Several high-performance and tunable erbium-doped fiber lasers are reviewed. They are constructed by using fiber Bragg gratings (FBGs) or short-wavelength-pass filters (SWPFs) as wavelength tunable components inside the laser cavity. Broadband wavelength tuning range including C- and/or S-band was achieved, and tunable laser output with high slope efficiency, high side-mode suppression ratio was obtained. These fiber lasers can find vast applications in lightwave transmission, optical test instrument, fiber-optic gyros, spectroscopy, material processing, biophotonic imaging, and fiber sensor technologies.
In recent years, fiber lasers have found a variety of applications in the testing of fiber components, fiber sensing and wavelength division multipling (WDM) systems, in which they are used to act as a backup source with ITU-T grids [
In this paper, we overview several works regarding tunable fiber lasers done by our groups. The first kind is the FBG-based linear-cavity tunable fiber laser using an optical circulator (OC) [
In principle, a wavelength shift in a FBG may be due to the changes in temperature, strain, pressure and/or other parameters. The shift in Bragg wavelength with strain and temperature can be expressed as [
where ε is the applied strain, Λ is the period of fiber, Pi,j are the Pockel’s (piezo) coefficients of the stress-optic tensor, and ν is the Poisson’s ratio. Note that n is the effective refractive index of the fiber core as defined in Equation (1), α is the thermal expansion coefficient of the silica fiber with a typical value of 0.015 nm/ºC, and ΔΤ is the temperature change in degree Celsius. The term (n2/2) [P12-ν(P11-P12)] has a numerical value of 0.22. The strain can be measured under a constant temperature according to the following equation:
where λB is the Bragg wavelength, and this value gives a “rule-of-thumb” measurement of wavelength shift for a FBG with strain of 1 nm per 1000 με at 1.31 μm. To design a strain tunable FBG, firstly, it is embedded in a strip of composite thermal plastic material and then is attached to L-shaped holders at both ends. The FBG is then mounted on a precision translational stage with a high-resolution micrometer. By strained or compressed tuning of the precise screw of the micrometer, we can apply both directions in the transverse displacement for increasing the tuning range up to ±8 nm. Two steel rods are attached to the sides of the FBG composite strip to confine the applied strain or stress to the longitudinal direction only. The micrometer has a resolution of 0.5 mm /turn and a full range of 5.0 mm in translational distance, therefore up to ten turns can be applied to tune the FBG reflection wavelength.
The lasing wavelength as a function of turns of screw is shown in
1552.7 nm. Fine tune resolution as precise as 0.2 nm FBG can be realized.
The S-band tunable erbium-doped fiber lasers were achieved by connecting the active fiber to the thermooptic tunable SWPFs. The mechanism of the proposed SWPFs [12,13] is to interact with the guiding optical fields to cause fundamental mode loss at long-wavelength, and the cutoff wavelength can be tuned when the heating temperature applying on the filter changes. The dispersion engineering methods had been employed by controlling the propagation losses of lights at different wavelengths. Both the side-polishing and the fused-tapering techniques were adopted in our previous works [12,13]. When the SWPFs are temperature-tuned to attenuate the wavelengths longer than 1530 nm, the C + L band ASE is suppressed and the S-band gain is obtained. The commonly used S-band erbium-doped optical fiber amplifiers (EDFAs) employ erbium-doped fiber (EDF) with depressed inner cladding to achieve fundamental-mode cutoff at the longer wavelengths [15,16]. The cutoff wavelength and the mode field diameter can be adjusted through bending and local heating, but the fabrication, insertion loss, crosstalk and cost of the filters using dispersive fibers show great difficulties for practical use. Thus, an alternative way to obtain SWPFs is to interact with the light through the evanescent field that is spread out of the waveguide with wavelength-dependent properties. When the optical fiber is side-polished or tapered, the mode field is expanded out of the fiber cladding. Using dispersive liquids surrounding the side-polished fiber/taper fiber, the device can be a SWPF if the dispersion relations are properly designed. The tunable SWPF was achieved by tuning the temperature of the dispersive liquids to change the dispersive curves for obtaining different cutoff wavelengths [17,18].
In
experimental results agree well with the simulation ones. The results include tuning efficiency of 50 nm/°C, cut- off efficiency of −1.2 dB/nm, and rejection efficiency of 55 dB. Based on these experimental and simulated results, the mechanism of the SWPF is proved to be qualified for developing wavelength tunable S-band erbium- doped fiber lasers.
The proposed configuration of linear cavity for the tunable laser is shown in
concentration EDF, a 1480/1550 nm WDM coupler, and one 1480 nm pumping source. The 3-port OC here acts as a wavelength router by connecting port 3 with port 1. In this way, the residual pumping power travels back to the EDF for twice amplification to increase its pumping efficiency up to 2 dB difference in laser output power. A piece of EDF is inserted into the cavity to act as pump absorber. At the right hand side of this cavity, there is one 1 × 2 optical switch (OSW) and two TFBGs (i.e., TFBG1, TFBG2) connected to the two switched ports of OSW. The tuning range could cover the whole C-band by switching between the two OSW ports connected to individual TFBG. The original reflected wavelengths of TFBGs are 1540.5 and 1552.68 nm, respectively. Two variable optical attenuators (VOAs) are used in each port for the power equalization.
The proposed BFM-based liner-cavity tunable fiber laser in a backward pump scheme is shown schematically in
6(b). The transfer efficiency versus pump power for different lengths of EDF is shown in
The transfer efficiency here is defined as
where η is the laser transfer efficiency, Ppin is the input pump power, and Ppth is the threshold power.
In this section, a continuously tunable erbium-doped fiber laser is demonstrated by incorporating a tunable SWPF into ring resonator. The wideband tunable SWPF is based on dispersive evanescent tunneling from a side- polished single-mode fiber and a dispersive optical polymer overlay structure. In fabrication, a portion of the fiber jacket was stripped off and the section was then embedded and glued into the curved V-groove on a silicon substrate, as shown in
To investigate the influences of the sharpness of the spectral cutoff curve, the Cargille liquids were applied on SWPF. The spectral responses of the fiber laser are shown in
In this subsection, wideband tunable high cutoff-effici- ency SWPFs were discretely located in standard silica- based C-band EDF to filter out the C + L band ASE so that the optical gain for S-band could be acquired to realize fiber laser. To investigate the amplification characteristics in the S-band, a 980-nm pump laser with 135- mW output power was launched into EDF in a forward pumping scheme. The high-cutoff-efficiency short-pass filters in the 17.5-m-long EDF could discretely suppress the unwanted C + L band ASE and pass the S-band signal
and 980-nm pump light. Subsequently, an input power of −25 dBm was launched into the EDF from distributed feedback laser signals in the S-band. The input signal spectra and amplified output signal spectra in the S-band at 28.6°C are shown in
The experimental set-up of the tunable EDF ring laser is shown in
dard silica-based C-band EDF to substantially suppress the ASE at the wavelengths longer than the lasing wavelength which can be tuned by varying the applied temperature on SWPFs. When a 980-nm laser with pump power of 208 mW launches into the EDF, the laser spectra at different temperatures are shown in
A versatile and cost-effective laser source should have the ability to allow the user to choose which wavelength is needed or the desired scanning range. The wavelength tunable FBG-based lasers we presented here can satisfy such requirement. It is well known that the cavity of a fiber laser may be designed based on a pair of FBGs that work as its end mirrors and determine the resonant wavelength. When one of the resonant wavelengths of the FBGs is changed slightly by tension or heating, the reflection power by the FBG pair at a new laser wavelength will decrease due to wavelength misalignment between them, thus it is difficult to fine-tune the FBG pair back to the same wavelength. Nevertheless, either the OC-based or BFM-based laser configuration could overcome such a problem because one FBG only is used to tune the lasing wavelength. Other advantages of FBG- based tunable fiber lasers are: 1) Narrow laser linewidth and near polarization-independent; 2) both the OC- based and BFM-based tunable fiber lasers improve the pumping efficiency by recycling the residual pump power back to the gain medium using backward pumping; 3) the TFBG could be used to tune the desired wavelength precisely and quickly; 4) the proposed FBG-based tunable fiber lasers may use one OSW pair and a plurality of tunable FBGs to expand the output wavelength range; 5) they are simpler and potentially less expensive than other commercial products; and 6) the sizes are compact and the weights are light.
It is advantageous to explore a widely tuning fiber laser with lasing wavelength down to S-band at a high tuning speed. Conventionally, the silica-based EDF at room temperature can only emit fluorescence at wavelengths longer than 1490 nm. Thus, achieving high-performance S-band lasers critically depends on the SWPFs. The side- polished SWPFs were adopted because they are mecha- nically strong, and the polishing depth and interaction length can be precisely determined. From a different point of view, SWPFs using the fused-tapering technique are easy, fast, and cost-effective fabrication processes. An optimized side-polishing/tapered fiber filter structure can attain high-cutoff efficiency and wide tuning range. Based on the proposed SWPFs, widely tunable, single- frequency rare-earth-doped fiber lasers can be achieved. Besides, the SWPF-based tunable fiber lasers have other advantages such as: 1) wide tuning range covering the S- and C-bands, 2) high power and low noise, 3) simplicity and cost-effectiveness, and 4) high index sensitivity up to 1 ´ 10−5 with high Q resonator.
To design a single-frequency tunable fiber laser, various kinds of methods such as multiple ring cavities, FBGs, microrings, spatial hole burning in unpumped EDF, and nonlinear loop mirror were proposed. Also, a short cavity length is usually required to enlarge the mode spacing. For the linear-cavity fiber lasers as mentioned, a simpler way to achieve single-longitudinal-mode (SLM) operation is to put a piece of EDF as pump absorber between the WDM coupler and 1 × 2 OSW for the OC-based linear-cavity tunable fiber laser as shown in
Two kinds of tunable fiber-filter-based EDF fiber lasers have been reviewed. Both of them have broadband wave- length tuning range including C- and/or S-band. Using FBG in strain mechanism, we have proposed and demonstrated a tunable FBG-based fiber laser that employs one OC, two homemade TFBGs. The configuration consists of a linear cavity to achieve a wavelength tuning range of 31.5 nm with 0.05 nm linewidth and over 60 dB SNR. The power variation over the entire tuning range is less than 0.1 dB with power equalization by using low-cost VOAs. Another way is to employ a BFM and tunable FBG at either cavity end of fiber cavity. The BFM acts as a broadband rear-end reflector both for lasing signal and pump source. For wavelength tunable demonstration, power variation over the whole C-band is less than ±1.0 dB without the usage of power equalization. The time to reach stable laser operation is less than 11 ms after switching between the two FBGs, and the continuous tuning resolution is less than 0.2 nm in the whole range. For the SWPF-based tunable fiber laser using temperature tuning mechanism, two tunable SPWFs based erbium-doped fiber lasers were reviewed. The side-polishing and fused-tapering techniques were used to achieve thermo-optic tunable short-wavelength pass function based on material dispersion discrepancy and variations of waveguide structures. The tuning efficiency is 50 nm/°C, cut-off efficiency is −1.2 dB/nm, and rejection efficiency is 55 dB, individually. The widely tunable SWPFs were applied to achieve broadband and high-tuning-efficiency S- and/or C-band EDF ring lasers, which can be tuned close to the short-wavelength edge of gain bandwidth, and the tuning range is 26 nm with the signal-to-ASE-ratio of around 40 dB, and the FWHM linewidth is about 0.5 nm. All of them have graceful features of simple structure, compactness, ease of connection to fiber components, high-efficiency, and continuous tunability. They are promising for vast applications in lightwave transmission, optical test instrument, fiber-optic gyros, spectroscopy, material processing, fiber sensing, WDM backup light sources, as well as in bio- photonics.
The authors were partially supported by the National Science Council (NSC) (Project Nos. NSC 98−2221−E- 011-017, NSC 97-2923−E-011-001-MY3, NSC 98−2218 −E−008-004, NSC 98-2221-E-239-001-MY2). We thank Jang W. Y., Wang C. J., Hung K. L., Jhong G. S., Chi S., Tseng S. M., Huang C. M., Lai Y. for discussion, T. Wang and Z. G. Shieh for kind help.
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