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Miniaturized lasers with multicolor output and high spectral purity are indispensable for various ultracompact photonic devices. Here, we propose an optically reconfigurable Forster resonance energ...

Miniaturized lasers with high spectral purity and wide wavelength tunability are crucial for various photonic applications. Here we propose a strategy to realize broadband-tunable single-mode lasing based on a photoisomerization-activated intramolecular charge-transfer (ICT) process in coupled polymer microdisk cavities. The photoisomerizable molecules doped in the polymer microdisks can be quantitatively transformed into a kind of laser dye with strong ICT character by photoexcitation. The gain region was tailored over a wide range through the self-modulation of the optically activated ICT isomers. Meanwhile, the resonant modes shifted with the photoisomerization because of a change in the effective refractive index of the polymer microdisk cavity. Based on the synergetic modulation of the optical gain and microcavity, we realized the broadband tuning of the single-mode laser. These results offer a promising route to fabricate broadband-tunable microlasers towards practical photonic integrations.

Random fiber lasers (RFLs), which employ random distributed feedback (RDFB) provided by the intrinsic Rayleigh scattering (RS) along the passive fiber, have gained wide attention in the last decade. A major advantage of Raman gain based RFL is wavelength flexibility, and the so-called cascaded Raman conversion can extend the operating wavelength considerably. However, frequently used silica fibers often need multiple Raman shifts to reach the final desired wavelength due to its peak frequency shift of ∼13.2 THz. A good alternative is the phosphosilicate fiber which has a gain peak at ∼39.8 THz. Nevertheless, the simultaneous existence of silica-related and phosphorus-related gain peaks in the phosphosilicate fiber intensifies the gain competition, further limiting the power scaling and the enhancement of spectral purity, even making it difficult to reach the desired wavelength. In this article, we experimentally explored the influences of the pump wavelength and bandwidth on the lasing performance of a phosphosilicate RFL, and measured the extreme frequency shift of the phosphorus-related Raman gain peak. Besides, by the aid of a modified power balance model, we qualitatively simulated the impacts of pump spectrum and boundary condition on the laser output. The simulation agrees well with the experimental results, and may also explain the parasitic lasing in the prior reports of phosphosilicate RFLs. As a result, the extreme frequency shift of phosphosilicate RFL is measured to be ∼1 THz. And through optimizing the pump wavelength and pump bandwidth, 99.24% high spectral purity random lasing at 1237 nm with 33.1 W output power is obtained. This work may help to reveal the gain competition in phosphosilicate RFL and guide the future laser design.

In this article, we present a tunable GaSb-based blazed grating external cavity laser (BG-ECL) with high spectral purity and high output power single-mode operation around 1940nm. The drastic increase in spectral selectivity and optical power results from the employment of a single-transverse-mode operating narrow ridge waveguide laser diode with an optimized AR coating on the front facet. The stable fundamental spatial mode output beam from the laser diode enables efficient collimation and high coupling efficiency with the blazed grating, leading to stronger wavelength-selective feedback. The AR coating with proper low reflectivity on the straight waveguide effectively suppresses the internal cavity mode lasing without causing extra optical loss. As a result, the BG-ECL device exhibits excellent comprehensive performance with a side mode suppression ratio (SMSR) over 50 dB with optical power exceeding 30 mW within a 70 nm tuning range. A maximum SMSR of 56.26 dB with 35.12 mW output power was observed in continuous-wave operation. By increasing the working temperature of the diode laser, the tuning range can be further extended to over 100 nm without noticeable degradation in spectral and output power performance.

We present performance manipulation of the squeezed coherent light source based on four-wave mixing (FWM) in alkaline-earth atoms. We investigate the dynamic response of the system and the spectroscopic feature of lasing generated by resonantly enhanced wave-mixing in coherently prepared system. In this method, the spectral purity and stability of the wave-mixing lasing can be manipulated at will by choosing optimal laser parameters. We also analyze the effect of Langevin noise fluctuations on the system and the relative-intensity noise spectrum of the wave-mixing lasing is well below the standard quantum limit (down to -4.7 dB). This work opens new possibilities for alternative routes to laser stabilization and provides a promising path to realize precision metrology.

An approach to generating microwave frequency combs (MFCs) with superior performance is proposed and experimentally demonstrated based on an optically injected semiconductor laser (OISL). In this scheme, an OISL subject to a short-delay optoelectronic feedback loop is firstly applied to produce the initial MFC based on Fourier domain mode-locking, and a long-delay loop is added to improve the signal quality based on the self-injection-locking technique. In the experimental demonstration, a K-band MFC (18–26 GHz) with a line-spacing of 8.45 MHz has been obtained, and the comb contrast ratio is over 45 dB. In addition, the generated MFC exhibited superior performance in terms of spectral purity. Each comb component has a linewidth below 500 Hz and a phase noise below −90 dBc/Hz at 10kHz offset, demonstrating an excellent phase coherence between each comb component.

The monochromator of the Kosmos synchrotron radiation metrology station is discussed. The results of testing the monochromator in the energy range 2000–6000 eV using Si (111) crystals are presented. A spectral resolution ΔE/E = 10−4 was obtained. A technique for checking the spectral purity of the monochromatic radiation is described. It is shown that the monochromator can be used for spectroscopic measurements in the indicated energy range.

Ultra-stable lasers of high spectral purity are the technological basis for optical atomic clocks, quantum measurements, stable microwave signal sources and high-resolution optical spectroscopy. For these purposes, lasers with a linewidth of the order of hertz are used. Such lasers are usually stabilized with Fabry-Perot resonators made of ultra-low expansion materials, which are very fragile and bulky. Therefore, there is a clear demand for reliable miniature lasers with narrow linewidths. Optical resonators with a "whispering gallery" mode (WGM) make it possible to create narrow-band microlasers on their basis using self-injection locking method. But they have not yet reached the stability determined by their fundamental noise. Here we calculate the noise characteristics of lasers self-injection locked to WGM microresonators and estimate the linewidth limitations due to thermorefractive, thermoelastic, photoelastic and Brownian noise. We also calculated the total noise in an integrated waveguide and obtained an estimate of the linewidth of 0.5 kHz at an averaging time of 1 s.

We propose a novel scheme for the expansion and comb densification of gain-switched optical frequency combs (GS-OFC). The technique entails mutual injection locking of two gain-switched lasers with a common master to generate a wider bandwidth OFC. Subsequently, the OFC is further expanded and/or densified using a phase modulator with optimum drive conditions. We experimentally demonstrate the generation of an OFC with 45 highly correlated lines separated by 6.25 GHz with an expansion factor ~3. In addition, operating in comb densification mode, the channel spacing of the OFC is tuned from 6.25 GHz to 390.625 MHz. Finally, a detailed characterization of the lines, across the entire expanded comb, is reported highlighting the excellent spectral purity with linewidths of ~40 kHz, a relative intensity noise better than −152 dB/Hz, and a high degree of phase correlation between the comb lines. The proposed method is simple, highly flexible and the architecture is suitable for photonic integration, all of which make such an OFC extremely attractive for the employment in a multitude of applications.

Optoelectronic oscillators (OEO) have become of paramount importance for the generation of high spectral purity, (optically-carried) microwave signals. The most widespread architecture uses a Mach-Zehnder Modulator as the generator of the RF modulation [1] . Direct modulation of the injection current of a semi-conductor laser, arguably the simplest possible OEO [2] , has been comparatively much less explored [3] .

Mid-infrared semiconductor lasers have emerged as indispensable compact coherent sources for military and commercial applications. While much of the historical emphasis has been on maximizing the output power and/or spectral purity, a recent new focus has been on engineering these lasers to operate as optical frequency combs (OFCs) for broadband real-time spectroscopy. In particular, the combination of low-drive-power and broad gain bandwidth has made interband cascade laser (ICL) OFCs an attractive complement to quantum cascade laser OFCs operating at longer wavelengths. Moreover, ICL combs can potentially be incorporated into fully-integrated dual-comb spectrometers that employ fast, room-temperature IC photodetectors processed on the same chip. However, the high refractive index of the ICL’s GaSb substrate poses some challenges to the optical waveguiding. Because the modal index is considerably lower than that of the substrate, the optical field can penetrate the bottom cladding layer and leak into the GaSb, inducing wavelength-dependent interference that modifies the gain and group velocity dispersion (GVD) profiles. Even when the effect on lasing threshold is small, the comb properties can be adversely affected. Using the sub-threshold Fourier transform technique, we studied ICL combs with various ridge widths, substrate thicknesses, and center wavelengths. This allowed us to evaluate the effects of modal leakage on the GVD. We find that the resonant nature of the substrate modes induces oscillations, which affect both the spectral bandwidth and the phase-locking properties above threshold. Strategies to mitigate the GVD’s undesired and unpredictable spectral variation will be presented.

Recent progress in developing tunable mid-IR (2.6 um range) integrated hybrid lasers, are demonstrated. the hybrid laser includes gain chips based on AlGaInAsSb/GaSb quantum wells and tunable reflectors based on micron-scale silicon on insulator integrated photonics platform. The devices exhibit milliwatt-level average power, 10 mW of peak powers and tuning ranges up to 60 nm. Factors limiting tuning range, spectral purity and maximum power are discussed. The demonstrated platform enables realization of tunable lasers for a wide range of applications requiring high performance light sources with emission wavelengths from 1.8 µm to 3 µm and beyond.

Infrared spectroscopy is a powerful tool for identification and quantification of functional molecular groups. Molecular gas lasers have long been used for such purposes due to their early development and mature manufacturing technology. Over the past few decades, Quantum Cascade Lasers (QCLs) have popularized due to their continuously tunable wavelength, climbing peak power, and steadily improving manufacturing economics. However, for many applications in the longwave infrared (LWIR) spectral region, carbon dioxide (CO2) lasers retain a clear advantage over QCLs by providing higher power, greater spectral purity and extended coherence length. This combination allows for detection of volatile organic compounds indicative of many diseases such as early-stage lung cancer, an advanced prognosis of which can increase the 5-year survival rate by 37.5%. In addition to medical applications, CO2 lasers also retain their niche in environmental sensing by providing a robust photoacoustic airborne measurement platform for air quality and climate research. Furthermore, recent advances in thermally balanced gas laser architectures have enabled reliable mobile leak monitoring of the insulation gas Sulfur Hexafluoride (SF6) used in medium to ultra-high-voltage electricity transmission lines. With the installed base of SF6 expected to grow 75% by 2030 due to the increase of green energy infrastructure via solar panels and wind turbines – both of which rely on electrical connections, switches, and circuit breakers – this task has gained new urgency due to an upper leak estimate of 15% over the full life cycle of the insulation gas.

Narrow linewidth laser diodes (LDs) emitting in the near-UV (NUV) are gaining attention for applications ranging from spectroscopy to atom cooling and interferometry or other applications requiring high spectral purity. InGaN edge-emitting LDs can exhibit a power of hundreds of mW in an unstable multimode regime detrimental to aforementioned uses. In this paper we report on a compact and robust design based on a low-cost blue LD, a beam shaping optical system and a fiber Bragg grating (FBG) acting as a wavelength selective reflector. One longitudinal mode of the non-antireflection coated laser diode is selected by a close to 30 pm bandwidth FBG allowing a few mW output power around 400 nm and a sidemode- suppression-ratio approaching 50 dB exceeding our last published results. Our previous studies showed that a single-frequency regime with a sub-MHz integrated linewidth and an estimated intrinsic linewidth of 16 kHz was possible by a carefully engineered external cavity. We will study the influence of the cavity length with different fiber types (SM or PM). Assessment will focus on the linewidth and a detailed intensity and frequency noise analysis of the emission. We will also investigate for the first-time the stability of several types of UV-FBG submitted to tens of mW of 400 nm light guided into the fiber core. This work demonstrates state-of-the-art performances by connecting low-cost components and opens the way to the fabrication of highly coherent laser sources that could meet the markets for the NUV applications.

Recently, imec has installed and commissioned an industrial, ultrafast EUV materials characterization and lithography lab, imec’s AttoLab, with a primary aim to explore limits of photoresist performance and their associated ultrafast chemistries. Here, we demonstrate, for the first time, the use of a table-top, high-harmonic EUV system (KM Labs, XUUS4) to perform interference lithography of sub-22-nm pitch patterns in an Inpria MOx resist via a Lloyd’s mirror interference lithography (IL) tool. Analysis of SEM images enables us to identify potential sources of image blur, which we attribute to out-of-sync vibrations, flare, spectral purity, and laser stability. Nevertheless, these results confirm the ability of table-top, high-harmonic EUV sources to print lithographic patterns below a 22-nm pitch. In future work, we plan to investigate sub-20-nm patterning in different resist formulations, as well as expand the lithographic capabilities in AttoLab to perform IL on full 300-mm wafers.

Narrow-bandwidth luminescent materials are already used in optoelectronic devices, superresolution, lasers, imaging, and sensing. The new-generation carbon fluorescence nanomaterials—carbon dots—have attracted considerable attention due to their advantages, such as simple operation, environmental friendliness, and good photoelectric performance. In this work, two narrower-bandwidth (21 and 30 nm) emission graphene quantum dots with long-wavelength fluorescence were successfully prepared by a one-step method, and their photoluminescence (PL) peaks were at 683 and 667 nm, respectively. These red-emitting graphene quantum dots were characterized by excitation wavelength dependence of the fluorescence lifetimes, and they were successfully applied to spectral and spatial superresolved sensing. Here, we proposed to develop an infrared spectroscopic sensing configuration based on two narrow-bandwidth-emission graphene quantum dots. The advantage of the method used is that spectroscopic information was extracted without using a spectrometer, and two narrow-bandwidth-emission graphene quantum dots were simultaneously excited to achieve spatial separation through the unique temporal “signatures” of the two types of graphene quantum dots. The spatial separation localization errors of the graphene quantum dots (GQDs-Sn and GQDs-OH) were 1 pixel (10 nm) and 3 pixels (30 nm), respectively. The method could also be adjusted for nanoscope-related applications in which spatial superresolved sensing was achieved. Graphene nanoparticles useful for high-resolution spectroscopy and imaging have been fabricated by scientists in China and Israel. Quantum dots are nanoparticles of semiconductor that can trap electrons and their positively charged counterparts, ‘holes’. The light emitted when the electron and hole combine is notable for its high spectral purity, i.e., its very narrow range of frequencies, or colors. Zheng Xie from the Technical Institute of Physics and Chemistry in Beijing, Zeev Zalevsky from Bar-Ilan University, Ramat-Gan, and their colleagues used a simple one-pot method to synthesize graphene quantum dots that emit ultranarrow frequency light in two different shades of red. The team proposed unique configurations for using those nanoparticles for spatial and spectral superresolved sensing specifically for field called nanoscopy, to create images at a resolution beyond that achievable with a microscope. Infrared spectral and spatial imaging configurations were developed based on near-infrared graphene quantum dots with ultranarrow half-width (FWHM = 21 nm). The spectral imaging is obtained without a spectrometer and the spatial imaging exceeds the limits of resolution (superresolved imaging). The superresolved sensing is obtained due to the unique temporal and spectral properties of the quantum dot.

We present a transportable photonic microwave oscillator, porting optical stability to a 10 GHz microwave signal. The system is composed of two main sub-units: a compact 8 height-unit (HU) 194 THz ultra-stable laser with Hz linewidth showing a fractional frequency stability (modified Allan deviation) of 8.5 × 10-15 at 1 s and a 3 HU ultra-low noise optical frequency comb used to phase-coherently divide the optical frequency reference’s spectral purity down to a 10 GHz microwave signal. Characterizing the synthesized microwave by means of a commercial digital cross-correlator, a phase noise power spectral density of -70 dBc/Hz at 1 Hz and -160 dBc/Hz at 10 kHz Fourier frequency is measured, maintaining a white plateau at -165 dBc/Hz up to 10 MHz offset.

Optical frequency transfer provides the means for high-fidelity frequency transfer across thousands of kilometers. A critical step in the further developing optical frequency transfer is its capability to transfer a high spectral purity feature from ultrastable lasers or optical clocks to any remote locations and, at the same time, its adaptability to incorporate the optical frequency transfer technique into any existing communication networks with different topologies. Here we for the first time report a technique that delivers optical-frequency signals to multiple independent remote hubs along a ring optical-fiber network with passive phase stabilization. The technique automatically corrects optical-fiber length fluctuations of arbitrary hubs along the loop by mixing and shifting optical signals. Without the help of an active phase tracker and a compensator, it could significantly mitigate some technical problems such as the limited compensation speed and phase recovery time, the phase jitter contamination caused by the servo bump in conventional phase noise cancellation. Moreover, by transmitting optical signals along both directions using the same optical source, it can improve the signal-to-noise ratio at each hub. This technique maintains the same delay-limited phase noise correction capability as in conventional techniques and, furthermore, improves the phase jitter by a factor of 3, opening a way to a broad distribution of an ultrastable frequency reference with high spectral purity and enabling a wide range of applications beyond metrology over a ring fiber network with the naturally impressive reliability and scalability., Comment: 9 pages, 6 figures

In this paper, an all-fiberized and narrow-linewidth 5 kW power-level fiber amplifier is presented. The laser is achieved based on the master oscillator power amplification configuration, in which the phase-modulated single-frequency laser is applied as the seed laser and a bidirectional pumping configuration is applied in the power amplifier. The stimulated Brillouin scattering, stimulated Raman scattering, and transverse mode instability effects are all effectively suppressed in the experiment. Consequently, the output power is scaled up to 4.92 kW with a slope efficiency of as high as approximately 80%. The 3-dB spectral width is about 0.59 nm, and the beam quality is measured to be M2∼1.22 at maximum output power. Furthermore, we have also conducted a detailed spectral analysis on the spectral width of the signal laser, which reveals that the spectral wing broadening phenomenon could lead to the obvious decrease of the spectral purity at certain output power. Overall, this work could provide a reference for obtaining and optimizing high-power narrow-linewidth fiber lasers.

External-cavity diode lasers are ubiquitous in atomic physics and a wide variety of other scientific disciplines, due to their excellent affordability, coherence length and versatility. However, for higher power applications, the combination of seed lasers, injection-locking and amplifiers can rapidly become expensive and complex. Here we present a useful, high-power, single-diode laser design with specifications: $>210\,$mW, $100\,$ms-linewidth ($427 \pm 7$) kHz, $>99\%$ mode purity, $10\,$GHz mode-hop-free tuning range and $12\,$nm coarse tuning. Simple methods are outlined to determine the spectral purity and linewidth with minimal additional infrastructure. The laser has sufficient power to collect $10^{10}$ $^{87}$Rb atoms in a single-chamber vapour-loaded magneto-optical trap. With appropriate diodes and feedback, the system could be easily adapted to other atomic species and laser formats., 4 pages, 3 figures

In a conventional semiconductor laser, light reciprocates in the lateral direction and resonates by the light-emitting region laid in the horizontal direction. Although the horizontal length is as small as 0.3 mm, it is still 300 times as large as the light wavelength of 0.001 mm. Therefore, analogous to a musical instrument, many harmonics appear, and the spectral purity of the light is lost. As will be described in detail later, this horizontal cavity laser (abbreviated as a stripe laser) could not be manufactured by a monolithic process by thin film formation such as a semiconductor large-scale integrated (LSI) circuit. Therefore, let us compare the conventional horizontal cavity semiconductor laser and the VCSEL, and consider why it is a surface emitting laser.

Fluorescent probes are widely used in biological imaging; however, their spectral properties often limit sensing and multiplexing. Instead, intracellular lasers, offer increased spectral purity, photostability and distinct spectral outputs enabling the unique tagging of multiple cells over long time periods. Here we report on the optimisation of low threshold miniature lasers, 1000-fold smaller than the eukaryotic nucleus (Vlaser

Optical frequency comb generation by continuous-wave-pumped cascaded quadratic nonlinearities (CQN) appears a promising alternative to well-established modelocked lasers, especially if access to the mid-infrared spectral region is needed. We report an experimental investigation of spectral properties of a CQN frequency comb, which is based on second-harmonic generation (SHG) and is pumped internally by a continuous-wave optical parametric oscillator. Our study focuses on SHG phase mismatch, which has drawn little attention in the previously reported CQN frequency comb research. The main observation of our study is that it is possible to improve spectral purity of the CQN frequency comb by adjusting the phase mismatch. In addition to the CQN process that generates a frequency comb in the near-infrared region, our experimental setup involves several other nonlinear processes. These processes lead to a composite comb, which extends from visible to mid-infrared.

International audience; This paper demonstrates the stabilization of an external cavity semiconductor laser at 1550 nm on an ultra-high Q factor fiber ring resonator (FRR). Based on a Pound-Drever-Hall technique, our system is optimized in order to take into account the properties of the FRR which features a large Q factor (2 10^9) but a relatively small free spectral range (2 MHz). The measured performances of the stabilized laser shows a frequency noise level as low as 0.4 Hz^2 /Hz in the 1 kHz to 10 kHz range and a linewidth lower than 30 Hz for 5 ms integration time.

We demonstrate 104 Watts of in-band output power from a cascaded Raman fiber laser operating around 1.7 μm with a spectral purity of over 90% operating in both continuous wave and pulsed regimes. Through the use of a filter fiber with its cutoff wavelength designed between the 6th and 7th Stokes orders, output above 1.8 μm is suppressed below threshold values. In the pulsed regime the laser produces output pulses ranging from 11.5 mJ pulses with 100 μs pulse width to 10 J pulses with 100 ms pulse width.

X-ray free-electron lasers in the oscillator configuration (XFELO) are future fully coherent hard X-rays sources with ultrahigh spectral purity. X-ray beams circulate in an XFELO optical cavity comprising diamond single crystals. They function as high-reflectance (close to 100%), narrowband (∼10 meV) Bragg backscattering mirrors. The average power density of the X-ray beams in the XFELO cavity is predicted to be as high as ∼10 kW mm−2. Therefore, XFELO feasibility relies on the ability of diamond crystals to withstand such a high radiation load and preserve their high reflectivity. Here the endurance of diamond crystals to irradiation with multi-kW mm−2 power density X-ray beams is studied. It is shown that the high Bragg reflectivity of the diamond crystals is preserved after the irradiation, provided it is performed at ∼1 × 10−8 Torr high-vacuum conditions. Irradiation under 4 × 10−6 Torr results in a ∼1 meV shift of the Bragg peak, which corresponds to a relative lattice distortion of 4 × 10−8, while the high Bragg reflectivity stays intact.

A stimulated thermal Rayleigh scattering (STRS)-based self-injection locking technique is proposed and utilized in fiber laser to improve its spectral purity. Both theoretical and experimental analyses of the STRS-based filter and the interaction mechanism between the STRS-based filter and self-injection locking are demonstrated in detail. These analyses verify effects of the proposed method on single-longitudinal-mode (SLM) selection and linewidth compression. A fiber laser employing STRS-based self-injection locking configuration is designed, and a significant improvement of spectral purity comparing to commercially available lasers is achieved. Experimental results indicate that this fiber laser operates at SLM regime and has a linewidth of 45 Hz, and can be widely applied in distributed sensing, optical communication, and high-resolution optical spectrum analysis.

Here we report the changes in performance of a Raman fiber laser subjected to step-by-step modifications. The laser scheme departed from a simple free-running distributed-feedback cavity that was firstly modified into a ring configuration, then a Fabry-Perot filter was inserted and it was finally completed by inserting a piece of Yb3+-doped. Every modification changed the emitted spectrum, increased the efficiency and improved the power stability. The main result of modifying the simplest two-band emitting configuration into a ring cavity was the emission of a single-band with improved stability. Then a Fabry-Perot filter was inserted to the ring cavity, obtaining in this way a single-line oscillation. The last step consisted on inserting a piece of Yb3+-doped fiber that mainly contributed to force the decrease of the oscillating threshold. We propose these schemes as alternatives for achieving more stable, more efficient and with superior spectral purity (single-line emission) in Raman fiber lasers.

This letter presents narrow-linewidth 780-nm edge-emitting semiconductor DFB lasers fabricated without re-growth using UV-nanoimprinted surface gratings. The third-order laterally coupled ridge-waveguide surface gratings enable single mode operation, excellent spectral purity (40–55 dB side mode suppression ratio and 10-kHz linewidth), and good light–current–voltage characteristics in continuous wave operation (~112-mA threshold current, ~1.55-V opening voltage, and 28.9-mW output power from one facet at 300-mA current for 2.4-mm-long devices), which are vital in various applications, such as rubidium spectroscopy and atomic clock pumping. The low fabrication cost, high throughput, structural flexibility, and high device yield make the fabrication method fully compatible with large-scale mass production, enabling the fabrication of low-cost miniaturized modules.

The phenomenon of self-pulsation (SP) in terahertz (THz) quantum cascade lasers (QCLs) due to optical feedback was reported recently. In this Letter, we propose a THz imaging modality using the SP phenomenon in a THz QCL. We explore the theoretical oscillation properties of the SP scheme and demonstrate its suitability to perform imaging experimentally. The SP imaging scheme operates in self-detection mode, eliminating the need for an external detector. Moreover, the scheme requires only a fixed current, meaning that one can avoid many of the pitfalls associated with high temperature operation of THz QCLs, including frequency chirp and mode hops caused by sweeping the laser current. This also means that one is free to locate the operating point at the maximum power, to produce the desired beam profile or for highest spectral purity, depending on the application. The SP imaging modality proposed in this work can be translated directly to high operating temperature THz QCLs.

Optical frequency combs 1,2 establish a rigid phase-coherent link between microwave and optical domains and are emerging as high-precision tools in an increasing number of applications 3 . Frequency combs with large intermodal spacing are employed in the field of microwave photonics for radiofrequency arbitrary waveform synthesis 4,5 and for the generation of terahertz tones of high spectral purity in future wireless communication networks 6,7 . Here, we demonstrate self-starting harmonic frequency comb generation with a terahertz repetition rate in a quantum cascade laser. The large intermodal spacing caused by the suppression of tens of adjacent cavity modes originates from a parametric contribution to the gain due to temporal modulations of population inversion in the laser 8,9 . Using multiheterodyne self-detection, the mode spacing of the harmonic comb is shown to be uniform to within 5 × 10−12 parts of the central frequency. This new harmonic comb state extends the range of applications of quantum cascade laser frequency combs 10–13 . Self-starting harmonic frequency comb generation with a THz repetition rate in a q uantum cascade l aser is demonstrated . T he mode spacing uniformity is verified to within 5 × 10−12 parts of the central frequency. The findings extend the range of applications of quantum cascade laser frequency combs.

Self-seeding is one of the most promising methods to improve the longitudinal coherence and spectral purity of free-electron lasers (FELs). Measurements of the multishot-averaged, soft x-ray self-seeding spectrum at the Linac Coherent Light Source (LCLS) FEL often have a pedestal-like distribution around the seeded wavelength. In this paper, a theoretical model based on the mechanism of spectral sideband generation is developed to explain the statistical behavior of the seed and pedestal. The model is in good agreement with statistical analysis of numerical FEL simulations and experimental measurements that show that pedestal fluctuations reflect the level of long-wavelength microbunching structures in the beam, and are driven by fluctuations of the seed.

International audience; The recent demonstration of a terahertz (THz) molecular gas laser pumped by a mid-infrared quantum cascade laser (QCL) has opened up new perspectives for this family of sources, traditionally relying on CO2-laser pumping. A so far open question concerning QCL-pumped THz molecular lasers (MLs) is related to their spectral purity. Indeed, assessing their frequency/phase noise is crucial for a number of applications potentially exploiting these sources as local oscillators. Here this question is addressed by reporting the measurement of the frequency noise power spectral density (PSD) of a THz ML pumped by a 10.3µm-wavelength QCL, and emitting 1mW at 1.1THz in continuous wave. This is achieved by beating the ML frequency with the 1080 th harmonic of the repetition rate of a 1560nm frequency comb. We find a frequency noise PSD < 10Hz 2 /Hz (-95dBc/Hz) at 100kHz from the carrier. To demonstrate the effect of the stability of the pump laser on the spectral purity of the THz emission we also measure the frequency noise PSD of a CO2-laser-pumped 2.5THz ML, reaching 0.1Hz 2 /Hz (-105dBc/Hz) at 40kHz from the carrier, limited by the frequency noise of the frequency comb harmonic. Finally, we show that it is possible to actively phase-lock the QCL-pumped molecular laser to the frequency comb repetition rate harmonic by controlling the QCL current, demonstrating a sub-Hz linewidth. 2

We present an integrated semiconductor-dielectric hybrid dual-frequency laser operating in the 1.5 μm wavelength range for microwave and terahertz (THz) generation. Generating a microwave beat frequency near 11 GHz, we observe an intrinsic linewidth as low as about 2 kHz. This is realized by hybrid integration of a single diode amplifier based on indium phosphide (InP) with a long, low-loss silicon nitride (Si3N4) feedback circuit to extend the cavity photon lifetime, resulting in a cavity optical roundtrip length of about 30 cm on a chip. Simultaneous lasing at two frequencies is enabled by introducing an external control parameter for balancing the feedback from two tunable, frequency-selective Vernier mirrors on the Si3N4 chip. Each frequency can be tuned with a wavelength coverage of about 80 nm, potentially allowing for the generation of a broad range of frequencies in the microwave range up to the THz range.

The degree of spectral coherence characterizes the spectral purity of light. It can be equivalently expressed in the time domain by the decay time τ or the quality factor Q of the light-emitting oscillator, the coherence time τ coh or length l coh of emitted light or, via Fourier transformation to the frequency domain, the linewidth Δν of emitted light. We quantify these parameters for the reference situation of a passive Fabry-Perot resonator. We investigate its spectral line shapes, mode profiles, and Airy distributions and verify that the sum of all mode profiles generates the corresponding Airy distribution. The Fabry-Perot resonator is described, as an oscillator, by its Lorentzian linewidth and finesse and, as a scanning spectrometer, by its Airy linewidth and finesse. Furthermore, stimulated and spontaneous emission are analyzed semi-classically by employing Maxwell′s equations and the law of energy conservation. Investigation of emission by atoms inside a Fabry-Perot resonator, the Lorentz oscillator model, the Kramers-Kronig relations, the amplitude-phase diagram, and the summation of quantized electric fields consistently suggests that stimulated and spontaneous emission of light occur with a phase 90° in lead of the incident field. These findings question the quantum-optical picture, which proposed, firstly, that stimulated emission occurred in phase, whereas spontaneous emission occurred at an arbitrary phase angle with respect to the incident field and, secondly, that the laser linewidth were due to amplitude and phase fluctuations induced by spontaneous emission. We emphasize that the first derivation of the Schawlow-Townes laser linewidth was entirely semi-classical but included the four approximations that (i) it is a truly continuous-wave (cw) laser, (ii) it is an ideal four-level laser, (iii) its resonator exhibits no intrinsic losses, and (iv) one photon is coupled spontaneously into the lasing mode per photon-decay time τc of the resonator, independent of the pump rate. After discussing the inconsistencies of existing semi-classical and quantum-optical descriptions of the laser linewidth, we introduce the spectral-coherence factor, which quantifies spectral coherence in an active compared to its underlying passive mode, and derive semi-classically, based on the principle that the gain elongates the photon-decay time and narrows the linewidth, the fundamental linewidth of a single lasing mode. This linewidth is valid for lasers with an arbitrary energy-level system, operating below, at, or above threshold and in a cw or a transient lasing regime, with the gain being smaller, equal, or larger compared to the losses. By applying approximations (i)-(iv) we reproduce the original Schawlow-Townes equation. It provides the hitherto missing link between the description of the laser as an amplifier of spontaneous emission and the Schawlow-Townes equation. Spontaneous emission entails that in a cw lasing mode the gain is smaller than the losses. We verify that also in the quantum-optical approaches to the laser linewidth, based on the density-operator master equation, the gain is smaller than the losses. We conclude this work by presenting the derivation of the laser linewidth in a nut shell.

Undulator based synchrotron light sources and Free Electron Lasers (FELs) are valuable modern probes of matter with high temporal and spatial resolution. Laser Plasma Accelerators (LPAs), delivering GeV electron beams in few centimeters, are good candidates for future compact light sources. However the barriers set by the large energy spread, divergence and shot-to-shot fluctuations require a specific transport line, to shape the electron beam phase space for achieving ultrashort undulator synchrotron radiation suitable for users and even for achieving FEL amplification. Proof-of-principle LPA based undulator emission, with strong electron focusing or transport, does not yet exhibit the full specific radiation properties. We report on the generation of undulator radiation with an LPA beam based manipulation in a dedicated transport line with versatile properties. After evidencing the specific spatio-spectral signature, we tune the resonant wavelength within 200–300 nm by modification of the electron beam energy and the undulator field. We achieve a wavelength stability of 2.6%. We demonstrate that we can control the spatio-spectral purity and spectral brightness by reducing the energy range inside the chicane. We have also observed the second harmonic emission of the undulator.

With optical spectral marks equally spaced by a frequency in the microwave or the radio frequency domain, optical frequency combs have been used not only to synthesize optical frequencies from microwave references but also to generate ultralow-noise microwaves via optical frequency division. Here, we combine two compact frequency combs, namely, a soliton microcomb and a semiconductor gain-switched comb, to demonstrate low-noise microwave generation based on a novel frequency division technique. Using a semiconductor laser that is driven by a sinusoidal current and injection-locked to microresonator solitons, our scheme transfers the spectral purity of a dissipative soliton oscillator into the subharmonic frequencies of the microcomb repetition rate. In addition, the gain-switched comb provides dense optical spectral emissions that divide the line spacing of the soliton microcomb. With the potential to be fully integrated, the merger of the two chipscale devices may profoundly facilitate the wide application of frequency comb technology.

The effect of laser-current delay on extreme ultraviolet emission by laser-triggered discharge-plasma has been investigated. Typical waveforms for current, voltage, laser signals, and X-ray signals have been compared. Theoretical tin spectra were simulated among the electron temperature ranges from 10 to 50 eV to compare with the experimental results. The results show that longer laser-current delay time is propitious to increase the steady-state time of plasma at high temperatures, and it increases the intensity and spectral purity of 13.5 nm emission in 2% band. The 13.5 nm radiation intensity increases about 120% with the delay increasing from 0.7 to 5 &mu, s, and the extreme ultraviolet (EUV) emission conversion efficiency (CE) increases from 0.5% to 1.1%.

The Free Electron Laser (FEL) application of Laser Plasma Acceleration (LPA) requires the handling of the energy spread and divergence. The COXINEL manipulation line, designed and built at SOLEIL for this purpose, consists of high gradient quadrupoles for divergence handling and a decompression chicane for energy sorting, enabling FEL amplification with baseline parameters. Installed at Laboratoire d’Optique Appliquee (LOA), it uses robust electrons generated and accelerated by ionization injection using a 30 TW laser. We report here on the work progress towards a FEL demonstration. The LPA measured electron beam characteristics deviates from the baseline reference case. After the installation of the equipment, the electron beam transport has first been optimized. The electron position and dispersion are independently adjusted. Then, undulator radiation has been measured. The spectral purity is controlled via the energy spread adjusted in the slit located in the chicane. FEL effect demonstration is within reach, with currently achieved performance on different LPA experiments.

Phosphosilicate fiber has the inherent advantage of generating dual-wavelength output owing to the two Raman gain peaks at the frequency shifts of ∼13.2 THz (silica-related) and 39.9 THz (phosphorus-related), respectively. The frequency shift of 39.9 THz is often adopted to obtain long wavelength laser, while the control of Stokes light at 13.2 THz has attracted much attention currently. In this paper, a dual-wavelength random distributed feedback Raman fiber laser (RDFL) with over 100 nm wavelength interval and continuously tunable power proportion was presented based on phosphosilicate fiber for the first time. Through using the filtered amplified spontaneous emission (ASE) source as the pump source, the spectral purity of the Stokes light could be as high as 99.8%. By tuning two manual variable optical attenuators (VOAs), the power proportion of the silica-related Stokes light could range from ∼0% to 99.0%, and the maximum value is limited by the generation of second order Stokes light. Although the power handling capability of the VOA is merely 2 W, over 23 W total output power of the Stokes light was obtained thanks to the particular power distribution property of RDFL. This experiment demonstrates the potential to achieve a flexible high-power and high-spectral purity dual-wavelength RDFL output.

Measurements of the soft x-ray, self-seeding spectrum at the Linac Coherent Light Source (LCLS) free-electron laser generally display a pedestal-like distribution around the central seeded wavelength. This pedestal limits the spectral purity and can negatively affect some user applications not employing a post-undulator monochromator. In this paper, we investigate the detailed experimental characteristics of both the amplified seed and its accompanying pedestal using data from a number of separate LCLS shifts over the 2015-2018 time period. We find that the amplified seed shows excellent wavelength stability and an exponential growth rate whose dependence upon energy detuning is consistent with theory. The pedestal's spectral distribution and integrated strength vary strongly shot by shot, independent of electron beam energy jitter. Its shot-averaged strength relative to that of the seed grows at least linearly with $z$ and can approach values of 15% or more. The pedestal is comprised of two separate components: (1) normal self-amplified spontaneous emission (SASE) whose total strength is nominally insensitive to energy detuning and laser heater (LH) strength; (2) sideband-like emission whose strength positively correlates with that of the amplified seed and negatively with energy detuning and LH strength. We believe this latter, non-SASE component arises from comparatively long wavelength (i.e., $\ensuremath{\lambda}\ensuremath{\sim}0.3--3\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$) amplitude and phase modulations of the main seeded radiation line. Its shot to shot variability and LH sensitivity suggests an origin connected to growth of the longitudinal microbunching instability on the electron beam.

Light detectors such as photodiodes and phototransistors are fundamental elements that compose imaging sensors and cameras devices. The photocurrent generated by photodiodes depends on the energy of light that is absorbed by semiconductor material, describing an important characteristic of this kind device. We present an optic instrumentation for measurement of normalized spectral responsivity of light detectors. The optics is composed by a single-grating monochromator, converting lens, a slit and a beamsplitter. Supercontinuum lasers provides a high-stability beam with ultra-broadband white-light spectrum. High spectral purity is achieved with only monochromator and the measurements are performed in terms of spectral power measured through a calibrated spectrometer. Uncertainties are examined at carefully for a specific confidence level considering Type A and Type B evaluations.

This article reports the development of 200-mW 1.55-μm DFB laser module with RIN below -162 dB/Hz which are well suited for microwave photonics or free space optical communication applications. Specific design has allowed reaching high power (>300 mW), low noise and high spectral purity laser chip. The chip has been packaged in Butterfly module optimized for reducing the module power consumption. DFB laser module system validations have been done on three laboratory test-beds representative of target applications, namely high-frequency optical LO distribution, photonic RF frequency conversion, and free space optical communication links.

Coherent beam combining (CBC) of high brightness tapered amplifiers (TPAs) is a promising approach for the development of high power semiconductor laser sources with excellent beam quality and offers a pathway to future direct diode high brightness system [1]. Operation of the amplifiers in quasi continuous wave (QCW) mode allows to overcome the thermal limitations in the amplifiers and to test their full power scaling potential. Additionally, high brightness QCW sources with high spectral purity are in demand as NIR pump sources for nonlinear frequency conversion to address medical applications of modulated high power visible laser sources. In this paper we discuss dynamic effects in pulsed high brightness tapered amplifiers and demonstrate their coherent superposition for brightness scaling. We use state-of the art TPAs with the so called ELOD2 epitaxial design, which allows us to maintain close-to-diffraction limited beam quality at high powers [2,3].

International audience; The experimental implementation of optically carried RF signals, in the 1-100 GHz range and beyond, is fundamental for microwave photonics applications. The technique of heterodyning two laser waves provides an interesting alternative to direct modulation of light. Indeed, it allows generating directly a single-sideband signal over an optical carrier, that is inherently insensitive to dispersion in a fiber link. It also features a 100% modulation depth, a broad and continuous tunability. Phase-locking to electronic local oscillators results in optical beatings of very high spectral purity; furthermore, high microwave frequencies 100 GHz) can be reached by suitable multiplication or downconversion techniques (at the expense of a degradation of the phase noise performances)

International audience; Standard laser linewidths typically range from 10 kHz for solid-state lasers up to a few MHz for semiconductors. Several techniques have been developed in the past in order to reduce lasers linewidths to sub-kHz level. It can be achieved by servo-locking the mean laser frequency to a reference frequency provided by an atomic or molecular absorption line, leading to long-term frequency stabilization. However, it does not generally offer significant reduction of short-term phase-noise. Other techniques consist in using the resonances provided by ultra-stable cavities or km-imbalanced interferometers. These methods are accurate, but are very sensitive to environmental noise which degrades the laser linewidth. Injection locking provides alternative solutions for linewidth narrowing, but the problem is merely deferred as they rely on the use of stabilized low-noise optical seeders. Besides Brillouin fiber lasers (BFL) employing the stimulated Brillouin scattering (SBS) in fibers are known to present intrinsic narrow sub-kHz linewidths [1]. We show here how the benefits of using a solidstate laser, in terms of power, and SBS, in terms of spectral purity, can be easily combined.

Fourier-limited, also called single-frequency or single-longitudinal-mode (SLM) nanoseconds lasers at 1 μm are key elements for several applications, such as optical communications, data storage and high-resolution spectroscopy. Some special application such as high spectral resolution LIDAR [1], require very high spectral purity (tens of MHz bandwidth), resulting in long pulse durations in the range of several tens of ns. Microchip lasers with large free spectral range (FSR) comparable to the active medium gain bandwidth (about 160 GHz for Nd:YAG) allow easy SLM selection, but with much shorter pulses. Recently, we demonstrated a Cr:YAG Passively Q-Switched (PQS) Nd:YVO 4 ring laser oscillator delivering 50-ns, 80μJ SLM pulses at 0.1–10 kHz repetition rate employing a specifically designed intracavity etalon to enhance SLM operation [2]. For SLM PQS operations, Nd:YAG offers some advantages if compared to Nd:YVO4. Namely, the higher saturation fluence relaxes the fulfilment of the second PQS threshold condition, the ∼2 times longer fluorescence lifetime enhances energy storage and the narrower gain bandwidth favours SLM operation.

Cascaded Raman fiber lasers (CRFLs) based on random distributed feedback are proven to be wavelength agile where, by simply adjusting the input pump power and wavelength, high-power continuous wave fiber lasers at arbitrary wavelengths were demonstrated [1–3]. However, these systems have limited degree of wavelength conversion or spectral purity defined as the percentage of output power in the desired wavelength band. Recently, use of fiber-based amplified spontaneous emission (ASE) sources have been proposed to increase the spectral purity [4]. In [4] authors attributed relative intensity noise (RIN) of the pump laser, as the reason for reduction in spectral purity. Here, we comprehensively investigate the role of RIN on spectral purity by using three kinds of pump lasers for cascaded Raman conversion. We demonstrate that by using high-power, phase-modulated (PM) narrow-linewidth fiber amplifier as the pump source, ultra-high spectral purity of >97% over the entire Raman stokes orders (from 1.1μm to 1.5μm) can be achieved.