Metrology Source Research Articles

Dual comb generation from a mode-locked fiber laser with orthogonally polarized interlaced pulses.

Ultra-high precision dual-comb spectroscopy traditionally requires two mode-locked, fully stabilized lasers with complex feedback electronics. We present a novel mode-locked operation regime in a thulium-holmium co-doped fiber laser, a frequency-halved state with orthogonally polarized interlaced pulses, for dual comb generation from a single source. In a linear fiber laser cavity, an ultrafast pulse train composed of co-generated, equal intensity and orthogonally polarized consecutive pulses at half of the fundamental repetition rate is demonstrated based on vector solitons. Upon optical interference of the orthogonally polarized pulse trains, two stable microwave RF beat combs are formed, effectively down-converting the optical properties into the microwave regime. These co-generated, dual polarization interlaced pulse trains, from one all-fiber laser configuration with common mode suppression, thus provide an attractive compact source for dual-comb spectroscopy, optical metrology and polarization entanglement measurements.

Iodine frequency references for space

Optical frequency references are a key element for the realization of future space missions. They are needed for missions related to tests of fundamental physics, gravitational wave detection, Earth observation and navigation and ranging. In missions such as GRACE follow-on or LISA the optical frequency reference is used as light source for high-sensitivity inter-satellite distance metrology. While cavity-based systems are current baseline e.g. for LISA, frequency stabilization on a hyperfine transition in molecular iodine near 532 nm is a promising alternative. Due to its absolute frequency, iodine standards crucially simplify the initial spacecraft acquisition procedures. Current setups fulfill the GRACE-FO and LISA frequency stability requirements and are realized near Engineering Model level. We present the current status of our developments on Elegant Breadboard (EBB) and Engineering Model (EM) level taking into account specific design criteria for space compatibility such as compactness (size iodine spectroscopy EM: 38 × 18 × 10 cm3) and robustness. Both setups achieved similar frequency stabilities of ∼ 1 · 10−14 at an integration time of 1 s and below 5 · 10−15 at integration times between 10 s and 1000 s. Furthermore, we present an even more compact design currently developed for a sounding rocket mission with launch in 2017.

Laser source for dimensional metrology: investigation of an iodine stabilized system based on narrow linewidth 633 nm DBR diode

We demonstrated that an iodine stabilized distributed Bragg reflector (DBR) diode based laser system lasing at a wavelength in close proximity to nm could be used as an alternative laser source to the helium-neon lasers in both scientific and industrial metrology. This yields additional advantages besides the optical frequency stability and coherence: inherent traceability, wider optical frequency tuning range, higher output power and high frequency modulation capability. We experimentally investigated the characteristics of the laser source in two major steps: first using a wavelength meter referenced to a frequency comb controlled with a hydrogen maser and then on an interferometric optical bench testbed where we compared the performance of the laser system with that of a traditional frequency stabilized He–Ne laser. The results indicate that DBR diode laser system provides a good laser source for applications in dimensional (nano)metrology, especially in conjunction with novel interferometric detection methods exploiting high frequency modulation or multiaxis measurement systems.

Droplet-based, high-brightness extreme ultraviolet laser plasma source for metrology

We report on the development of a high brightness source of extreme ultraviolet radiation (EUV) with a working wavelength of 13.5 nm. The source is based on a laser-produced plasma driven by pulsed radiation of a Nd:YAG laser system. Liquid droplets of Sn-In eutectic alloy were used as the source fuel. The droplets were created by a droplet generator operating in the jet break-up regime. The EUV emission properties of the plasma, including the emission spectrum, time profile, and conversion efficiency of laser radiation into useful 13.5 nm photons, have been characterized. Using the shadowgraphy technique, we demonstrated the production of corpuscular debris by the plasma source and the influence of the plasma on the neighboring droplet targets. The high-frequency laser operation was simulated by usage of the dual pulse regime. Based on the experimental results, we discuss the physical phenomena that could affect the source operation at high repetition rates. Finally, we estimate that an average source br...

Laser Sources for Metrology and Machine Vision

AbstractThe laser sources shown in Fig. 1 combine a laser diode with refractive and sometimes diffractive optics to achieve the required beam shape. The form and shape of the beams reflect the broad variety of their application. Even within the basic groupings of laser line, laser spot or pattern, there are variants that differ widely in their physical characteristics.

Visible-to-near-infrared octave spanning supercontinuum generation in a silicon nitride waveguide.

The generation of an octave spanning supercontinuum covering 488-978 nm (at -30 dB) is demonstrated for the first time on-chip. This result is achieved by dispersion engineering a 1-cm-long Si3N4 waveguide and pumping it with an 100-fs Ti:Sapphire laser emitting at 795 nm. This work offers a bright broadband source for biophotonic applications and frequency metrology.

Techniques for improving overlay accuracy by using device correlated metrology targets as reference

The performance of overlay metrology as total measurement uncertainty, design rule compatibility, device correlation, and measurement accuracy has been challenged at the 2× nm node and below. The process impact on overlay metrology is becoming critical, and techniques to improve measurement accuracy become increasingly important. We present a methodology for improving the overlay accuracy. A propriety quality metric, Qmerit, is used to identify overlay metrology measurement settings with the least process impacts and reliable accuracies. Using the quality metric, a calibration method, Archer self-calibration, is then used to remove the inaccuracies. Accuracy validation can be achieved by correlation to reference overlay data from another independent metrology source such as critical dimension–scanning electron microscopy data collected on a device correlated metrology hybrid target or by electrical testing. Additionally, reference metrology can also be used to verify which measurement conditions are the most accurate. We provide an example of such a case.

Co-phasing of a diluted aperture synthesis instrument for direct imaging

Context. Amongst the new techniques currently developed for high-resolution and high-dynamics imaging, the hypertelescope architecture is very promising for direct imaging of objects such as exoplanets. The performance of this instrument strongly depends on the co-phasing process accuracy. In a previous high-flux experimental study with an eight-telescope array, we successfully implemented a co-phasing system based on the joint use of a genetic algorithm and a sub-aperture piston phase diversity using the object itself as a source for metrology. Aims. To fit the astronomical context, we investigate the impact of photon noise on the co-phasing performance operating our laboratory prototype at low flux. This study provides experimental results on the sensitivity and the dynamics that could be reached for real astrophysical observations. Methods. Simulations were carried out to optimize the critical parameters to be applied in the co-phasing system running in the photon-counting regime. We used these parameters experimentally to acquire images with our temporal hypertelescope test bench for di erent photon flux levels. A data reduction method allows highly contrasted images to be extracted. Results. The optical path di erences have been servo-controlled over one hour with an accuracy of 22.0 nm and 15.7 nm for 200 and 500 photons/frame, respectively. The data reduction greatly improves the signal-to-noise ratio and allows us to experimentally obtain highly contrasted images. The related normalized point spread function is characterized by a 1:1 10 4 and 5:4 10 5 intensity standard deviation over the dark field (for 15 000 snapshots with 200 and 500 photons/frame, respectively). Conclusions. This laboratory experiment demonstrates the potential of our hypertelescope concept, which could be directly transposed to a space-based telescope array. Assuming eight telescopes with a 30 cm diameter, the I-band limiting magnitude of the main star would be 7.3, allowing imaging of a companion with a 17.3 mag.

Q-switched Er-doped fiber laser with low pumping threshold using graphene saturable absorber

We propose a Q-switched Er-doped fiber laser (EDFL) with a threshold pumping power as low as 7.4 mW, and demonstrate using graphene polyvinyl alcohol (PVA) thin film as a passive saturable absorber (SA). The SA is fabricated from graphene flakes, which is synthesized by electrochemical exfoliation of graphite at room temperature in 1% sodium dodecyl sulfate aqueous solution. The flakes are mixed with PVA solution to produce a thin film, which is then sandwiched between two ferrules to form a SA and integrated in the EDFL ring cavity to generate a stable Q-switched pulse train. The pulse train operates at 1560 nm with a threshold pump power of 7.4 mW. At maximum 1480 nm pump power of 33.0 mW, the EDFL generates an optical pulse train with a repetition rate of 27.0 kHz and pulse width of 3.56 μs. The maximum pulse energy of 39.4 nJ is obtained at a pump power of 14.9 mW. This laser can be used as a simple and low-cost light source for metrology, environmental sensing, and biomedical diagnostics.

Laser triggered Z-pinch broadband extreme ultraviolet source for metrology

We compare the extreme ultraviolet emission characteristics of tin and galinstan (atomic %: Ga: 78.35, In: 14.93, Sn: 6.72) between 10 nm and 18 nm in a laser-triggered discharge between liquid metal-coated electrodes. Over this wavelength range, the energy conversion efficiency for galinstan is approximately half that of tin, but the spectrum is less strongly peaked in the 13–15 nm region. The extreme ultraviolet source dimensions were 110 ± 25 μm diameter and 500 ± 125 μm length. The flatter spectrum, and −19 °C melting point, makes this galinstan discharge a relatively simple high radiance extreme ultraviolet light source for metrology and scientific applications.

Development of Pure Oxygen Certified Reference Material Determined by Subtraction Method

Certified reference materials of pure oxygen were produced by National Metrology Institute of Japan mainly for the metrological traceability source of JCSS oxygen in nitrogen standard gases. The property value of the certified reference material is the amount of substance of oxygen which was determined by the subtraction method described in ISO/IEC 6142:2001. The expected impurity components in the certified reference material were CO, CO2, CH4, N2O, H2O, Ar and N2. CO, CO2, CH4 and N2O in the certified reference material were determined by FT-IR with a long-path gas cell. Ar and N2 were determined by GC-TCD with an oxygen absorber, and H2O was determined by a capacitance hygrometer. The stability of the amount of substance of impurities was monitored. The impurities except for CO2 were not detected and were regarded as being stable. The amount of substance of CO2 seemed to increase. The uncertainty for long-term stability of the property value was determined from the expected increase of the CO2 amount of substance on the expiration date. The typical certified value and its expanded uncertainty (k = 2) are 0.9999993 mol mol−1 and 0.9 μmol mol−1, respectively. The dominant uncertainty for the certified value was the uncertainty for the impurity analysis and long-term stability. The stability was also monitored after the certification. The increase in the CO2 amount of substance is smaller than the uncertainty for the long-term stability, and drift of the amount of substance of oxygen is smaller than the expanded uncertainty of the certified value.

Generation of a spectrum with high flatness and high bandwidth in a short length of telecom fiber using microchip laser

In this work, we studied experimentally the generation of a supercontinuum spectrum induced in a piece of standard single-mode fiber using pulses from a microchip laser. For different values of fiber length, we obtained spectra with high flatness in visible and IR regions. The possibility to generate a spectrum with a high flatness and spectral width of more than ∼1100nm (600nm to over 1700nm) in relatively short lengths of telecom fiber (∼57m), using as the pump pulses with no more than a few kW peak power at a non-zero-dispersion wavelength, is attributed to the peculiar properties of the pulses generated by the pump source. The physical processes leading to the formation of the supercontinuum spectrum were studied by monitoring the growth of the spectrum while increasing the input power. The coupling efficiency between the microchip laser and the telecom fiber helped us obtain a very wide spectrum. This work shows that the use of conventional fiber for supercontinuum generation can be viewed as a cheap and efficient option, in particular for applications like optical metrology, coherence tomography and low noise sources for the characterization of devices.

Recent results on the development of extreme ultraviolet sources for lithography and metrology at 6.x nm

Recent increases in reflectance at 6.x nm of La/B4C multilayer mirrors has led to the adoption of this wavelength for the development of future sources for lithography. The emission from laser produced plasmas of Gd and Tb emit strongly in this wavelength region. The transitions responsible and the optimum plasma conditions for optimum emission have been identified both theoretically and experimentally.

Probing quasiparticle excitations in a hybrid single electron transistor

We investigate the behavior of quasiparticles in a hybrid electron turnstile with the aim of improving its performance as a metrological current source. The device is used to directly probe the density of quasiparticles and monitor their relaxation into normal metal traps. We compare different trap geometries and reach quasiparticle densities below 3 μm−3 for pumping frequencies of 20 MHz. Our data show that quasiparticles are excited both by the device operation itself and by the electromagnetic environment of the sample. Our observations can be modelled on a quantitative level with a sequential tunneling model and a simple diffusion equation.

High-radiance extreme-ultraviolet light source for actinic inspection and metrology

Actinic mask defect inspection and metrology requires high-brightness extreme-ultraviolet (EUV) sources. The self-absorption of radiation limits the in-band EUV radiance of the source plasma and makes it difficult to attain the necessary brightness and power from a conventional single unit EUV source. One possible solution is through multiplexing of multiple low etendue sources. NANO-UV is delivering a new generation of EUV light source, the CYCLOPS, in which a micro-plasma-pulsed discharge source is integrated to a photon collector based on an in situ active plasma structure. The source module is characterized by high brightness, low etendue, and high irradiance at moderate output power without the use of external physical optics. Such a source could form the basic building block of EUV source through spatial-temporal multiplexing of several units to deliver the brightness and power required for actinic mask metrology. We report on the EUV source development including the extensive numerical modeling, which provided the basic parameters required for high irradiance operating regimes. A new Sn-alloy cathode material enhances the output. Based upon the multiplexing concept, a family of specially configured multiplexed source structures, the HYDRA design, is being introduced to address the mask metrology needs.

Development of Coherent Extreme-Ultraviolet Scatterometry Microscope with High-Order Harmonic Generation Source for Extreme-Ultraviolet Mask Inspection and Metrology

In extreme-ultraviolet (EUV) lithography, defect-free mask production is one of the critical issues for the high-volume manufacturing of semiconductor devices. We developed a coherent EUV scatterometry microscope (CSM), which is a simple lensless system. The CSM records diffraction from mask patterns with a charge-coupled-device (CCD) camera directly, which is illuminated with a coherent EUV light. Since a practical standalone system is required by the industry, we developed a standalone CSM system employing a high-order harmonic generation (HHG) EUV source. The 59th high-order harmonic generation of 13.5 nm wavelength is pumped by a tabletop, 6 mJ, 32 fs, Ti:sapphire laser system. The EUV output energy of 1 µW is successfully achieved. We performed the observation of an EUV mask using the HHG-CSM system. The detection limit of the line defect size is improved to 2 nm for the high output power of the HHG EUV source.

Development of Coherent Extreme-Ultraviolet Scatterometry Microscope with High-Order Harmonic Generation Source for Extreme-Ultraviolet Mask Inspection and Metrology

In extreme-ultraviolet (EUV) lithography, defect-free mask production is one of the critical issues for the high-volume manufacturing of semiconductor devices. We developed a coherent EUV scatterometry microscope (CSM), which is a simple lensless system. The CSM records diffraction from mask patterns with a charge-coupled-device (CCD) camera directly, which is illuminated with a coherent EUV light. Since a practical standalone system is required by the industry, we developed a standalone CSM system employing a high-order harmonic generation (HHG) EUV source. The 59th high-order harmonic generation of 13.5 nm wavelength is pumped by a tabletop, 6 mJ, 32 fs, Ti:sapphire laser system. The EUV output energy of 1 µW is successfully achieved. We performed the observation of an EUV mask using the HHG-CSM system. The detection limit of the line defect size is improved to 2 nm for the high output power of the HHG EUV source.

Photon number statistics of NV centre emission

Optical sources that deterministically produce single photons with a high suppression of multi-photon emission and a negligible background component are promising candidates for standard sources for quantum metrology, quantum communication and foundations of quantum mechanics. In this paper, the photon number distribution of non-classical light emitted by nitrogen vacancy (NV) centres in nano-diamonds is studied by three different experimental techniques. The photon number resolving transition edge sensor (TES) detector and the On/Off detection method are applied to determine the diagonal elements of the optical density matrix. From the data measured by the two methods the second order correlation function at time delay zero is calculated and compared with the g2(0)-values obtained by Hanbury Brown–Twiss (HBT) interferometric measurements. Among the g2(0)-values evaluated with the three techniques, we found good agreement in the results for a single photon emitter with a g2(0)-value close to zero and a multi-photon source with a g2(0)-value of approximately 0.5.

Ab Initio Frequency Measurement and Characterisation of Frequency Doubled Fibre Laser Utilised for Precision Oscillators

An ab initio measurement of a free running, frequency doubled erbium fibre laser is made with an optical frequency comb in concert with an internally calibrated wavemeter. The measurement is validated via a Monte Carlo uncertainty analysis. The operation and characteristics of a new high performance optical metrology source for an optical frequency standard is also verified in the process.

5 μm laser source for frequency metrology based on difference frequency generation

A narrow-linewidth cw 5 μm source based on difference frequency generation of a 1.3 μm quantum dot external cavity diode laser and a cw Nd:YAG laser in periodically poled MgO:LiNbO(3) has been developed and evaluated for spectroscopic applications. The source can be tuned to any frequency in the 5.09-5.13 μm range with an output power up to 0.1 mW, and in the 5.42-5.48 μm range with sub-microwatt output. The output frequency is stabilized and its value determined by measuring the frequency of the two lasers with a remotely located frequency comb. A frequency instability of less than 4 kHz for long integration times and a linewidth smaller than 700 kHz were obtained.