Amplified wavelength–time transformation for real-time spectroscopy

Real-time spectroscopy provides invaluable information about the evolution of dynamical processes, especially non-repetitive phenomena. Unfortunately, the continuous acquisition of rapidly varying spectra represents an extremely difficult challenge. One method, wavelength –time mapping, chirps the spectrum so that it can be measured using a single-shot oscilloscope. Here, we demonstrate a method that overcomes a fundamental problem that has previously plagued wavelength –time spectroscopy: fine spectral resolution requires large dispersion, which is accompanied by extreme optical loss. The present technique uses an optically amplified wavelength –time transformation to beat the dispersion-loss trade-off and facilitate high-resolution, broadband, real-time applications. We show that this distributed amplification process can even be pumped by broadband noise, generating a wide gain bandwidth using a single pump source. We apply these techniques to demonstrate real-time stimulated Raman spectroscopy. Amplified wavelength– time Raman spectroscopy creates new opportunities for the study of chemical and physical dynamics in real time. Real-time spectroscopy is a powerful tool for studying dynamic chemical and physical systems. Unfortunately, conventional spectrometers are relatively slow, and do not permit real-time measurements. Recently, a technique has emerged known as wavelength–time mapping, which uses chromatic dispersion to acquire spectra directly in the time domain. Inspired by photonic time-stretch analog-to-digital conversion, wavelength– time mapping has been applied to infrared absorption spectroscopy of gaseous compounds. Wavelength–time spectroscopy circumvents conventional spectrometers and permits real-time single-shot measurements of rapidly evolving or fluctuating spectra. The technique—chirped wavelength encoding and electronic time-domain sampling (CWEETS)—transforms the temporal envelope of a signal into its spectrum using group-velocity dispersion (GVD) to chirp the signal (that is, to map wavelength into time). Once the temporal profile is mapped into its spectral shape, the spectrum can be acquired directly in the time domain using a single detector and a real-time oscilloscope. Unfortunately, real-time wavelength–time spectroscopy has been fundamentally limited by the high loss of dispersive elements. In a real-time measurement, signal averaging is not possible and, thus, a signal can only be dispersed so much before it falls beneath the single-shot noise floor. On the other hand, substantial dispersion is required because the amount of dispersion determines the spectral resolution in the time domain. The trade-off between dispersion and loss is the key limiting factor in single-shot wavelength–time spectroscopy. Furthermore, the Kramers–Kronig relations (fundamental expressions linking dissipation and refraction) impose restrictions on the dispersionto-loss ratio. In other words, dispersion is inexorably linked to loss in passive media. Here, we show that this problem can be solved using distributed Raman amplification directly in the dispersive medium. The amplified wavelength–time transformation overcomes the fundamental trade-off between dispersion and loss. We simultaneously use the medium’s linear dispersion and one of its nonlinear properties—Raman gain—to overcome the dispersionloss limitation, and raise weak signals above the detection noise floor. We also show that broadband amplification can be achieved using incoherent pump light. We demonstrate the utility of the amplified wavelength–time transformation in real-time stimulated Raman spectroscopy, a nonlinear technique suitable MLL