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MEMS-Tunable VCSEL OCT

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MEMS-VCSEL Swept Source OCT




  • Long Imaging Range of Over 12mm
  • Imaging Speed of 100kHz

Features

  • Out-of-the-Box Functionality
  • 2D Cross-Sectional Imaging at 100,000 Lines per Second
  • High-Speed 2D and 3D Acquisition
  • 25 µm (Lateral) x 16 µm (Depth) Resolution
  • Includes Handheld Probe with Integrated Video Camera
    (See Product Details tab for a Description of all Items Included with each System)
  • Easy Data Acquisition and Scan Control (2D and 3D)
    Using Included Software (See Software tab for more Details)

Thorlabs' OCS1310V1 Swept Source Optical Coherence Tomography (SS-OCT) System is based on a patented Micro-Electro-Mechanical (MEMS)-tunable Vertical Cavity Surface Emitting Laser (VCSEL) that is specially designed for optimal performance in OCT applications. Developed in partnership with Praevium, a strategic partner of Thorlabs, this MEMS-VCSEL OCT system provides high-speed imaging at unprecedented imaging depth ranges of over 12 mm. The 1300 nm central wavelength and greater than 50 mm coherence length of this swept laser source enable imaging through highly scattering samples with an imaging range of over 12 mm (current imaging range capability is solely limited by data acquisition electronics). Compared to our OCS1300SS swept source system, the MEMS-VCSEL SS-OCT system provides nearly 8 times the imaging speed and 5 times the imaging range. The system includes a handheld probe, probe stand, and computer with user software. Details are provided in the Product Details tab.

Optical Coherence Tomography (OCT) is a noninvasive optical imaging modality that provides real-time, 1D depth, 2D cross-sectional, and 3D volumetric images with micron-level resolution and millimeters of imaging depth. OCT images provide structural information of a sample, based on light backscattered from different layers of material within that sample. Although it is considered to be the optical analog to ultrasound, OCT achieves higher resolution through the use of near infrared wavelengths, at the cost of decreased penetration depth. In addition to high resolution, the non-contact, noninvasive advantage of OCT makes it well suited for imaging samples such as biological tissue, small animals, and industrial materials.

Compared to Spectral Domain OCT technology, Swept Source Optical Coherence Tomography does not suffer from inherent sensitivity degradation at longer imaging depths. Thorlabs' MEMS-VCSEL OCT system is based on our MEMS-VCSEL Swept Laser Source, which boasts a greater than 50 mm coherence length and over 100 nm wavelength tuning range. Using this laser, the OCS1310V1 is capable of providing over 12 mm imaging depth with a nominal amount of signal degradation.

System Specificationsa
Center Wavelength 1300 nm
A-Scan Line Rate 100 kHz
Imaging Range >12 mm
Axial Resolution  
Lateral Resolution
(with LSM03 Scan Lens)
25 µm at Focus

a. Specifications are subject to change.

 

OCS1310V1 Processor

The OCS1310V1 includes a Dell Precision T5600* workstation including a 24" monitor. This processing system is set up with all the necessary data acquisition hardware, drive electronics, and software to begin imaging upon arrival.

Computer Specifications*

  • Six Core 2.3 GHz Processor
  • Windows 7 Profesional, 64-Bit
  • 32 GB DDR3 RAM
  • 1.0 GB NVIDIA Quadro 600 Graphics Card
  • 1 TB Hard Drive

Data Acquisition Specifications

  • A/D Conversion Rate: 500 MS/s
  • A/D Resolution: 12-bit

 

 

OCS1310V1 Laser and Imaging Module

The SL1310V1 is a compact design that is built into two units. The Swept Source Laser (bottom unit) contains Thorlabs' Swept Source Laser and all associated drive electronics and controllers. The Imaging Module, which conveniently sits on top of the laser, houses the OCT interferometer module, hand-held probe drive electronics, aiming beam, and user-adjustable reference arm and polarization control.

Dimensions

  • Swept Source Laser: 321 mm x 320 mm x 150 mm
  • Imaging Module: 321 mm x 320 mm x 65 mm

 

Hand-Held Probe and Stand

All Thorlabs OCT systems include a hand-held probe and stand, as shown here. The probe provides X-Y scanning for three-dimensional data acquisition. A camera that is integrated in the probe provides live video imaging during OCT data acquisition. The probe easily slips onto the stand for imaging of small samples.
The probe stand consists of a post-mounted focus block which is attached to a specially designed 12" x 14" aluminum breadboard using a Ø1.5" P14 Stainless Steel Post.

Stand Features

  • 3/4" Thick Aluminum Breadboard Provides Increased Stability
  • Breadboard Base has Side Grips and Recessed Feet for Easy Lifting and Transportation of the Stand
  • Includes a Sample Stage with 1" X and Y Travel as well as Rotation

 

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 ov VCSELs became evident, leading to them 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 micro-electromechanical 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 laser sources 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. In addition, the short cavity design enables nearly identical spectral in the forward and backward sweeps. 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 laser source. 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.

 

 

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 50 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 semiconductor optical amplifier (SOA) to achieve greater than 25 mW of power. An additional advantage of this post-amplification scheme is that the SOA 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.

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