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Lasers & Sources

Compact, short lasers perform well in sensing applications

Ultra-short fiber-grating lasers less than 1cm long have proven robust for sensing uses.
27 April 2010, SPIE Newsroom. DOI: 10.1117/2.1201004.002694

Short-fiber grating lasers are attractive for several different applications because of their high optical signal-to-noise ratio, reliability, low noise, and compact size. We have demonstrated two laser configurations: a distributed feedback (DFB) structure consisting of a single Bragg grating written in active fiber with a phase shift incorporated into the grating structure, and a distributed Bragg reflector (DBR) structure consisting of two wavelength-matched Bragg gratings at both ends of a short section of active fiber. The length of DFB and DBR fiber lasers are typically several centimeters, but certain applications require shorter lasers.

Many efforts to develop short-fiber grating lasers have been reported. These include splicing two fiber Bragg gratings to a short piece of active fiber or directly writing fiber Bragg gratings in an active fiber treated with hydrogen for strengthening. Unfortunately, fusion splicing introduces intercavity loss, and hydrogen treatment degrades laser efficiency. A longer active fiber is therefore necessary to provide more optical gain to overcome the laser cavity losses. We fabricated ultra-short fiber grating lasers with a total length of ~8mm by directly writing fiber Bragg gratings in the active fibers without hydrogen treatment.

The gratings were inscribed in the active fiber using a 193nm excimer laser. Because it induces index grating in the fiber core by a two-photon excitation process, it does not require hydrogenation to photosensitize the fiber. This not only avoids laser efficiency degradation, but also simplifies fabrication. We have made ultra-short DBR fiber lasers with a total length of less than 1cm in both erbium (Er)/ytterbium (Yb) co-doped fibers and Er-doped fibers.

In our experimental setup for writing an ultra-short fiber grating laser (see Figure 1), we used phase-mask and beam-scanning techniques. We first wrote the high reflectivity grating, then the low reflectivity one. We monitored the laser output during laser fabrication and stopped the UV inscription when the laser output power reached the maximal value. Figure 2 shows the photograph of the ultra-short fiber grating laser fabricated in Er/Yb co-doped fiber. The green section is the Er/Yb co-doped fiber, which emits green upconversion fluorescence. Two gratings with lengths of 4.6mm and 2.8mm are written inside the doped fiber. The grating spacing is 1mm. The entire laser is, therefore, only 8.4mm long.

Figure 1. Experimental setup for the distributed Bragg reflector (DBR) fiber laser inscription. BBS: Broad band source. WDM: Wavelength division multiplexer. ISO: Optical isolator. OSA: Optical spectrum analyzer.

Figure 2. Photograph of the ultra-short DBR fiber laser. The green section is the Er/Yb co-doped fiber. Two Bragg gratings were written inside the doped fiber.

The ultra-short cavity length ensures a robust single mode operation of the laser. However, a major challenge that limits practical applications of DBR fiber lasers is the issue of mode hopping. Typical DBR fiber lasers are several centimeters long, so the longitude mode spacing is much smaller than the grating reflection bandwidth. As a result, there are multiple modes that satisfy the lasing conditions. The dominant mode oscillates, and other modes are suppressed. As a result, the lasers normally operate in single longitude mode. However, these lasers are susceptible to mode hopping when subjected to external perturbations. In our ultra-short DBR laser, the longitude mode spacing is comparable to the grating bandwidth, so the cavity supports only one longitude mode.1 This obviates the possibility of mode hopping when the laser is subjected to external perturbations.

Our laser operates around 1539.5nm with signal-to-noise ratio better than 70dB, as seen in Figure 3. It has slope efficiency of ~0.86% and pump threshold of less than 1mW. It emits two orthogonal polarization modes, which generate a beat signal in the radio frequency range with signal-to-noise ratio of ~70dB, and 3dB line width of ~3kHz (see Figure 4). These lasers are well suited for use in polarimetric sensors,2,3 which translate the change in the laser fiber's birefringence (in response to external perturbations) into a beat frequency change of the two polarization modes. The advantages of this type of sensor include ease of signal extraction, absolute frequency encoding, and capability to multiplex a number of sensors on a single fiber by using frequency-division multiplexing.

Figure 3. Output spectrum of the ultra-short DBR fiber laser in Er/Yb co-doped fiber.

Figure 4. Polarization-mode beat signal spectrum of the ultra-short DBR fiber laser in Er/Yb co-doped fiber. The inset shows the enlarged view of the beat signal.

The ultra-short cavity length limits the laser output power by lower pump absorption. The use of Er/Yb co-doped fiber increases the laser efficiency because the Yb ions exhibit strong absorption at 980nm and transfer their energy to the Er ions with high efficiency. However, it is a drawback for applications requiring multiplexed sensors, because the lasers will rapidly deplete the pump light. For sensor multiplexing, it is desirable to fabricate lasers with active fibers without Yb co-doping. We have also fabricated an ultra-short DBR laser with a total length of 8mm in Er-doped fiber. Including the splicing loss with a standard single mode fiber, an ultra-short DBR laser in Er-doped fiber induces transmission loss of only 0.9dB at 980nm. This means that one pump laser could excite many ultra-short DBR lasers connected in series. Our next step will be to multiplex ultra-short DBR lasers with a different polarization-mode beat-frequency on a single fiber.

This work was supported by the Key Project of National Natural Science Foundation of China (60736039), the Program for New Century Excellent Talents in University (NCET-06-0271), and the Research Fund for the Doctoral Program of Higher Education (20070141041).

Bai-Ou Guan
Institute of Photonics Technology
Jinan University
Guangzhou, China

Bai-Ou Guan is professor and director of the institute. His current research interests include photonic crystal fiber devices, fiber lasers, fiber-optic sensors, microwave photonics, and fiber-optic components for telecommunication and sensing systems.

Yang Zhang
Dalian University of Technology
Dalian, China
Hwa-Yaw Tam
The Hong Kong Polytechnic University
Hong Kong, China

Hwa-Yaw Tam is chair professor of photonics and director of the Photonic Research Center. His current work includes fabrication of specialty glass and polymer fibers, fiber gratings, fiber amplifiers, optical fiber communication, and fiber sensor systems.