Spectral Interrogation Based on Swept or Tunable Lasers

The scheme in Figure 5.9 could be theoretically used to interrogate multiple sensors, if the light intensity is high enough. In fact, power density of a broadband light source is relatively low, which is difficult to support multiple branches. A swept or tunable laser could be used to realize multiple sensor interrogation by power splitting due to its high optical intensity at a certain single wavelength, as shown in Figure 5.10. The coherent length of the laser is much longer than the OPD of the FFP sensor. The key point of such a scheme is how to construct a swept or tunable laser.

Generally, the swept laser is a ring fiber laser with a tunable FFP filter to select the output wavelength [14]. Erbium-doped fiber amplifier or semiconductor amplifier can be used as gain medium. When the light source scans a cycle in a certain wavelength range, each wavelength of light reflected from the FFP sensor is detected by a photodetector, which could be used to recover the spectrum of the sensor.

On the other hand, a semiconductor tunable laser is also used to obtain the full spectrum of an FFP sensor by scanning its output wavelength. Unlike the swept fiber ring laser, the output wavelength of a semiconductor tunable laser is shifted by changing its driving current to modulate the resonate wavelength of the laser cavity. The semiconductor laser could achieve much faster tuning speed and more stable laser output due to its shorter laser cavity [15].

Spectra interrogation system based on swept or tunable laser

Figure 5.10 Spectra interrogation system based on swept or tunable laser.

Table 5.1 Comparison of Different Spectral Interrogation Methods

METHOD

TYPICAL RESOLUTION

WAVELENGTH RANGE (NM)

REFERENCES

Diffraction grating + CCD

0.14 -7 nm

400-1100 or 900-2500

9

VSP + CCD

1 pm

1510-1590

10

FTS

0.02 nm

600-1700

12

TFPF

0.02-0.2 nm

1460-1620

14

Swept laser

1 pm

1510-1590

14

Tunable laser

1 pm to 0.1 nm

1250-1360

15

The advantages of the scheme based on swept or tunable laser are easy to realize multiple parallel spectral interrogations with high dynamic range and SNR.

Comparison of Different Spectral Interrogation Methods

Table 5.1 gives a comparison of different spectral interrogation methods. It can be seen that spectral interrogators based on VSP, swept laser, or tunable laser could offer superior performance.

 
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