Broadband and High-Speed Swept Sources
High-speed wavelength-swept laser technology offers versatile applications in optical coherence tomography (OCT), bio-chemical spectroscopy, and fiber-optic sensing. Particularly in recent years, swept laser technology has been critical in advancing OCT applications in the field of biomedical imaging and industrial imaging, imparting the advantages of real-time in-vivo diagnostics and high sensitivity. To realize real-time clinical imaging and broad-base commercial deployment, compact, small, and stable swept sources are of utmost importance. To meet such demands, wavelength-flexible swept sources between 400 and 1700 nm with sweep rates from 1 to 150 kHz can be realized in a miniaturized optical butterfly package that also allows the integration of optical reference (k-clock) interferometers or other optical reference filters for spectral calibration. The salient advantages of EXALOS’ swept source technology include:
(1) Wavelength flexibility: currently covering wavelengths from 800 nm to 1650 nm; potentially reaching the visible spectrum down to 400 nm.
(2) High phase stability: suitable for phase-sensitive applications such as Doppler-OCT.
(3) Ultra-stable long-term operation: enables spectral calibration with fixed remapping vectors.
(4) Clean imaging performance: no secondary coherence peaks as well as sharp and narrow PSF peaks without sidelobes throughout the imaging range.
(5) Symmetric bi-directional sweeps: allows high duty cycle and efficiency.
(6) Customizable bi-directional or uni-directional operation.
(7) Small size: optical module in compact butterfly package; packaged with electronics in hard-disk drive (HDD) form factor.
For biomedical OCT applications, different imaging system requirements would demand an individual combination of spectral region, sweep range, sweep frequency, coherence length, average power, and frequency calibration (k-space remapping) methods.
Spectra of swept sources in different spectral regions
(The spikes at the spectra edges are artefacts due to longer integration times in the optical spectrum analyzer at the turn-around points of the spectral sweep)
Swept Source Architecture
EXALOS has developed a miniature external-cavity swept laser based on a micro-optic integrated platform that allows for compact embodiments, performance flexibility, field reliability, and economy of scale. This laser architecture integrates a broadband semiconductor optical amplifier (SOA) gain chip, a high-speed resonant 1D Micro-Electro-Mechanical-System (MEMS) scanning mirror, and a proprietary diffraction grating on a temperature-controlled optical bench inside a 26-pin butterfly package. In the category of longitudinal multi-mode lasers, this is a truly self-contained compact packaging. Contrary to monolithic devices such as the Vernier-Tuned Distributed Bragg Reflector (VT-DBR) laser and MEMS-tuned Vertical Cavity Surface Emitting Laser (MEMS-VCSEL), this hybrid platform has the flexibility to generate lasers in different spectral regions (from 400 nm to 1700 nm) and at different sweep frequencies (currently 1 kHz to 150 kHz).
Schematic of external cavity swept laser with Littrow configuration.
The dashed line represents the optical butterfly module, indicating
that the entire laser cavity is contained inside the module.
3D model of the external cavity swept laser in a butterfly package.
Numerous design parameters for the critical components must be well-matched in order to generate a high-performance swept source. First, the high-performance SOAs are designed in-house for a wide spectral gain and linear behavior to enable long-coherence sources in various spectral regions. Second, the wavelength scanning mechanism is based on a MEMS mirror. The long-term stability of the MEMS is extremely high as it is based on mono-crystalline electro-static MEMS scanners that do not degenerate or degrade over time. Changes in resonance frequency are mainly due to temperature effects (e.g., warm-up effects from light on/off) but are minimized by the temperature-controlled optical platform. The custom-designed MEMS scanners offer high mechanical stability (shock resistance >5,000g), high phase stability and low jitter. Novel diffraction gratings are designed for ultra-high effective resolvance to achieve narrow filtering and hence long coherence lengths while maintaining high diffraction efficiency over a wide spectral range. Optical retarders are used to achieve the right cavity length in order to optimize laser dynamics and minimize mode hopping noise. A free-space k-clock interferometer followed by balanced detection simplifies post-processing in OCT-systems. The A-scan trigger is directly derived from the MEMS clock and therefore is always in sync with the MEMS movement. The sweep is sufficiently stable such that one can create a remapping vector for initial calibration and continue to use the same re-mapping vector for hours of continuous use. The electronic A-scan trigger used in the swept source is derived from a crystal oscillator, which gives a timing jitter down to a few picoseconds. Due to the high Q-factor, there is little noise transferred from the electronics driver to the MEMS scanner.
The laser cavity is manufactured in an automated micro-optic assembly station. This packaging platform offers highest flexibility, alignment accuracy, reproducibility, long-term stability and high production rate.
Hybrid optical packaging platform
An optical packaging platform is critical in the process of realizing the construction of complex optical system that can only exhibit the intended functionalities in a miniature embodiment. EXALOS has co-developed a unique hybrid optical packaging platform (HOPP) machine for (semi-) automated micro-optic assembly with unprecedented alignment accuracy (down to 50 nm or 1 arcsec) for free-space propagating beams. This HOPP machine has 21 motorized stages, six cameras, epoxy dispenser, machine vision, custom programming interfaces, and performs active or passive alignment of all optical elements inside the butterfly package. Advanced design rules are embedded into the process programming and machine training (alignment algorithms) for the laser cavity and its optical components to achieve required optical performance (low wave-front distortions, suppression of unwanted reflections, etc.). During the optical assembly process, the HOPP machine can perform active alignment of optical components, characterize each micro-optical component (size, flatness, optical axis, etc.), and measure optical near/far field and beam propagation through lenses, filters, etc.. Once the machine has learned the alignment procedure, fabrication of swept source modules is highly repeatable.
Automated alignment process of optical components inside butterfly package.
The swept sources are activated while being built on the HOPP machine and certain target performance parameters like SNR or coherence fall-off can be used as active feedback signals to align critical components. This means that the laser performance is monitored throughout the whole manufacturing process, allowing high yield manufacturing on module level.
Compact OEM Swept Source Module
The swept source optical modules (SSOMs) described above are assembled on electronic driver boards. Those fiber-coupled turn-key swept sources are available either as a bench-top version or as an OEM module in 3.5” HDD format (footprint 101.6mm x 147.0mm) that can be mounted inside a disk cage of a PC. The OEM module operates on 12V and 24V DC with a typical power consumption of 10 Watt.
Swept source optical module (SSOM) mounted on a compact electronic driver board, enabling OEM swept sources (with metal case) in 3.5” HDD form factor, as shown for comparison on the right side.