Time-domain diffuse optics

Time-Domain Diffuse Optics

Introduction

Time-Domain Diffuse Optics is branch of Functional Near-Infrared Spectroscopy which deals with light propagation in diffusive media. There are three main approaches to diffuse optics namely Continuous wave Frequency Domain and Time-Domain. In Time-domain approach, a narrow pulse of light is injected into the medium and the photon arrival times is recorded.

Instrumentation

Instrumentation in Time-Domain Diffuse optics includes a pulsed laser source, a single photon detector and a timing electronics

Sources

Sources in TD-DOS must have the following characteristics emission wavelength in the optical window i.e. between 650 to 1350 nanometre (nm); a narrow full width at half maximum (FWHM), ideally a delta function; high repetition rate (>20MHz) and finally, sufficient laser power (>1 mW) to achieve good signal to noise ratio.

In the past bulky tunable Ti:sapphire Lasers were used. They provided a wide wavelength range of 400 nm, a narrow FWHM (< 1 ps) high average power (up to 1W) and high repetition (up to 100 MHz) frequency. However, they are bulky, expensive and take a long time for wavelength swapping.

In recent years, pulsed fiber lasers based on super continuum generation have emerged. They provide a wide spectral range (400 to 2000 ps), typical average power of 5 to 10 W, a FWHM of < 10ps and a repetition frequency of tens of MHz. However, they are genrally quite expensive and lack stability in super continuum generation and hence, have been limited in there use.

The most wide spread sources are the pulsed diode lasers. They have a FWHM of around 100 ps and repetition frequency of up to 100 MHz and an average power of about a few milliwatts. Even though they lack tunability, there low cost and compactness allows for multiple modules to used in a single system.

Detectors

Single photon detector used in Time-Domain Diffuse Optics require not only a high photon detection efficiency in the wavelength range of optical window, but also a large active area as well as large numerical aperture (N.A.) to maximize the overall light collection efficiency. They also require sharp timing response and a low noise background.

Traditionally, fiber coupled Photomultiplier tubes (PMT) have been the detector of choice for diffuse optical measurements, thanks mainly due to there large active area, low dark count and excellent timing resolution. However, they are intrinsically bulky and prone to electromagnetic disturbances. Moreover, they require a high biasing voltage and are quite expensive. Single photon avalanche diodes have emerged as an alternative to PMTS. They are low cost, compact and can be placed in contact, while needing a much lower biasing voltage. However, they have a much lower active area and hence a lower photon collection efficiency and a larger dark count. Silicon photomultipliers (SiPM) are an arrays of SPADs with a global anode and a global cathode and hence have a larger active area while maintaining all the advantages offered by SPADs. However, they suffer from a larger dark count and a broader timing response.