Multi-wavelength gain-switched Yb-doped fiber laser

Laser sources emitting multiple wavelengths simultaneously (or multi-wavelength lasers) may have applications in wavelength division multiplexed (WDM) systems [1], sensors [2], microwave photonics [3], and component testing and research. Usually different optical comb-like filtering devices, like WDMs and arrayed waveguide gratings [4], Mach–Zehnder filters [5], Lyot filters [6, 7], or fiber Bragg gratings (FBGs) [8], are selected for their lasing wavelengths. The mentioned papers are focused on maximization of the laser line quantity and improvement of their stability. Rare-earth-doped fibers have large gain bandwidth, so multi-wavelength oscillation is potentially feasible for use in lasers based on the fibers. The main problem for multi-wavelength lasing is connected with homogeneous gain saturation of an active medium at room temperature. As a result, the wavelength competition is very strong and dissallows a multi-wavelength generation regime. There are a number of different techniques to achieve multi-wavelength lasing operation: cooling with liquid nitrogen [9], four-wave mixing in photonic crystal fibers having high nonlinearity [10], nonlinear polarization rotation [11], introducing the stimulated Brillouin scattering effect [12] and so on. The ideas of mode competition suppression based on nonlinear effects are associated with the introduction of intensity-dependent loss. As a result, lasing at many wavelengths with low power in each line becomes more energetically favorable than at a single wavelength with high power. It should be noted that a significant part of the mentioned multi-wavelength fiber lasers is based on Er-doped fibers (EDFs). This fact is connected with the importance of these lasers for telecommunications. The lasers based on fibers with other dopants have been less investigated, but there are some examples. A multi-wavelength praseodymium fiber laser operating in the 1300 nm reegime. There are a number gion is demonstrated in [13]. An all-fiber multi-wavelength Tm-doped laser assisted by four-wave mixing in highly nonlinear fiber has been experimentally demonstrated [14]. Until now, there have been several reports about multi-wavelength Yb-doped fiber (YDF) lasers [15–17]. YDF has a broader gain spectrum in comparison with EDF. As a result, simultaneous oscillation at a large number of wavelengths can be expected for YDF. However, the early obtained results demonstrate lasing at several wavelengths only for the YDF laser (2 [15], 6 [16], 4 [17]) with rather large wavelength spacing (>1 nm) as compared with the EDF-based one. The homogeneous gain broadening is the main obstacle to achieving multi-wavelength operation at room temperature with YDF. Mode competition enhancement with reduction of wavelength spacing is expected in lasers. Moreover, the effects of spatial population inversion inhomogeneity (spatial hole burning as well as dynamical grating) are strongly manifested themselves in YDF lasers. This inhomogeneity contributes to a laser wavelength instability, which is known as the self-sweeping operation [18, 19].

In this paper, we present a method to prevent wavelength competition in the YDF laser and demonstrate multi-wavelength lasing at up to hundreds of laser lines simultaneously with the reduction of wavelength spacing down to 0.05 nm with spectral selection based on an internal Lyot filter. The method for mode competition suppression is based on a gain-switching technique with a modulated pump. At certain modulation parameters (amplitude, duration, and repetition rate), the laser generates a stable sequence of microsecond pulses consisting of many longitudinal modes. We believe that the effect of multi-wavelength lasing is connected with the small contribution of lasing pulses in developing saturation effects due to the low energy of generated pulses.

The laser scheme shown in figure 1 is based on polarization maintaining (PM) components. The active medium—PM double-clad Yb-doped fiber (PM-YDF-5/130, Nufern) with a length of 2.1 m—is pumped with multimode laser diode at a wavelength of 968 nm and maximal power of 3 W through a (2 + 1) × 1 pump combiner with signal feed-through. The main feature of the laser is active gain switching via pump power modulation. For this purpose the power of the laser diode is modulated by rectangular pulses of a signal generator (DG 4162, Rigol) applied to the laser driver. The unabsorbed multimode pump radiation is filtered with a home-made cladding pump stripper at a splice point between the active fiber and a piece of a 20 cm long PM passive fiber (PMF1). The cavity is formed with a fiber loop mirror (FLM1) based on a polarizing filter coupler 50/50 with a reflectivity of R₁ ~ 100% and an output mirror. In the experiments two types of output mirrors are used: a cleaved fiber end (CFE) with Fresnel reflection (R₂ ~ 3.5%) and the second fiber loop mirror (FLM2) based on a fused coupler 10/90 with a reflectivity of R₂ ~ 36%. An additional fused coupler 95/5 is inserted into the fiber cavity for output coupling of a small portion of the generated radiation for analysis. Another PM passive fiber (PMF2) is spliced with the PMF1 with a 45° offset as related to the birefringence axis of the fibers to form a Lyot filter at the output of the laser cavity for spectral component selection. The CFE of the PMF2 is used as the output mirror or the FLM2 is spliced at the laser output in two main cavity configurations. The PMF2 fiber length L is varied between the experiments from 0.75 to 24.8 m to control the free spectral range (FSR) of the Lyot filter. The other side of the laser cavity as related to the 45° splicing point has a total length of L₀ = 6.9 m and is kept fixed during the experiments. An effective length of the output loop mirror FLM2 is L₂ = 1.9 m. The optical spectrum is analyzed at fiber output of FLM1 with an optical spectrum analyzer (OSA AQ6370, Yokogawa) at a spectral resolution of 20 pm. It has been shown that the optical spectra at the output of FLM1 and the output mirror have the same shape. The intensity dynamics are analyzed with a fast InGaAs photodetector and digitized with an oscilloscope (Wavepro 725i-A, LeCroy) with bandwidths of 1 GHz and 2.5 GHz, correspondingly.

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It should be noted that the laser at the CW pump (i.e. without power modulation) operates in an irregular self-sustained pulsation regime. In this case, the optical spectrum is unstable and the regime of the laser operation is similar to an unstable self-sweeping one [19]. The stabilization of the pulse repetition rate is observed at a certain configuration of pump modulation parameters such as the repetition rate, duty cycle and peak power. For example, the stable pulsed operation is obtained at repetition rate of 10–20 kHz and duty cycle of about 43%–50%. At the same time, the peak pump power value can be varied over a range from 2 to 3.1 W (up to the maximum available pump level). Then the experiments are performed at fixed peak power of 3.1 W. The modulation parameters are varied in the abovementioned range and are fixed during spectrum measurements for each laser configuration, when the stable generation of pulses is observed. However, it should be noted that the spectra shape is independent of modulation parameters, when conditions of the stable pulse train generation are found.

The typical stable intensity dynamics measured at repetition rate of 10 kHz and duty cycle of 50% are presented in figure 2(a). Each pulse (figure 2(b)) has a bell-shaped envelope and chaotic amplitude modulation of ±25% with a FWHM duration of about 4 µs. The pulse duration decreases with cavity length shortening down to 2 µs with other parameters fixed. The average output power is about 40 mW and is independent of cavity length elongation with fixed modulation parameters. However, the power increases with duty cycle and repetition rate. The last fact is associated with the increase of the average pump power. The peak power is estimated as ~1 W for the results presented in figure 2. The pulse stability is confirmed by the RF spectrum (figure 2(c)), which consists of a series of narrow peaks corresponding to higher harmonics of the repetition rate. The chaotic pulse amplitude modulation in figure 2(b) is connected with a huge number of longitudinal modes generating simultaneously and having random phases (in contrast to the mode-locking operation). Simultaneous generation on different longitudinal modes is also observed in the RF spectrum recorded in the high-frequency range (figure 2(d)). It contains many peaks corresponding to beating between longitudinal modes. The similar intensity dynamics in RF spectra are observed in all laser configurations differing based on the length of PMF2 and for both types of output mirrors.

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The optical spectrum (figures 3 and 4) is broadband and is much more stable in the modulated pump in contrast to the CW pump regime. The form and position of the spectrum envelope depends on reflectivity of output mirror. The spectrum is centered near 1065 and 1075 nm at the configuration, with CFE (figure 3) and FLM2 (figure 4) as the output mirror respectively. It is well known that the generation spectrum can be shifted to long wavelengths with a Q factor increase (see, for example [20],). Bandwidths of the spectrum envelopes can be estimated as ~14 and 10 nm at −10 dB level for the scheme with CFE and FLM2 respectively. The spectrum has high-frequency modulation with the spectral period defined by the length of the Lyot filter. As a result of the spectral selection, stable multi-wavelength operation is observed at gain switching. The spectra for four different lengths L of PMF2 in configuration with CFE are shown in figure 3: (a) 0.75 m, (b) 2.5 m, (c) 5 m and (d) 24.3 m. The similar spectra for configuration with FLM2 as output mirror are shown in figure 4. It should be noted that the loop mirror has a considerable effective length L₂ of 1.9 m as compared with the zero physical length of CFE. That is why the spectra of figures 3(b) and 4(a) have similar line spacing. The positions of the laser lines slightly drift over time due to the absence of the thermostabilization of laser elements (especially of the Lyot filter). The problem of the laser line stabilization has not been studied here.

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The period of modulation as well as the linewidth of separate peaks @−3 dB level decrease with the increase of the PMF2 length. A summary of the spectra measurements for all laser configurations is presented in table 1. The linewidth decreases with a decreasing modulation period due to the sine transmission function of Lyot filters (i.e. the spectral width of individual transmission peaks is equal to one-half of the spectral period). It should be noted that the linewidth of each line is much narrower than the wavelength spacing for the short configurations (L = 0.75, 2.5, 5 m) and these values are comparable to each other for the longest one (L= 24.3 m). However, in the last case the measured linewidth is comparable with the spectral resolution of the OSA used. This means that the actual linewidth should be less than the measured one. The total number of simultaneously generating wavelengths can be estimated as the ratio between the envelope bandwidth and the line spacing (period). As a result, the total number of wavelengths @−10 dB level increases from ten to several hundred with the Lyot filter length increase. However, at the same time, the modulation depth decreases from >30 dB (figures 3(a) and (b) & 4(a) and (b)) to 10 dB (figures 3(d) and 4(d)) with the length increase. In addition to these experiments we also checked that the addition of a passive fiber between the highly reflective FLM1 and the 45° splice (i.e. elongation of length L₀) does not change the line spacing.

The laser is made of PM components and it contains a polarizer inside its cavity. Thus, on the one hand, the laser generates linearly polarized radiation. On the other hand, operation of the Lyot filter at the output of the laser leads to equal separation of the generated power between the two polarization modes of the output fiber. As a result, the radiation coming out of CFE or FLM2 has a mixed polarization state in spite of the all-PM laser configuration. The polarization state is qualitatively analyzed with the rotation of a bulk polarizer at the laser output. Total suppression of transmitted power can be achieved if the polarization state is linear and orthogonal with respect to the polarizer's transmission axis. In our case, there is no considerable transmitted power variation during the rotation of the polarizer. This means that the polarization state of the generated radiation at the output of the main output mirror of the laser is not linear. Nevertheless, the polarization state at the output of polarizing loop mirror is linear.

In the presented cavity configuration, the Lyot filter is formed by a double pass between the 45° splice and the output mirror. The polarized radiation reflected from FLM1 is divided into two polarization modes at the 45° splice point. Then the two polarization modes propagate in the forward direction with different group velocities due to fiber birefringence, reflect from the output mirror, propagate in the backward direction and then combine at the 45° splice point in two different polarization modes. Only one of the later modes is supported by the laser cavity due to presence of polarizing FLM1. In this case, the effective length of Lyot filter is

$$\begin{equation*}{L_{eff}} = {\text{ }}2\left( {L + {L_{mir}}} \right),\end{equation*}$$

where L_mir =0 or L₂ for the case with CFE or FLM2 as the output mirror respectively. The FSR for the Lyot filter with the length of L_eff and birefringence of δn near the wavelength of λ is

$$\begin{equation*}FSR = {\lambda ^2}/\left( {\delta n\cdot{L_{eff}}} \right).\end{equation*}$$

In other words, the modulation frequency v =1/FSR is proportional to effective length

$$\begin{equation*}v = {L_{eff}}\delta n/{\lambda ^2},\end{equation*}$$

with slope δn/λ². Taking into account the birefringence of the fiber used of δn≈ 4.4 × 10⁻⁴ and the central wavelength of λ= 1064 nm the slope can be estimated as 0.39 (m⁻¹ nm⁻¹). The obtained value is in good agreement with experimental results of figure 5(a), where a slope of 0.41 (m⁻¹ nm⁻¹) is observed. As a result, we have shown that the wavelength spacing can be controlled by changing the fiber length L inside the cavity.

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The amplitude of spectral modulation in decibels linearly decreases with the effective length (figure 5(b)). The linear extrapolation allows one to estimate the effective length for the zero modulation (i.e. when the modulation disappears) as ~60–65 m. The modulation frequency can be estimated as 25 nm⁻¹ at this length, which corresponds to modulation period of ~40 pm. It should be noted that this value is close to the spectral resolution of the OSA used of about 25 pm. This means that the amplitude reduction at long fibers can be associated with spectral smoothing due to approaching the instrumentation function of the OSA. The proper spectral instrument should be used for correct characterization of the optical spectra with small wavelength spacing.

The spectral modulation of laser cavity losses itself does not necessarily result in multi-wavelength lasing because of mode competition in homogeneously broadened gain mediums. As mentioned earlier, the CW pumping results in an irregular self-sustained pulsation lasing regime with an unstable optical spectrum. The operation can be associated with the effects of population inversion spatial inhomogeneity resulting in spatial hole burning as well as in dynamically changing gain and refraction index gratings. In particular, these effects result in laser line self-sweeping spectral dynamics [19]. However, gain-modulation at suitably chosen modulation parameters leads to stable multi-wavelength lasing. The pump pulse duration for stable operation in the mentioned configurations lies in the range from 25 to 50 µs. As a result, we observe generation pulses with durations of microseconds (2–4 µs), which is much shorter than the upper state lifetime for the ytterbium-doped active medium used of ~800 µs. In this case, for correct description of the short pulse amplification the effects of gain saturation should be taken into account. The saturation energy is defined as [21]:

$$\begin{equation*}{E_{sat}} \approx {E_p}A/\left( {{\sigma _e} + {\sigma _a}} \right),\end{equation*}$$

where E_p = ħω is the single photon energy where the frequency of ω, ħ is Planck's constant, A is the mode area and σ_e and σ_a are emission and absorption cross sections, correspondingly. All of these values are taken at the lasing wavelength. The saturation energy is estimated as E_sat~ 14 μJ at the central wavelength of 1060 nm. The generated pulse energy in our laser can be estimated as P/ν~ 2–4 μJ, where P= 40 mW is the average power and ν= 10–20 kHz is the pulse repetition rate. In this case, the generated pulse energy is below saturation energy level. As a result, the lasing mode competition is rather weak at least during the first pulse formation.

Obviously, the seed for pulse formation (at least during the laser start) is spontaneous emission modulated with the transmission spectral function of the incorporated filter (a Lyot filter in our case). Waves at many wavelengths are amplified in the gain medium during pulse formation. The pulse formation time as well as pulse duration time (τ= 2–4 µs) both considerably exceed the cavity round trip time (τ₀= 80–300 ns). The nature of the pulse radiation is close to the amplified spontaneous emission (ASE), i.e. superluminescence, except for two points. The first point is the mode cavity structure. The analysis of the RFspectrum shows that the optical spectrum consists of a huge number of longitudinal modes. This also means that the effects of spatial inhomogeneity along the gain fiber (associated with spatial hole burning) are also negligibly small. This uniform gain saturation also prevents mode competition during generation of subsequent pulses. The second point is the narrow linewidth for each generated lines: the linewidth of each line is much narrower than the wavelength spacing (period) in experimentally measured spectra. We believe that an amplified spontaneous emission originating from the active medium will obtain sinusoidal spectral modulation (because of the Lyot filter transmission function) after a single roundtrip pass through the cavity. Multiple transmission through the filter results in a narrowing of the transmission function. In our case, the multiplicity is provided by a high gain factor in spite of the low feedback. As a result, the cavity plays an important role in radiation formation in contrast to ASE sources operating without any feedback. We believe that our source can be considered as a laser. However, some additional investigation is required to answer the questions about the coherence properties of the generated radiation.

In the present paper, we presented a technique for the suppression of wavelength competition in rare-earth-doped fiber lasers and for obtaining simultaneous lasing at tens (up to several hundred) laser lines with wavelength spacing down to 50 pm. An ytterbium-doped fiber (YDF) laser is used for experimental demonstration. Spectral selection is based on a built-in Lyot filter, whose length can be selected to set suitable wavelength spacing. The method for the mode competition suppression is based on the gain-switching technique with a modulated pump. At certain modulation parameters (amplitude, duration, and repetition rate), the laser generates a stable sequence of microsecond pulses with a peak power of ~1 W consisting of many longitudinal modes. The measured linewidth for each generating line is less than 100 pm and can be as little as 20 pm. The output polarization state is not linear, despite the all-PM configuration of the presented source. It is expected that the polarization state of the output radiation can be changed to a linear one in modified schemes. For example, a Lyot filter can be incorporated into highly reflective or output fiber loop mirrors. We believe that the effect of multi-wavelength lasing is connected with appropriate gain control and small gain saturation level in the laser. It can be expected that a source with amplitude control of each spectral line can be designed using this technique, for example, in a scheme where an intracavity filter is based on a set of fiber Bragg gratings [2]. In this case, the proposed multi-wavelength laser source can be used for sensing.

The work was performed within the framework of the State assignment of the IA&E SB RAS (No. AAAA-A19-119112990054-4) with the employment of the facilities of the Multiple-Access Centre 'High-resolution Spectroscopy of Gases and Condensed Matter' at the IA&E SB RAS, Novosibirsk, Russian Federation.