Shot-Noise Limited Time-Encoded Raman Spectroscopy

Figure 1

The schematic of the shot-noise limited TICO-Raman spectroscopy system is presented. (a) The FDML probe laser comprises a fiber Fabry-Pérot tunable filter (FFP-TF), optical isolators (ISO) for unidirectional lasing, a semiconductor optical amplifier (SOA), a fiber-coupler (FC) output, a resonant time delay, and a polarization controller (PC) paddle. (b) The pump laser is a home-built master oscillator fiber power amplifier (MOFPA) with additional Raman shifter. It consists of a narrow-line laser diode at 1064 nm, followed by a fast electro-optical amplitude modulator (EOM). This EOM is used to digitally modulate the TICO pulse pattern and to adjust the pulse length to the detection bandwidth. The pulses are amplified by ytterbium-doped fiber amplifiers (YDFA). A wavelength-division multiplexer (WDM) can be used to couple in additional 1122 nm seed light for Raman shifting inside the double-clad YDFA power amplifier. (c) The pump and probe lasers are collimated and combined by dichroic mirrors (DM) before being focused onto the Raman active sample (S). After recollimation, the pump light is blocked by additional dichroic filters and detected on a photodiode (PD1). A second photodiode (PD2) is used to detect reference light for differential, balanced detection. (e) The differential transimpedance amplifier subtracts FDML offset light and common mode noise, so only intensity changes induced in the Raman-active sample are digitized by the data acquisition (DAQ). (d) The whole system is synchronized by an arbitrary waveform generator (AWG).