A turnkey phase-sensitive swept-source optical coherence tomography (OCT) system requires stabilization of both the swept source and main signal interferometer. We have provided stable clocking and triggering of a 1050 nm swept micro-electromechanical system (MEMS) vertical cavity surface emitting laser (VCSEL) by mounting a solid clock etalon and volume Bragg grating optical trigger inside a temperature-stabilized butterfly package. A photonic integrated circuit (PIC) provides a phase-stable main interferometer near the sample. This system is only sensitive to the motion of the sample, not to the fibers leading to and from the PIC. We provide optical frequency domain reflectivity measurements of the PIC performance. We also demonstrate OCT imaging as well as static and dynamic phase measurements of a Kindle e-book reader electrophoretic display.

A sub-wavelength grating has been added to an optically-pumped MEMS tunable VCSEL structure to control polarization. This "P_VCSEL" is a thermocompression-bonded sandwich of a 1/2-VCSEL with gain and semiconductor DBR attached to a silicon MEMS mirror on an electrostatically actuated membrane for wavelength control. The sub-wavelength pitch grating is etched into the surface of the 1/2-VCSEL. Optical layer stack simulations have accurately predicted the tuning behavior of the P_VCSELs but not the polarization behavior. The polarization selection does not occur through threshold gain asymmetries as originally believed, but rather through polarization-dependent loss in the grating. We believe the rough sidewalls of the grating teeth are responsible for the polarization-dependent loss. While the selected-against polarization loss is very high, the selected-for polarization loss is still higher than desirable. This reduces power output, especially at short wavelength. However, 91 nm tuning in the 1050 nm band was achieved, suitable for application in optical coherence tomography.

The flexible membranes used in MEMS tunable VCSELs are so small and light that thermally induced vibrations can impact laser performance. We measure the thermal vibration spectrum of such a membrane showing peaks at the spatial vibration mode resonant frequencies of the membrane/plate. These vibrations result in a theoretical floor to the linewidth of the VCSEL. Frequency domain LiDAR and optical coherence tomography systems can get around this thermal linewidth limit with adequate clock measurement and processing. Essentially an OCT/LiDAR sweep with a concomitantly measured clock is a feed-forward linewidth reduction scheme. This can be achieved because the membrane resonances are relatively low frequency. LiDAR ranging out to 9 meters has been demonstrated with a resolution of 13 μm, close to the transform limit for the 70 nm sampling range.

Low NEP balanced receivers generally do not result in better sensitivity in a shot-noise-limited swept source OCT system. However, there is an advantage if RIN is significant. A lower NEP, even in the shot-noise-limited case, does allow for lower reference arm powers. This, in turn, reduces fixed pattern artifact signals caused by stray optical component reflections inside a swept source laser cavity. An NEP reduction of √10 allows the reference power to be reduced 10 dB while maintaining SNR. This reduces the pattern to noise ratio by 10 dB because pattern noise is directly detected (20 dB scaling per decade reference power), whereas the image signal is heterodyne detected (10 dB scaling per decade of reference power). We present sensitivity and fixed pattern measurements taken with six commercial balanced receivers, including an APD receiver. We also present an NEP survey of 23 commercial receivers over a wide range of bandwidths and transimpedances.

Optical coherence tomography (OCT) can be an important diagnostic tool in optics R&D and manufacturing. We illustrate this with four applications. (1) Imaging of vibration modes of MEMS membranes. (2) Strain mapping of VCSEL chips. (3) Optical frequency domain reflectometry (OFDR) of laser cavities. (4) Combined responsivity and 3D mapping of photodiode packages.

Side lobe artifacts on point spread functions can be traced back to (1) fringe visibility variation across the spectrum, (2) errors in sampling instances, and (3) window functions. We demonstrate signal processing methods for correcting for all three of these issues. These methods require a system calibration step. If the systems slowly age, the recalibration step could be performed in the field with a fixtured target.

A 1060 nm optically pumped tunable VCSEL was formed from an InGaAs/AlGaAs/GaAs half-VCSEL bonded to a MEMS movable mirror on a silicon substrate. The VCSEL was co-packaged in a 14-pin butterfly module with an 825 nm pump laser and a 1060 nm semiconductor optical amplifier. The co-packaged device exhibited shot-noise-limited sensitivity with up to 50 mW output power and 75 nm tunability. Ophthalmic OCT, especially whole-eye imaging and ocular biometry, is considered the primary application of this device. However, we have also investigated LiDAR to greater than 10 meter ranges with non-mechanical beam steering through angular diffraction from a grating. A new generation of photonic integrated circuit LiDARs work this way and we have investigated the depth resolution limitations due to time dispersion from the grating. Distributed fiber temperature sensing was also demonstrated.

MEMS tunable lasers are not inherently phase stable because Brownian motion and drive electronics noise make the starting wavelength of the sweep unstable with respect to the electrical sweep trigger. A typical solution to the problem is to use a fiber Bragg reflector wavelength trigger. That is a sub-optimal solution since environmental changes can move both the Bragg peak and the k-clock phase. We have packaged temperature controlled trigger and clock etalons in a butterfly package to solve this environmental problem. By making the wide FSR trigger etalon from silicon and the narrow FSR clock etalon from fused silica, the relative spectral positions of the trigger and clock can be adjusted through temperature control. The system has applications in background subtraction, phase-sensitive and Doppler sensing, synthetic aperture imaging, and long-term averaging to increase SNR. It can be used for direct hardware clocking of a DAQ board, as well as in a software resampling context.

Most swept external cavity diode lasers tune in the short-to-long wavelength direction (red tuning). Lower relative intensity noise (RIN) and higher output power are typically possible in this direction. We show here that long-to-short tuning (blue tuning) is possible for a short, linear cavity laser that has both low noise and high power. This mode of operation is made possible by nonlinear frequency broadening in the semiconductor optical amplifier (SOA) followed by clipping of the red portion of the spectrum by the micro-electro-mechanical systems (MEMS) tunable Fabry-Perot filter. Blue shifting during gain recovery is an important broadening mechanism. There is an approximate 50% advantage in coherence length for the same filter bandwidth for blue over red tuning, which allows deeper imaging in optical coherence tomography (OCT) applications. Calculations contrasting the blue tuning mechanism with red tuning are presented. The accuracy of the blue-tuning model is confirmed by coherence and coherence revival measurements and simulations.

It has been shown theoretically and experimentally that short cavity swept lasers are passively mode locked. We develop a mathematical model of these lasers and the light field solutions are used to predict the coherence length and coherence revival behavior. The calculations compare favorably with data from a 990-1100 nm laser swept at 100 kHz suitable for optical coherence tomography applications.

A back-to-back comparison of a tunable narrow-band SLED (TSLED) and a swept laser are made for OCT applications. Both are 1310 nm sources sweeping at 50 kHz over a 100 nm tuning range and have similar coherence lengths. The TSLED consists of a seed SOA and two amplification SOAs. The ASE is filtered twice by a tunable MEMS Fabry Perot in a polarization multiplexed double-pass arrangement on either side of the middle SOA. This allows very long coherence lengths to be achieved. A fundamental issue with a SLED is that the RIN is proportional to 1/Linewidth, meaning that the longer the coherence length, the higher the RIN. High RIN also leads to increased clock jitter. Most swept source SNR calculations assume that the noise is independent of the amplitude of the signal light: The higher the signal, the higher the SNR. We show that in the case of the TSLED, that the high signal RIN and clock jitter give rise to additional noises that scale with signal power. This leads to an SNR limit in the case of the TSLED: The higher the signal, the higher the noise, so the SNR reaches a limit. While the TSLED has respectable sensitivity, the SNR limit causes noise streaks in an image where the A-line has a high reflectivity point. The laser, which is shot noise limited, does not exhibit this effect. This is illustrated with SNR data and side-by-side images taken with the two sources.

A back-to-back comparison of a tunable narrow-band-filtered SLED (TSLED) and a swept laser are made for OCT applications. The two sources are similar in terms of sweep speed, tuning range and coherence length. A fundamental issue with a TSLED is that the RIN is proportional to 1/linewidth, meaning that the longer the coherence length, the higher the RIN and clock jitter. We show that the TSLED has an SNR limit that causes noise streaks at points of high reflection in images. The laser, which is shot noise limited, does not exhibit this effect. We add noise terms proportional to the sample power times reference power to standard swept source SNR expressions to account for the SNR limit.

In the early to mid-2000s, it became apparent that swept-source optical coherence tomography (SS-OCT) offers significant advantages over both time-domain (TD-OCT) and spectral-domain OCT (SD-OCT). Since that time, significant academic and commercial effort has been focused on developing SS-OCT lasers and sources capable of enabling high-quality SS-OCT imaging. While there are many important components in an SS-OCT system, it can reasonably be argued that the swept source is the most critical as its properties form the basis for most of the major system performancemetrics. The detection and data acquisition electronics, together with the source, ultimately define the SS-OCT system performance specifications such as imaging speed, SNR and sensitivity, axial resolution, and imaging depth. In this chapter, we describe a commercially available 1,060 nm swept-source OCT “engine” developed at Axsun Technologies suitable for ophthalmic and other OCT imaging applications. The engine consists of a swept laser module, control electronics, k-clock, balanced receiver, and data acquisition board which samples on k-clock transitions. The OCT engine provides optical system performance for shotnoise-limited imaging. The engine is designed to simplify construction of OCT imaging systems; the final user provides the optical probe/interface, application

It has been known for quite some time that spinning polygon, and similar, swept lasers used in OCT favor the short to long wavelength sweep direction because of four wave mixing in the gain medium. Here we have reformulated the problem in the time domain and show experimentally and through numerical simulation that these lasers are pulsed. The emitted pulses modulate the gain medium refractive index to red shift the light. Instead of new wavelengths being built up slowly from spontaneous emission, each pulse hops to a longer wavelength by nonlinear means, tracking the tunable filter. This allows high speed, low noise tuning in the blue to red direction. Based on this model, we make the first coherence length calculations for a swept source.

An optical teardown, or reverse engineering, of an Amazon Kindle Paperwhite electrophoretic display was performed by Optical Coherence Tomography at 1060 nm. The display incorporates an optical diffuser, lightguide and scattering layers for white light illumination, capacitive touch sensing, and an electrophoretic display. All these layers can be imaged by OCT as well as the thin film transistor array on the back side for driving the pixels. Phase sensitive OCT is used to measure motion of the pigment particles as the display changes between black and white.