Feature Of The Week 6/6/10: Electronically Controlled Coherent Optical Sampling For Optical Coherence Tomography
Feature Of The Week 6/6/10: Researchers from the Institute of Semiconductor Electronics at the RWTH Aachen University in Germany have published a variety of interesting papers in OCT. This includes novel work on electronically controlled coherent optical sampling. In optical sampling, a transient waveform under study is gated with time delayed laser pulses to sample this waveform. This gating mechanism can be achieved easily by coherent detection and is commonly performed in low coherence interferometry (LCI) and optical coherence tomography (OCT), where the electric field of the backscattered light of the sample is interfered with time delayed light of a reference arm.
Classical approaches either use time domain detection, employing a scanning reference arm or spectral domain detection based on spectrometers or wavelength sweeping lasers. While time domain systems based on mechanical scanning allow high imaging depths, the spectral domain systems are limited by the achievable spectral resolution. The latter systems suffer from a depth dependent degradation in signal to noise ratio, autocorrelation noise and mirror terms.
These limitations are circumvented by coherent asynchronous optical sampling (ASOPS), recently introduced by the research group for LCI and OCT imaging . ASOPS allows a non-mechanical scan of the time delay by employing two pulsed lasers with slightly detuned repetition rates. A constant difference fREP between the repetition rates of two lasers leads to an increasing mutual time delay. Thus, the pulses of the first laser sweep the pulses of the second laser, scanning large ranges without mechanical action. However, the scanning rate (that is the rate used for scanning this range) is strongly limited by the discrete timings of the laser pulses. At least three pulses per coherence length are necessary to generate a correlation signal. Thus, the scanning rate dramatically suffers from low repetition rate lasers <<1 GHz, limiting the difference frequency in ASOPS to only 350 Hz for a standard 100 MHz fiber laser . Furthermore, the scanning range is always fixed to half of the cavity length (1.5m here). Hence, ASOPS is not scalable to low repetition rate lasers.
These limitations are removed by dynamically changing the repetition rate in dependence of a user adjustable phase control signal. This so called electronically controlled coherent linear optical sampling (ECOPS) allows for flexible scanning parameters. The scanning range, the scanning rate and the offset position of the scanning process can be chosen by full electronic control. Thus, the user can define regions of interest within the sample.
This setup is realized by two femtosecond fiber lasers (FFS.BU, Toptica Photonics AG, Gräfelfing, Germany) which are arranged in parallel. The pulse trains of both lasers are internally measured and the phase difference to the user adjustable phase control signal is determined. The repetition rate of laser 1 can be adjusted with the help of a stepper motor and a piezoelectric transducer. A phase locked loop (PLL) control (FFS.SYS, Toptica Photonics AG) regulates the cavity length of laser 1 by locking the phase difference to zero. The output of laser 1 is guided to the sample under investigation. The backscattered light is overlapped at the photodetector with the light of laser 2. The photodetector signal is bandpass filtered and recorded by a digital transient recorder (Saturn, AMO GmbH, Aachen, Germany).
In this manner, flexible scanning parameters are possible. Small scanning ranges of some millimeters up to several centimeters can be covered by full electronic control. Our novel method allows for in vivo measurements as well as subsurface analysis of large volumes .
Thus, coherent ECOPS is a novel technique for LCI and OCT imaging. The approach described here breaks the repetition rate limitations of ASOPS and allows defining regions of interest within the sample. The advantages of time domain detection such as high dynamic range and large scanning dimensions are exploited without having to accept the disadvantages of spectral domain systems. Further applications such as en-face OCT imaging, interferometric metrology or future applications including non-linear effects might be exploited.
For more information see references below. Courtesy of Stefan Kray. Click "Full-Screen" for better viewing.
 S. Kray, F. Spöler, M. Först, and H. Kurz, "Dual femtosecond laser multiheterodyne optical coherence tomography", Optics Letters 33, 2092-2094 (2008).
 S. Kray, F. Spöler, T. Hellerer, and H. Kurz, "Electronically controlled coherent linear optical sampling for optical coherence tomography", Optics Express 18, 9976-9990 (2010).