Spectral line‐by‐line shaping for optical and microwave arbitrary waveform generations

Spectral line‐by‐line shaping is a key enabler towards optical arbitrary waveform generation, which promises broad impact both in optical science and technology. In this paper, generation of optical and microwave arbitrary waveforms using the spectral line‐by‐line shaping technique is reviewed. Compared to conventional pulse shaping, significant new physics arises in the line‐by‐line regime, where the shaped pulse fields generated from one laser pulse now overlap with those generated from adjacent pulses. This leads to coherent interference effects related to the properties of optical frequency combs which serve as the source in these experiments. We explore such effects in a series of experiments using several different high‐repetition‐rate optical combs, including harmonically mode‐locked lasers and continuous‐wave lasers that are externally phase modulated either with or without the help of an optical cavity. As an application of line‐by‐line pulse shaping, we describe generation of microwave electrical arbitrary waveforms that can be reprogrammed at rates approaching 10 GHz.     

光频链接的双光梳气体吸收光谱测量

双光梳光谱技术以其无运动部件快速采样、高分辨率探测等优势成为宽带激光光谱测量中的热点技术.但受限于常用微波锁定双光梳光源间的噪声特性,双光梳光谱技术仍难以发挥其探测潜能.本文报道一种光频域互相链接的双光梳光谱探测方案.通过将两台激光器的偏置频率同时锁定到一个窄线宽激光器上,既免去了结构复杂且成本高昂的非线性自参考系统,又将双光梳间的共同参考点设置到了光频范围,抑制了双光梳光谱采样抖动,实现光谱探测性能的提升.~(13)C_2H_2的ν_1+ν_3 P支光谱数据测量数据分析结果表明:谱线位置与文献结果符合良好,光谱分辨率为0.086 cm~(-1),信噪比200:1(62.5 ms,100幅平均),相应的秒均噪声等效吸收系数达6.0×10~(-6)cm~(-1)·Hz~(-1/2).该工作为双光梳光谱测量的实际应用提供了一种高精度、低成本、易于实现的解决方案.     

Interplanetary laser communications and precision ranging

Future interplanetary missions, including robotic probes and human travel, will require significantly enhanced communications bandwidth that will be difficult to realize with current radio/microwave frequency links. Besides satisfying this requirement, optical (laser) communications has the potential for substantially lowering mass, power, and volume burden on the host spacecraft; is free from spectrum allocation issues; and can potentially support tracking functions resulting in improved spacecraft navigation. Primary challenges of optical communications include precision laser beam pointing over the huge planetary range, inefficiency of laser transmitters, quantum noise limited detection, especially in presence of additive background during periods of near Sun pointing, atmospheric degradation due to attenuation and turbulence and weather outages. This paper will briefly review the status of technologies that have been devised to meet these challenges and system‐level developments for bi‐directional telecommunications and ranging to distant probes.     

Near‐room‐temperature photon‐noise‐limited quantum well infrared photodetector

With the modern development of infrared laser sources such as broadly tunable quantum cascade lasers and frequency combs, applications of infrared laser spectroscopy are expected to become widespread. Consequently, convenient infrared detectors are needed, having properties such as fast response, high efficiency, and room‐temperature operation. This work investigated conditions to achieve near‐room‐temperature photon‐noise‐limited performance of quantum well infrared photodetectors (QWIPs), in particular the laser power requirement. Both model simulation and experimental verification were carried out. At 300 K, it is shown that the ideal performance can be reached for typical QWIP designs up to a detection wavelength of 10 µm. At 250 K, which is easily reachable with a thermoelectric Peltier cooler, the ideal performance can be reached up to 12 µm. QWIPs are therefore suitable for detection and sensing applications with devices operating up to or near room temperature.     

Microresonator Kerr frequency combs with high conversion efficiency

Microresonator‐based Kerr frequency comb (microcomb) generation can potentially revolutionize a variety of applications ranging from telecommunications to optical frequency synthesis. However, phase‐locked microcombs have generally had low conversion efficiency limited to a few percent. Here we report experimental results that achieve conversion efficiency ( on‐chip comb power excluding the pump) in the fiber telecommunication band with broadband mode‐locked dark‐pulse combs. We present a general analysis on the efficiency which is applicable to any phase‐locked microcomb state. The effective coupling condition for the pump as well as the duty cycle of localized time‐domain structures play a key role in determining the conversion efficiency. Our observation of high efficiency comb states is relevant for applications such as optical communications which require high power per comb line.

     

Dynamic optical arbitrary waveform generation and detection in InP photonic integrated circuits for Tb/s optical communications

This article presents recent results in the development of optical arbitrary waveform generation (OAWG) technologies based on optical frequency combs and indium phosphide devices. A novel spectral-slice dynamic-OAWG approach and waveform shapers with customized spectral multiplexers and modulators, enable continuous generation of high fidelity optical waveforms accessing bandwidths in excess of 1 THz. We show results for two types integrated waveform shapers, a 100 GHz electrically controlled device with 10 channels spaced at 10 GHz and a 1 THz optically controlled device with 100 channels spaced at 10 GHz. Additionally, we include results from a 640 GHz waveform measurement device with 16 channels and 40 GHz spacing.     

Precise Underwater Distance Measurement by Dual Acoustic Frequency Combs

Underwater acoustics is of fundamental importance for marine science and technology. However, acoustic waves transmitted by state‐of‐the‐art underwater acoustic systems are not inherently phase locked, which hinders the development of underwater acoustic technology. For example, the precision of underwater distance measurement can only achieve centimeter level. As a versatile tool, optical frequency combs have enabled revolutionary progress in optical metrology and precision measurement. In parallel with optical frequency combs, here, the generation of fully stabilized, underwater acoustic frequency combs is reported, in which equidistant acoustic modes are produced via a hydroacoustic transducer. The precision of each individual acoustic mode is measured to be 10^?9 at 1 s and 10^?12 at 1000 s averaging times. Underwater distance measurements are carried out in an anechoic pool using a dual‐comb scheme. Comparison with reference values shows consistency within 50 µm (7 × 10^?6 in relative). The relatively long‐duration experiments at 7 m distance yield an Allan deviation of 1.8 µm (2.6 × 10^?7 in relative) at 1 s and further 480 nm (6.8 × 10^?8 in relative) at 40 s averaging times. The approach to acoustic frequency comb generation offers a promising and powerful platform for future underwater distance measurement, positioning, and navigation.     

Lossless equalization of frequency combs

Frequency combs obtained by sinusoidal phase modulation of narrowband cw lasers are widely used in the field of optical communications. However, the resulting spectral envelope of the comb is not flat. We propose a general and efficient approach to achieve flat frequency combs with tunable bandwidth. The idea is based on a two-step process. First, efficient generation of a train with a temporal flat-top-pulse profile is required. Second, we use large parabolic phase modulation in every train period to map the temporal intensity shape into the spectral domain. In this way the resulting spectral envelope is flat, and the size is tunable with the chirping rate. Two different schemes are proposed and verified through numerical simulations.     

Few‐optical‐cycle light pulses with passive carrier‐envelope phase stabilization

One of the most advanced frontiers of ultrafast optics is the control of carrier‐envelope phase (CEP) ϕ of light pulses, which enables the generation of optical waveforms with reproducible electric field profile. Such control is important for pulses with few‐optical‐cycle duration, for which a CEP variation produces a strong change in the waveform, so that strongly nonlinear optical phenomena, such as multiphoton absorption, above‐threshold ionization and high‐harmonic generation become CEP‐dependent. In particular, CEP control is the prerequisite for the production of isolated attosecond pulses. Standard laser systems generate pulses that are CEP unstable; the CEP can be stabilized using either active or passive methods. Passive, all‐optical schemes rely on difference‐frequency generation (DFG) between two pulses sharing the same CEP: in this process the phases of the two pulses add up with opposite signs, leading to cancellation of the shot‐to‐shot CEP fluctuations. This paper presents an overview of passive CEP stabilization schemes, starting from the basic concepts and progressing to the details of the practical implementations of the idea. The passive approach allows the generation of CEP‐controlled few‐optical‐cycle pulses covering a very broad range of parameters in terms of carrier frequency (from visible to mid‐IR), energy (up to several mJs) and repetition rate (up to hundreds of kHz)     

Femtosecond pulse processing

We discuss Fourier methods for shaping and processing femtosecond optical signals. Examples of applications in optical communications and in generation of terahertz radiation are presented.     

Enabling arbitrary wavelength frequency combs on chip

A necessary condition for generation of bright soliton Kerr frequency combs in microresonators is to achieve anomalous group velocity dispersion (GVD) for the resonator modes. This condition is hard to implement in the visible as well as ultraviolet since the majority of optical materials are characterized with large normal GVD in these wavelength regions. We overcome this challenge by borrowing ideas from strongly dispersive coupled systems in solid state physics and optics. We show that photonic compound ring resonators can possess large anomalous GVD at any desirable wavelength, even if each individual resonator is characterized with normal GVD. Based on this concept, we design a mode‐locked frequency comb with thin‐film silicon nitride compound ring resonators in the vicinity of the rubidium D₁ line (794.6 nm) and propose to use this optical comb as a flywheel for chip‐scale optical clocks.

     

A novel frequency sextupling scheme for optical mm-wave generation utilizing an integrated dual-parallel Mach-Zehnder modulator

A novel scheme is proposed for frequency sextupling mm-wave generation based on a laser and an integrated dual-parallel Mach-Zehnder modulator (MZM) without optical filter. Theoretical analysis is presented to suppress the undesired optical sidebands for the high quality generation of frequency sextupling mm-wave signal. The performance of the proposed scheme is evaluated by simulations. Utilizing the integrated MZM consisted of two sub-MZMs with extinction ratio of 30 dB, the optical sideband suppression ratio (OSSR) is as high as 29.9 dB and the radio frequency spurious suppression ratio (RFSSR) exceeds 24 dB without any optical or electrical filter. The impact of the nonideal RF driven voltage and phase difference of RF driven signal applied to two sub-MZMs of the integrated MZM on OSSR and RFSSR is discussed and analyzed. After transmission over fiber, the generated optical mm-wave signal demonstrates good performance. Furthermore, the performance of two cases for the proposed scheme is also compared.     

覆盖可见光波长的掺Er光纤飞秒光学频率梳

飞秒光学频率梳波长覆盖范围向可见光波长扩展对于碘稳频激光的绝对频率测量以及光钟研究中钟激光的绝对频率测量都具有十分重要的意义. 本文在自行研制掺Er光纤飞秒光学频率梳的基础上, 采用放大-倍频-扩谱的方案, 实现了激光输出波长向可见光波长的扩展. 掺Er光纤飞秒光学频率梳输出的一部分光激光脉冲, 功率约为8 mW, 首先经掺Er光纤放大器将功率提高到531 mW, 此后利用MgO: PPLN晶体倍频, 倍频后激光的功率为170 mW, 倍频效率为32%, 脉冲宽度为85 fs. 倍频后的激光通过光子晶体光纤进行光谱展宽. 通过优化入射光偏振状态可以实现波长覆盖500-1000 nm, 输出功率为85 mW, 耦合效率为50%. 采用小型化碘稳频532 nm Nd: YAG激光器输出激光与光学频率梳光谱展宽后的激光进行拍频可以获得30 dB的拍频信号. 覆盖可见光波长的掺Er光纤飞秒光学频率梳为可见光范围内激光的绝对频率测量提供了技术手段.     

Stable multi-frequency generator based on phase-locked optical frequency combs

A photonic generation of multi-frequency source based on the heterodyne of two phase-locked optical frequency combs(OFCs) is proposed and demonstrated.By applying an optical phase-locked loop,the phase noise induced by optical links is decreased by approximately 70,66,and 35 dB at 0.01,0.1,and 1.0 Hz offset frequencies,respectively.The proposed scheme provides 8 radio frequency signals,the frequencies of which span from 540 to 4040 MHz,with a 500-MHz interval.The number of generated signals can be readily scaled by using OFCs with broader bands,whereas the frequencies can be scaled by tuning the repetition rates of OFCs.     

Development and Performance Analysis of a Photonics-Assisted RF Converter for 5G Applications

This article presents a simple, ultra-wideband and tunable radiofrequency (RF) converter for 5G cellular networks. The proposed optoelectronic device performs broadband photonics-assisted upconversion and downconversion using a single optical modulator. Experimental results demonstrate RF conversion from DC to millimeter waves, including 28 and 38 GHz that are potential frequency bands for 5G applications. Narrow linewidth and low phase noise characteristics are observed in all generated RF carriers. An experimental digital performance analysis using different modulation schemes illustrates the applicability of the proposed photonics-based device in reconfigurable optical wireless communications.     

Einstein Lecture – Passion for precision

Optical frequency combs from mode‐locked femtosecond lasers have link optical and microwave frequencies in a single step, and they provide the long missing clockwork for optical atomic clocks. By extending the limits of time and frequency metrology, they enable new tests of fundamental physics laws. Precise comparisons of optical resonance frequencies of atomic hydrogen and other atoms with the microwave frequency of a cesium atomic clock are establishing sensitive limits for possible slow variations of fundamental constants. Optical high harmonic generation is extending frequency comb techniques into the extreme ultraviolet, opening a new spectral territory to precision laser spectroscopy. Frequency comb techniques are also providing a key to attosecond science by offering control of the electric field of ultrafast laser pulses. In our laboratories at Stanford and Garching, the development of new instruments and techniques for precision laser spectroscopy has long been motivated by the goal of ever higher resolution and measurement accuracy in optical spectroscopy of the simple hydrogen atom which permits unique confrontations between experiment and fundamental theory. This lecture recounts these adventures and the evolution of laser frequency comb techniques from my personal perspective.     

Fiber-laser frequency combs with subhertz relative linewidths

We investigate the comb linewidths of self-referenced, fiber-laser-based frequency combs by measuring the heterodyne beat signal between two independent frequency combs that are phase locked to a common cw optical reference. We demonstrate that the optical comb lines can exhibit instrument-limited, subhertz relative linewidths across the comb spectra from 1200 to 1720 nm with a residual integrated optical phase jitter of approximately 1 rad in a 60 mHz to 500 kHz bandwidth. The projected relative pulse timing jitter is approximately 1 fs. This performance approaches that of Ti:sapphire frequency combs.     

Growth of epitaxial c ‐plane ZnO film on a ‐plane sapphire by radio frequency reactive magnetron sputtering

In this Letter, we report the successful growth of high quality c ‐plane oriented epitaxial ZnO films on a ‐plane sapphire substrates by using radio frequency reactive magnetron sputtering. The effect of substrate temperature on the structural and optical properties has been investigated. X‐ray diffraction (XRD) studies reveal that the ZnO film is grown epitaxially on a ‐plane sapphire substrate, and the film quality is improv‐ ed as the substrate temperature is increased. Photoluminescence (PL) results manifest that screw dislocations can exert great influence on the optical properties. It is found that the line width of the near‐band‐edge emission of PL decreases linearly with increase in screw density. In addition, a simple and effective method is proposed to assess the defect density in epitaxial ZnO films by performing PL measurement. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photonic time‐stretch digitizer and its extension to real‐time spectroscopy and imaging

Real‐time wideband digitizers are the key building block in many systems including oscilloscopes, signal intelligence, electronic warfare, and medical diagnostics systems. Continually extending the bandwidth of digitizers has hence become a central challenge in electronics. Fortunately, it has been shown that photonic pre‐processing of wideband signals can boost the performance of electronic digitizers. In this article, the underlying principle of the time‐stretch analog‐to‐digital converter (TSADC) that addresses the demands on resolution, bandwidth, and spectral efficiency is reviewed. In the TSADC, amplified dispersive Fourier transform is used to slow down the analog signal in time and hence to compress its bandwidth. Simultaneous signal amplification during the time‐stretch process compensates for parasitic losses leading to high signal‐to‐noise ratio. This powerful concept transforms the analog signal's time scale such that it matches the slower time scale of the digitizer. A summary of time‐stretch technology's extension to high‐throughput single‐shot spectroscopy, a technique that led to the discovery of optical rouge waves, is also presented. Moreover, its application in high‐throughput imaging, which has recently led to identification of rogue cancer cells in blood with record sensitivity, is discussed.     

Few‐cycle,broadband, mid‐infrared optical parametric oscillator pumped by a 20‐fs Ti:sapphire laser

A few‐cycle, broadband, singly‐resonant optical parametric oscillator (OPO) for the mid‐infrared based on MgO‐doped periodically‐poled LiNbO₃ (MgO:PPLN), synchronously pumped by a 20‐fs Ti:sapphire laser is reported. By using crystal interaction lengths as short as 250 µm, and careful dispersion management of input pump pulses and the OPO resonator, near‐transform‐limited, few‐cycle idler pulses tunable across the mid‐infrared have been generated, with as few as 3.7 optical cycles at 2682 nm. The OPO can be continuously tuned over 2179‐3732 nm (4589‐2680 cm^‐1) by cavity delay tuning, providing up to 33 mW of output power at 3723 nm. The idler spectra exhibit stable broadband profiles with bandwidths spanning over 422 nm (FWHM) recorded at 3732 nm. The effect of crystal length on spectral bandwidth and pulse duration is investigated at a fixed wavelength, confirming near‐transform‐limited idler pulses for all grating interaction lengths. By locking the repetition frequency of the pump laser to a radio‐frequency reference, and without active stabilization of the OPO cavity length, an idler power stability better than 1.6% rms over >2.75 hours is obtained when operating at maximum output power, in excellent spatial beam quality with TEM₀₀ mode profile. Photograph shows a multigrating MgO:PPLN crystal used as a nonlinear gain medium in the few‐cycle femtosecond mid‐IR OPO. The visible light is the result of non‐phase‐matched sum‐frequency mixing between the interacting beams.