| Fujikura Ltd. White Paper |  |
Novel 1.8-um-band light sources composed of thulium-doped fiber for in vivo imaging
Junji Yoshida, Takeshi Segi and Keiji Kaneda
Fujikura Ltd., 1440 Mutsuzaki, Sakura, Chiba, 285-8550, JAPAN
ABSTRACT
We have developed a single mode fiber-light source at 1.8um band, which composes of thulium (Tm)-doped silica fiber and optical devices used in telecommunication fields. This laser can be operated as a laser and broadband light source selectively. The laser wavelength is tuned from 1765nm to 1812nm by adjusting an angle of a built-in band pass filter (BPF). A maximum output power is 115mW at 1812nm, in the case that pump power is 363mW. The broadband light source has a maximum power of 10mW, the peak wavelength of 1840nm and the full width at half maximum of 50nm. By using these light sources, we have succeeded in obtaining images of test target by observing reflected light through opaque liquid.
Keywords: 1.8um light source, Thulium-doped fiber, tunable fiber laser, in vivo imaging
1. INTRODUCTION
Recently, optical systems have been utilized in the fields of medical diagnosis and treatments, because the technology of light sources and optical detectors have advanced. Optical systems using methods of optical absorption, fluorescence and scattering are applied to measurements of oxygen in blood and in anatomy, cancer diagnosis and blood flow measurement. Especially infrared light sources have been developed vigorously and utilized for various non-invasive diagnosis such as OCT and fMRI.
To our knowledge, restricted infrared regions, shorter wavelength than 1.1um and around 2.8um, have been utilized for medical applications. In the former region water has low absorption coefficient of the order of 10-1cm-1 and in the latter water has high absorption coefficient of the order of 103cm-1. On the other hand, water has comparatively lower absorption coefficient ~101cm-1 in a wavelength region from 1.6um to 1.9um. Moreover, this infrared region has lower scattering comparing to the shorter wavelength region. Therefore infrared in this region is promising candidate for medical diagnosis as in vivo imaging.
As for light sources emitting around 1.9-um wavelength light, thulium (Tm) has been investigated as a gain medium [1-3]. The Tm+3-doped fluoride fiber is one of the mediums for obtaining high power output, because of high doping level and long lifetime at excited state [4–7]. However, the fluoride fiber oes not comply with the requirement of durability and reliability. This is one of the factors that the developments of fiber based light sources emitting around 1.6um to 1.9um for the medical application have not been focused on.
In this paper, we report a single mode tunable continuous wave (CW) laser and incoherent broadband light source at 1.8um band composed of Tm-doped silica fiber. Because this light source consists of silica-based fiber and optical devices established in telecommunication fields, arc fusion splice is used to connect the optical fibers of optical devices, this newly developed light source is durable and reliable. We have evaluated optical performances of 1.8um light source and obtained images of test target through opaque liquid.
2. EXPERIMENTAL
2.1 Evaluation of 1.8um light source
Tm+3 ion doped in fiber core is used as a gain medium for 1.8um light. The energy level diagram of Tm+3 ion is shown in figure 1. Each arrow in figure1 represents optical absorption and emission. Pumping light at 1.6um excites the ion from ground state 3H6 to lower excite state 3H4. Then radiative decay at 1.8um occurs and the characteristic lifetime is few 100us. Because degenerate energy level at 3H6 is separated by means of Stark effect due to crystal electric fields, the ground state 3H6 forms energy band. Therefore the emission light has wide fluorescence from 1.7um to 1.9um with center peak of 1.8um.
Fig. 1. Energy level diagram of Tm+3 ion.
The configuration of 1.8um band light source is shown in figure 2. The light source consists of a master oscillator and an optical power amplifier. The former generates seed light, which has wavelength of 1.8um, and the latter amplifies optical output power from the master oscillator. The master oscillator is indicated by closed dotted area I in figure 2, which consists of optical devices. Tm-doped silica fibers are used as gain mediums. Two laser diodes (LDs) emitting 1.62um light are used as pumping sources and the total maximum optical output power is 257mW. The wavelength division multiplexing (WDM) couplers combine the pump light and seed light. The pump light is launched into Tm-doped fiber. The 10dB coupler splits light power ten-ninety, the power of 10% is brought back to ring cavity and the power of 90% is launched into the optical power amplifier. Fibers of optical devices are connected by arc fusion splicing technique. The BPF is used for tuning the oscillation wavelength by adjusting an angle between the BPF and incident light. As pump power is above lasing threshold of the master oscillator, the lasing wavelength is controlled by the BPF.
On the other hand, as pump power is under threshold, Tm-doped fiber emits amplified spontaneous emission (ASE) light which is incoherent and broad. In this case, the BPF is removed to suppress oscillation. Closed dotted area II in figure 2 indicates the optical power amplifier consisted of Tm-doped fiber, LD and WDM coupler. The LD has the maximum optical power of 106mW. The generated 1.8um seed light is amplified through the power amplifier. If the fiber end is cleaved with flat facet, the power amplifier oscillates by the reflected light from the fiber end at low pump power. The output facet is angle-cleaved to suppress the reflection.
The optical spectra and output powers are evaluated by monochromator and by power meter.
Fig. 2. Configuration of 1.8um-band light source composed of Tm-doped silica fiber. Part I is master oscillator. Part II is optical power amplifier.
2.2 Image observation by 1.8um-band light
Considering advantage that 1.8um light has lower scattering than the shorter wavelength light, 1.8um light is one of the candidates for illumination in opaque liquid. We demonstrate an observation of test target images illuminated by 1.8um light. The images are observed by using infrared vidicon camera in darkroom because the camera is sensitive to optical background noise. Experimental setup is shown in figure 3. A glass cell filled with an opaque liquid (milk) is placed in front of the test target. The transmittance of glass cell filled with milk is 32%. After the 1.8um light goes through the glass cell twice, the light is detected by infrared vidicon camera. Hence the total transmittance from the light source to the detector except for the reflectance of the test target is 10%. To investigate influence of coherency for illumination, we compare the images obtained by the laser light to broadband light.
Fig. 3. Schematic view of experimental setup.
3. RESULTS AND DISCUSSION
3.1 Evaluation of 1.8um light source
Figure 4 shows the output power of the optical power amplifier. Here the launched pump power in the master oscillator is varied from 0mW to 257mW and the pump power of the amplifier is kept at 106mW. The oscillation wavelength fixed by BPF is 1810nm. The maximum output power of 115mW is obtained in the case that the launched pump power is 257mW. The optical energy conversion efficiency is 32% and the lasing threshold is about 20mW.
Figure 5 shows laser spectra and output powers. The launched pump powers of the master oscillator and the power amplifier are 257mW and 106mW, respectively. The oscillation wavelength is changed from 1765nm to 1812nm in continuity. This tuning range is limited by the present specification of the BPF. BPFs with different wavelength region or wider tunable range are useful for extending the tunable range of the master oscillator.
The spectra of broadband light are shown in figure 6. To obtain broadband light, the BPF is removed from the ring cavity. However same configuration of the master oscillator remains except for the BPF. The maximum output power, the peak wavelength and the full width at half maximum are 10mW, 1840nm and 50nm, respectively. The optical energy conversion efficiency is 6.5%.
Fig. 4. Output power of optical power amplifier as a function of the launched pump power in the master oscillator. The launched pump power in the power amplifier is 106mW.
Fig. 5. Laser spectra and output powers at lasing wavelength of 1765nm, 1786nm, 1799nm and 1812nm. The launched pump powers in the master oscillator and the power amplifier are 257mW and 106mW, respectively.
Fig. 6. A spectrum of broadband light source. The launched pump powers in master oscillator and power amplifier are 40mW and 106mW, respectively.
Fused couplers used here are not designed appropriately. Therefore by optimizing these devices for 1.8um light, optical loss in this equipment can be reduced resulting in higher output power. An optical isolator optimized at 1.8um wavelength band is the most effective device to improve this light source. By inserting a built-in optical isolator between the master oscillator and the power amplifier, the optical interference is suppressed. Therefore higher output power of broadband light can be expected.
3.2 Image observation by 1.8um-band light
Figure 7 (a) shows an obtained image of test target illuminated by 1812nm laser light and (b) shows the schematic view of the test target image. The output power is 115mW. A clear image is observed through opaque liquid. This result indicates that 1.8um light is applicable in high scattering environment such as in vivo illumination.
Here we compare illumination ability between laser light and broadband light. Figure 8 and figure 9 show images of test target illuminated by laser light and incoherent broadband light, respectively. Because the illuminated optical power is 10mW, which is lower than previously used power of 115mW, the images are smaller than that of figure 7 (a). It is possible that light coherency affects the observed image. Therefore it is speculated that there are differences between the images obtained by laser light and that by incoherent broadband light. Since almost same images are observed it seems that this result is caused by coherence loss of the laser light. The detected light is strongly scattered by the fine particles in opaque liquid resulting in the loss.
Sharpness of image depends on the sensitivity of detector. High-quality image can be obtained by using a high-sensitivity detector.
Fig. 7. Image of test target illuminated by 1812nm laser light at 115mw output power. The observed image is (a). Schematic view of (a) is (b).
Fig. 8. Image of test target (laser light). Output power is 10mW.
Fig. 9. Image of test target (broadband light). Output power is 10mW.
4. CONCLUSION
We have developed novel 1.8um band light source composed of Tm-doped silica fiber. Durability and reliability are realized because this light source is based on silica-based optical fiber technique established in telecommunication field. This light source generates both laser and broadband lights selectively. The laser light has tunable wavelength range from 1765nm to 1812nm and maximum output power of 115mW at 1812nm. On the other hand, the broadband light has maximum power of 10mW, the peak wavelength of 1840nm, and full width at half maximum of 50nm.
By using 1.8um light, we demonstrate an observation of test target images through an opaque liquid. We have succeeded in obtaining clear test target images, as laser output is 115mW at 1812nm. It is confirmed that there is no differences between the images obtained by laser light and that by incoherent broadband light.
It is concluded that 1.8um band light source composed of Tm-doped silica fiber is a promising candidate for illumination of in vivo imaging system. In addition, this tunable 1.8um light source has possible application as laser spectrum light source.
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