A powerful new tool for medical imaging and industrial measurement

While ultrasonic imaging has been widely and effectively used in medical imaging, one of its disadvantages is poor resolution due to the comparatively long wavelength of ultrasonic waves. By using light waves instead of ultrasonic waves, a recently developed imaging technique called optical coherence tomography (OCT) has achieved a resolution as high as a few micrometers.^1–4

The light source is one of the most important components in an OCT imaging system. Two types of light sources are commonly used: broadband light sources with very short coherence lengths, such as SLED (superluminescent light-emitting diode) or other ASE (amplified spontaneous emission) sources; and lasers with a wide wavelength tuning range, such as swept light sources. Swept light sources offer a number of benefits over broadband light sources in terms of improving system performance, but various practical problems have hindered their use in certain applications. We have now found solutions to some of these problems and developed swept light sources that can be used for medical imaging and industrial measurement.

Figure 1. Optical spectrum of a 1300nm swept light source (tuning speed 20KHz).

One key part of swept light sources is the optical gain medium, which provides the gain required for the lasing process. Rare-earth-doped fiber amplifiers such as the semiconductor optical amplifier (SOA) have achieved great success as optical gain media in telecom applications. However, they may not be good candidates for swept light source systems due to their long fiber length (tens of meters). The long round-trip time of the ring cavity introduced by this inherent fiber length limits the tuning speed.⁷ Although techniques can be employed to avoid this problem, they tend to make the system too complex.⁸ Nevertheless, SOAs have a very short length of gain medium (∼1mm) with comparable bandwidth and power, making them highly suitable for high-speed applications and optical integration.

Inphenix has now developed swept light sources at 1300nm (IPSSM13xx series: for a typical spectrum, see Figure 1) and 1550nm (IPSSM15xx series: for a typical spectrum, see Figure 2) wavelength ranges, based on its own SOAs operating at 1300 and 1550nm windows. The IPSSM13xx series (1320nm) was developed for medical OCT imaging systems, while the IPSDM15xx series (1550nm) targets industrial imaging and measurement applications. Because of the material properties of different biological tissues, medical imaging OCTs operating at nonconventional telecom wavelengths demand swept light sources at 820 and 1060nm windows. Inphenix is now developing swept light sources that can operate at these wavelengths.

Figure 2. Optical spectrum of a 1550nm swept light source (tuning speed 1.2KHz).

Ideally, the axial imaging in OCT systems can be obtained by conducting a fast Fourier transform (FFT) of the A-scan data, as long as the data is collected evenly in k-space. Unfortunately, that is not the case in most swept source OCT systems, due to the nonlinearity of the light source. This nonlinearity can introduce test error if the A-scan data is not properly processed.⁹ To resolve this issue, certain data interpolation and rescaling techniques have been developed,^9,10 but these techniques usually require substantial system processing power and additional hardware, such as a frequency clock. This may be acceptable for medical imaging systems, where performance is the most important criterion. But for industrial measurement applications, cost is critical.

For this reason, Inphenix has developed the IPSSM1550 series module. This is a 1550nm swept light source designed to minimize nonlinear effects at a system level, thereby removing the need for an extra frequency clock and signal processing before FFT. A typical curve of the tuning wavelength versus scanning time is shown in Figure 3. Providing a close-to-linear wavelength scan reduces or eliminates the need for data interpolation.

Figure 3. Tuning wavelength versus scanning time for a 1550nm swept light source.

In summary, Inphenix has developed swept light sources for broad use in medical imaging and other test areas. These include a low-cost swept light source system with good linearity, which avoids the need for additional hardware.