Broad Band Light Sources
200 kHz Swept Light Source for OCT Imaging
- High-speed: 200 kHz A-line repetition rate
- Voltage-controlled sweeping formats: k-linear scan available
- Coherence length: >7mm at 200 kHz Scan
- All solid state: no moving parts
- Wavelength sweep span larger than 100 nm
- Average light output of 15 mW
DescriptionHigh-speed lightwave sweep enabling high-speed, high-resolution imaging – New Laser Technology reduces the impact of diagnostic imaging on patients
The newly developed light source uses a high-speed KTN light deflector and high-speed wavelength swept variable laser technology, both developed by NTT for used in telecommunications. This is currently the fastest light source commercially available. The wavelength used is in the 1.3 μm band, the band used for OCT-based examination of the coronary arteries.
In addition to the high-speed sweep at 200 kHz, the product has the following performance features:
- wavelength sweep span larger than 100 nm
- average light output of 15 mW
- coherence length longer than 7 mm
OCT systems that incorporate this product can take high-resolution cross-sectional images of living tissue at high speed. This feature will not only reduce the time required to complete an examination and hence reduces impact on patients but also broaden the application of OCT systems, such as innovative medical diagnosis using real-time 3D imaging, and clinical study applications in the field of R&D.
The maximum wavelength sweep speed that can be achieved with conventional products is 100 kHz. These products use a MEMS mirror as the light source to achieve a high-speed sweep. However, a MEMS mirror has a moving part, which limits the maximum speed to below 100 kHz. The high-speed KTN light deflector used in the newly developed product has no moving parts. It deflects light using an electro-optic effect. This can dramatically increase the operating speed of the product. The new product operates at 200 kHz, double the speed of conventional products. This allows instantaneous imaging taking, thereby reducing examination time. This also means that twice as many images can be taken per unit time. Therefore, a large number of images can be rapidly acquired to produce high-resolution images with reduced noise. This high-speed operation also makes it possible to take high-resolution images with 4000 voxels at a rate of 50 fps, allowing an OCT system using this light source to produce a high-resolution 3D image of a living body.
High-performance wavelength swept light source using a KTN light deflector
The newly developed wavelength swept light source uses a laser with an external resonator, the so-called Littman-Metcalf configuration. The laser includes a highly efficient diffraction grating and KTN light deflectors. This compact and optimized structure has made it possible to achieve high-speed operation, a wide wavelength sweep span, and sufficient coherence length.
This product is designed for sale as a stationary light source to be incorporated in OCT systems for development purposes. The goals of the combined efforts of the two companies over the next year include development of a light source in the 1.05-μm band, which is being increasingly used in OCT examination of the fundus, expansion of their product portfolios, and enhancement of the performance of the light source. The companies also aim to develop a high-speed optical detector that will allow high-quality images to be taken, and products that combine their light source with a 2D sweep mirror.
1: Swept Source Optical Coherence Tomography (SS-OCT)
Two OCT systems are currently attracting interest: SD-OCT (spectral-domain OCT) and SS-OCT (swept-source OCT). A feature of SD-OCT is that its signal detector is a spectroscope while SS-OCT’s light source is a wavelength swept coherent light. In particular, SS-OCT has grabbed the limelight because image taking can be sped up by increasing the wavelength sweep speed. An OCT system constructs a 2D cross-sectional image by sweeping the wavelength of the light source to capture images in the depth direction (A-line scan) and sliding the laser beam horizontally (B-line scan). With SS-OCT, one image in the depth direction (A-line) can be taken by scanning a single wavelength. Therefore, the time needed to take an OCT image depends on the wavelength sweep speed.
2: KTN light deflector
A light deflector is also called an optical scanner. When the refractive index distribution of a KTN crystal, which is an electro-optic crystal, is varied by the superposition of the space charge injected inside the crystal and the external electrical field, the light transmitted to the crystal is deflected. This mechanism allows high-speed operation of 200 kHz (double the speed of conventional products) or higher. Since a KTN crystal can respond to an electrical field of hundreds of MHz, it will be possible to develop a light deflector that operates at several MHz
3: Electro-optic effect
This is a phenomenon in which the refractive index varies with the applied voltage. There are two types of effect: Pockels effect, in which the refractive index is proportional to the voltage applied, and the Kerr effect, in which the refractive index is proportional to the square of the voltage applied. KTN crystals exhibit the Kerr effect.