Mid‐IR supercontinuum noise reduction using a short piece of normal dispersion fiber ‐ a general mechanism

Mid-infrared (IR) supercontinuum (SC) lasers are important in applications such as pollution detection, stand-off detection, and non-destructive testing. The performance in many applications is limited by the noise level of the supercontinuum laser. High noise typically results in low sensitivities or a need for long integration times. In this paper, a simple technique to reduce the noise of high noise soliton-based SC sources is introduced by adding a short piece of normal dispersion fiber to force the spectrally distributed solitons to spectrally broaden through self-phase modulation and thereby overlap to average out the noise. The noise reduction is demonstrated experimentally and numerically using a ZBLAN fiber based mid-IR SC source and adding a short piece of highly nonlinear arsenic-sulfide fiber. However, the method is generally applicable to any soliton-based near-IR or mid-IR SC source. Its efficiency is underlined by experimentally comparing it to SC generation in fibers in which a second zero-dispersion wavelength provides the spectral alignment noise reduction mechanism

Ultra-Violet Supercontinuum Sources for Scatterometry

There is a general and growing interest in UV sources to improve resolution in imaging, provide access to endogenous markers, probe light matter interactions in the UV,and so forth. UV supercontinuum sources based on gas-filled hollow core fibers is a promising candidate for many such applications. However, despite impressive results in terms of tuning range, pulse duration, and noise, the number of demonstrations on such applications are remarkably few. Three key issues persist that effectively prevent UV supercontinuum sources from repeating the success that modulation instability supercontinuum sources have had in the visible and near infrared spectrum. First, the UV supercontinuum is not nearly as flat, making it a less versatile source. UV supercontinuum sources compensate by allowing tunability of the resonant dispersive wave across several octaves, but at the cost of changing the pressure and filling gas. Second, the lack of an all fiber-based solution makes any mechanical adjustments risky, as it can necessitate realignment or in worst case result in catastrophic damage to the fibers. Third, many applications rely on high repetition rates. While some might be well out of reach, even at 100 kHz (well below the record) the stability starts to suffer at the hand of long-lived refractive index changes brought on by plasma instabilities. This work introduces a well known concept for UV supercontinuum sources, pump modulation, in a particularly simple form that only modifies the peak power of the pulse train. The key result of the pump modulation is to flatten the spectrum from −26 dB (typical) and −20 dB (best) to only −14 dB difference in intensity across the spectrum from 250 nm to 860 nm, hence amending the first issue. In spectrometerbased applications, a uniform spectrum facilitates better noise performance by making it possible to maximise the signal to noise ratio for all wavelengths simultaneously. Pump modulation enables us to demonstrate spectroscopic transmission scatterometry. This is critical to many applications, including scatterometry, and indeed results in on-par performance with a state of the art setup. More importantly, the unmodulated source fails completely for scatterometry.While the improved noise performance is key to the performance, the reason the unmodulated source fails turns out to have a different origin: A chaotic drift of the mean intensity. The drift evolves significantly on the 10-100 ms time scale, depends critically on peak power and repetition rate, but not on average power. The drift is thoroughly analysed, and mitigation strategies are discussed. Scatterometry is an interesting use case in itself, but represents a stepping stone to other and more advanced techniques. I demonstrate the pump modulation framework for both UV and modulational instability based supercontinuum sources, both experimentally and numerically. Importantly, it can be used both in the design phase of a product and by the end user to retrofit the source for their specific experiment

Real-time Monitoring of DNA Binding by Plasmonic Chip Using an Optical Fiber

In this study, we developed a label-free plasmonic biosensor chip platform to detect DNA binding in real-time. Our aim is to integrate this chip with an optical fiber to make the sensor more compact