Performance index of fast sweep frequency light source

Fast sweep light source is a kind of laser that can achieve a wide range of operating frequency and wavelength and periodic continuous tuning. Because the wavelength can be quickly tuned, it is more suitable for application in SS-OCT. All types of sweep light source systems can be viewed as consisting of a gain medium providing gain and light amplification, a filter device providing frequency and wavelength tuning, and a resonator containing this filter. The key parameters are as follows.

1) Tuning range, which determines the range of frequencies and wavelengths of light that the laser can output, will affect the axial resolution of SS-OCT. The axial resolution of SS-OCT can be expressed as Δz= (2ln2 · λ0²) ÷ (π · nΔλ) (1), where λ0 represents the central wavelength of the sweeping light source; ∆λ indicates bandwidth; n is the refractive index of the sample. The larger the sweep range, the wider the bandwidth, and the higher the corresponding axial resolution.

2) Sweep speed, which determines how fast the laser can change the output light frequency and wavelength. The sweep rate of the sweep light source directly determines the imaging speed of SS-OCT. The faster the sweep rate, the faster the imaging speed. In addition, it also affects the signal-to-noise ratio and sensitivity of SS-OCT systems. The noise of SS-OCT is divided into thermal noise, shot noise and light source relative intensity noise. Among them, shot noise accounts for most of the total noise, and the proportion of thermal noise and light source relative intensity noise is almost negligible. If these two kinds of noise are not considered, the signal-to-noise ratio of the system can be expressed as RSN ≈ηPS TS÷hν, where (2) : TS is the frequency sweep period of the light source; PS represents the light power returned from the sample to the detector. The slower the sweep rate, the longer the sweep period of the light source, the higher the signal-to-noise ratio, and the clearer the generated image. The sensitivity of the SS-OCT system can be expressed as S=-10log (ηPS T0÷hν) (3), where P0 is the sample optical power, the higher the value, the higher the sensitivity. The cavity structure of the sweep frequency light source has an external cavity type, a ring cavity type and an integrated shape, and the principle of laser formation is similar, and the laser needs to accumulate enough energy in the cavity to achieve the emission. The number of cycles that the laser needs to run in the cavity is n’ =[log(Psta÷Pspt)]÷logβ, (4) where: the energy required by Psta to form a stable laser; Pspt is the energy of spontaneous radiation. β is the small signal gain of the gain medium. It is assumed that t is the time interval at which the filter wavelength changes, which represents the drive frequency of the filter, and the shorter t is, the faster the sweep rate is. When t>n’·Ts, the laser with saturated energy can be output at this time; When t<n’·Ts, because the filter drive frequency is too fast, the laser cannot run in the cavity in a short time to achieve the sufficient number of turns required for saturation energy, so the output laser energy does not reach saturation, and the faster the sweep speed, the lower the output energy. When t<Ts, the laser fails to run in the cavity for a complete week, and the output energy is only the energy of spontaneous radiation. According to formula (4), when n’ is constant, the shorter the time the laser runs in the cavity, the faster the sweep rate theoretically can be achieved.

3) Output power, which is the power of the laser output by the tunable laser, has an impact on the signal-to-noise ratio and sensitivity of the SS-OCT system. As can be seen from equation (2), the higher the output power, the higher the light power returned from the sample, and the clearer the image generated. According to formula (3), the higher the output power, the higher the sensitivity.

4) The central wavelength, which determines which band the laser works in, will affect the axial resolution, transverse resolution and maximum imaging depth of SS-OCT. As can be seen from equation (1), the shorter the central wavelength, the higher the axial resolution. The transverse resolution of SS-OCT can be expressed as Δx= (4λ0÷π) × (f÷d) = λ0÷2πNA, where NA is the numerical aperture of the lens. When the numerical aperture is constant, the lateral resolution is only related to the central wavelength, and the shorter the wavelength, the higher the resolution. The central wavelength also affects the maximum imaging depth of SS-OCT. The maximum imaging depth of SS OCT can be expressed as Lmax = λ02 ÷4nδλ = λ02 N÷4nΔλ (6), where δλ is the wavelength distance between adjacent sampling points of the spectrum, which can be called spectral resolution. N is the number of sampling points in the spectrum bandwidth. It can be seen that the maximum imaging depth of SS-OCT is related to the central wavelength and the number of sampling points in the bandwidth range, and the larger the two, the deeper the maximum imaging depth. Therefore, if you want to increase the maximum imaging depth, you can also set the acquisition card during the axial scan to obtain enough sampling points.

5) The dynamic coherence length, which determines the distance that the laser can propagate to maintain a certain coherence degree, will affect the imaging depth of SS-OCT. Table 1 shows the corresponding relationship between the performance specifications of SS-OCT and those of the sweep frequency light source.