution vector spectrum analyzer for integrated photonics

The analysis of optical spectra—emission or absorption—has been arguably the most powerful approach for discovering and understanding matter.The invention and development of many kinds of spectrometers have equipped us with versatile yet ultra-sensitive diagnostic tools for trace gas detection,isotope analysis,and resolving hyperfine structures of atoms and molecules.With proliferating data and information,urgent and demanding requirements have been placed today on spectrum analysis with ever-increasing spectral bandwidth and frequency resolution.These requirements are especially stringent for broadband laser sources that carry massive information and for dispersive devices used in information processing systems.In addition,spectrum analyzers are expected to probe the device’s phase response where extra information is encoded.Here we demonstrate a novel vector spectrum analyzer(VSA)that is capable of characterizing passive devices and active laser sources in one setup.Such a dual-mode VSA can measure loss,phase response,and dispersion properties of passive devices.It also can coherently map a broadband laser spectrum into the RF domain.The VSA features a bandwidth of 55.1 THz(1260–1640 nm),a frequency resolution of 471 kHz,and a dynamic range of 56 dB.Meanwhile,our fiber-based VSA is compact and robust.It requires neither high-speed modulators and photodetectors nor any active feedback control.Finally,we employ our VSA for applications including characterization of integrated dispersive waveguides,mapping frequency comb spectra,and coherent light detection and ranging(LiDAR).Our VSA presents an innovative approach for device analysis and laser spectroscopy,and can play a critical role in future photonic systems and applications for sensing,communication,imaging,and quantum information processing.

Frequency-comb-linearized, widely tunable lasers for coherent ranging

Tunable lasers,with the ability to continuously vary their emission wavelengths,have found widespread applications across various fields such as biomedical imaging,coherent ranging,optical communications,and spectroscopy.In these applications,a wide chirp range is advantageous for large spectral coverage and high frequency resolution.Besides,the frequency accuracy and precision also depend critically on the chirp linearity of the laser.While extensive efforts have been made on the development of many kinds of frequency-agile,widely tunable,narrow-linewidth lasers,wideband yet precise methods to characterize and linearize laser chirp dynamics are also demanded.Here we present an approach to characterize laser chirp dynamics using an optical frequency comb.The instantaneous laser frequency is tracked over terahertz bandwidth at 1 MHz intervals.Using this approach we calibrate the chirp performance of 12 tunable lasers from Toptica,Santec,New Focus,EXFO,and NKT that are commonly used in fiber optics and integrated photonics.In addition,with acquired knowledge of laser chirp dynamics,we demonstrate a simple frequency-linearization scheme that enables coherent ranging without any optical or electronic linearization unit.Our approach not only presents novel wideband,highresolution laser spectroscopy,but is also critical for sensing applications with ever-increasing requirements on performance.

Recent advances in laser self-injection locking to high-Q microresonators

The stabilization and manipulation of laser frequency by means of an external cavity are nearly ubiquitously used in fundamental research and laser applications. While most of the laser light transmits through the cavity, in the presence of some back-scattered light from the cavity to the laser, the self-injection locking effect can take place, which locks the laser emission frequency to the cavity mode of similar frequency. The self-injection locking leads to dramatic reduction of laser linewidth and noise. Using this approach, a common semiconductor laser locked to an ultrahigh-Q microresonator can obtain sub-Hertz linewidth, on par with state-of-the-art fiber lasers. Therefore it paves the way to manufacture high-performance semiconductor lasers with reduced footprint and cost. Moreover, with high laser power, the optical nonlinearity of the microresonator drastically changes the laser dynamics, offering routes for simultaneous pulse and frequency comb generation in the same microresonator. Particularly, integrated photonics technology, enabling components fabricated via semiconductor CMOS process, has brought increasing and extending interest to laser manufacturing using this method. In this article, we present a comprehensive tutorial on analytical and numerical methods of laser self-injection locking, as well a review of most recent theoretical and experimental achievements.

Foundry manufacturing of tight-confinement,dispersion-engineered,ultralow-loss silicon nitride photonic integrated circuits

The foundry development of integrated photonics has revolutionized today’s optical interconnect and datacenters.Over the last decade,we have witnessed the rising of silicon nitride(Si_(3)N_(4)) integrated photonics,which is currently transferring from laboratory research to foundry manufacturing.The development and transition are triggered by the ultimate need for low optical loss offered by Si_(3)N_(4),which is beyond the reach of silicon and III-V semiconductors.Combined with modest Kerr nonlinearity,tight optical confinement,and dispersion engineering,Si_(3)N_(4) has today become the leading platform for linear and Kerr nonlinear photonics,and it has enabled chip-scale lasers featuring ultralow noise on par with table-top fiber lasers.However,so far all the reported fabrication processes of tight-confinement,dispersion-engineered Si_(3)N_(4) photonic integrated circuits(PICs)with optical loss down to few dB/m have only been developed on 4-inch(100 mm diameter)or smaller wafers.Yet,to transfer these processes to established CMOS foundries that typically operate 6-inch or even larger wafers,challenges remain.In this work,we demonstrate the first foundry-standard fabrication process of Si_(3)N_(4) PICs with only 2.6 dB/m loss,thickness above 800 nm,and near 100%fabrication yield on 6-inch(150 mm diameter)wafers.Such thick and ultralow-loss Si_(3)N_(4) PIC enables low-threshold generation of soliton frequency combs.Merging with advanced heterogeneous integration,active ultralow-loss Si_(3)N_(4) integrated photonics could pave an avenue to addressing future demands in our increasingly information-driven society.

Experimental test of generalized Hardy's paradox

Since the pillars of quantum theory were established, it was already noted that quantum physics may allow certain correlations defying any local realistic picture of nature, as first recognized by Einstein,Podolsky and Rosen. These quantum correlations, now termed quantum nonlocality and tested by violation of Bell's inequality that consists of statistical correlations fulfilling local realism, have found loophole-free experimental confirmation. A more striking way to demonstrate the conflict exists, and can be extended to the multipartite scenario. Here we report experimental confirmation of such a striking way, the multipartite generalized Hardy's paradoxes, in which no inequality is used and the conflict is stronger than that within just two parties. The paradoxes we consider here belong to a general framework [S.-H. Jiang et al., Phys. Rev. Lett. 120(2018) 050403], including previously known multipartite extensions of Hardy's original paradox as special cases. The conflict shown here is stronger than in previous multipartite Hardy's paradox. Thus, the demonstration of Hardy-typed quantum nonlocality becomes sharper than ever.