Tunable lasers cover diverse application fields comprising medical technology, telecommunications, and the research and industrial sectors [1–4]. Based on the adjustable and stabilized emission spectrum, such lasers are increasingly employed in biophotonics and spectroscopy, including atmospheric sensing and nonlinear optical analysis methods such as coherent anti-Stokes Raman scattering (CARS) [5].

Fiber lasers provide a perfect framework for applications with an excellent beam quality and high efficiency due to the waveguide structure. Rare-earth doped active fibers usually feature ultra-broad gain regions spanning tens and hundreds of nanometers providing a large operation window for spectral tuning. Additionally, by maintaining a fiber-integrated structure, application-driven laser development significantly benefits from a rugged and compact setup with low maintenance requirements and no alignment instabilities. However, most common tuning concepts prevent an all-fiber structure using free-space coupled filters like diffraction gratings [6]. On the other hand, a standard fiber Bragg grating (FBG) as a narrowband reflector tuned by temperature or strain provides limited spectral freedom to exploit the complete gain region of active fibers due to the mechanical destruction limit of the fiber [7].

One approach to extend the spectral tuning bandwidth of fiber lasers is based on chirped FBG structures covering a broader feedback range due to a changing grating period [8]. The enhanced bandwidth has been utilized for pulsed tunable lasers by a sigma ring resonator layout [9]. Based on the distributed response of the chirped FBG structure, the emission wavelength ${\lambda }_L$ can be tuned within the feedback range of the grating by changing the repetition rate of the laser. The scheme relies on optical gating with a modulator switching transmission losses with the corresponding frequency. Hence, the effective feedback position of the laser pulse is shifted within the grating. While this concept was implemented and commercialized at first based on a continuously chirped FBG with limited spectral bandwidth [10,11], recently, an implementation with discretely chirped FBG arrays has been introduced [12,13]. Such a discrete filter array comprises many standard gratings spatially separated along the fiber with dissimilar feedback wavelengths ${\lambda }_{\mathrm{FBG}}$, respectively [14]. While FBG arrays are usually employed for quasi-distributed sensing [15] or spectral pulse analysis [16], the discrete spectral sampling uniquely enables tailored tuning features with application-driven laser emission lines. The feedback wavelength of each FBG can be designed almost independently. Based on an inline inscription of FBGs during the fiber drawing process [14,17], array sizes with even hundreds of FBGs are feasible, covering large bandwidths. Accordingly, this approach enabled a record tuning range of 74 nm for a fiber-integrated Ytterbium (Yb)-doped laser featuring also inherently narrow linewidths, excellent spectral signal contrast, and programmable operation [12].

However, wavelength tuning in a sigma ring resonator fundamentally works with a changing repetition rate over the tuning range, which, especially with the large filter lengths of discretely chirped FBG arrays, shows considerable impact on pulse properties with changing durations and pulse energies. Furthermore, the varying repetition rate over the tuning range interdicts applications with synchronized processes like synchronized detectors or an interaction with other pulsed sources.

In this Letter, we present a novel laser design fusing the advantages of FBG array tuned fiber lasers with a constant repetition rate over the full tuning range. The concept is based on a novel cavity layout following the scheme of a theta ring resonator (see Fig. 1). While a similar resonator layout has been used previously, realizing an intra-cavity pulse stretcher and compressor with a continuously chirped FBG [18], we use a discretely chirped FBG array in the middle branch of the cavity to achieve wavelength tuning with a constant repetition rate. The principle relies on two counter-propagating passes through the reflective spectral filter canceling the characteristic time delays for each wavelength over one full round trip. In this work, we demonstrate the proof of principle of this tuning method. Based on an Yb-doped laser system, we have realized a tuning range of 25 nm with steady emission characteristics over the tuning range, including high signal contrast and adjustable pulse durations in the nanosecond (ns) range.

 

Fig. 1. Principle layout of the theta ring resonator. The middle branch of the tunable laser, which is coupled to the ring structure by two circulators, includes the FBG array as spectral filter and the modulator driven by an arbitrary function generator. The modulator switches two transmission gates with variable separation ${\tau }_{1-2}$ as the spectral tuning parameter selecting the emission wavelength ${\lambda }_L$.

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The cavity of the theta ring resonator comprises an outer ring structure, including the gain medium with the pump coupler, a first output coupler OC1, optionally a second output coupler OC2, and two circulators (1 and 2), which ensure unidirectional propagation in the ring and couple the signal to the middle branch. This middle part contains the FBG array as a reflective spectral filter and a modulator in order to indirectly control the emission wavelength ${\lambda }_L$ by optically gating pulses.

The scheme in Fig. 1 includes three insets sketching spectral graphs of the propagating signal at different positions along one round trip in order to illustrate pulse formation. Starting clockwise from the gain medium, the active fiber typically emits a broadband amplified spontaneous emission (ASE) signal as depicted in graph 1. Passing through circulator 1, this signal couples to the FBG array with each grating reflecting only the corresponding wavelength component. The spectral responses propagate through circulator 1 along the upper branch highlighted as a comb of different wavelengths in the spectrum of graph 2. Transmitting through circulator 2, this signal couples again into the middle branch being reflected a second time in the FBG array at the corresponding FBG before the signal couples back to the lower branch, completing the round trip. Because both filtering processes of the laser signal in the FBG array are connected to counter-propagating directions, the characteristic time delay for each reflected wavelength caused by the distributed feedback is balanced over a full round trip, giving identical round trip times $T_{\mathrm{MP}}$ (dispersion effects are disregarded here).

In order to generate a pulsed operation and promote one single wavelength out of the ensemble provided by the FBG array, as well as prohibit a subcavity formed by the lower loop of the theta ring resonator, a modulator is implemented in the middle branch temporally gating the laser pulses. Using a LabVIEW driven arbitrary function generator, the transmission of the modulator is periodically modulated with the frequency $1/T_{\mathrm{MP}}$. Within one period, two transmission windows are applied with an adjustable separation ${\tau }_{1-2}$ and electrical gate width ${\tau }_{\mathrm{GW}}$ (see Fig. 1). While the first transmission window allows the laser pulse to enter the FBG array from the right side, the second gate collects the spectrally filtered feedback of a distinct grating to finish the round trip as highlighted in Fig. 1 graph 3. Accordingly, ${\tau }_{1-2}$ dictates the time of flight, i.e., response time, of the pulse in the FBG array and thus promotes the feedback of a single grating for laser oscillations determining ${\lambda }_L$. With ${\tau }_{1-2}$ acting as the spectral tuning parameter, the laser is electrically tuned, enabling a programmable system.

In the experimental realization, a fiber-coupled acousto-optic modulator (AOM) with a rise time of 25 ns is used for pulse gating. The complete tunable system is realized in a fiber-integrated configuration based on an in-house developed cladding-pumped Yb-doped fiber as gain medium. The output couplers extract 90% (OC1—high power port) and 1% (OC2—low ASE port) of the laser signal, respectively. The FBG array comprises 11 gratings covering 1060 nm–1085 nm with spectral and spatial step sizes of 2.5 nm and 2.5 m, respectively. The FBGs are inscribed with a pulsed Excimer laser at 248 nm in a hydrogen loaded 1060XP fiber (Thorlabs) using a phase mask interferometer similar to Ref. [17]. For achieving low insertion loss of the reflective filter while also maintaining a narrow grating linewidth ($<200\mathrm{pm}$), the design reflectivity of the FBGs is about 90%. On the cost of a lower reflectivity ($<40\%$), FBG arrays inscribed during fiber drawing [17] may be used containing more gratings for tailored features.

Working with a modulation period $T_{\mathrm{MP}}$ matched to the cavity length of the resonator, the first step to experimentally verify the tuning principle is based on measuring the emission spectra along a gradual scan of the tuning parameter ${\tau }_{1-2}$. The results are plotted in the tuning spectrogram in Fig. 2. With the x-axis designating ${\tau }_{1-2}$ as the tuning parameter and the $y$-axis the wavelength scale, the intensity graph illustrates the evolution of the emission spectrum over the tuning range.

 

Fig. 2. Graph depicts the tuning spectrogram plotting the emission spectra of the tunable laser at the OC1 port in an intensity grading along a scan in fine increments of ${\tau }_{1-2}$. Following the bright staircase-like shape of high intensity, the emission wavelength shifts in discrete steps matching the characteristics of the FBGs with a tuning range of 25 nm.

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Matching the characteristics of the FBG array, the emission wavelength shifts in discrete steps from 1060 nm to 1085 nm depending on the tuning parameter ${\tau }_{1-2}$. Over the full tuning range, ${\lambda }_L$ is locked to the feedback wavelength of a corresponding grating from the filter without any parasitic laser lines. This proves that the tuning mechanism works efficiently. The linewidth (FWHM) of the laser lines nearly matches the 3 dB width of the FBGs in the filter measuring less than 150 pm ($\approx 40\mathrm{GHz}$). The average output power of the system for this measurement was about 100 mW. Analyzing the ASE background peaking at about 1050 nm, the laser maintains an excellent spectral contrast of 35–45 dB. A minor increase of the ASE background becomes visible in the transition zone of ${\tau }_{1-2}$ in between two gratings, which can be easily avoided by proper tuning.

While the covered spectral tuning range is sampled here in rough and equidistant steps, it is important to keep in mind that the discrete filter structure with an individually adjustable feedback wavelength for each FBG also enables customized tuning ranges, including high resolution, calibrated wavelengths, and large tuning bandwidths. Additionally, these features can be scaled via the number of gratings in the filter.

In order to analyze the pulse evolution in the resonator similar to the schematic graphs in Fig. 1, a tap monitor OC2 has been incorporated into the upper branch of the laser cavity. Figure 3 shows four plots corresponding to the spectral and temporal emission behavior of output OC1 and OC2, respectively. The measurements are recorded with an optical spectrum analyzer (Yokogawa AQ6370C) and an oscilloscope (bandwidth of photodiode: 2 GHz) for a fixed ${\tau }_{1-2}$ of 109.5 ns (FBG 3 at 1065 nm). Hence, the trace in graph (a) refers to the data of a single column in Fig. 2 at ${\tau }_{1-2}=109.5\mathrm{ns}$, emphasizing the excellent spectral emission properties of the system with a high signal contrast and narrow linewidth. Comparing the emission spectra of OC1 and OC2, i.e., graphs (a) and (b), the broadband ASE background vanishes at OC2, leaving only the main laser emission line and the spectrally filtered ASE responses of the other FBGs. This observation conforms with the spectral features sketched in graph 2 of Fig. 1. Due to the first interaction with the FBG array, the signal at output OC2 offers an enhanced spectral contrast of about 47 dB without any broadband ASE background, which is blocked at the modulator. Hence, OC2 is labeled as a low-ASE port with enhanced spectral purity, whereas OC1 provides efficient power extraction right behind the amplification path. As shown in graphs (c) and (d) of Fig. 3, plotting the temporal emission behavior, the laser emits parabola-like pulses with a duration of about 10 ns.

 

Fig. 3. Emission behavior of the laser analyzed at two output positions OC1 and OC2 for a fixed tuning parameter of ${\tau }_{1-2}=109.5\mathrm{ns}$ (FBG 3 at 1065 nm) and ${\tau }_{\mathrm{GW}}=35\mathrm{ns}$. Graphs (a) and (b) plot the emission spectra measured at OC1 and OC2, respectively. Graphs (c) and (d) highlight the averaged temporal emission properties showing a parabola-like pulse shape. The evolution of these emission characteristics with gradual tuning of ${\tau }_{1-2}$ is highlighted in a video sequence (see Visualization 1).

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The evolution of the emission characteristics shown in Fig. 3 is illustrated in a video (see Visualization 1) for a gradually tuned ${\tau }_{1-2}$. The segment covers tuning from FBG 2 to FBG 5 of the spectral filter, also highlighting the transition zones between adjacent gratings. Both the emission spectrum and pulse properties are quite insensitive to ${\tau }_{1-2}$, giving an operation window of about 10 ns per wavelength channel. Additionally, the emission characteristics in both domains are very consistent among the different emission wavelengths, maintaining a narrow linewidth, high signal contrast, and parabolic pulse shape. With the laser strongly following the discrete characteristics of the FBG array without any parasitic laser line, the tuning concept works very efficiently in locking the spectral laser characteristics. Some variations are visible only in the temporal amplitude of the pulses, which may be linked to varying grating reflectivities along the array structure. The steady shift of the pulse position within a single wavelength channel is connected to a fixed triggering point at the electrical gating signal. However, the reiterating pulse position along the different wavelength channels shows the constant repetition rate over the tuning range.

The gradual scan of ${\tau }_{1-2}$ also resolves the transition zones between adjacent wavelengths, e.g., at ${\tau }_{1-2}=96.5\mathrm{ns}$, the emission spectra on both ports show two neighboring spectral lines at the same time. This also results in two distinct pulses in the temporal domain. Since ${\tau }_{1-2}$ is in between the response time of both gratings with respect to the modulator, the laser starts to operate on both lines due to comparably slow rise and fall times of the AOM. However, in normal operation of the laser, this regime in ${\tau }_{1-2}$ can be easily avoided.

With the AOM transmission window generating pulsed operation, Fig. 4 investigates the impact of the gate width ${\tau }_{\mathrm{GW}}$ on the pulse duration ${\tau }_{\mathrm{pulse}}$ (FWHM). The graph proves an increasing ${\tau }_{\mathrm{pulse}}$ for rising ${\tau }_{\mathrm{GW}}$, enabling independent electrical tuning of the pulse properties that can be useful for applications. This relation directly originates from the optical gating forming the pulses. However, compared to the FBG array tuned sigma ring laser [12], in which, based on similar parameters, ${\tau }_{\mathrm{pulse}}$ has been adjusted over a broader range (10 ns to about 60 ns), the theta resonator configuration allows for tuning only between 4.5 ns to about 15 ns. We attribute the shortened pulses to the two consecutive temporal filter passes in the theta ring resonator, which also may cause the reduced tuning window in ${\tau }_{\mathrm{pulse}}$.

 

Fig. 4. Mean pulse duration ${\tau }_{\mathrm{pulse}}$ (FWHM) for all wavelength channels is plotted depending on the electrical gate width ${\tau }_{\mathrm{GW}}$ of the transmission windows of the AOM. ${\tau }_{\mathrm{pulse}}$ can be electrically tuned over a small range from about 4.5 ns to almost 15 ns.

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A prerequirement for operating the tunable laser in the theta ring configuration is a matched modulation period $T_{\mathrm{MP}}$ with respect to the actual pulse round trip time in the cavity. Figure 5 investigates the sensitivity and accuracy required for proper operation by plotting the ASE background level (peaking around 1050 nm), the spectral peak amplitude, and the difference between both, giving the spectral contrast for different $T_{\mathrm{MP}}$. The measurement was performed at output OC1 with ${\tau }_{\mathrm{GW}}=35\mathrm{ns}$. The graph is centered around $T_{\mathrm{MP},0}$, given by the modulation period enabling the highest signal contrast. $\mathrm{\Delta }{T}_{\mathrm{MP}}$ designates the deviation from $T_{\mathrm{MP},0}$.

 

Fig. 5. ASE background level (analyzed around 1050 nm), the spectral peak amplitude and, resulting from the difference, the spectral signal contrast are plotted, depending on $T_{\mathrm{MP}}$, whereby $\mathrm{\Delta }{T}_{\mathrm{MP}}$ designates the mismatch to the actual resonator length. The signal contrast is maximized at $\mathrm{\Delta }{T}_{\mathrm{MP}}=0\mathrm{ns}$ due to minimized round trip losses.

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For $|\mathrm{\Delta }{T}_{\mathrm{MP}}|\gtrsim 14\mathrm{ns}$ i.e., significant deviations in $T_{\mathrm{MP}}$, the large losses at the modulator over consecutive round trips prohibit any lasing giving only a background ASE signal. On the other hand, for $\mathrm{\Delta }{T}_{\mathrm{MP}}=0\mathrm{ns}$, the signal contrast is maximized, coinciding with the smallest ASE background level and the largest spectral laser amplitude. In comparing the different traces, the peak amplitude level is rather insensitive to an accurate match in $T_{\mathrm{MP}}$, whereas the ASE background level significantly reduces in the vicinity of $\mathrm{\Delta }{T}_{\mathrm{MP}}=0$ due to optimized round trip losses and the highest laser efficiency. The resulting spectral signal contrast increases symmetrically and in a nearly linear dependency (in logarithmic scale) towards $\mathrm{\Delta }{T}_{\mathrm{MP}}=0$. Even though $T_{\mathrm{MP}}$ should be accurately matched to obtain the best emission characteristics, the operation window for the analyzed configuration is about 20 ns depending on the lasing threshold criteria. This corresponds to a relative acceptance range in $T_{\mathrm{MP}}$ of about $\pm 1\%$. This decent operation window is especially important towards large tuning ranges, when dispersion influences cannot be neglected for the entire operation bandwidth.

In conclusion, we have presented a novel tuning concept employing versatile FBG arrays as spectral filters, which for the first time, to the best of our knowledge, works with a constant repetition rate over the full tuning range based on a theta ring resonator. The discrete filter structure enables the spectral and temporal laser pulse properties to be modified independently. With a realized tuning range of 25 nm, which was solely limited by the employed FBG array, the system featured excellent emission properties, including large spectral signal contrast (35–45 dB), narrow emission linewidths ($<150\mathrm{pm}$), and adjustable pulse durations in the ns regime. Accordingly, this concept combines the general advantages of FBG array tuned lasers, like the possibility of tailored tuning ranges, a fiber-integrated as well as compact setup with robust design and programmable operation, and an excellent beam quality with a constant repetition rate, enabling consistent emission properties over the tuning range.

In the future, this concept will be analyzed for larger tuning ranges and transferred to other spectral regions. Additionally, the constant repetition rate is not only beneficial for some applications, but also enables an oscillation of several pulses at the same time with different wavelengths in the oscillator. A novel operation regime based on this theta ring resonator will be investigated working with at least two independently tunable emission wavelengths emitted synchronously from this oscillator. This may target novel applications, such as building tunable light sources for other spectral domains based on nonlinear frequency generation with two pump wavelengths.