| Popis: |
Mid-infrared optical coherence tomography (MIR OCT) has shown promise in the last few years in applications such as spectroscopy and non-destructive testing. Previously, we have successfully demonstrated a MIR swept-source OCT and measured its noise from three main sources: quantization noise from the ADC, shot noise from the detectors and relative intensity noise (RIN) of the laser. Of these sources, RIN places an upper limit on the SNR of swept-source OCT systems. We attempt to characterize RIN in greater depth and determine whether it can be reduced through normalization. The pulsed laser used (Block Engineering Lasertune) is tunable within the wavelength range of 5.4-12.8μm. The laser output was held at a fixed wavelength, repetition rate and pulse width. Each laser pulse was integrated to find its average power along the pulse duration. A Fourier transform of the result was used to calculate the ratio of the AC power to DC power, giving a value for RIN. By using a beamsplitter and aspheric lenses to carefully focus the beam onto two detectors (Vigo System’s PVMI-4TE), the two pulse trains can be normalized. Through normalization, RIN was reduced from -74dB/Hz to -92dB/Hz. Increasing the repetition rate and pulse width leads to a decrease in RIN, but an upper limit on the laser duty cycle constrains improvements to RIN via this method. As the swept laser has four integrated quantum cascade laser (QCL) chips, we also examine the effect of different emission wavelengths on RIN. |
