Sampled Grating Distributed Bragg Reflector (SGDBR) monolithic tunable lasers are now entering the production phase in telecommunications applications. These tunable lasers are unique in that they offer wide wavelength tuning (1525 to 1565 nm), fast wavelength tuning (5 ns) and high speed amplitude modulation all on the same monolithic chip 1,2,3,4 . This work studies the applicability of SGDBR monolithic tunable laser diodes for biomedical imaging using swept-wavelength or Fourier domain optical coherence tomography. This paper will present our work involved with utilizing the strengths (table 1) of this SGDBR laser class and mitigating the weaknesses (table 2) of this device for swept-wavelength imaging applications. The strengths of the laser are its small size (portable solutions), wide wavelength range (good distance resolution), fast switching speeds (improved update rates), wide choice of center wavelengths, and lower power consumption. The weaknesses being addressed are the complicated wavelength tuning mechanism (3 wavelength control currents), wider laser linewidth (10s of MHz), moderate output power (10mW ), and the need for improved laser packaging. This paper will highlight the source characterization results and discuss an initial measurement architecture utilizing the SGDBR measurement engine.

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Fast wavelength tunable sampled grating distributed Bragg reflector (SG-DBR) lasers are used to generate fast, linear, continuous wavelength sweeps. High resolution wavelength sweeps in excess of 45 nm are demonstrated at a 100 kHz repetition rate. The front mirror, back mirror and phase segment tuning segments can be modulated at very fast rates, which allows for very fast wavelength ramp rates. This sweep is generated through three time synchronized current versus time waveforms applied to the back mirror, front mirror and phase sections of the laser. The sweep consists of fifty separate mode-hop-free tuning segments which are stitched together to form a near continuous wavelength ramp. The stitching points require a maximum of 60 ns for amplitude, wavelength, and thermal settling time to allow the laser to equilibrate. Wavelength tuning non-linearities, output power wavelength dependency, and wavelength discontinuities are defects in the wavelength sweep that result from properties of the wavelength tuning mechanism as well as limitations of the signal generators that produce the time varying bias currents. A Michelson Interferometer is used to examine the effects of these defects for optical coherence tomography (OCT). The OCT measurements demonstrate spectral broadening of the source and interference signal reduction as the penetration depth increases. However, these effects are not very severe for delay differences less than 2 mm even without correction for sweep nonlinearities.

Millimeter wave difference frequencies are easily available from the SG-DBR.Temperature robustness of SG-DBR self-heterodyne signals was also investigated. While each laser wavelength by itself changes by 0.12 nm/C (21 GHz/C), the difference frequency temperature coefficient is well less than 30MHz/C typically.

An edge illumination technique has been designed using a monochromatic light source that improves the identification of surface flaws on optics. The system uses a high-resolution CCD camera to capture images of the optics. Conventional edge illumination methods using white light sources have been plagued by light leaking around the optics causing high background levels. The background combined with lower resolution cameras has made it difficult to determine size and intensity characteristics of the flaws. Thus photographs taken of the optics are difficult to analyze quantitatively and do not allow for the detection of small, faintly illuminated sites.Infrared diodes have been utilized to illuminate large-scale (43 cm x 43 cm) fused silica optics, and a two-dimensional array CCD camera has been used to collect the image data. Flaw sizes as small as ~10 µm have been detected. A set of frames has been built to support the infrared sources where one diode array per side is magnetically attached to the frame. The diodes inject light into the optic causing the sites to illuminate, which can be detected by the camera. A customized mounting design has been implemented to secure the frames to the stage, or base, for image acquisition. The design uses a dual bracket assembly to support the frames. With this design for optical illumination, quantitative data has been obtained of the surface flaws.A comparison of the peak intensity, total integrated intensity and size of the flaws measured in these images and the size of the flaws as measured using a microscope will be presented.