Frequently Asked Questions
Axsun lasers do not typically contain polarization controllers or employ polarization maintaining fiber. The laser output's degree of polarization is high, but is in an arbitrary elliptical state when emitted from single mode optical fiber.
Axsun lasers are insensitive to the typically low return losses which would be expected from a connected OCT interferometer using APC fiber connectors, and thus optical isolators are not integrated into the laser engine nor required to be added by the user in most cases. Consider an external isolator and/or contact Axsun if your application requires abnormally high (>4%) optical feedback into the laser's source output fiber.
Q: Why does the swept laser spectrum measured using an Optical Spectrum Analyzer (OSA) have a dramatically different shape than the optical power output plot shown in the Laser Test Report?
The optical power output plot in the Laser Test Report shows power as a function of time for two individual sweeps measured with an oscilloscope. An OSA measurement shows optical power integrated over a significantly longer time period (many sweeps) and with the horizontal axis as a function of wavelength. The laser tuning naturally slows down at the extreme edges of the sweep (i.e. it spends more time and thus integrates more total energy at those wavelengths), artificially distorting the spectral shape to give exaggerated "batman ears" on the extreme edges of the OSA spectrum.
Q: How is the instantaneous output wavelength related to the sweep trigger?
The instantaneous output wavelength of the laser at a time point corresponding to the sweep trigger rising edge is estimated at time of manufacture. The sweep trigger output is not derived directly from the instantaneous laser wavelength and is expected to vary slightly from sweep-to-sweep (fractions of a nanometer) as well as over temperature and laser lifetime (~nanometers). SS-OCT systems do not typically need absolute wavelength calibration for each individual laser sweep. However, applications which require precise wavelength vs. time information can generate this by introducing a fixed wavelength reference trigger (e.g. fiber Bragg grating) and extrapolating to other time points by counting k-clock pulses relative to the reference trigger location.
Q: In what direction along the total tuning range is my laser sweeping?
Lasers are configured at manufacture to sweep their emitted spectrum in either long-to-short wavelength fashion, short-to-long wavelength fashion, or in both directions. Generally, 1310 nm lasers sweep long-to-short and 1060 nm lasers sweep short-to-long, but there are some exceptions. Please inquire with our technical support team for more information if you are unsure about your laser's sweep direction.
The effect on the OCT image of the electronic K-clock delay setting can be fairly subtle, especially in the shallower regions of the OCT scan depth. However, in deeper regions of the OCT image where interference fringe frequency is higher, the constant phase error from a sub-optimal delay setting represents a larger portion of the interference fringe period and has a larger adverse effect on the axial point spread function (PSF) width. Therefore, experimentally optimizing the electronic K-clock delay setting for a given interferometer should be undertaken while observing a PSF signal near the bottom of the OCT image.1) Run your OCT image display software in "live" update mode so that you can view real-time processed OCT images. For Axsun Ethernet/PCIe DAQ users, use the Image Capture Tool.2) Place an object such as a mirror or flat glass interface approximately 90% of the way to the bottom of the OCT image scan depth, and align/tilt it so that its signal is not saturating the photoreceiver or ADC, but is aligned sufficiently to see a single strong PSF shape rising above the background noise. Zoom into this PSF using your display software so that you can see subtle changes in the PSF width.3) Watch the width of the PSF change as you adjust the electronic K-clock delay setting from its minimum to its maximum and back using either OCT Host or the Hardware Control Tool.4) You will likely find an optimal K-clock delay setting which results in the narrowest possible PSF width, with any values higher or lower than the optimal setting resulting in a wider or degraded PSF.5) If the optimal setting (narrowest PSF) occurs at either the minimum or maximum of the adjustable electronic K-clock delay range, this means that your OCT interferometer and the K-clock interferometer (which is built into the Axsun engine) have total pathlengths which are too different to be accommodated by the electronic delay adjustment. One of the interferometers will need to be physically modified in order to more closely match their total fiber pathlengths.6) Note, this experimental adjustment process is similar to that which can be used to optimize dispersion compensation parameters. Iteratively adjust the electronic K-clock delay and the dispersion compensation parameters to fully optimize the OCT system's axial resolution (PSF width).