We present a novel method to discretely tune the emission wavelength of pulsed fiber-integrated lasers. As spectral filter, a step-chirped fiber Bragg grating (FBG) array is employed combining a monolithic structure with an unrivaled design freedom enabling large tuning bandwidths as well as tailored spectral characteristics towards fingerprint tuning features. Together with an electrical control mechanism ensuring programmable operation, this tuning method promotes fiber-integrated lasers to access new fields of applications e.g. in biophotonics and distributed sensing. The potential of this tuning concept is investigated based on an Ytterbium-doped fiber laser. The system shows superb emission properties including excellent wavelength stability, high spectral signal contrast (up to 50dB) and narrow linewidth (15GHz) as well as adjustable pulse durations in the nanosecond range with peak powers up to 100W. Additionally, the unique spectral potential of this method is demonstrated by realizing filter designs enabling e.g. a record tuning range of 74nm for fiber-integrated lasers.
© 2015 Optical Society of America
Tunable lasers address a wide range of applications which, besides the fields of telecommunications  and material processing , particularly includes the rapidly growing branch of biophotonics and spectroscopy [3–6]. In addition to absorption and transmission measurements, novel analysis methods such as Raman and CARS spectroscopy or LIDAR  show an increasing demand for bright tunable light sources. Common solutions based on solid-state lasers (e.g. Titanium-Sapphire lasers) are costly and rely on complicated and sensitive setups. A promising alternative is provided by fiber lasers. They not only offer numerous attractive properties including excellent beam quality and high efficiency , but for all-fiber based systems, these advantages are combined with the prospect for compact and robust setups as well as low maintenance requirements paving the way for a fast technology transfer to industrial and clinical applications. Furthermore, based on the properties of rare-earth doped fibers showing wide absorption and emission cross sections , fiber lasers provide the perfect basis to realize wavelength tunable light sources adjustable over broad spectral regions which may cover even hundreds of nanometers. However, in order to benefit from these characteristics, a suitable tuning mechanism is required.
In general, wavelength tunability is realized by implementing an adjustable spectral filter into the laser cavity. Even though, common approaches based on free-space coupled diffraction gratings , prisms , multiple-prism grating combinations  or Fabry-Pérot etalons  offer good spectral flexibility for large tuning ranges, they are not fiber integrated and thus eliminate most benefits that arise from monolithic geometries. On the other hand, fiber Bragg gratings (FBGs) as narrow band reflectors inscribed in the core of the fiber enable fiber-integrated filter designs . However, the spectral response of an FBG can only be tuned indirectly via strain  or temperature . Due to the induced mechanical stress, this approach challenges the destruction limit of the waveguide restricting the accessible tuning range usually to a few nanometers for standard fibers. Still, for fiber lasers, a tuning range of up to 45nm in the Ytterbium (Yb)-band has been reported using an elaborate FBG compression configuration .
A solution to overcome the bandwidth limit of this fiber-integrated filter is given by chirped FBG structures . These designs are characterized by a changing grating period along the fiber axis enabling wider spectral coverage of FBGs. Mostly, such broadband filters are realized by a single continuously chirped FBG, which is characterized by a steady evolution of the grating period along the fiber axis. Accordingly, the local feedback wavelength changes along the FBG introducing a structural dispersion that causes different spectral components to have dissimilar delay or response times in the reflected signal. Hence, such gratings are mostly used for managing the dispersion in fiber lasers [18, 19]. However, in 1998, Li et al. utilized a continuously chirped FBG to tune the emission wavelength of a pulsed fiber laser using an electro-optic modulator to mode-lock the system and shift the emission wavelength λL over the feedback spectrum of the FBG . The limited tuning range of the laser of 7.2nm is connected to the challenges of inscribing broadband FBGs with a continuous chirp. In a simple picture, the feedback bandwidth scales with the length of the grating impeding the realization of wide spectral coverage. In order to circumvent this issue, Burgoyne et al. have combined the feedback of 4 continuously chirped FBGs achieving a total tuning bandwidth of 80nm in the Erbium band around 1.55 μm .
An alternative approach for chirped FBG structures is to emulate the continuous evolution of the grating period by a stack of easy-to-inscribe standard FBGs whereby each narrow-band reflector provides feedback at a different wavelength . Accordingly, the complete set of FBGs may cover a broader spectral range. These discretely chirped or so called step-chirped FBG arrays are usually employed in quasi-distributed sensing . As an analogue to bulk diffraction gratings, such filters have also been used to realize fiber-integrated spectrometers for simultaneous spectral and temporal laser pulse characterization . The main advantage of this monolithic filter structure arises from the discrete spectral sampling. In general, the feedback wavelength of each FBG can be designed independently providing a unique spectral freedom to tailor the response spectrum. The included spectral features scale with the number of incorporated gratings. Accordingly, the huge potential of this filter is enabled by a highly productive inscription of the FBGs directly during fiber drawing [22, 25]. Based on so called draw-tower gratings, array designs are feasible containing hundreds or even thousands of FBGs with independently adjustable feedback wavelengths. This easy-to-scale filter layout not only allows ultra-broad spectral coverage but, most notably, also enables customized designs with fingerprint features as well as discrete and calibrated filter lines.
In this work, we investigate the promising properties of step-chirped FBG arrays as discrete spectral filters for tunable lasers. Similar to the spectroscopic application , we utilize the time-encoded spectral feedback to narrow down the effective filter wavelength and tune it within the broadband response. After introducing FBG arrays as spectral filters and the implemented tuning principle for pulsed systems, we present the experimental study based on a fiber-integrated Yb-doped laser, investigating the spectral and temporal emission properties. In order to highlight the spectral flexibility in the design of step-chirped FBG arrays, different filter configurations are evaluated. This includes the demonstration of a tuning range covering 74nm, which, to the best of our knowledge, is the largest tuning range reported for a fiber laser relying on a fiber-integrated setup.
2. Novel Tuning principle based on FBG arrays
The presented tuning principle is based on a step-chirped FBG array selecting the emission wavelength of the laser. The general structure of this spectral filter is sketched in Fig. 1. It contains N standard FBGs with different feedback wavelengths, respectively, whereby the i-th grating reflects the component λFBG,i. The grating positions are spatially distributed along the fiber axis. Usually, an equidistant spacing is selected with a separation of Δz between adjacent FBGs, controlling the induced dispersion by setting the time delay between different wavelength channels. Assuming a step-wise evolution of the feedback wavelengths along the fiber, Fig. 1 also includes a sketch of the principle reflectivity spectrum. The graph highlights the discrete sampling of the wavelength regime as well as the enhanced spectral coverage based on stacking many FBGs.
In order to utilize the broad spectral response of this filter for tunable lasers, a control mechanisms is required locking laser oscillations only to a small part of the feedback spectrum. The target is to effectively select a specific FBG dominating the feedback for lasing and determining the emission wavelength. With the discrete character of the filter, wavelength tuning is achieved by switching the laser emission between the response of different FBGs. This principle is realized with a control mechanism in the time domain taking advantage of the structural dispersion, which relies on the distributed feedback of the chirped FBG structure (Fig. 2).
For the simple case of a continuously chirped FBG in a single mode fiber, the spatial dependence of the reflected wavelength λFBG along the fiber axis position z is described byEq. (1) also holds for step-chirped FBG arrays with equidistant spectral spacing ΔλFBG = λFBG,i − λFBG,i−1 and spatial spacing Δz of the gratings. However, the discrete sampling enables much larger chirp parameters inducing stronger structural dispersion.
Due to the distributed feedback, the response time of the filter Tfilter for a pulsed signal reflected at the position z is given byEq. (1) and Eq. (2), the response time of the chirped FBG structure is connected to the local feedback wavelength λFBG of the filter giving the relation 26].
This characteristic is explored with a sigma ring resonator configuration as shown in Fig. 2. Besides the chirped filter whose feedback is coupled to the cavity by a circulator, the sketch also contains principle laser components comprising the output coupler (OC) and the gain element. Additionally, an optical modulator is included as temporal gate to ensure pulsed operation by periodically switching the resonator losses. In this configuration the cavity round trip time TRT for laser pulses is composed of the wavelength-dependent response time of the chirped FBG filter Tfilter(λFBG) and the propagation time Tloop in the rest of the resonator resulting in
The wavelength dependence in TRT provides access to control the effective spectral feedback of the chirped FBG structure for laser operation. The idea is to adapt the modulation frequency at the modulator in order to stipulate a specific TRT for circulating pulses. Based on the induced transmission losses at the modulator, efficient laser operation is ensured over consecutive round trips forEq. (4) and Eq. (5), TMP works as the tuning parameter controlling the effective filter spectrum and, therefore, determining the emission wavelength λL of the laser.
In order to get a unique tuning behavior for an unambiguous relation between TMP and λL, the resonator configuration has to prevent simultaneous oscillations of various pulse modulation orders m. Hence, different feedback wavelengths of the filter should not exhibit multiples of the round trip time compared to others. Based on the minimum and maximum TRT supported by the filter length, the cavity design has to ensure (m + 1) · min[TRT (λFBG)] > m · max[TRT (λFBG)] for prohibiting the (m+1)th modulation order within the tuning range. Working in the fundamental modulation order m = 1, Eq. (4) gives the condition Tloop > max[Tfilter] for an unambiguous tuning behavior in TMP. In case of Tloop ≤ max[Tfilter], multiple wavelength operation may occur. This condition can be fulfilled e.g. by including a delay line in the resonator.
3. Experimental setup
The experimental study of the tuning principle is conducted based on an Yb-doped fiber laser emitting in the wavelength range around 1060nm. Figure 3 illustrates the principle setup of the sigma ring resonator implemented with polarization-insensitive components.
The gain medium is implemented based on an in-house fabricated 7m long Yb-doped fiber with an Yb concentration of 0.18mol%, core and cladding diameters of 7 μm and 125 μm, respectively and an NA of 0.12 ensuring single mode operation. In order to enhance pump light absorption in the double-clad structure, the active fiber is coiled in a kidney shape . This arrangement is pumped with a temperature-controlled laser diode working at around 975nm with a maximum output power of 25W. In order to not distort the tuning bandwidth of the system, a wavelength-insensitive pumping scheme is employed using a pump-signal combiner to couple in the pump light. The laser signal is extracted with a 10dB output coupler (OC) feeding 10% of the signal back to the resonator loop.
Showing an excellent broad-band extinction ratio and proper power handling, a fiber-coupled acousto-optic modulator (AOM, Gooch & Housego Fiber-Q) is used to periodically modulate the resonator losses and generate pulsed operation. The insertion loss and rise time are measured to be 2dB and 25ns, respectively. The AOM is temporally controlled with a function generator (Tektronix AFG3252) that electrically sets the gate width τGW and modulation period TMP governing the spectral and temporal emission properties.
The feedback of the FBG array as the spectral filter is coupled back to the cavity by a circulator ensuring, together with an isolator, unidirectional operation. An additional delay fiber is incorporated, balancing the paths lengths in the cavity in order to avoid any ambiguity in the tuning characteristics (m = 1). The major part of the study is conducted with the design of FBG array A containing 73 FBGs equidistantly distributed over a spectral range of 18nm. With a proper reflectivity of around 40%, the linewidth (FWHM) of a single grating response is about 50 pm exhibiting an excellent side lobe suppression of well above 10dB. Based on the rise time of the AOM, the spatial spacing Δz between adjacent gratings has been adjusted to 1.5m accumulating to a total fiber length of about 108m. The detailed spectral features of this filter have been discussed previously .
4. Experimental results
The general characteristics of this tuning method are analyzed based on a tuning spectrogram as shown in the top graph of Fig. 4. While the abscissa depicts TMP as the tuning parameter applied at the AOM, the ordinate covers the emitted spectrum. Accordingly, the intensity plot pictures variations in the spectral emission behavior depending on the modulation period with bright regions indicating strong amplitudes. As spectral filter, FBG array A is used, comprising 73 gratings in descending order that cover the spectral range between 1052nm and 1070nm in steps of 250 pm. In an automated procedure, the emission spectrum is recorded with an optical spectrum analyzer (OSA, Yokogawa AQ6370C).
As proven by the bright trace in the tuning spectrogram of Fig. 4, the emission wavelength of the laser shifts depending on TMP. The tuning range of 18nm corresponds to the specification of FBG array A. The linear evolution of the emission wavelength over the modulation period arises from an equidistant spectral and spatial distribution of the FBGs along the filter structure. Emphasizing the reliability of the tuning principle, inset Fig. 4(a) indicates the laser emission to be always locked to the feedback of a single grating resulting in a sharp laser peak. This confirms the temporal gating mechanism to efficiently narrow down the broadband filter response for lasing. Highlighted in inset Fig. 4(a) and 4(b), the feedback of adjacent FBGs only becomes visible in a weak horizontal fringe pattern in the amplified spontaneous emission (ASE) background.
The tuning behavior of the system is dominated by the discrete nature of the filter feedback. As indicated in inset Fig. 4(a), the wavelength changes in discrete increments jumping between the feedback of adjacent gratings without intermediate steps. This behavior is highlighted in the graph at the bottom of Fig. 4 plotting the peak emission wavelength λL over the tuning range. The stepwise evolution perfectly resembles the discrete character of the FBG array. Still, besides sharp spectral transitions, λL is efficiently locked to the spectral feedback of the target grating showing no wavelength fluctuations. This insensitive connection between TMP indicates a strong coupling between filter feedback and laser oscillations.
The fine scan of TMP unveils further dynamics in the tuning behavior. In the transition region between two grating wavelengths, an elevated ASE background becomes visible forming a vertical fringe pattern in the tuning spectrogram of Fig. 4. Based on elevated round trip losses with a detuned modulation period, the transition regime is also linked to unstable temporal emission properties comprising degrading pulse-to-pulse variations. However, under normal operation of the laser, the mismatch between the modulation period and the actual pulse round trip time is easily avoided. Hence, for further scans of the tuning range, TMP is changed in increments adapted to the spatial spacing of the gratings in the array structure, skipping these transition regions.
As an example, the shape of a single emission spectrum is highlighted in Fig. 5(a). Corresponding to a column in the tuning spectrogram of Fig. 4, the graph pictures a single acquisition measurement with the OSA for an particular value of TMP. For this setting, the emission peak is at λL = 1053.35nm and exhibits a narrow 3dB linewidth of about 55 pm (≈ 15GHz), which coincides well with the specification of a single FBG. The spectral signal contrast of the main laser peak to the weak ASE background exceeds 35dB, which can be further enhanced by fine-tuning of TMP.
The temporal emission properties are analyzed with a 2GHz photodiode connected to an oscilloscope (Tektronix TDS7254). Based on the periodic modulation of the resonator losses, the laser works in pulsed operation. A typical pulse shape is plotted in Fig. 5(b). The emission is characterized by parabola-shaped pulses. Due to the changing repetition rate over the tuning range, the pulse properties including the duration τpulse and the amplitude slightly vary with λL. While τpulse in this example is measured to be 20ns (FWHM), the tuning concept features variable pulse lengths adjustable by the gate widths τGW at the AOM.
The impact of this parameter on τpulse as well as on the peak power Ppeak is highlighted in Fig. 6. With an almost linear relation, the black trace confirms shorter τGW to generate shorter pulse durations. This relation is almost reversed for the peak power indicated by the red trace. For τGW > 8ns, the peak power rises towards smaller gate widths. The pulse energy is simply concentrated over a shorter pulse duration causing a stronger amplitude. However, in this measurement, the peak power is maximized at τGW = 8ns and decreases rapidly for even shorter gate widths. This decline is connected to the limited rise time of the AOM of 25ns introducing severe insertion losses for such short gate widths (≈ 11dB at τGW = 10ns). The degradation in laser efficiency restricts the shortest pulse duration to about 10ns. Shorter pulses are possible with faster modulators.
For an optimization of the peak power of the system, this characteristic has to be considered by adapting τGW for short pulses. With τGW = 10ns and increased pump power, the peak power has been scaled to about 100W as shown in Fig. 5(b). This results in a pulse energy in the μJ range. Further scaling of power caused degradations in the emission spectrum. Besides a rising ASE background (≈ 30dB), the linewidth of the laser broadens to values exceeding 100 pm due to nonlinear interactions such as self phase modulation (SPM). For an output peak power of 100W the B-integral for the resonator is estimated to be larger than 1 proving the significance of SPM . However, together with an average power of 1.1W, this power regime is remarkable for a tunable oscillator that can be readily boosted by an additional amplifier.
Based on the broad operation window of the laser, the wavelength stability of the tuning concept is investigated considering different power levels as well as pulse durations and temporal fluctuations. Figure 7 exemplarily pictures four tuning traces recorded with the laser working in different regimes. Because TMP is scanned in adapted increments, the emission wavelength evolves in a linear manner. Comparing the traces, there are no visible deviations in λL. Only with strong magnification as shown in the inset, small differences become visible, which, however, are below the resolution limit of the used OSA (20 pm). Accordingly, the tunable emission wavelength exhibits a superb wavelength stability independent of the pulse duration and power level. This results in a user-friendly behavior arising from the sealed fiber-integrated configuration as well as the efficient coupling of the filter feedback. However, for highest precision demands, the FBG array is required to be operated in a temperature-controlled environment in order to avoid detuning of the FBGs (expected temporal shift in λFBG,i ≈ 10 pm/K, ). On the other hand, this characteristic of the tunable laser could be explored for novel solutions in distributed sensing.
The favorable tuning characteristics of this concept are combined with vast design freedom in the spectral characteristics of FBG arrays. Based on the broad gain region of Yb-doped fibers , three additional filter designs have been realized demonstrating the flexibility to tailor the tuning range. The corresponding tuning spectrograms are shown in Fig. 8 revealing also an excellent spectral signal contrast with an ASE suppression of mostly 50dB. Due to an all-fiber structure, the FBG arrays are easily exchanged using a commercial fusion splicer.
In order to demonstrate the potential to implement large tuning bandwidths, FBG array B covers a spectral range of 75nm sampled with a resolution of 1nm. The corresponding tuning spectrogram is illustrated in the left graph of Fig. 8. The tuning trace proves a bandwidth of 74nm ranging from 1045nm to 1119nm, which, to the best of our knowledge, is the largest tuning bandwidth for fiber-integrated lasers using a monolithic filter design. Indicated by the rising ASE level on the left side of the graph, the gain bandwidth is not exhausted towards shorter wavelengths. However, on the longer wavelength end, the last FBG at 1120nm does not experience sufficient gain for lasing. By further modifying the filter design and optimizing the length of the active fiber, the tuning range may be even further extended.
In contrast, FBG array C targets high spectral resolution working with a spectral spacing of 100 pm between adjacent gratings while covering a rather small tuning range of 6nm. The tuning spectrogram in the center graph of Fig. 8 demonstrates a resolution in the range of the peak width giving a quasi-continuous tuning behavior. This would be beneficial for wavelength-sensitive applications that require fine spectral adjustment.
On the right-hand side of Fig. 8, the design of FBG array D highlights the possibility to stack different spectral features in a single FBG array design. The tuning spectrogram demonstrates two sub-tuning ranges of 5nm, respectively separated by a spectral gap of 15nm. While in Fig. 8, the FBGs are aligned in an descending order with the wavelength, in general, the feedback wavelength of each grating can be tailored allowing arbitrary arrangements. Accordingly, specific spectral features may be realized with FBG arrays providing the possibility to implement tuning ranges that are customized for particular applications. Together with the excellent wavelength stability, this tuning concept also enables several discrete and calibrated emission lines to be implemented targeting e.g. sharp absorption bands for measurements without covering a continuous range.
We have demonstrated a novel tuning method for pulsed fiber-integrated lasers using FBG arrays as versatile spectral filters. The discrete sampling of the tuning range provides an excellent spectral flexibility to implement different tuning characteristics e.g. ultrabroad bandwidths as well as quasi-continuous tuning in wavelength regions of interest. In general, the feedback wavelength of each grating can be designed independently enabling filter configurations with unique features tailored for specific applications. This unprecedented design freedom can be fully exploited by a highly efficient fabrication procedure inscribing the gratings directly during fiber drawing. Accordingly, large FBG arrays are feasible enabling customized tuning ranges.
The potential of this filter design is demonstrated by a record tuning range of 74nm in the Yb band, which, to the best of our knowledge, is the broadest tuning bandwidth using a monolithic filter design. Together with the possibility to also implement quasi-continuous tuning steps and varying spectral resolutions (down to 100pm) as well as calibrated emission lines, this concept significantly outperforms the spectral freedom of other fiber-integrated solutions offering only small tuning bandwidths. For some applications, the discrete structure may even provide benefits compared to common free-space coupled filter designs. For the first time, a tuning concept offers a combination of huge spectral freedom to exploit the broad gain regions of active fibers and an all-fiber setup. This allows robust and compact systems to be implemented with an user-friendly design and low-maintenance requirements. Additionally, the convenient electrical control mechanism facilitates programmable and automated operation.
As shown by this study, these features are combined with favorable emission properties. The emitted pulses are characterized by an excellent wavelength stability over an broad operation regime of the laser as well as narrow linewidth (15GHz) and high signal contrast (up to 50dB). This is completed by adjustable pulse durations in the nanosecond range.
In the future, based on faster modulators, pulse durations might even be shortened to the sub-nanosecond range enabling higher peak powers. Furthermore, the whole resonator can be implemented based on polarization-maintaining components in order to comply with the demands of applications in the fields of e.g. nonlinear microscopy or nonlinear frequency conversion. Still, the characteristic variation of the pulse round trip time over the tuning range introduces small spectral dependencies in the pulse properties. Future work concentrates on extending this concept towards a spectrally independent repetition rate in order to overcome this limitation. Nevertheless, the beneficial properties of this fiber-integrated tuning concept show an excellent potential to replace common tunable solid-state lasers by cost-efficient and easy-to-use fiber lasers with great prospects in e.g. spectroscopic applications and distributed sensing.
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