Frequently Asked Questions
Last updated
Last updated
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.
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.
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.
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).
The user has an ability to switch between four different pre-programmed modes of sweeping operation (e.g. different tuning speeds or tuning ranges) using a software command but only if additional modes are requested for programming at the time of manufacture. The user cannot control the laser behavior by adjusting its low-level drive signals directly. Please inquire with our technical support team for more information if you need additional sweep modes programmed after a unit's original shipment.
No. Axsun SS-OCT lasers are designed to be tuned in a continuous fashion based on an internal time-base at repetition rates from kilohertz to hundreds of kHz, and across a spectral range from a few nanometers up to ~140 nm.
Q: What happens when a laser engine baseplate exceeds the specified maximum operating temperature?
The Axsun SS-OCT laser engine contains a feedback control system which maintains the laser module's interior using a thermo-electric cooler (TEC) at a temperature set point designed for optimal laser performance. The TEC is cooling the laser module by moving excess heat (mainly generated by the laser's optical amplifier) into the laser engine baseplate where it can be passively or actively removed according to the user's external system design. If heat removal from the laser engine baseplate is insufficient or if other external heat sources cause it to exceed ~55°C, the TEC will be unable to effectively maintain the target temperature and the laser emission will be automatically disabled rather than allowing the laser to operate with suboptimal performance. This latching "TEC ERROR" state can be queried from the engine firmware (via the Control APIs or GUI tools) and can be cleared by lowering the baseplate temperature and then restarting the engine. In addition to stopping laser emission and any heat directly generated by the laser's optical amplifier, the TEC ERROR state will also disable the temperature feedback control system and any excess heat generated by the TEC itself. If no significant external sources of heat are present, the laser engine will therefore avoid reaching temperatures that might risk long-term damage (>70°C).
