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Wavelength Sweep Range (-10 dB)
Duty Cycle (Unidirectional Sweep)
Average Output Powerb
Ripple Noise Suppression
Relative Intensity Noise (RIN)
These are typical values. See the Specs tab for more details.
Measured at laser output aperture. Valid over the entire wavelength range.
1300 nm Benchtop Swept-Wavelength Laser Source
100 nm Mode-Hop-Free Sweep Range
User-Adjustable k-Clock Signal Delay
Trigger and k-Clock Signal Output Provided
Thorlabs' SL13 Series 1300 nm Swept-Wavelength Laser Sources are available with 100 kHz or 200 kHz sweep rates and operate mode-hop-free over the full 100 nm wavelength range. They have record-breaking coherence lengths of over 100 mm. These single mode benchtop laser sources are designed primarily for high-speed and long-range optical coherence tomography (OCT) systems requiring superior sensitivity, as well as being well-suited for metrology, spectroscopy, and other applications. These laser sources are based on a patented microelectromechanical system (MEMS) tunable vertical cavity surface emitting laser (VCSEL) and include an active power control that maintains constant output power over the lifetime of the laser.
All drive electronics and trigger signals needed to easily integrate an SL13 series laser into custom swept-source OCT systems are provided. An output digital “k-clock” signal generated by the integrated Mach-Zehnder interferometer (MZI) and drive electronics can be used as a data acquisition sampling clock, with no further resampling in k-space required. The k-clock signal delay can be adjusted up to ±8 ns, and the output power of the laser up to a maximum of ±5% by the user.
OCT Imaging with a MEMS-VCSEL Swept Source OCT systems built using our SL13 series lasers provide high-quality images and are capable of significant imaging depths. The OCT cross-sectional image of the anterior eye shown below (left) was obtained using the equivalent of an SL131090 integrated into a Vega OCT system. The laser was swept over a wavelength range of 100 nm and had a 100 kHz sweep rate. This Vega system had a 44 mm Mach-Zehnder interferometer delay, an axial resolution in air better than 16 μm, and was capable of high-resolution imaging to a maximum depth of 11 mm. The OCT measurement signal was digitized with a 500 MSPS A/D converter, and the system provided 100 000 A-scans/s. Details of this system and its performance can be found here.
Point spread function (PSF) is a measurement of the optical performance of an imaging system, and narrower peaks indicate better performance. The PSF shown below (right) was obtained for the same Vega system as was used for the image to the left, but is typical of any OCT system using one of our swept-source lasers. For example, a system with an integrated SL132120 would show similar performance, but the imaging depth would be truncated to 8 mm.
Click to Enlarge OCT Cross-Sectional Image of the Anterior Eye, from an OCT System with Integrated SL13 MEMS-VCSEL Swept Source
Click to Enlarge Click Here to Download the Raw Data PSF Obtained for an OCT System with Integrated SL13 Series MEMS-VCSEL Swept Source, Using Integrated k-Clock
k-Clock Max Frequency (Typical)
OCT Imaging Depth Range
Center Wavelength, λc
Wavelength Sweep Range (-10 dB), Δλ
Duty Cycle (Unidirectional Sweep)
Average Output Power, Po
Coherence Length, Lcoh
Scan Linearization Ratio (SLR)
Spectral Ripple Noise Suppression, dP
Relative Intensity Noise (RIN)
Fiber Bragg Grating Trigger Wavelength
Output Fiber Numerical Aperture, NA
Laser Classification (IEC 60825-1:2014)
MZI Delay (±2% Tolerance)
k-Clock Max Frequency (Typical)
OCT Imaging Depth Range
Click to Enlarge Click Here to Download the Raw Data Optical Output Spectrum of the Swept-Wavelength Laser Source
Thorlabs' MEMS-VCSEL benchtop systems incorporate all the necessary drive electronics, temperature controllers, trigger signals, and optical isolators for easy operation and integration into any swept-source OCT system. Additionally, these benchtop laser sources utilize a specially designed Mach-Zehnder Interferometer (MZI) "k-clock" that provides a digital output signal for triggering data acquisition.
The figure to the right shows a schematic of the MEMS-VCSEL benchtop system. These systems consist of a MEMS-VCSEL cavity, a booster optical amplifier (BOA), a fiber-optic monitoring network, and signal generation circuits. The optical output of the MEMS-VCSEL cavity module is connected to the optical input of the BOA. Integrated optical isolators in the benchtop laser sources eliminate the need for additional isolators external to the laser. Three electrical output signals, shown at the bottom of the schematic, can be used via SMA connectors to synchronize a data acquisition system to the swept optical output.
Sweep Trigger The Sweep Trigger provides an electronically generated line trigger at the beginning of the sweep. The rising edge of the pulse is synchronized to the beginning of the sweep. The sweep trigger is generally used as a trigger for data acquisition systems. The signal is series terminated with 50 Ω.
Wavelength Trigger (λ Trigger) "λ Trigger" is an optically generated line trigger that indicates when the laser output has crossed a certain wavelength. This 1310 nm fiber-Bragg-grating-based optical trigger usually represents the middle of the sweep. Due to its higher wavelength stability, it is used as an alternative to the sweep trigger and can be used for phase-sensitive applications where sweep-to-sweep stability is required. The signal is series terminated with 50 Ω.
k-Clock The k-clock signal is used as a data acquisition sampling clock, internally generated from an MZI. The digital output from the MZI "k-Clock" is linear in wavenumber, allowing optically generated signals to be sample linearly in "k-space," rather than in the time domain. Electrically, the signal is an AC-coupled digital clock with an amplitude between 0.7 and 1.0 Vpp. During the back-scan, the k-clock circuit will switch over to a "dummy clock", which operates at a fixed frequency and is not correlated to the optical subsystem. The dummy clock runs at approximately 250 MHz.
Click to Enlarge Figure 1: Praevium's MEMS-Tunable VCSEL is an innovative design that offers high-speed and broadband emission with long coherence length. This is an ideal combination for an OCT swept laser source.
Click to Enlarge Figure 2: MEMS-tunable VCSELs can be densely packed on a single wafer to increase the potential yield. The inset shows a single MEMS-tunable VCSEL device after fabrication. The overall size of the MEMS-tunable VCSEL is approximately 600 µm x 600 µm square.
VCSEL Overview Vertical Cavity Surface Emitting Lasers (VCSELs) are semiconductor-based devices that emit light perpendicular to the chip surface, as shown in Figure 1. VCSELs were originally developed as low-cost, low-power alternatives to edge-emitting diodes, mainly for high-volume datacom applications. Quickly thereafter, the advantages of VCSELs became evident, leading them to being preferred light sources over edge-emitters in many applications. Compared to edge-emitting sources, VCSELs offer superior output beam quality and single mode operation.
MEMS-tunable VCSELs utilize microelectromechanical mirror systems (MEMS) to vary the cavity length of the laser, thereby tuning the output wavelength. MEMS-tunable VCSELs have existed for several years; however, the limited tuning range and output power of these devices have precluded them from being used in OCT applications. Praevium Research, in cooperation with Thorlabs and MIT, has since developed a MEMS-tunable VSCEL design that overcomes these previous limitations.
In order for a MEMS-tunable VCSEL to be successful for applications in OCT, it needs to meet certain standards:
Rapid Sweep Speed
Broad Tuning Range
Long Coherence Length
High Laser Output Power
Rapid Sweep Speed Applications using OCT demand high-speed imaging without sacrificing imaging quality. Fast imaging rates allow better time resolution, dense collection of 3D datasets, and decreased laser exposure times to the sample.
Currently, there exist a few swept-source lasers that offer high-speed scanning. Fourier domain mode-locked lasers, for example, achieve extremely high imaging speeds but require the use of very long fiber optic delays in the laser cavity and can only operate in wavelength ranges where the fiber loss is low. Of the commercially available high-speed swept lasers, many operate with multiple longitudinal modes or have long cavity lengths, which limit coherence length or tuning speed, respectively.
The low mass of the MEMS-tuning mirror in a MEMS-based tunable VCSEL and the short cavity length both contribute to its high-speed operation. The short cavity length also places only one mode in the gain spectrum, enabling single-mode continuous operation. We have recently measured greater than 500 kHz sweep rates using a MEMS-tunable VCSEL prototype, without using optical multiplexing to increase the sweep speed.
Broad Tuning Range High-resolution imaging depends on the overall tuning bandwidth of the swept-source laser. Praevium boasts the broadest bandwidth MEMS-tunable VCSEL that has ever been developed. A unique design incorporating broadband, fully oxidized mirrors, as well as wideband gain regions and thin active regions, has currently resulted in greater than 100 nm of continuous mode-hop-free tuning, centered around 1300 nm. For details, please see Figure 3.
Click to Enlarge Figure 3: MEMS-tunable VCSELs are capable of tuning over 100 nm. Here we show single-mode operation over a 110 nm spectral tuning range centered at 1300 nm.
Click to Enlarge Figure 4: Spectrum of MEMS-tunable VCSEL operating at 200 kHz, with a center wavelength around 1310 nm, and post amplification using a BOA.
Long Coherence Length A significant limitation to most OCT systems is the depth of view (maximum imaging depth range). Especially in clinical applications, where sample thickness, patient motion, and sample location cannot be controlled, a long depth of view is advantageous. A long coherence length alone, however, is not enough. Image sensitivity needs to be virtually unaffected throughout the entire depth. Due to the micron-scale cavity length of the VCSEL and single mode, mode-hop-free operation, we have measured coherence lengths of greater than 100 mm from our MEMS-tunable VCSEL with nearly no signal degradation. Currently limited by detector bandwidth, we are confident that the MEMS-tunable VCSEL is able to achieve even longer imaging depths than have been measured to date. This remarkable depth of view will not only benefit the medical imaging community but also open doors to other applications such as large objective surface profiling, fast frequency domain reflectometry, and fast spectroscopic measurements with high spectral resolution.
High Output Power Increased imaging speed often comes at the cost of decreased output power and/or optical power on the sample. One advantage of edge-emitting light sources over VCSELs is that they can emit greater output powers. As a general rule, most OCT imaging applications need a minimum of 20 mW of laser output power to maintain image quality when operating at faster scan rates. To reach this goal, the MEMS-tunable VCSEL is coupled with a booster optical amplifier (BOA) to achieve greater than 25 mW of power. An additional advantage of this post-amplification scheme is that the BOA reshapes the MEMS-VCSEL output spectrum such that it is much more uniform.
Additional Considerations and Manufacturing Capabilities A special feature of the MEMS-tunable VCSEL is that it is scalable for different wavelengths. Through innovative combinations of gain materials and dielectric mirrors, a wide wavelength range in the visible or near infrared can be reached, enabling expansion of this new family of light sources.
As we further develop this light source, we look forward to finding new and exciting applications for its use. Please contact us to discuss how a MEMS-tunable VCSEL may advance your research.
Finally, a dielectric mirror (L) is deposited and patterned. The top MEMS contact is further patterned to complete creation of the actuator. The sacrificial layer is undercut to leave a suspended, moveable top mirror above the MQW structure, producing a VCSEL with a MEMS-based tuning element in a single device.
Laser Safety and Classification
Safe practices and proper usage of safety equipment should be taken into consideration when operating lasers. The eye is susceptible to injury, even from very low levels of laser light. Thorlabs offers a range of laser safety accessories that can be used to reduce the risk of accidents or injuries. Laser emission in the visible and near infrared spectral ranges has the greatest potential for retinal injury, as the cornea and lens are transparent to those wavelengths, and the lens can focus the laser energy onto the retina.
Safe Practices and Light Safety Accessories
Thorlabs recommends the use of safety eyewear whenever working with laser beams with non-negligible powers (i.e., > Class 1) since metallic tools such as screwdrivers can accidentally redirect a beam.
Laser goggles designed for specific wavelengths should be clearly available near laser setups to protect the wearer from unintentional laser reflections.
Goggles are marked with the wavelength range over which protection is afforded and the minimum optical density within that range.
Post appropriate warning signs or labels near laser setups or rooms.
Use a laser sign with a lightbox if operating Class 3R or 4 lasers (i.e., lasers requiring the use of a safety interlock).
Do not use Laser Viewing Cards in place of a proper Beam Trap.
Lasers are categorized into different classes according to their ability to cause eye and other damage. The International Electrotechnical Commission (IEC) is a global organization that prepares and publishes international standards for all electrical, electronic, and related technologies. The IEC document 60825-1 outlines the safety of laser products. A description of each class of laser is given below:
This class of laser is safe under all conditions of normal use, including use with optical instruments for intrabeam viewing. Lasers in this class do not emit radiation at levels that may cause injury during normal operation, and therefore the maximum permissible exposure (MPE) cannot be exceeded. Class 1 lasers can also include enclosed, high-power lasers where exposure to the radiation is not possible without opening or shutting down the laser.
Class 1M lasers are safe except when used in conjunction with optical components such as telescopes and microscopes. Lasers belonging to this class emit large-diameter or divergent beams, and the MPE cannot normally be exceeded unless focusing or imaging optics are used to narrow the beam. However, if the beam is refocused, the hazard may be increased and the class may be changed accordingly.
Class 2 lasers, which are limited to 1 mW of visible continuous-wave radiation, are safe because the blink reflex will limit the exposure in the eye to 0.25 seconds. This category only applies to visible radiation (400 - 700 nm).
Because of the blink reflex, this class of laser is classified as safe as long as the beam is not viewed through optical instruments. This laser class also applies to larger-diameter or diverging laser beams.
Lasers in this class are considered safe as long as they are handled with restricted beam viewing. The MPE can be exceeded with this class of laser, however, this presents a low risk level to injury. Visible, continuous-wave lasers are limited to 5 mW of output power in this class.
Class 3B lasers are hazardous to the eye if exposed directly. However, diffuse reflections are not harmful. Safe handling of devices in this class includes wearing protective eyewear where direct viewing of the laser beam may occur. In addition, laser safety signs lightboxes should be used with lasers that require a safety interlock so that the laser cannot be used without the safety light turning on. Class-3B lasers must be equipped with a key switch and a safety interlock.
This class of laser may cause damage to the skin, and also to the eye, even from the viewing of diffuse reflections. These hazards may also apply to indirect or non-specular reflections of the beam, even from apparently matte surfaces. Great care must be taken when handling these lasers. They also represent a fire risk, because they may ignite combustible material. Class 4 lasers must be equipped with a key switch and a safety interlock.
All class 2 lasers (and higher) must display, in addition to the corresponding sign above, this triangular warning sign