1. Introduction

A fiber laser -based fiber-optic sensing system has the advantages of a high signal-to-noise ratio (SNR) and wavelength encoding of the measurands [1]. Erbium (Er) doped fiber is most commonly used in such fiber laser systems for the sensing field, due to the wide availability of optical components operating at those wavelengths, as a result of the rapid developments in the optical communications industry. The line-width of the laser output can be very narrow and the output power of the laser is stable if the laser can be made to oscillate in a single mode and this is particularly useful for a fiber laser-based sensing system, allowing it to be highly sensitive to small perturbations of measurands [2].

To assure a stable, single mode operation of the fiber laser, a ring cavity configuration is often used [3]. As the laser field in a ring cavity configuration acts as a traveling wave, it is comparatively easy to keep it working in single mode. However, in the ring configuration, a circulator or an isolator is usually required to achieve only one direction of oscillation of the laser, thus making the structure comparatively complicated. It is thus more difficult to incorporate such a complex laser system directly into a sensor application, especially when high spatial resolution is required for the sensor system. For a long-haul fiber laser system with a ring cavity structure (in which a fiber Bragg grating (FBG) is spliced to a long piece of normal communication silica fiber and connected to the ring cavity through a circulator) the FBG works as the cavity mirror of the fiber laser and also as the sensing element [4]. Though the gain fiber and other laser cavity components are all far from the sensing position in such a configuration, it is still difficult to eliminate the effect of different kinds of interference on the long transmission fiber. Any resulting change of the conditions along this length of fiber may result in changes to the fiber laser cavity conditions and thus limit the achievable sensing resolution. (For example, the temperature change of some parts of the transmission fiber may result in change of the cavity length and any mechanical oscillations to the fiber may have a similar effect). In addition to this, it is difficult to configure a multi-wavelength oscillation laser system, which is often valuable in a multi-point sensor configuration, using the ring-cavity structure, except when additional polarization controlling components are used [5].

Besides the choice of a ring-cavity structure, a linear-cavity structure is also possible for single mode fiber laser operation [6]. A single mode fiber laser with high power output has been developed in recent years by using a highly (Er) doped phosphate glass fiber as the gain medium [7]. As the cavity length of the fiber laser is significantly limited by the requirement for single mode operation, use of a rare-earth ion doped fiber, with high unit length gain, is necessary. As reported, the use of phosphate glass fiber with a high (Er) doping concentration can offer high unit length gain and has shown superior performance. However, due to the limitation of the phosphate glass itself (with poor resistance to high temperature and corrosion), it is impossible to use such a fiber laser directly in a hostile environment for sensing application. For in-situ sensing applications, a silicate-based fiber laser is clearly advantageous and to be preferred. It is thus important to develop a novel Er doped silica fiber with excellent fluorescence gain characteristics to meet the requirement of single mode fiber laser operation under a linear cavity configuration.

In this letter, a newly developed Er doped silicate fiber with excellent gain characteristics is reported, which was then used in a short length (2 cm long of gain fiber) linear cavity fiber laser with single mode operation. Two FBGs written into a Bi-Ge codoped photosensitive fiber, which are specially designed to be high temperature sustainable [8][9], were used as the laser mirrors. This compact linear cavity fiber laser can oscillate stably when pumped by a laser diode (LD) working at 976 nm. By putting the fiber laser into a tube-oven, tests were carried out to evaluate the laser (and thus the potential sensor) performance over a wide temperature range from room temperature to 400 °C. It has been confirmed that the laser system can maintain single mode oscillation over the whole temperature range discussed and thus is especially suitable for high resolution sensing applications.

2. Development of the highly Er doped silica fiber

The high Er-dopant silica fiber used for the laser described was fabricated by means of the modified chemical vapor deposition (MCVD) technique. The details of fiber fabrication were similar to those formerly reported [10]. The fiber has a heavily doped Er concentration (about 8000 ppm by weight of Er₂O₃) together with co-dopants of Al and Ge. This concentration was much higher than that in most commercially available Er doped silicate fibers and thus the issue of ion clustering would be a severe problem if no special measures were taken. To ease the clustering problem, the solvent composition used for soaking the silica tube during the process of preform fabrication was changed. Some parts of bismuth oxide were also added to the solvent (hydrochloric acid and de-ionized water) together with erbium oxide and aluminum oxide before the silica tube, on which was deposited a loose layer of SiO₂—GeO₂ was put into the solvent for soaking. It was noted that by a careful control of the solvent acidity and viscosity, the Er ion clustering problem could be mitigated and better fluorescence performance obtained. Analysis of the composition of the preform indicated that there was almost no bismuth oxide left in the glass after the preform was finally fabricated as it would appear most of it would have evaporated in the subsequent process of preform fabrication.

The absorption spectrum of the fiber was measured and is shown in Fig. 1. It is clear that the absorption coefficient at 976 nm and 1480 nm is greater than 40dB/m while at 1530 nm, it is greater than 100 dB/m. The absorption band around 1380 nm, resulting from the presence of the OH⁻ ion in the silicate fiber, was also controlled to remain low to allow for a high fluorescence quantum efficiency. The fluorescence decay lifetime was measured to be 7.8 ms at room temperature, through excitation by light from a modulated LD working at 976 nm. This shortened Er lifetime compares with that seen for typical commercial Er doped fiber, in which the lifetime is normally 11~ 12 ms [11], and likely indicates that Er ion clustering may already be happening and that will result in a comparatively lower quantum efficiency. However, when compared with the situation of heavily Er doped phosphate glass fiber (in which several per cent Er dopant was often included [7]) it is reasonable to assume that the strong absorption of the developed silica fiber at the specific pumping wavelength used may result in a high unit length gain if the fluorescence efficiency is high enough. This makes the fiber especially suitable for short cavity fiber laser operation, the details of which are discussed in the next section.

 

Fig. 1. Absorption spectrum of the silicate fiber developed with a high Er³⁺ concentration

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3. Short cavity single frequency fiber laser for in-situ sensing applications

A short piece (of 2 cm length) of the Er-doped fiber developed was spliced to a pair of wavelength-matched FBGs, one at either end. The FBGs were written into a specially developed Bi-Ge co-doped photosensitive fiber that had excellent high temperature sustainability [9] with a length of 6.5 mm and a spectral full width at half maximum (FWHM) of less than 0.1 nm. As the grating (reflectivity) will decay to some extent at high temperature, a high initial reflectivity of greater than 99.5% at room temperature was chosen. The writing conditions of the excimer laser operating at 248 nm for the FBG fabrication were 10mJ per pulse (which corresponds to the power fluence of 180 mJ/cm²) at a repetition rate of 200 Hz, for 5 minutes. Annealing at 400 °C for hours will result in a high residual reflectivity, of about 92% [9] and this reflectivity is believed to be sufficiently high for the fiber laser oscillations to occur. It was pumped by light from a LD working at 976 nm through a WDM configuration (see Fig. 2) and the output was monitored by using an Optical Spectrum Analyzer (OSA). It is clear that the fiber laser oscillated stably on a single mode and the power output of the laser was measured to be in the tens of microwatt region when pumped at 60 mW input power The linewidth of the laser output was measured to be narrower than 1 kHz (see Fig. 3) by use of a heterodyne interference approach [12]. This narrow linewidth, indicating a stable single mode oscillation of the fiber laser, is extremely important for high sensitivity and high resolution performance of a sensing system relying upon accurate measurement of the laser wavelength.

 

Fig. 2. Experimental setup of the linear cavity fiber laser, in which the pumping laser at 976 nm and the fiber laser output at 1550 nm band are configured to be in one direction via a 980/1550 WDM

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Fig. 3. The fiber laser output recorded by using a heterodyne interference approach, showing its narrow linewidth of less than 1 kHz

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The choice of a short piece of Er doped fiber is important in maintaining single mode oscillation over the whole temperature range, from room temperature to 400 °C. The choice of such a short cavity arose from the need to eliminate the remaining gain along the Er fiber which has resulted from the spatial hole-burning effect of the standing wave mode operation in the linear cavity configuration. When a longer fiber length (3 or 4 cm) was used, a higher power output (at the level of hundreds of microwatt was realized, but a self-pulsing phenomenon was observed, especially at low pumping levels. This was consistent with the results published one decade ago [13] [14]. Indications of a mode hopping phenomenon (normally changing between single mode and two modes) could also be observed (from the data recorded on the OSA) if no stable temperature control was applied to the fiber laser and this would inevitably affect the performance of any fiber laser based sensor system.

The fiber laser section used (including the Er fiber and the FBGs) was put into a silica tube and the tube was then placed in the center of a tube oven for temperature evaluation tests. The fiber laser output was also monitored using an OSA when the oven was heated up to a maximum temperature of 420 °C. It was found that the laser output kept stably to a single mode over the whole temperature range and no mode hopping phenomenon was observed. After annealing at 420 °C for twenty minutes and then at 400 °C for two hours, the oven was allowed gradually to cool to room temperature and then heated up again to 400 °C for several such temperature cycles. The laser output spectra at several specific temperatures were recorded using the OSA and these are shown together in Fig. 4. The tuning characteristics of the laser output wavelength with temperature are illustrated in Fig. 5. From Fig. 4, it can be seen that the output power of the fiber laser varied between -15 dBm and -20 dBm at different temperatures but always was sufficiently intense to be measured accurately. From Fig. 5, it can be seen that after the first round of temperature cycling, the tuning characteristics of the output wavelength changed a little in the second cycle. This is believed to be the result of FBG reflectivity decay after the annealing. The decay of the FBG reflectivity at high temperatures is always accompanied by a shift of the peak reflectance wavelength. Further annealing at 400 °C (or even higher temperatures) for longer times will result in a more stable performance of the laser output as the FBGs themselves will become stabilized. The relationship of the laser tuning characteristics observed seemed similar to that between the peak reflectivity of FBG and the temperature [8].

 

Fig. 4. Fiber laser output spectra when cycling from room temperature to 400 °C, showing the gradual wavelength change of the laser output and their strong signal when compared to background (30 dB above background)

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4. Discussion

As presented above, the fiber laser developed using the specialist fiber described can be operated in single mode over the whole temperature range from room temperature to 400 °C, monitored by using an OSA. As the mode spacing in such a fiber laser is as large as 0.04 nm, it is easy to clarify the mode numbers of the laser oscillation when the resolution of the OSA is as high as 0.01 nm (Ando Model AQ6317C). It was demonstrated that the laser output can be tuned smoothly with temperature. To achieve this, both the Er doped fiber and the high temperature sustainable FBGs play important roles. The newly developed Er doped fiber with high doping concentration and high fluorescence efficiency is essential to the short cavity laser operation. It would be very difficult to be assured of single mode oscillation over the whole temperature range if the fiber laser cavity was not short enough.

 

Fig. 5. Tuning characteristics of the fiber laser output when experiencing heating and cooling from room temperature to 400 °C for the first two rounds

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Besides, the use of a pair of FBGs written into the same fiber material with essentially the same parameters of peak reflectance wavelength and similar (small) FWHM is also very important to enable wide temperature tuning to occur. In test carried out, it was much more difficult to keep the fiber laser in single mode oscillation if a chirped grating instead of normal FBG was used as the cavity mirror [1]. The high temperature sustainability of the gratings enables the FBGs to retain a high reflectivity after high temperature annealing and still meet the requirements of laser oscillation. It is noted from Fig. 4 that the power output did not drop much as the temperature increased, though the power output showed oscillation, to some extent, with temperature. It seems that the fluorescence efficiency of the Er doped fiber changed little over the whole temperature range. The oscillation of the laser power output may possibly result from the mismatch between the accurate frequency position of the laser mode and the peak reflection wavelength of the FBGs due to the gradual shifting of the mode position with temperature. This mismatch may result in a different reflectivity of the cavity mirrors and as a result, the difference in power output.

As the fibers used in this fiber laser system, including the Er doped gain fiber and the photosensitive fiber for the FBG fabrication, were all silica-based, the fiber laser system is inherently stable and resistant to the harsh environment, such as high temperature and corrosion effects, and is thus suitable for practical sensing applications. The WDM-based pumping configuration (see Fig. 2) enables a series of fiber lasers with similar structure but working at different wavelengths and installed at discrete positions to be easily spliced together along a fiber network. Only one LD with a moderate power output is required to be the pump source for all these fiber lasers because every fiber laser requires a small percentage of the total output power available for pumping. For such a quasi-distributed fiber-optic sensing system, both the narrow line-width and high resolution in the determination of the measurand can be readily realized with the help of a high resolution OSA. Though the laser spectra shown in Fig. 4 exhibits a 3dB bandwidth of nearly 0.1 nm, which mainly resulted from the relatively poor resolution of OSA (0.06 nm, HP86140 from Agilent), the actual linewidth of the laser output is much narrower (the 1 kHz linewidth corresponds to about 10^-8 nm). Currently, commercially available tunable fiber filters based on the fiber Fabry-Perot cavity can realize a spectral resolution of 1 pm [15] or better, and this would help the fiber laser-based sensing system to demonstrate the potential to measure at a series of monitoring points simultaneously and maintain a high temperature resolution of better than 0.1°C.

Though the experimental results presented above are limited to a maximum temperature of 400 °C, it is possible for the laser system to operate at higher temperatures by further optimization of the laser cavity parameters. Obviously, the fiber laser based sensing system presented in this letter may also be used to sense strain or other parameters through a suitable configuration of the fiber laser. It is, of course, important to distinguish the effect of temperature and strain separately and thus to enable the simultaneous measurement of these parameters. For a single point fiber laser based sensing system, use of the combined techniques of wavelength detection and the fluorescence intensity ratio [16] detection would be a feasible approach to achieving this, as has been demonstrated for fluorescence-based sensing.