We present results and analysis of 5 Gb/s On-Off-Keyed (OOK) data transmission at ~ 4.6-micron wavelength using, at room temperature, a directly modulated, single-mode DFB-QCL transmitter and a Resonant-Cavity Infrared Detector (RCID) receiver. The DFB-QCL design enables relatively high 165-mW CW output power. We used a 3-mm device with ~ 4-micron aperture for stable single-spatial-mode operation, operating at 65 mW. The RCID detector suppresses background radiation while providing enhanced quantum efficiency, ~ 60%, with low polarization dependence and low dark current at room temperature. The data transmission was achieved with no transmitter pulse shaping, consistent with lower-cost transceiver implementation.
KEYWORDS: Optical coherence tomography, Laser dentistry, Stereoscopy, In vivo imaging, Biomedical applications, Visualization, Tissues, Ranging, Point spread functions, Laser frequency
A framework presented in this study demonstrates an advanced high-speed and long-range wide-field swept-source OCT/OCTA system for imaging the oral cavity. By employing a k-clock calibration strategy, the system's point spread function performance is significantly improved. The achieved wide FOV of 42×42 mm² for OCT and OCTA imaging demonstrates a feasibility for imaging of oral cavity morphology and vascular distribution in a clinical setting. This advanced system provides promising potential for efficient and comprehensive in vivo oral cavity imaging, further expanding OCT's clinical applications in dentistry.
Resonant cavity infrared detectors (RCIDs) can reduce the noise in sensing a laser signal by strongly suppressing background photocurrent at wavelengths outside the narrow spectral band of interest. We recently reported an RCID with 100-nm-thick InAsSb/InAs absorber, GaAs/AlGaAs bottom mirror, and Ge/SiO2 top mirror. At T = 300 K, the external quantum efficiency reached 58% atλres ≈ 4.6 μm, with linewidth δλ = 27 nm. The characteristics at 125 K implied a specific detectivity of 5.5 × 1012 cm Hz½/W, which is more than 3× higher than for a state-of-the-art broadband HgCdTe device operating at that temperature. However, a prominent variation with mesa diameter of the deposited Ge spacer thickness made it difficult to predictably control λres for devices processed with a given diameter. This has been addressed by measuring the reflectivity spectrum following deposition of the spacer, so that thicknesses of the top mirror’s SiO2 and Ge layers could be adjusted appropriately to attain a targeted resonance. This was especially beneficial in matching the λres for a small mesa, needed to minimize the capacitance in high-frequency measurements, to the emission wavelength of a given ewquantum cascade laser.
We demonstrate the first 1050nm MEMS-eVCSEL co-packaged with a wideband amplifier to achieve over 70nm wavelength tuning at over 30mW of output power and SMSR greater than 40dB. Ophthalmic Optical Coherence Tomography Angiography (OCTA) images acquired at 800kHz A-scan rates showcase the telecom grade 14pin butterfly co-package as a path to low cost swept source OCT engines. Device design employs a strain-compensated InGaAs/GaAsP gain region disposed on a wideband fully oxidized GaAs/AlxOy back mirror capable of tuning ranges beyond 100nm. It has been suggested the wideband fully oxidized GaAs/AlxOy back mirror may pose risk to device lifetime reliability. However, over 9000hrs of lifetime testing validates reliability and projects device lifetimes exceed 20,000hrs under continuous use.
In recent years, MEMS-tunable VCSELs have emerged as a leading swept source for optical coherence tomography imaging. At the ophthalmic imaging wavelength of 1050nm, optically pumped MEMS-VCSELs (MEMS-oVCSELs) have previously achieved >100nm tuning range and repetition rates approaching 1Mhz, enabling high-resolution and high-speed eye imaging. Electrically pumped MEMS-VCSEL technology (MEMS-eVCSEL) is a critical need for many emerging low-cost high-volume applications, but thus far tuning range has lagged substantially behind optically pumped devices. In this work, we demonstrate 97nm continuous tuning range in a MEMS-eVCSEL operating near 1050nm, and >100nm total tuning range, representing the widest tuning ranges achieved to date, and rivaling the performance of optically pumped devices. Our devices employ a strain-compensated InGaAs/GaAsP gain region disposed on a wideband fully oxidized GaAs/AlxOy back mirror. A deposited top mirror rests on a flexible dielectric membrane separated by a variable airgap from the underlying gain region. Application of voltage between the dielectric membrane and a bottom actuator contact on the top of the gain region creates an electro-static force which pulls the suspended mirror down, contracting the airgap and tuning the device to shorter wavelengths. In this 3-terminal device, the bottom actuator contact doubles as the laser anode. Current injection proceeds from the anode to the cathode at the back of the GaAs substrate through a lithographically defined low-loss current aperture, enabling reproducible aperture size and reproducible single-mode performance. These devices offer promise for many emerging high-volume imaging applications.
Over the last two years, our group has reported the first room-temperature continuous-wave (RTCW) fixed wavelength VCSELs operating above 3 microns, in both optically pumped and electrically pumped devices. Our optically pumped 3.3um devices employ one or two wafer-bonded GaAs/AlGaAs mirrors, in conjunction with a type I InGaAsSb/AlInGaAsSb quantum well active region. Our electrically pumped 3.3um devices employ a bottom waferbonded GaAs/AlGaAs mirror, top deposited ZnSe/ThF4 mirror, and type II interband cascade (ICL) active region. These fixed wavelength devices lay a foundation for tunable devices in the spectrally rich 3-5um region. Narrowly tunable devices can use thermal tuning, by variation of pump power (optically pumped devices), bias current (electrically pumped devices), or device temperature (both electrically and optically pumped devices). In this paper, we describe tunable CW optically pumped devices with >4nm of tuning near 3.3um using variation of pump power. CW electrically pumped devices show ~2nm tuning near 3.3um using variation of bias current. These results are a critical first step towards an inexpensive and high-speed methane sensing source. A first generation of MEMS-tunable optically pumped devices has achieved 70nm tuning range near 3.34um.
Endoscopic optical coherence tomography (OCT) angiography enables volumetric coregistered architectural and microvasculature imaging of the human gastrointestinal tract in vivo. In this talk, we will discuss technical advances and clinical gastroenterology applications with the endoscopic OCT angiography technique.
Tunable vertical cavity surface emitting lasers (VCSELs) offer a potentially low cost tunable optical source in the 3-5 μm range that will enable commercial spectroscopic sensing of numerous environmentally and industrially important gases including methane, ethane, nitrous oxide, and carbon monoxide. Thus far, achieving room temperature continuous wave (RTCW) VCSEL operation at wavelengths beyond 3 μm has remained an elusive goal. In this paper, we introduce a new device structure that has enabled RTCW VCSEL operation near the methane absorption lines at 3.35 μm. This device structure employs two GaAs/AlGaAs mirrors wafer-bonded to an optically pumped active region comprising compressively strained type-I InGaAsSb quantum wells grown on a GaSb substrate. This substrate is removed in processing, as is one of the GaAs mirror substrates. The VCSEL structure is optically pumped at room temperature with a CW 1550 nm laser through the GaAs substrate, while the emitted 3.3 μm light is captured out of the top of the device. Power and spectrum shape measured as a function of pump power exhibit clear threshold behavior and robust singlemode spectra.
Since the first demonstration of swept source optical coherence tomography (SS-OCT) imaging using widely tunable micro-electromechanical systems vertical cavity surface-emitting lasers (MEMS-VCSELs) in 2011, VCSEL-based SSOCT has advanced in both device and system performance. These advances include extension of MEMS-VCSEL center wavelength to both 1060nm and 1300nm, improved tuning range and tuning speed, new SS-OCT imaging modes, and demonstration of the first electrically pumped devices. Optically pumped devices have demonstrated continuous singlemode tuning range of 150nm at 1300nm and 122nm at 1060nm, representing a fractional tuning range of 11.5%, which is nearly a factor of 3 greater than the best reported MEMS-VCSEL tuning ranges prior to 2011. These tuning ranges have also been achieved with wavelength modulation rates of >500kHz, enabling >1 MHz axial scan rates. In addition, recent electrically pumped devices have exhibited 48.5nm continuous tuning range around 1060nm with 890kHz axial scan rate, representing a factor of two increase in tuning over previously reported electrically pumped MEMS-VCSELs in this wavelength range. New imaging modes enabled by optically pumped devices at 1060nm and 1300nm include full eye length imaging, pulsatile Doppler blood flow imaging, high-speed endoscopic imaging, and hand-held wide-field retinal imaging.
We developed an ultrahigh speed endoscopic swept source optical coherence tomography (OCT) system for clinical
gastroenterology using a vertical-cavity surface-emitting laser (VCSEL) and micromotor based imaging catheter, which
provided an imaging speed of 600 kHz axial scan rate and 8 μm axial resolution in tissue. The micromotor catheter was
3.2 mm in diameter and could be introduced through the 3.7 mm accessory port of an endoscope. Imaging was
performed at 400 frames per second with an 8 μm spot size using a pullback to generate volumetric data over 16 mm
with a pixel spacing of 5 μm in the longitudinal direction. Three-dimensional OCT (3D-OCT) imaging was performed in
patients with a cross section of pathologies undergoing standard upper and lower endoscopy at the Veterans Affairs
Boston Healthcare System (VABHS). Patients with Barrett’s esophagus, dysplasia, and inflammatory bowel disease
were imaged. The use of distally actuated imaging catheters allowed OCT imaging with more flexibility such as
volumetric imaging in the terminal ileum and the assessment of the hiatal hernia using retroflex imaging. The high
rotational stability of the micromotor enabled 3D volumetric imaging with micron scale volumetric accuracy for both en
face and cross-sectional imaging. The ability to perform 3D OCT imaging in the GI tract with microscopic accuracy
should enable a wide range of studies to investigate the ability of OCT to detect pathology as well as assess treatment
response.
In the last 2 years, the field of micro-electro-mechanical systems tunable vertical cavity surface-emitting lasers (MEMS-VCSELs)
has seen dramatic improvements in laser tuning range and tuning speed, along with expansion into unexplored
wavelength bands, enabling new applications. This paper describes the design and performance of high-speed ultra-broad
tuning range 1050nm and 1310nm MEMS-VCSELs for medical imaging and spectroscopy. Key results include
achievement of the first MEMS-VCSELs at 1050nm and 1310nm, with 100nm tuning demonstrated at 1050nm and
150nm tuning at shown at 1310nm. The latter result represents the widest tuning range of any MEMS-VCSEL at any
wavelength. Wide tuning range has been achieved in conjunction with high-speed wavelength scanning at rates beyond 1
MHz. These advances, coupled with recent demonstrations of very long MEMS-VCSEL dynamic coherence length,
have enabled advancements in both swept source optical coherence tomography (SS-OCT) and gas spectroscopy.
VCSEL-based SS-OCT at 1050nm has enabled human eye imaging from the anterior eye through retinal and choroid
layers using a single instrument for the first time. VCSEL-based SS-OCT at 1310nm has enabled real-time 3-D SS-OCT
imaging of large tissue volumes in endoscopic settings. The long coherence length of the VCSEL has also enabled, for
the first time, meter-scale SS-OCT applicable to industrial metrology. With respect to gas spectroscopy, narrow dynamic
line-width has allowed accurate high-speed measurement of multiple water vapor and HF absorption lines in the 1310nm
wavelength range, useful in gas thermometry of dynamic combustion engines.
Recent advances in swept-source / Fourier domain optical coherence tomography (SS-OCT) technology enable in vivo ultrahigh speed imaging, offering a promising technique for four-dimensional (4-D) imaging of the eye. Using an ultrahigh speed tunable vertical cavity surface emitting laser (VCSEL) light source based SS-OCT prototype system, we performed imaging of human eye dynamics in four different imaging modes: 1) Pupillary reaction to light at 200,000 axial scans per second and 9 μm resolution in tissue. 2) Anterior eye focusing dynamics at 100,000 axial scans per second and 9 μm resolution in tissue. 3) Tear film break up at 50,000 axial scans per second and 19 μm resolution in tissue. 4) Retinal blood flow at 800,000 axial scans per second and 12 μm resolution in tissue. The combination of tunable ultrahigh speeds and long coherence length of the VCSEL along with the outstanding roll-off performance of SS-OCT makes this technology an ideal tool for time-resolved volumetric imaging of the eye. Visualization and quantitative analysis of 4-D OCT data can potentially provide insight to functional and structural changes in the eye during disease progression. Ultrahigh speed imaging using SS-OCT promises to enable novel 4-D visualization of realtime dynamic processes of the human eye. Furthermore, this non-invasive imaging technology is a promising tool for research to characterize and understand a variety of visual functions.
KEYWORDS: Optical coherence tomography, Vertical cavity surface emitting lasers, Endoscopy, Imaging systems, Data acquisition, In vivo imaging, 3D acquisition, Colon, Image resolution, 3D image processing
We developed a micro-motor based miniature catheter with an outer diameter of 3mm for ultrahigh speed endoscopic optical coherence tomography (OCT) using vertical cavity surface-emitting laser (VCSEL) at a 1MHz axial scan rate. The micro-motor can rotate a micro-prism at 1,200-72,000rpm (corresponding to 20- 1,200fps) with less than 5V driving voltage to provide fast and stable scanning, which is not sensitive to the bending of the catheter. The side-viewing probe can be pulled back for a long distance to acquire three-dimensional (3D) dataset covering a large area on the specimen. VCSEL provides high a-line rate to support dense sampling under high frame rate operation. With the use of a C++ based high speed data acquisition (DAQ) system, in vivo three-dimensional OCT imaging in rabbit GI tract with 1.6mm depth range, 11μm axial resolution, 8μm lateral resolution, and frame rate of 400fps is demonstrated.
This paper demonstrates new wavelength swept light source technology, MEMS tunable VCSELs, for OCT imaging.
The VCSEL achieves a combination of ultrahigh sweep speeds, wide spectral tuning range, flexibility in sweep
trajectory, and extremely long coherence length, which cannot be simultaneously achieved with other technologies. A
second generation prototype VCSEL is optically pumped at 980nm and a low mass electrostatically tunable mirror
enables high speed wavelength tuning centered at ~1310nm with ~110nm of tunable bandwidth. Record coherence
length >100mm enables extremely long imaging range. By changing the drive waveform, a single 1310nm VCSEL was
driven to sweep at speeds from 100kHz to 1.2MHz axial scan rate with unidirectional and bidirectional high duty cycle
sweeps. We demonstrate long range and high resolution 1310nm OCT imaging of the human anterior eye at 100kHz
axial scan rate and imaging of biological samples at speeds of 60kHz - 1MHz. A first generation 1050nm device is
shown to sweep over 100nm. The results of this study suggest that MEMS based VCSEL swept light source technology
has unique performance characteristics and will be a critical technology for future ultrahigh speed and long depth range
OCT imaging.
MEMS tunable vertical cavity surface emitting laser (MEMS-VCSEL) development, over the past two decades, has
primarily focused on communications and spectroscopic applications. Because of the narrow line-width, single-mode
operation, monolithic fabrication, and high-speed capability of these devices, MEMS-VCSELs also present an attractive
optical source for emerging swept source optical coherence tomography (SSOCT) systems. In this paper, we describe the
design and performance of broadly tunable MEMS-VCSELs targeted for SSOCT, emphasizing 1310nm operation for
cancer and vascular imaging. We describe the VCSEL structure and fabrication, employing a fully oxidized
GaAs/AlxOy mirrors in conjunction with dielectric mirrors and InP-based multi-quantum well active regions. We also
describe the optimization of MEMs speed and frequency response for SSOCT. Key results include 1310 nm VCSELs
with >120nm dynamic tuning range and imaging rates near 1MHz, representing the widest VCSEL tuning range and
some of the fastest swept source imaging rates thus far obtained. We also describe how low-noise semiconductor optical
amplification boosts average optical power to the required levels, while maintaining superior OCT imaging quality and
state of the art system sensitivity. Finally, we present measured multi-centimeter dynamic coherence length, and discuss
the implications of VCSELs for OCT.
This paper discusses the design and the internal device physics of novel high-performance vertical-cavity surface-emitting lasers (VCSELs) emitting at 1.32 µm wavelength. Our VCSEL design features intra-cavity ring contacts, strain-compensated AlGaInAs quantum wells, and an AlInAs/InP tunnel junction. The tunnel junction is laterally confined forming an aperture for current injection and wave guiding. Undoped AlGaAs/GaAs mirrors are bonded on both sides to the InP-based active region. These devices have recently demonstrated continuous-wave (CW) lasing at stage temperatures up to 134°C, the highest temperature reported thus far for any long-wavelength VCSEL. In order to increase the single mode output power at high temperatures, we simulate, analyze, and optimize our VCSEL using advanced numerical software tools. The two-dimensional model self-consistently combines electrical, optical, thermal and gain calculations. It gives good agreement with measurements after careful calibration of material parameters. Design optimization promises single mode output power of 2mW in CW operation at 80°C ambient temperature.
We introduce a scheme incorporating wafer bonding and tunnel junctions to improve the performance long-wavelength Vertical Cavity Surface Emitting Lasers (VCSELs). Through careful design of PL-mode offset, mirror reflectivity, and aperture definition, we achieve lasing to 134°C, output power above 2 mW, single-mode output power at 80°C above 1 mW, and differential efficiencies of 46%. We achieve lasing at wavelengths as high as 1336 nm and show a versatile design that can be applied to any VCSEL functioning at long wavelengths.
Standards activities for the next generation of Ethernet, 10 Gigabit Ethernet, are underway. Vertical Cavity Surface Emitting Lasers (VCSELs) offer significant advantages for realizing cost-effective, high speed optical data links. The progress towards achieving 10 Gb/s VCSEL-based links is reviewed.
Vertical cavity surface emitting lasers (VCSELs) operating near 1310 or 1550 nm have been the subject of intensive research by multiple groups for several years. In the past year at Gore, we have demonstrated the first 1300 nm VCSELs which operate with useful power, high modulation rate, and low voltage over the commercial temperature range of 0 - 70 degree(s)C. These results have been achieved using a new structure in which an 850 nm VCSEL optical pump is integrated with the 1300 nm VCSEL. Electrical drive is applied to the 850 nm pump, and 1300 nm light is emitted from the integrated structure. This approach has resulted in over a milliwatt of single transverse mode power at room temperature, and several hundred microwatts of single transverse mode power at 70 degree(s)C. In addition, these devices demonstrate multi-gigabit modulation and excellent coupling efficiency to single-mode fiber.
Wavelength-division-multiplexing (WDM) has long been recognized as an attractive way to exploit the bandwidth of optical fiber. Such systems have been slow to gain commercial acceptance, because of the high cost of WDM components such as Distributed Bragg Reflector lasers and/or Distributed Feedback lasers. We propose that 1300/1550 nm long-wavelength vertical cavity lasers (VCSELs) can be manufactured inexpensively in WDM arrays. VCSELs at 1300/1550 nm have progressed slowly, relative to their 850/980 nm counterparts. In this paper, we review progress in long-wavelength VCSELs, and introduce a new device structure which can provide high CW output power, and wide wavelength operation. We then introduce a method for fabricating VCSELs in WDM arrays, and show initial results.
We present a technique for monolithic integration of vertical cavity lasers and detectors with refractive microlenses etched on the back side of the semiconductor substrate in a wafer-scale process. This integration provides collimated or focused laser beam sources for applications in free-space interconnections or for coupling to optical fibers, and it improves the collection efficiency of detectors.
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