Optical frequency-modulated continuous-wave (FMCW) interferometry using a swept laser source allows for highly precise distance measurements. The FMCW technique has the distinct capability to perform both diffuse surface and sub-surface ranging with micron-level accuracy over meters of depth. Similar to swept-source optical coherence tomography, this technique can also produce detailed images of internal structures over large volumes. Recent advances such as active sweep linearization and laser phase-noise suppression techniques have greatly improved the ranging depth and accuracy, allowing for high-resolution 3D imaging of complex objects. Understanding how these techniques can affect the FMCW signal is key in determining system performance limits. The application of an opto-electronic phaselocked loop (OPLL) can greatly reduce both the laser phase noise and deterministic sweep errors. The OPLL uses optical feedback to suppress sweep nonlinearities while also controlling drifts that can be extremely detrimental to the accuracy of the interferometric beat signal. In this work, an analysis of various error sources encountered in activelylinearized FMCW ranging systems is presented.
Swept-source optical coherence tomography (SS-OCT) has been widely employed in the medical industry for the high resolution imaging of subsurface biological structures. SS-OCT typically exhibits axial resolutions on the order of tens of microns at speeds of hundreds of kilohertz. Using the same coherent heterodyne detection technique, frequency modulated continuous wave (FMCW) ladar has been used for highly precise ranging for distances up to kilometers. Distributed feedback lasers (DFBs) have been used as a simple and inexpensive source for FMCW ranging. Here, we use a bandwidth-combined DFB array for sub-surface volume imaging at a 27 μm axial resolution over meters of distance. 2D and 3D tomographic images of several semi-transparent and diffuse objects at distances up to 10 m will be presented.
Coherent optical frequency-domain reflectometry (C-OFDR) is a distance measurement technique with significant sensitivity and detector bandwidth advantages over normal time-of-flight methods. Although several swept-wavelength laser sources exist, many exhibit short coherence lengths, or require precision mechanical tuning components. Semiconductor distributed feedback lasers (DFBs) are advantageous as a mid-to-long range OFDR source because they exhibit a narrow linewidth and can be rapidly tuned simply via injection current. However, the sweep range of an individual DFB is thermally limited. Here, we present a novel high-resolution OFDR system that uses a compact, monolithic 12-element DFB array to create a continuous, gap-free sweep over a wide wavelength range. Wavelength registration is provided by the incorporation of a HCN gas cell and reference interferometer. The wavelength-swept spectra of the 12 DFBs are combined in post-processing to achieve a continuous total wavelength sweep of more than 40 nm (5.4 THz) in the telecommunications C-Band range.
We present a compact, side pumped passively Q-switched Yb:YAG laser that was operated in a burst mode with pump durations of 2-4 ms at low duty cycles. Intra-pump pulse Q-switched pulse repetition frequencies varied from 5-20 kHz depending on the transmission of the Cr:YAG saturable absorber, which was varied from 70% to 94%. Pump duration, pulse repetition frequency and output coupler reflectivity were optimized to yield maximum Yb:YAG laser average power and laser efficiency, while providing sufficient peak intensity, typically 0.3-1 MW, to enable efficient forth harmonic generation (FHG). Pulse energies and durations were in ranges of 0.3-1.8 mJ and 1.5-7ns, respectively, dependent on the unbleached transmission of the Cr:YAG saturable absorber. We achieved an optical efficiency of greater than 15% for the Yb:YAG laser. Extra-cavity 515 nm second harmonic generation (SHG) was achieved using a 5mm long KTP crystal. The 515 nm light was then frequency doubled by focusing it into a 7mm long BBO crystal, resulting in a 15% conversion efficiency from 1030nm to 257.5 nm, with an average UV power greater than 100 mW.
A compact 1030nm fiber laser for ultraviolet generation at 257.5nm is presented. The laser employs a short length of highly-doped, large core (20μm), coiled polarization-maintaining ytterbium-doped double-clad fiber pumped by a wavelength-stabilized 975nm diode. It is passively Q-switched via a Cr<sup>4+</sup>:YAG saturable absorber and generates 2.4W at 1030nm in a 110μJ pulse train. Lithium triborate (LBO) and beta-barium borate (BBO) are used to achieve 325mW average power at the fourth harmonic. The laser's small form factor, narrow linewidth and modest power consumption are suitable for use in a man-portable ultraviolet Raman explosives detection system.
To address the issue of pulse-to-pulse timing jitter in a passively Q-switched Cr:YAG/Nd:YAG laser, we have
developed a technique for optical triggering, where the energy from a single bar diode was used to bleach a thin sheet
within the Cr:YAG saturable absorber from a direction orthogonal to the lasing axis. A strong anisotropy for bleaching
effect was observed; with appropriate polarization of the bleaching light the transmission through the saturable absorber
was increased from 45% to 63%. This technique was applied to a monolithic Cr:YAG/Nd:YAG laser operating under
steady state conditions. By placing the Q-switched pulse at the time corresponding to the steepest slope for change in
transmission during bleaching, which occurs ~1μs after the bleaching diode trigger, we measured an 12.5X reduction in
the pulse-to-pulse timing jitter, from 100ns for free running operation to 8ns with optical triggering.