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We report a lateral-drilled DBR fiber laser which contains a defective parabola-like opening inside the cavity fabricated by the CO2-laser exposure and study the laser responsivity to external refractive index (RI). Surrounding materials can readily reach the vicinity of the fiber core via the opening and interact with the laser mode. Research shows that the laser emission power mainly relies on changes of external RI while the lasing wavelength on temperature. The effects of structural parameters, pump power, and external refractive index on the RI responsivity of the device are demonstrated. The lasing threshold condition is also concerned. This work provides an opportunity for controlling emission characteristics of the DBR fiber laser through modification of external RI value, of which the results are valuable for the potential applications in optical sensing, tunable lasing, and etc.
© 2016 Optical Society of America
1. Introduction
Distributed Bragg reflector (DBR) fiber lasers, which consist of a section of active fiber sandwiched between two highly reflective in-fiber Bragg gratings, have been extensively studied in the areas of wavelength-division multiplexing communication, optical microwave signal generation, optical sensing, and etc, due to its advantages such as compactness, single-longitudinal-mode operation, and multiplexing capability [1–10]. A stringent lasing condition, including high concentration and gain of the active optical fiber, high reflection of the fiber Bragg gratings, and low transmission loss of the cavity, is in general required in contrast to those long-cavity counterparts [11]. Excellent efforts have previously been made to develop high performance DBR fiber laser structures that are sensitive to transverse pressure [6], torsion [7], magnetic field [8], and high-frequency ultrasound [9,10]. The structure can exhibit the advantages including high power incidence, narrowed bandwidth, large sensitivity, and etc [12,13], in comparison with those passive counterparts such as fiber Bragg gratings [14]. However, almost all of the existing configurations are based on utilization of a “complete” DBR structure without defect in cavity [1–10]. The excited laser modes are confined in the fiber core and thus are normally insensitive to external refractive index (RI) variation. Recently, A. C. L. Wong et al. have realized the RI sensing by writing a blazed Bragg grating inside the DBR fiber laser cavity [5]. The optical loss varies due to the spectral shift of the grating with the surrounding RI, resulting in the variation of output power at the lasing wavelength. The measured RI sensitivity is ~39.4 dB/RI-unit. However, the difficulty is the alignment of the resonance wavelength of the grating to the operating wavelength of the laser.
In this paper, we present a lateral-drilled DBR fiber laser which contains a parabola-like opening at the scale of tens of micrometer inside the cavity, fabricated by use of the CO2-laser exposure technique, and study the responsivity to external RI. The effects of physical parameters on the structure performance are studied. The lasing threshold condition and also the temperature influence are concerned. The proposed device is thought to be valuable for the potential applications in tunable lasing, optical sensing, and etc.
2. Configuration and principle
Figure 1 shows the schematic of the proposed lateral-drilled DBR fiber laser, which consists of a section of active fiber acting as the plus medium and two highly-reflective fiber Bragg gratings (FBGs) acting as reflectors. We use an erbium-ytterbium co-doped fiber (Coractive, EY305), which contains the core diameter of ~7μm, the outer cladding diameter of 125μm, and the peak absorption of ~22dB/m@1535nm. Two FBGs are fabricated in the active fiber by use of the 193nm ArF excimer laser exposure technique and a phase mask with a period of 1057.22 nm. High reflection of FBG is achieved due to the high writing efficiency associated with two-photon excitation at wavelength 193 nm. In our experiment, one FBG is L1 = 3.5 mm long with reflectivity of about 99.8%, acting as an output coupling mirror, and the other FBG is L2 = 4.0 mm long with reflectivity of more than 99.9%, acting as a complete reflecting mirror. And the distance between the two FBGs is ~5.0mm. A 980-nm pump light source is used to generate the laser emission around wavelength of 1550nm. This present configuration can produce a stable single-longitudinal-mode lasing operation without mode hopping in contrast to those long-cavity counterparts [11].
Fig. 1 The proposed DBR fiber laser configuration. A parabola-like opening is formed inside the cavity by CO2 laser exposure.
We fabricate the opening inside cavity of the DBR fiber laser through the CO2-laser exposure technique. A pulsed CO2 laser, SYNRAD 48-5, with the maximum output of 50W, is used. When the CO2 laser beam is perpendicularly irradiated onto the surface of the fiber through a ZnSe lens with diameter of ~50 μm, a dramatic temperature promotion occurs due to the photon absorption effect, resulting in the sputtering of fiber material and the formation of structural opening [15,16]. Owing to the thermal effect of the CO2 laser, the opening can have a relatively smooth surface and thus a low light transmission loss. The decreasing in the CO2 laser energy at the edge of the laser beam can induce a gradually-varied parabola-like opening, as shown in Fig. 1. A typical transmission loss of the opening is measured to be 0.5dB, which may be attributed to light diffraction with large index contrast between the fiber and the opening. This opening is characterized by width W and depth h and can be optimized by modifying the irradiation time and the scan range of the CO2 laser beam in practice. The fabrication procedure is monitored using a CCD camera. In our experiment, a series of samples with W = 100μm~150μm and h = 40μm~60μm are fabricated and studied. As shown in Fig. 2(a) is a microphotograph of the fabricated opening in the laser cavity. Figure 2(b) indicates typical emission spectra of the DBR fiber laser before and after CO2 laser machining, with W = 136.5μm and h = 59.3μm, recorded by an optical spectrum analyzer with a wavelength resolution of 0.02nm. The presence of the opening induces a dramatic drop of laser power (P) from −19.301dBm to −31.453dBm with a total change of 12.152dB and a blueshift of lasing wavelength from 1533.961nm to 1533.834nm with a shift of ~127pm only.
Fig. 2 (a) Microphotograph of the fabricated opening in the cavity of DBR fiber laser. The opening width and depth are in general given by W = 100μm~150μm and h = 40μm~60μm, respectively. (b) Typical emission spectra of the DBR fiber laser before and after CO2 laser machining, with parameters W = 136.5μm and h = 59.3μm, respectively.
3. Response to external RI
3.1 Experiment
Owing to the existence of the defective opening inside laser cavity, the surrounding material can readily reach the vicinity of the fiber core and have strong interaction with the laser mode. The opening acts as a variable attenuator that varies the optical loss upon changes of external RI. We measure responsivity of the laser to external RI by immersing the structure into an aqueous solution of sucrose, with the value of RI modified by tuning the sucrose concentration in the room temperature (~26°C). Figure 3(a) plots the emission spectra of the laser at different RIs and Fig. 3(b) details the laser output power and the lasing wavelength as a function of external RI, with W = 139.7μm, h = 58.9 μm, and pump power of Ppump = 26mw. With an increase of RI from 1.333 to 1.450, the peak power rises gradually from −27.327dBm to −20.036dBm in a good linear relationship, producing a total change of ~7.291dB. Linear fitting to the experimental data produces a responsivity of Sp = 61.9 dB/RI-unit. Such high responsivity enables a possibility for the structure to be used in optical sensing and tunable lasing. The variation of external RI may also results in the fluctuation of lasing wavelength. From Fig. 3, the whole lasing wavelength variation is ~35.0pm, which is quite small and could be omitted in many situations for the power-dependent sensing or laser tuning.
Fig. 3 (a) Laser emission spectra at different RIs, with W = 139.7μm, h = 58.9 μm, and Ppump = 26mw. Inset shows the enlarged image of the spectra near the laser power peak. (b) Laser emission power and lasing wavelength as a function of external RI.
We measure the stability of the output power for the lateral-drilled DBR fiber laser inside an aqueous liquid at the room temperature (~26°C) as similarity to Fig. 3 for over two hours and obtain a power variation range of ~0.29dB around −27.0dBm. The standard deviation is 0.09dB and the power fluctuation is 1.07%. Such stability performance is close to that of a typical DBR fiber laser [4]. Considering the power variation range of ~0.29dB and the RI responsivity of 61.9 dB/RI-unit as shown in Fig. 3, for the RI sensing, the measurement precision is estimated to be ~4.68 × 10−3 RI-units. It should be noted that bending or vibration to the laser may introduce additional disturbance to the output power. Thereby the fiber device should be kept straight and stable in practice. Besides, better room temperature control would also help to improve the output power stability [4].
3.2 Influence of structural parameters and pump power
To understand the influence of geometrical parameters of the opening, Table 1 gives the measured RI responsivities with respect to different opening sizes at Ppump = 26mw. When the depth is small sufficiently or the opening is far from the fiber core, e.g. W = 140.6μm and h = 40.2μm in Table 1, the variation of external RI only produce unobvious influence on the laser mode. Otherwise, any change of width W or depth h may alter the RI responsivity. Investigation shows that the depth can dominate compared to the width of the opening. Figure 4 plots the variation RI responsivity with pump power, where W = 139.7μm and h = 58.9 μm. It is shown that the RI responsivity drops gradually with an increase of pump power in a nonlinear relationship. To understand the influence of pump power, we note that the steady-state laser power satisfies a general relation: P ∝ (γ0/α−1) [17], where γ0 denotes the small-signal gain coefficient and is determined by the pump power, and α denotes the loss coefficient with γ0>α. If assuming that the RI variation only has impact on α to simplify the problem, we can obtain d[10⋅lg(P)]/dα ∝ (−1/α)[1/(1−α/γ0)], which indicates that the lower gain γ0 (or the smaller pump power) generates a larger responsivity of laser power in dB to the loss (or RI) variation, consistent to the observation shown in Fig. 4. From Fig. 4, the RI responsivity tends to equal ~57.7dB/RI-unit when the pump power approaches 50mW.
Table 1. Measured RI responsivities with respect to different opening sizes at Ppump = 26mw.
3.3 Lasing threshold condition
From above discussion, the lower pump power generates the higher RI responsivity. However, too low pump power may induce the ceasing of laser emission. Figure 5(a) plots the measured laser emission power function of the pump power Ppump at different external RIs near the threshold point. From Fig. 5(a), the lasing threshold pump power occurs at Ppumpth = 9.05mw for RI = 1.33 and Ppumpth = 7.41mw for RI = 1.36, respectively. The laser emission power increases gradually once the pump power exceeds the threshold point. From Fig. 5(a), comparison shows that the larger external RI can generate the higher laser emission power, consistent to the observation shown in Fig. 3, and the smaller lasing threshold pump power. Figure 5(b) plots the laser emission power function of external RI at different pump powers. The threshold RI emerges at RIth = 1.389 for Ppump = 4mw and RIth = 1.368 for Ppump = 6mw, respectively. When the external RI is beyond the threshold, the laser emission power increases monotonously with the RI value in a good linear relationship. The lower pump power can generate the smaller lasing power, as seen in Fig. 5(b), but the higher threshold index. From Fig. 5(b), the measured maximum RI responsivity is 92.8dB/RI-unit, occurring at Ppump = 4mw. Utilizing the experimental data, the threshold pump power Ppumpth can be expressed with external RI, as Ppumpth = −85.10*RI + 122.32. The threshold pump power decreases gradually with a rate of −85.10mw/RI-unit with the increasing of external RI.
Fig. 5 (a) Laser emission power function of pump power at different external RIs. (b) Laser emission power function of external RI at different pump powers.
4. Temperature influence
We investigate the temperature influence by placing the structure into a resistance furnace in air. Figure 6 records (a) laser emission spectra at different temperatures and (b) the lasing wavelength and the laser output power as a function of temperature. With an increase of temperature from 30°C to 100 °C, the lasing wavelength redshifts continuously from 1533.98nm to 1534.79nm with a total shift of ~813pm. Linear fitting to the experiment data produces a coefficient of 11.79 pm/°C, which is analogous to that of a Bragg grating [14] due to their comparable thermo-expansion and thermo-optic coefficients in the fiber structure. Simultaneously, the laser emission power fluctuates about a mean value of −27.8dBm with a fluctuation of ~0.46dB as shown in Fig. 6(b). Only the lasing wavelength varies with temperature whereas the power closely relies on external RI as previously described. The temperature should be kept in a stable situation for the structure to be used in the power-dependent RI sensing or tunable lasing.
Fig. 6 (a) Laser emission spectra of the laser at different temperatures with Ppump = 26mw. (b) Variations of the lasing wavelength and the output power with temperature varying from 30°C to 100 °C.
5. Conclusion
In conclusion, we present a lateral-drilled DBR fiber laser that contains an active fiber sandwiched between two fiber Bragg gratings and a defective parabola-like opening inside the cavity machined by the CO2-laser exposure technique. The external material can easily reach the vicinity of fiber core via the opening and interact with laser mode, making the structure sensitive to changes of external RI. Study shows that the laser emission power mainly relies on the surrounding RI and the lasing wavelength on temperature. A good linearity of the RI-dependent laser power with the maximal sensitivity of ~92.8 dB/RI-unit is experimentally obtained. The influences of structural parameters, pump power, and refractive index on RI responsivity are investigated. The lasing threshold condition is also discussed. This research provides a novel approach for controlling emission characteristics of the DBR fiber laser through modification of external RI value, which is valuable for the potential applications in optical sensing, tunable lasing, and etc.
Acknowledgments
This work is supported by the National Science Fund for Distinguished Young Scholars of China (61225023), the National Natural Science Foundation of China (61235005 and 61575083), and the Guangdong Natural Science Foundation (S2013030013302 and 2014A030313364).
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