1. Introduction

Motivated by the vision of extended life expectancy and advanced healthcare for society, international research efforts are increasingly focused on the broad fields of biophotonics, life sciences and medical technology. Novel analysis techniques and enhanced concepts in spectroscopy as well as nonlinear microscopy enable faster and more precise diagnostics for deeper insights into molecular processes. This progress is often strongly linked to research for suitable light sources that on the one hand need to match the demand of new methods and on the other hand also facilitate technological transfer to clinical applications.

Tunable lasers are an important key technology in this process providing intense light sources with tunable wavelengths and tailored emission properties for diverse spectroscopic applications that crucially depend on the exciting wavelengths [1,2]. Fiber lasers provide the perfect platform to develop suitable sources. They combine an excellent beam quality, high efficiency and typically broad gain regions of rare-earth doped fibers, facilitating broad operation windows, with the feasibility of fiber-integrated systems [3]. Hence, the benefits of all-fiber networks are utilized for lasers enabling compact and robust systems with low operation cost as well as user-friendly layouts setting the benchmark for leading-edge industrial designs.

While common tuning concepts either prohibit an all-fiber design or are limited to small tuning bandwidths by the spectral filter design, a tuning concept for fiber-integrated lasers has been studied using a discretely chirped fiber Bragg grating (FBG) array as a versatile spectral filter [4]. It comprises many standard FBGs as narrowband reflectors distributed along the fiber with dissimilar feedback wavelengths, respectively. Discrete spectral sampling uniquely allows tailored tuning ranges with huge bandwidths as well as fully customized emission lines and varying resolutions. While this filter structure is known from distributed sensing [5] based on a highly productive fabrication during fiber drawing (i.e. draw tower gratings, DTG [6]), a record tuning range of 74 nm for fiber-integrated Yb-doped single-wavelength lasers has been demonstrated with excellent spectral emission properties via FBG array tuning [4].

Motivated by a broader application window involving synchronized processes and steady pulse properties over the tuning range, a theta-ring resonator layout has recently been presented for FBG array tuned lasers [7]. The theta layout facilitates a constant pulse repetition rate over the discrete tuning range of the reflective filter. The distributed feedback of the FBG array is balanced over one full round trip based on two inverse filter interactions ensuring the same round trip time in the cavity for each wavelength. The emission wavelength is tuned by optical gating of pulses controlling the time of flight in the FBG array with a modulator. A discrete tuning range of 25 nm has been shown in a single-wavelength theta cavity fiber laser (TCFL) [7].

The programmable operation as well as the constant pulse repetition rate over the tuning range point the way to a new operation mode of the TCFL enabling freely tunable multi-wavelength emission of inherently synchronized pulses. In general, multi-wavelength sources have been reported with various filtering techniques including free-space coupled diffraction gratings [8], interferometric transmission filters based, e.g., on an all-fiber Mach-Zehnder configuration [9], cascaded FBG reflectors [10, 11] and superstructured FBGs [12]. In [11], Li et al. employ a similar ring cavity design as presented in this work, however, they rely on polarization-controlled tuning to switch between single-wavelength emission and fixed dual-wavelength emission.

Most research efforts have been directed towards tunable dual-wavelength lasers facing a broad application range that covers, e.g., atmospheric sensing [13] spectroscopic measurements [14] and nonlinear biomedical applications [15] including coherent anti-Stokes Raman (CARS) scattering microscopy [16]. Additionally, by exploiting nonlinear frequency conversion pumped by tunable dual-wavelength sources, tunable laser sources may open up other spectral ranges that are difficult to reach with common gain media. Addressing diverse applications in spectroscopy, dual-wavelength lasers are used to realize tunable THz [17–19] and mid-Infrared (MIR) [20–22] sources. Accordingly, tunable dual-wavelength sources are reported with various approaches including the combination of two laser oscillators [21] or configurations with a free-space-coupled optical parametric oscillator (OPO) [19, 23], all-fiber OPO [15], polarization multiplexing combined with birefringent filters [24,25], and two FBGs with a variable attenuator [26,27].

In this report, based on the broad prospective application window, we have studied tunable dual-wavelength emission with a new approach relying on a discretely tunable TCFL with an FBG array as the spectral filter. Simply by changing the optical gating settings of the driving electrical signal, the tunable laser can be switched between single and dual-wavelength emission. This highly flexible approach exhibits unique advantages. It features independent tuning of both wavelengths with tailored spectral coverage of the FBG array, enabling easy to scale bandwidths as well as customized wavelength separations. Additionally, without relying on any moving parts, the fiber-integrated and programmable system ensures robust and user-friendly operation. Finally, due to the single oscillator design in the theta configuration, also allowing even more emission wavelengths, the pulsed emission is inherently synchronized.

Fusing all these advantages, this experimental study investigates the programmable TCFL with an independently switchable dual-wavelength emission. Using an Yb-doped fiber laser, 55 wavelength pairs are demonstrated, covering a large tuning range of 25 nm at around 1μm. Because prospective applications mostly rely on a simultaneous emission of both wavelengths, a modified time-delay spectrometer as presented in [28] is applied to investigate pulse synchronicity of the dual-wavelength TCFL, including the measurement of single-pulse spectra. With optimized optical gating parameters, simultaneous emission of both wavelengths has been achieved with full overlap of both pulses, which proves the suitability of this approach for diverse applications.

2. Concept of a tunable dual-wavelength theta oscillator

The operation scheme of the independently tunable dual-wavelength TCFL is sketched in Fig. 1.

 

Fig. 1 Design principle of the pulsed discretely tunable dual-wavelength theta cavity fiber laser (TCFL). The laser resonator comprises a middle branch with the FBG array as versatile spectral filter and a modulator to control the filter response by optical gating. The modulator is driven by an electrical signal generated by an arbitrary function generator (AFG) applying a periodic sequence (period T_MP) with three transmission windows. The corresponding gating parameters (amplitudes A₁, A₂, A₃ and delays τ_2–3 and τ_1–3) control both emission wavelengths λ_L1 and λ_L2.

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The oscillator setup is similar to the one presented in [7], enabling tunable single-wavelength emission. The cavity comprises an outer ring structure with the active fiber as gain medium coupled with a pump-signal combiner, an output coupler OC1 to extract the laser signal, an optional output coupler OC2 giving an ASE background-free signal (not further considered in this work) and two circulators connecting the middle branch. This middle path comprises the FBG array as a reflective discrete spectral filter as well as an amplitude modulator controlled by an arbitrary function generator (AFG). As described in detail in [7], the modulator controls the emission wavelength of the laser by optical gating presetting the response time, i.e. time of flight, of laser pulses in the FBG array. For tunable single-wavelength operation, two transmission windows are applied periodically with the period T_MP matching the pulse round trip time. The temporal delay of the gates determines the emission wavelength.

For tunable dual-wavelength emission presented in this report, an additional third transmission window has to be applied per period T_MP to promote the response times of two different FBGs in the spectral filter for low round trip losses, and thus for lasing. The corresponding electrical gating signal is sketched in the green box of Fig. 1, labeled AFG.

In order to illustrate pulse formation over one cavity round trip, Fig. 1 also depicts schematic signal spectra at three different positions along one round trip (blue shaded boxes). The typically broad ASE spectrum from the active fiber as shown in Graph 1 is coupled to the middle branch by Circulator 1. The filter response comprises the corresponding spectral feedback peaks of the FBG array propagating in the upper loop as shown in Graph 2. Passing through Circulator 2, the signal is reflected a second time by the FBG array to complete the round trip. The actual selection of the emission wavelengths is facilitated by the modulator controlling the filter feedback and pulsing the laser. In an illustrative picture, the first transmission window gates the spectral feedback peaks to the FBG array, whereas the second and third transmission windows are each applied with a specific delay to pick the desired wavelength pulses from the distributed filter response determining λ_L1 and λ_L2 (Graph 3). The time delay has to be chosen according to the time of flight difference of both wavelengths in the FBG array. In the experimental demonstration aiming at a synchronous emission at OC1, the system is actually operated with inverse timing. The first two transmission windows gate the corresponding pulses of the desired wavelengths λ_L1 and λ_L2, respectively, and the third gate collects the joint filter response, with both wavelength pulses being synchronized to complete the round trip and get amplified in the active fiber. Hence, the delays τ_2–3 and τ_1–3 select λ_L1 and λ_L2, respectively. The spectral amplitudes are conveniently balanced by the normalized gate amplitudes A₁, A₂ and A₃. Following this scheme, the pulses of λ_L1 and λ_L2 propagate synchronously in the lower branch (i.e. at OC1) and, due to the distributed feedback in the FBG array, delayed in the upper branch (i.e. at OC2). In principle, this relation can also be switched by the gating function.

To summarize this concept: By changing the electrical gate signal, the laser can be switched between discretely tunable single-, dual-or potentially even multi-wavelength operation. i + 1 transmission gates could enable i synchronized wavelengths. The experimental setup remains unchanged.

For the experimental demonstration of independently tunable dual-wavelength operation, an Yb-doped TCFL is used with an acousto-optic modulator (AOM, rise time 25 ns) for optical gating. Within the study, two FBG arrays have been used as filters. The design data are listed in Table 1. Both filters are inscribed with an FBG reflectivity of > 90 % by means of a fs-UV laser as reported in [29].

Table 1. Design specifications of FBG arrays for the TCFL

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FBG array A is used to establish the tunable dual-wavelength emission, enabling a tuning range of 25 nm. Comprising 11 FBGs, this filter provides $\left(\begin{array}{c}11\\ 2\end{array}\right)=55$ wavelength pairs in dual-wavelength operation. While FBG array A works with a constant wavelength spacing of 2.5 nm between adjacent FBGs, using a tailored FBG array with varying wavelength spacings would enable 55 dissimilar wavelength pairs having an incrementally changing Δλ = λ_L2 − λ_L1, which would be beneficial, e.g., for the generation of new frequencies (e.g. THz range) by difference frequency generation (DFG).

FBG array B is optimized for the characterization of the dual-wavelength mode with a TDS which is presented in section 4. It covers a reduced spectral range adapted to the measurement range of the TDS setup. Additionally, the spatial separation of adjacent FBGs is increased, considering the AOM rise time of 25 ns, to address a larger range of optical gating parameters for the investigation of pulse dynamics.

3. Demonstration of independently tunable dual-wavelength emission

The tunable dual-wavelength mode is established with FBG array A implemented in the TCFL. The control of the laser is based on a LabVIEW program that has been developed to generate tailored arbitrary signals with an AFG (Tektronix AWG 3252) as gating functions. Thus, the dual-wavelength mode is conveniently controlled by the gating parameters τ_2–3, τ_1–3, A₁, A₂ and A₃ as well as the constant modulation period T_MP approximately matched to the pulse round trip time of the TCFL.

Figure 2 shows two graphs plotting an electrical gating signal applied to the AOM in the graph at the top as well as the corresponding emission spectrum measured at OC1 at the bottom. In this random example, the gating parameters τ_2–3, τ_1–3 have been set to the filter response times of FBG 2 and FBG 8 promoting their feedback wavelengths for lasing. The wavelength separation is Δλ ≈ 15 nm. As covered in the top graph, the electrical gate amplitude A₁ of gate 1 had to be lowered to about 71 % in order to ensure equal spectral amplitudes for both wavelengths. This empirical value balances differences in the effective spectral gain of λ_L1 and λ_L2 in the resonator, also considering the spectral losses and FBG reflectivity. It is not linearly connected to the actual optical transmission of the gate window at the AOM.

 

Fig. 2 The top graph sketches an example electrical gating function applied to the modulator to achieve dual-wavelength emission. The gating parameters are listed in the green box in the lower graph that shows the corresponding emission spectrum of the laser demonstrating two emission lines at around 1062 nm and 1077 nm. Correspondingly, a scan over different wavelengths pairs is highlighted in a video sequence (see Visualization 1).

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Looking at the measured spectrum at the bottom of Fig. 2, it shows the two emission lines corresponding to the selected FBG wavelengths without any parasitic laser peaks. The spectrum features the typical characteristics of FBG array tuned lasers with narrow linewidths fixed by the FBG response (FWHM ≈ 150 pm, 40 GHz) and high signal contrast (≈ 40dB). The excellent emission properties show the efficient locking of laser oscillations to the feedback of the FBG array. The amplitude difference between λ_L1 and λ_L2 is about 0.23 dB (about 5 %).

In order to visualize the dynamics between different gating parameter sets and the actual emission spectrum, a short video sequence is added in the style of Fig. 2 scanning through 25 different wavelength pairs (see Visualization 1). In the first part, wavelength λ_L1 is fixed to the first grating (≈ 1060nm) and λ_L2 is tuned from FBG 2 to FBG 11 covering the complete tuning range. Subsequently, the procedure is repeated for λ_L1 locked to FBG 2 and λ_L2 scanned over FBG 3 to FBG 11 and so forth. The video highlights the changing electrical gating parameters, which are adapted for each wavelength pair ensuring balanced spectral amplitudes. Along the scan, the emission spectrum maintains its excellent characteristics as discussed before. Only for some wavelength pairs, two sidebands occur on both sides of the main spectral lines. They are caused by four-wave mixing (FWM) of the overlapping pulses, which will be discussed later.

Covering all $\left(\begin{array}{c}11\\ 2\end{array}\right)=55$ dual-wavelength combinations possible with 11 FBGs, Fig. 3 illustrates the complete spectral emission properties over the tuning range as an intensity plot. It follows the same scanning order as Visualization 1. Demonstrating each possible wavelength combination accessible in this configuration proves the independent tunablity in λ_L1 and λ_L2 working over a total tuning range of 25 nm. With a different FBG array design, even larger tuning ranges merely limited by the gain bandwidth and tailored wavelength separations Δλ are possible. The ASE background stays well below −35 dB throughout the scan. However, Fig. 3 also highlights two parasitic effects visible in the spectra. As shortly mentioned before, some dual-wavelength spectra are accompanied by FWM creating sidebands, as highlighted for some examples with white dashed boxes. This phase matching sensitive effect seems to be the strongest with small wavelength separations Δλ where the pulses propagate with the closest match of their phase velocities (considering dispersion). Nevertheless, this effect already indicates temporal overlap of the propagating pulses in the lower branch.

 

Fig. 3 Tuning spectrogram for dual-wavelength emission for different sets of gating parameters (τ_1–3, τ_2–3, A₁, A₂, A₃, T_MP). The graph covers 55 wavelength pairs within a total tuning range of 25 nm based on FBG array A.

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The second parasitic effect is exemplarily highlighted in Fig. 3 by blue dashed boxes. The corresponding spectral peaks belong to a single-wavelength pulse regime gated by only the first two gates (A₁, A₂) of the driving signal. With fully optimized gating parameters, introducing additional losses by lowering A₁ and A₂, the parasitic pulses, which occur delayed to the main pulses, can be suppressed completely.

In dual-wavelength mode, the TCFL works with similar temporal emission properties as presented in [7] for single-wavelength emission giving output powers of about 100 mW and pulse durations of 5 – 10 ns. For boosted output powers, a successive amplifier can be utilized [30].

4. Analysis of pulse synchronicity with a time-delay spectrometer (TDS)

Most applications of dual-wavelength sources demand a simultaneous emission of both wavelength pulses. However, with the use of a typical scanning optical spectrum analyzer (OSA, e.g. Yokogawa AQ6370C) that measures averaged emission spectra, as well as an oscilloscope with a photodiode that only records the temporal pulse shape, it cannot be clarified whether a single pulse even contains both wavelengths λ_L1 and λ_L2. In order to characterize pulse synchronicity as well as to measure single-pulse spectra of the dual-wavelength TCFL, a modified time-delay spectrometer (TDS) has been developed for this analysis, based on the idea presented in [28]. It uniquely enables synchronous temporal and spectral pulse characterization down to a single shot analysis for nanosecond (ns) pulses. The concept relies on a finely sampling DTG array as discrete spectral probe, slicing out different spectral components of the pulse under investigation and returning them with a time delay due to the distributed feedback of the grating array. Accordingly, each spectral component can be analyzed with regards to its temporal behavior by measuring the TDS signal with an oscilloscope. The idea is synonymous to a diffraction grating based spectrometer decomposing the emission spectrum in the spatial domain.

The experimental setup of the TDS is shown in Fig. 4 with the tunable TCFL and the AFG included from Fig. 1. The signal of the tunable dual-wavelength laser is split up by a 3dB coupler into a Reference channel measuring the pulse shape and into the DTG array as a spectral probe. It decomposes the spectral content in a reflected time-encoded signal measured by the TDS Signal channel. As highlighted in the small inset, each spectral component λ_Di is delayed relative to its adjacent component λ_Di±1 by the time Δt, depending on the spatial DTG spacing. Correcting the measured TDS signal with the calibration data of the DTG array, it is post-processed to retrieve the spectral and temporal emission characteristics. Further details about this analysis are given in [28].

 

Fig. 4 Experimental setup of the time-delay spectrometer (TDS). The input signal of the tunable dual-wavelength TCFL is split up by a 3dB coupler into a reference branch and an analysis branch with a draw tower grating (DTG) array to spectrally decompose the laser signal in a time-encoded trace. The signals are measured with an oscilloscope (Tektronix DPO70604C, bandwidth 6 GHz) and postprocessed to reconstruct the joint spectral and temporal pulse characteristics.

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The DTG array for the TDS investigation has been inscribed during fiber drawing, following the procedure in [6]. Here, the design comprises n = 61 gratings covering the spectral range between 1061 nm to 1067 nm, with equidistant spectral spacing of 100 pm between adjacent FBGs. The spatial separation between adjacent gratings is 2 m, each FBG having a linewidth of about 50 pm and a reflectivity of about 30 %. Due to the high repetition rate of the TCFL, the actual spectral measurement range has been numerically clipped during postprocessing to n = 52 gratings ranging from 1061.5 nm to 1066.6 nm. Aiming at an optimized spectral resolution of about 100 pm and working with a reasonable DTG array length, the limited spectral measurement range of the TDS explains the necessity of using FBG array B with three gratings (see Table 1) as spectral filter in the TCFL tailored for the TDS analysis. With similar behavior for each wavelength pair, the following discussion is exemplarily focused at the case of λ_L1 and λ_L2 being locked to FBG 1 and FBG 2 of FBG array B.

The analysis of the TDS signal results in a pulse spectrogram as plotted in Fig. 5 for an example set of gating parameters. With the x-axis depicting the time domain, the pulse spectrogram illustrates the relation between spectral and temporal pulse properties, revealing which wavelength occurs at which time.

 

Fig. 5 Pulse spectrogram (logarithmic) measured with the TDS with an acquisition averaged over 5000 pulses. The two emission wavelengths λ_L1 and λ_L2 are locked to grating 1 and grating 2 of FBG array B. The right graph pictures the reference spectrum recorded with an OSA as well as the reconstructed emission spectrum measured with the TDS (blue). The graph at the bottom shows the reference pulse shape (green) measured with the oscilloscope as well as the reconstructed temporal pulse shape from the TDS analysis, plus separate plots for each wavelength channel.

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By integrating along the lines of the pulse spectrogram, the emission spectrum is reconstructed from the TDS signal for each wavelength channel. In the projected graph on the right-hand side of Fig. 5, the result (blue trace) is compared to the reference measurement with an OSA (green trace). Even though the TDS measurement samples the spectrum in discrete steps without considering continuous wave (cw) components, and though it has a much lower dynamic range than an OSA, it resembles the reference trace quite well. Not only both emission wavelengths, but also a sideband from FWM on the longer wavelength side is measured accurately, verifying the application with the dual-wavelength laser.

By integrating the pulse spectrogram along the columns, the temporal pulse shape can be reconstructed as shown in the graph at the bottom (blue trace). Analyzing only the corresponding wavelength channels (dashed white lines) retrieves the particular normalized temporal pulse shape of λ_L1 and λ_L2 (red and orange traces). The reference pulse shape (green trace) is measured with the reference channel of the TDS. The good agreement between the reference trace and TDS reconstruction in both the time and spectral domains verifies the application with the dual-wavelength TCFL.

Analyzing the pulse spectrogram, only the TDS reveals detailed pulse dynamics. The spectrum measured with an OSA cannot reveal temporal variations but only confirms two balanced emission wavelengths. Vice versa in the time domain, the reference signal measured with the oscilloscope only shows a unified pulse shape and thus indicates proper synchronous dual-wavelength emission. However, as revealed in Fig. 5, the laser pulse of λ_L1 is delayed relative to the pulse of λ_L2, confirming only a weak temporal overlap between the two wavelengths. The asymmetric shape between the pulse of λ_L1 and λ_L2 may arise from gain competition dynamics between both wavelengths in the interaction zone. The TDS signal now can explain features like deviations from the typical single-wavelength emission pulse shape, e.g., the small elevated peak in the center of the reference pulse in the time domain. This arises from the small overlap region between the pulses of λ_L1 and λ_L2. Confirming the measurement, this is supported by the weak FWM trace at around 1066 nm arising in the interaction area of both wavelengths. However, for dual-wavelength applications demanding overlapping pulses, this setting, generating slightly delayed pulses, would not work properly. Because the TDS analysis provides a much deeper insight into the dual-wavelength characteristics than common measurements, it is used to investigate different gating parameters and optimize the pulse overlap of the tunable TCFL.

With mainly the gating parameters τ_2–3 and τ_1–3 slightly readjusted to shift the timing of the two wavelength channels, Fig. 6 refers to an example that has been adjusted for an optimized pulse overlap of λ_L1 and λ_L2 as demanded by many applications. Both, λ_L1 and λ_L2, are emitted at the same time, proving a fully synchronized tunable dual-wavelength emission of the TCFL. The optimized pulse overlap also manifests itself in an enhanced FWM signal in the spectrum (compare to Fig. 5). The pulses of λ_L1 and λ_L2 show similar characteristics and a pulse duration of about 5 ns. The temporal pulse amplitudes differ by only about 10 %, which is in the range of the measurement accuracy, considering the rough spectral resolution as well as the discrete sampling of the TDS.

 

Fig. 6 Pulse spectrogram (logarithmic) measured with the TDS with an acquisition averaged over 5000 pulses. The two emission wavelengths λ_L1 and λ_L2 are locked to grating 1 and grating 2 of FBG array B. The gating parameters are optimized for simultaneous emission of both wavelengths.

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The measurements in Fig. 5 and Fig. 6 highlight the application of the TDS as a unique tool to sensitively track down slight deviations from optimized gating parameters of the TCFL, ensuring maximized pulse overlap of the two emission wavelengths. On the other hand, even specific delays between the λ_L1 and λ_L2 pulse may be adjusted and verified based on the highly flexible programmable operation of the TCFL.

In order to prove the synchronized dual-wavelength emission from Fig. 6 down to a single-pulse level, Fig. 7 shows the single shot acquisition of the TDS with the same setting of the TCFL. Hence, no averaging is involved which raises the noise level and degrades the dynamic range. Still, the graph shows two emission wavelengths simultaneously emitted with consistent pulse shape. The spectral amplitudes differ by about 1.2dB. The agreement between averaged and single-pulse analysis are, in general, very good which has been established for many parameter settings. This confirms the dual-wavelength emission for single laser pulses, indicating a stable operation.

 

Fig. 7 Pulse spectrogram (logarithmic) measured with the TDS in single-pulse acquisition for an exemplary dual-wavelength emission with λ_L1 and λ_L2 locked to grating 1 and grating 2 of FBG array B. The gating parameters are optimized for simultaneous emission of both wavelengths

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5. Conclusion

We have presented a novel design for a discretely tunable dual-wavelength pulsed fiber oscillator using a theta resonator layout as well as FBG arrays as variable filters enabling tailored tuning ranges. Based on a novel electrical gating scheme, the tunable laser can be switched between single-and multi-wavelength operation. The experimental demonstration of the switchable dual-wavelength regime with an Yb-doped TCFL is based on an FBG array with 11 gratings. All possible 55 wavelength pairs have been realized covering a total tuning range of 25 nm. For each wavelength pair, the laser features the typical emission characteristics of FBG array tuned lasers including narrow emission linewidths (< 40GHz) as well as high signal contrast (mostly about 40dB).

A deeper insight into the ns pulse properties in dual-wavelength operation has been provided by the measurement with a time-delay spectrometer. The TDS enables a simultaneous measurement of spectral and temporal pulse properties as well as their relation that can even be applied to analyze single laser pulses. With the programmable operation of the TCFL providing a fine control over pulse delays between λ_L1 and λ_L2, the TDS analysis verifies optimal gating parameters for, e.g., maximum pulse overlap. Proving this even on a single-pulse regime demonstrates good stability of this operation mode. Because many prospective applications, e.g., those relying on a nonlinear interaction of both wavelengths, require simultaneous emission of both wavelengths, the TCFL as a single oscillator perfectly satisfies the requirements enabling efficient operation.

Adding up to the fiber-integrated structure, the advantages of this concept are also related to the FBG arrays as tailored filters for various applications. They can be customized also to meet the demands placed on a dual-wavelength source. Hence, even larger tuning ranges and more wavelength pairs are easily implemented by using different FBG array designs. This is of particular interest for applications using tunable dual-wavelength laser sources to pump nonlinear frequency conversion and realize, e.g., a tunable THz source. With an FBG array using non-equidistant spectral step sizes, each wavelength pair may target a different frequency separation. Hence, based on the independent tunability of λ_L1 and λ_L2 in the TCFL, a filter design with only 11 FBGs could already generate 55 dissimilar THz frequencies. More gratings scale accordingly, giving the unique prospective of tailored discrete tuning ranges in spectral regions that are hardly accessible by cost-efficient solutions.

Another big advantage of this approach relates to the convenient electrical gate control. As discussed in this report, the programmable operation creates high flexibility to target various operation regimes. This not only comprises switching between single-wavelength and multi-wavelength emission but also includes the sensitive temporal control of each emission wavelength. While in the presented regime, output coupler OC2 would inherently deliver delayed pulses, even the pulses at OC1 can be generated with a slight temporal offset as demonstrated during the measurements made with the TDS. With the ability to slightly tune the temporal offset of both wavelengths, this could enable a novel light source for pump-probe experiments. The precise control of the emission characteristics by an electrical gating signal without any moving parts in the cavity is a highly flexible and easy to scale approach with huge potential to realize novel functionality and target new applications.