The short-term jitter and longer-term wander of the frequency sweep profile of a Vernier-Tuned Distributed Bragg Reflector (VT-DBR) laser at 1550 nm used in OCT applications is characterized in this work. The VT-DBR has demonstrated success in source-swept OCT (SSOCT), performing both intensity [1] and phase-sensitive [2] OCT. The purpose of this paper is to investigate one of the unique aspects of the VT-DBR laser that makes it successful in OCT: the stability of the linear optical frequency sweep of the source. Jitter measurements of the optical frequency sweep are recorded using a 3-cavity 100 GHz free spectral range (FSR) solid etalon. A gas absorption reference cell is used for wander characterization. We report that the VT-DBR jitters by no more than 82 MHz RMS in optical frequency while sweeping at an 8 kHz repetition rate. Longer-term wander provides insight into the accuracy of the VT-DBR selfcalibration routine which produces an intrinsically linear optical frequency sweep. Over an 8-hour data collection period, the system maintains a linear sweep with an optical frequency step of 105 MHz per 2.5 ns with +/- 3 kHz per 2.5 ns (+/- 0.03%) peak-to-peak deviation. We find that the absolute frequency drifts by 325 MHz (2.6pm) over the same 8- hour period with ambient temperature fluctuations of no more than 5 °C. Results show that using calibration with a gas reference cell, picometer absolute wavelength accuracy of the laser can be achieved at any time for a single sweep. Stability and accuracy limits are thought to be due to electronic drive circuitry in the current design.
We design and discuss an impedance matching solution for a hybrid silicon mode-locked laser diode
(MLLD) to improve peak optical power coming from the device. In order to develop an impedance
matching solution, a thorough measurement and analysis of the MLLD as a function of bias on each of the
laser segments was carried out. A passive component impedance matching network was designed at the
operating frequency of 20 GHz to optimize RF power delivery to the laser. The hybrid silicon laser was
packaged together in a module including the impedance matching circuit. The impedance matching design
resulted in a 6 dB (electrical) improvement in the detected modulation spectrum power, as well as
approximately a 10 dB phase noise improvement, from the MLLD. Also, looking ahead to possible future
work, we discuss a Step Recovery Diode (SRD) driven impulse generator, which wave-shapes the RF drive
to achieve efficient injection. This novel technique addresses the time varying impedance of the absorber as
the optical pulse passes through it, to provide optimum optical pulse shaping.
Vernier-tuned distributed Bragg reector (VT-DBR) lasers in source swept OCT (SS-OCT) have previously been demonstrated at 1550 nm and 1600 nm.1 Many OCT applications prefer 1310 nm operation. This work describes the first demonstration of a VT-DBR operating at 1310 nm in the O-band, ideal for use in SS-OCT. This paper addresses the device characterization of such lasers, illustrating they are capable of fast amplitude and frequency sweeps necessary for SS-OCT applications. Equivalent circuit models for each of the five ports are also created to determine their electrical parasitics. Narrow linewidths of the VT-DBR indicate coherence length of several centimeters are possible during fast wavelength sweeps.
We demonstrate a new swept-wavelength laser for optical coherence tomography using a monolithic semiconductor
device with no moving parts. The laser is based on a Vernier-Tuned Distributed Bragg Reflector (VTDBR) structure. We
show highly-linear sweeps at 200 kHz sweep repetition rates, with peak output power of 20 mW. Using a test
interferometer, we demonstrate point-spread functions with 45-55 dB dynamic range. The source provides long
coherence length (> 40mm) at up to 200 kHz sweep rates. The laser system has sufficient linearity in optical frequency
and stability over time to provide an electronic sample trigger clock (an Electronic K-Clock) that denotes equal optical
frequency intervals during the sweep. The laser tuning mechanism is all-electronic, easily adjustable and programmable.
We demonstrate both flat and Gaussian power vs. wavelength profiles, programmable sweep rates with the same device,
and an adjustable duty cycle of up to 85% at full speed. Because the laser is a monolithic semiconductor structure based
on reliable, wafer-scale processes, the manufacturing cost of the laser will decrease rapidly in volume production.
A novel swept-wavelength laser for optical coherence tomography (OCT) using a monolithic semiconductor device with
no moving parts is presented. The laser is a Vernier-Tuned Distributed Bragg Reflector (VT-DBR) structure exhibiting a
single longitudinal mode. All-electronic wavelength tuning is achieved at a 200 kHz sweep repetition rate, 20 mW
output power, over 100 nm sweep width and coherence length longer than 40 mm. OCT point-spread functions with 45-
55 dB dynamic range are demonstrated; lasers at 1550 nm, and now 1310 nm, have been developed. Because the laser's
long-term tuning stability allows for electronic sample trigger generation at equal k-space intervals (electronic k-clock),
the laser does not need an external optical k-clock for measurement interferometer sampling. The non-resonant, allelectronic
tuning allows for continuously adjustable sweep repetition rates from mHz to 100s of kHz. Repetition rate
duty cycles are continuously adjustable from single-trigger sweeps to over 99% duty cycle. The source includes a
monolithically integrated power leveling feature allowing flat or Gaussian power vs. wavelength profiles. Laser
fabrication is based on reliable semiconductor wafer-scale processes, leading to low and rapidly decreasing cost of
manufacture.
Wavelength tunable sampled grating distributed Bragg reflector (SG-DBR) lasers used for telecommunications
applications have previously demonstrated the ability for linear frequency ramps covering the entire tuning range of the
laser at 100 kHz repetition rates1. An individual SG-DBR laser has a typical tuning range of 50 nm. The InGaAs/InP
material system often used with SG-DBR lasers allows for design variations that cover the 1250 to 1650 nm wavelength
range. This paper addresses the possibility of concatenating the outputs of tunable SGDBR lasers covering adjacent
wavelength ranges for enhancing the resolution of OCT measurements. This laser concatenation method is
demonstrated by combining the 1525 nm to 1575 nm wavelength range of a "C Band" SG-DBR laser with the 1570nm
to 1620 nm wavelength coverage of an "L-Band" SG-DBR laser. Measurements show that SGDBR lasers can be
concatenated with a transition switching time of less than 50 ns with undesired leakage signals attenuated by 50 dB.
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.
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
chip1,2,3,4. 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.
California Polytechnic State University (Cal Poly) has an active photonics-related program. The thrusts of the program are coursework, extensive photonic educational laboratories, an SPIE student branch chapter, and a new Project-Based Learning Institute (PBLI) to promote joint projects with industry. This paper will describe our program for a multidisciplinary approach to photonics education at the undergraduate and master’s degree level.
California Polytechnic State University (Cal Poly) is one of 23 campuses comprising the California State University, the nation's largest four-year comprehensive public undergraduate university system. Cal Poly has a photonics program, photonics student club, and photonics laboratory within the Electrical Engineering Department that dates back to 1985. This laboratory is dual-use for both teaching and as a photonics center of excellence for the newly established Project-Based Learning Institute (PBLI) (http://pbl.calpoly.edu/). Our photonic education program at Cal Poly emphasizes four main educational tools. A. Lecture Classes. B. Photonics Laboratory Classes C. Student Photonics Club, and D. PBLI Design Projects. In this paper, we will describe the above four aspects with emphasizing on our new initiatives for part B and D.
This presentation will discuss advancements in thin-film filter technology for telecommunications applications in the following key topic areas: Narrow Band Channel Spacing: Several years ago, it was thought by industry analysts that thin-film filter technology would not be practical for channel spacings below 100 GHz due to loss and center wavelength stability requirements. Thin-film solutions for 50 GHz and 25 GHz channel spacings have been demonstrated with state of the art low-loss thin-film fabrication and new packaging strategies. Chromatic dispersion and dispersion power penalty for narrow band filters will also be discussed. Wide band filters with fewer dropped channels and near theoretical performance: Optical Add-Drop Multiplexing (OADM) is a key application for wide-band thin-film filters. Application requirements call for a minimization of the number of dropped channels with very steep filter sidewalls. The reflection notch function must have very deep isolation to minimize coherent cross-talk effects. This class of filter requires advancements in total film thickness, accurate layer thickness control, and center wavelength temperature control. High angle of incidence filters: Filter designs that use fixed angle of incidence in the 8 degree to 15 degree range are enjoying resurgence in popularity especially for CWDM applications. Wavelength tunable optical add/drop filters using angle-tuning methods are also emerging as an important application. Special designs have been created to optimize performance in these applications.
Commercial telecommunication systems are now being offered at the OC-192/STM-64 (10 Gb/s) data rates. Research results show that even higher time domain multiplexed rates are feasible. With these increasing data rates, measurement technology is being challenged to keep up the pace. This presentation discusses several new measurement techniques and technologies that will enable measurement and verification of these state of the art systems.
High speed photodetectors are required for use in high speed optical interconnects. Recent results on high speed photodetectors are reviewed, and devices with important implications for future high speed photodetector operation are considered.
Modelocked semiconductor lasers are a source of subpicosecond pulses at high repetition rates with
very low phase noise. This makes them ideal sources for clock/strobe signals for OEIC's and optical
computing, for high data rate input devices and for use in optical digital-to-analog converters. These
applications will be described in this paper, together with an outline on the state of the art in
modelocked semiconductor lasers.
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