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Abstract
Frequency doubling of random fiber lasers could provide an effective way to realize visible random lasing with the spectrum filled with random frequencies. In this paper, we make a comprehensive study on the efficiency and spectral manipulation of a green random laser generated by frequency doubling of an ytterbium-doped random fiber laser (YRFL). To tailor the efficiency of green random lasing generation, the ytterbium-doped random fiber lasing is filtered at different spectral positions, and then amplified to watt-level to serve as the fundamental laser source for frequency doubling in a periodically poled lithium niobate (PPLN) crystal. We found that by selecting different spectral components of ytterbium-doped random fiber lasing, the temporal intensity fluctuations of the filtered radiations vary dramatically, which plays an important role in enhancing the efficiency of frequency doubling. By fixing the filtering radiation wavelength at 1064.5 nm and tuning the central wavelength of YRFL, we experimentally demonstrate that, compared to the filtered radiation in the center of the spectrum, the efficiency of frequency doubling can be nearly doubled by utilizing the filtered ytterbium-doped random fiber lasing in the wings of the spectrum. As a result, the conversion efficiency of the generated green random laser at 532.25 nm can be more than 11% when the input power of the polarized 1064.5 nm fundamental light is 2.85W. For spectral manipulation, we realize a spectral tunable green random laser in the range of 529.9 nm to 537.3 nm with >100 mW output power for the first time by tuning the wavelength of YRFL and the temperature of PPLN simultaneously. The system can be naturally modified to simultaneously realize the efficiency enhancement and wavelength tuning, thus providing a new route to generate high efficiency and tunable visible random laser via frequency doubling that are potentially useful for imaging, sensing and visible light communication applications.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Random fiber lasers (RFLs) have attracted extensive investigations in the past decade due to their unique physical properties and wide range of potential applications [1,2]. RFLs have shown their high laser performance in terms of compact and simple cavity, high efficiency/high power output [3–6], wavelength agility [7–10], multi-wavelength lasing with flat amplitude [11,12], narrowband [13–15] and low noise [16–18]. With these properties and advantages, RFLs have found emerging applications in fiber-optic communications [19], sensing [20,21], imaging [22] and pump sources for mid-infrared laser [23].
Although the wavelengths of the demonstrated RFLs based on silica fiber are limited in the near-infrared region, assisted by single-pass frequency doubling, RFLs could still perform as the promising sources to generate visible random lasing thanks to their high output power and broad wavelength tuning capability in different spectral bands [24–26]. Frequency doubling of RFLs could not only provide an alternative way to realize visible random laser with the spectrum filled with random frequencies, but also extend the applications of RFLs such as temporal ghost imaging [27] and speckle-free imaging [22,28] to visible regions. Random laser in the visible based on frequency doubling of RFL was first reported in [29] with the generation of 110 mW 654 nm wavelength in a periodically poled lithium niobate (PPLN) crystal. Recently, random laser at 532 nm was realized by frequency doubling of polarized 1064 nm ytterbium-doped random fiber laser (YRFL) [30]. However, in these studies, the wavelengths of the visible random laser are fixed, and the bandwidth of RFLs exceeds the quasi-phase matching (QPM) bandwidth of the used PPLN crystal, which limits the conversion efficiency of frequency doubling. On the other hand, the efficiency of frequency doubling is not only dependent on the bandwidth of the laser but also the laser temporal dynamics and statistics [29,31], by considering that the frequency-doubled intensity fluctuations are proportional to the square of the intensity fluctuations at the fundamental frequency. In [31], N. Valero et al. demonstrated the chaotic temporal events of amplified spontaneous emission (ASE) source can enhance the efficiency of frequency doubling compared to the single-frequency laser. However, the intensity probability density function (PDF) of the ASE source is fixed with the exponential distribution [31–33], which means the enhancement factor of the frequency doubling efficiency with ASE source is limited to some extent. Regarding to the RFLs, the differences in radiation statistics at different spectral positions of RFLs have been experimentally revealed for both the Raman gain [33–35] and erbium-ytterbium co-doped gain RFLs [36], which can be exploited to tailor the efficiency of frequency doubling.
In this paper, we make a thorough investigation on the efficiency and spectral manipulation of green random laser generated by frequency doubling of an YRFL. For efficiency enhancement, the YRFL is filtered at different spectral positions to tailor the temporal dynamics and statistics of the filtered radiations, and then the filtered radiation with 0.04 nm of -3 dB bandwidth is amplified to watt-level to serve as the fundamental laser source for frequency doubling in a PPLN crystal. Therefore, the filtered radiation features both the narrowband characteristics and the tailored statistics, which could significantly enhance the efficiency of frequency doubling. We experimentally verify that, compared to the filtered radiation in the center of the spectrum, the efficiency of frequency doubling can be nearly doubled by utilizing the filtered ytterbium-doped random fiber lasing in the wings of the spectrum. For spectral manipulation, we realize the frequency doubling of a tunable YRFL, and generate the continuously tunable green random laser in the range of 529.9 nm to 537.3 nm with >100 mW output power for the first time, to the best of our knowledge.
2. Experimental setup
The schematic experimental setup for tailoring the efficiency and spectrum of green random laser generated by frequency doubling of YRFL is shown in Fig. 1. To tailor the efficiency of frequency doubling, we use the filtered YRFL as the fundamental laser source. The YRFL is cladding pumped by a 976 nm laser diode (LD) in a laser cavity which combines a 5 m-long ytterbium-doped double-cladding fiber (YDF, Nufern LMA-YDF-10/130) and a 5 km-long standard single-mode fiber (SMF, Corning G.652). We choose 5 km-long SMF because it can provide sufficient random Rayleigh scattering feedback to realize the quasi-CW ytterbium-doped random fiber laser (YRFL) with stable and modeless spectrum. To generate the wavelength tunable YRFL, a tunable filter with 0.1 nm bandwidth is incorporated into the 1:1 optical coupler-based fiber loop mirror, which provides the wavelength-selectable point feedback. By combining the wavelength-selecting point feedback at the signal port of pump combiner and the Rayleigh scattering based random distributed feedback in SMF, tunable ytterbium-doped random fiber lasing can be realized with the output power >300 mW at 3 W LD pump power. Then, a narrowband filter consisting of an optical circulator and a fiber Bragg grating (FBG) with 1064.5 nm center wavelength is used to filter the specified spectral component in the YRFL radiation. The -3 dB bandwidth of the FBG is 0.04 nm and the peak reflectivity is ∼ 40%. We carefully tune the wavelength of YRFL to control the wavelength detuning between the central wavelength of YRFL and the filtering wavelength. In this way, we can select different spectral components of ytterbium-doped random fiber lasing, e.g. in the central part of the YRFL spectrum or in the wings of the spectrum. Since the power of the filtered radiations are different and only in mW level, a pre-amplifier (PA) is used to boost the power of the filtered radiations to 300 mW. The filtered YRFL seed is then further amplified in the main amplifier with a 6 m-long YDF (Nufern LMA-YDF-10/130), and finally output after an isolator for frequency doubling.
Fig. 1. Schematic experimental setup for tailoring the efficiency and spectrum of green random laser generated by frequency doubling. LD, laser diode; Com, pump combiner; YDF, ytterbium-doped fiber; SMF, single mode fiber; ISO, isolator; Cir, circulator; FBG, fiber Bragg grating; PA, pre-amplifier; L, lens; P, polarizer; DM, dichroic mirror; PM, power meter
To generate green random lasing, the frequency doubling is carried out by using a PPLN crystal in a single-pass configuration. The filtered 1064.5 nm YRFL radiation with random polarization is first collimated with a fiber collimator and made linearly polarized by a polarizer. The polarized beam is focused into the PPLN crystal (HC Photonics, SHVIS-MD) by using a 75-mm focal lens. The estimated beam waist radius inside the crystal is ∼ 18.2 μm. The dimensions of the crystal are 0.5 (height) × 1 (width) × 10 (length) mm. The quasi-phase matching (QPM) period of the PPLN is 6.86 μm, and the QPM bandwidth is ∼ 0.21 nm. After frequency doubling, the residual infrared radiation is filtered by using a dichroic mirror, and the generated green random lasing is detected by a power meter (Ophir, 3A) and a spectrometer (Ocean Optics, Maya2000). The spectrum of YRFL is measured by the optical spectrum analyzer (OSA) (YOKOGAWA, AQ6370D).
For spectral manipulation, the experimental setup is similar except that the spectral filtering components shown in the dash box in Fig. 1 are removed. The tunable YRFL seed is then directly amplified to watt-level in the main amplifier. At the frequency doubling stage, the temperature of the PPLN crystal is adjusted by a crystal oven to fulfil the QPM condition for the given YRFL wavelength. In this way, by tuning the wavelength of YRFL and the temperature of PPLN simultaneously, the tunable green random laser can be realized via frequency doubling.
It should be mentioned that although in this paper the manipulation of efficiency and spectrum are demonstrated individually due to the limitation of the available experimental apparatuses, the realization of both efficiency enhancement and spectral tuning is naturally achievable by using another tunable narrowband filter instead of the FBG based filter.
3. Tailoring the efficiency of frequency doubling with filtered YRFL
To select different spectral components of YRFL, the central wavelength of YRFL is continuously tuned from 1064.5 nm to 1064.7 nm, which is shown in Fig. 2(a). With the relatively high LD pump power (3 W) which is well above the threshold (0.8 W) and the long SMF (5 km), the YRFL can operate in the modeless regime without spurious modes [27,30], Similar to our previous work [27], we have checked the temporal intensity autocorrelation function of YRFL to ensure its modeless operation. The -3 dB spectral bandwidth of YRFL is 0.2 nm. The dash line in Fig. 2 (a) represents the spectral position of the spectral filter, and we can see that by increasing the detuning between the central wavelength of YRFL and that of the filter, the filtered spectral component of YRFL shifts from the central part to the wings of the spectrum. The filtered radiation is pre-amplified to 300 mW before entering the main amplification. Figure 2(b) represents the spectra evolution of the random lasing filtered in the wings of the spectrum (detuned by 0.2 nm from the center) with the increase of LD power in the main amplifier. The filtered radiation has a >35 dB signal-to-noise ratio and a narrow -3 dB bandwidth down to 0.04 nm, which is much narrower than the QPM bandwidth of the used PPLN. We also verified in Fig. 2 (b) that the bandwidth of the random lasing remains unchanged with the increase of the LD power. Since the amplified radiation is depolarized, the incident power for frequency doubling is measured after the polarizer, and the output power of filtered YRFL after polarizer versus LD power of the main amplifier is depicted in Fig. 2(c). The polarized 1064.5 nm random laser radiation can be boosted up to 2.82 W with the slope efficiency of ∼ 31%.
Fig. 2. (a) Spectra of tunable YRFLs for spectral filtering, the dash line represents the position of the spectral filter; (b) spectra evolution of the filtered radiation detuned by 0.2 nm from the center with the increase of LD power in the main amplifier;(c) output power of filtered YRFL after polarizer versus LD power of the main amplifier
To characterize the temporal dynamics and statistics of the filtered random lasing sources, the filtered radiations are detected by a photodetector (PD) with 20 GHz bandwidth and then recorded by the oscilloscope (OSC, R&S RTP016) with 16 GHz bandwidth and 40 GSamples/s sampling rate. The −3 dB bandwidth of the filtered radiation is 0.04 nm (10.6 GHz for the wavelength of 1064.5 nm), which is less than the bandwidth of the PD and OSC. Therefore, there is no effect of frequency average and we can measure the actual intensity dynamics and statistics for different spectral components of ytterbium-doped random fiber lasing. The results are shown in Fig. 3. The intensity profiles of the filtered radiations are quasi-continuous with fast fluctuations. One can see in Fig. 3(a) that the temporal intensity fluctuations are becoming stronger with the increase of the detuning between the central wavelength of YRFL and the filtering wavelength. For the filtered radiation at the left edge of the spectrum detuned by 0.2 nm from the center, intense peaks with the power more than 40 times the average value can be observed.
Fig. 3. (a) Temporal intensity dynamics and (b) intensity PDF of filtered ytterbium-doped random lasing radiations at different spectral locations. The black dash line represents the exponential distribution.
The intensity probability density functions (PDFs) of the filtered radiations intensity I(t) normalized to the mean value < I(t)> at different spectral locations are calculated from the time traces containing 2×108 samples and are plotted in Fig. 3(b). The probability of intense events becomes higher when the filtering wavelength is far away from the center of the spectrum, and this phenomenon is also observed in random Raman fiber laser [33] and erbium-ytterbium co-doped RFL [36], which is associated with turbulent four-wave mixing interactions between different frequency components [35–38]. The black dashed line in Fig. 3 (b) represents the exponential distribution. We can clearly see that compared to the ASE source in [31] which obeys the exponential distribution, the intensity PDF of the filtered YRFL can be tailored with different detuning between filtering wavelength and the central wavelengths of YRFL. Therefore, the proposed filtered YRFL provides an effective way to tailor the statistics distribution and can generate the intensity PDF with the probability of extreme events much higher than that defined by the exponential distribution, which is crucial to manipulate and further enhance the efficiency of frequency doubling. In addition, by considering that the temporal dynamics of frequency-doubled light is highly correlated with the fundamental light [39], the use of filtered YRFL can also provide an effective way to regulate the statistics and temporal dynamics of the green random laser.
It is worth mentioning that we have also verified the statistics of the filtered radiation mainly depends on the detuning between the filtering wavelength and the central wavelength of YRFL, rather than the change of central wavelength of YRFL in the range of several nanometers. Therefore, although in our experiment the selection of filtered radiation at different spectral positions is realized at the fixed filtering wavelength by tuning YRFL’s central wavelength, it has the similar effect as the selection of different spectral components from the fixed YRFL spectrum with the help of a tunable filter [33,36,37].
Due to the various radiation statistics, although the incident optical powers of the filtered random laser radiations at different spectral positions are the same, the optical powers of the generated quasi-continuous wave (CW) green random lasers via frequency doubling can be significant different. The green laser output power as a function of the filtered 1064.5 nm YRFL power is illustrated in Fig. 4. The incident power of the fundamental source is measured after the polarizer. One can clearly see the significant enhancement of the generated green random laser power with the radiation filtered in the wings of the spectrum, compared to the case with the radiation filtered in the center of the spectrum. The insert in Fig. 4 shows the conversion efficiency of frequency doubling at maximum incident power versus the detuning between the central wavelength of YRFL and the filtering wavelength. The conversion efficiency for an input power of 2.85 W increases from 6% to 11.5% by increasing the wavelength detuning from 0 nm to 0.2 nm. Therefore, compared to the filtered radiation in the center of the spectrum, the efficiency of frequency doubling can be nearly doubled by utilizing the filtered ytterbium-doped random fiber lasing in the wings of the spectrum. It can be explained as the frequency-doubled intensity fluctuations are proportional to the square of the intensity fluctuations at the fundamental frequency, the temporal dynamics of the filtered radiation in the wings of the spectrum with high contrast fluctuations and higher probability of extreme events is more favorable for efficient frequency doubling. Therefore, compared to the previous works on the frequency doubling of RFLs [29,30], the spectral filtering technique in our scheme enables not only the narrowband but also the statistics tailored radiation, which could significantly enhance the efficiency of frequency doubling. There is no sign of the output power saturation at the maximum available input power, and it should be noted that the power performance of the frequency doubling with our proposed scheme could be further improved by using higher YRFL power and optimizing the focal length and the waist diameter in the crystal.
Fig. 4. Green random laser output power versus incident power of YRFL filtered at different spectral positions. Insert: Conversion efficiency versus the detuning between the central wavelength of YRFL and the filtering wavelength.
4. Wavelength tunable green random laser via frequency doubling
In this section, the widely tunable YRFL is used as the fundamental laser source to generate the wavelength tunable green random laser. As shown in the experimental setup, the tunable YRFL seed is directly injected into the main amplifier for power amplification. The LD power is set as 9 W in the main amplifier, and the spectra are measured by an OSA with 0.02 nm resolution. It is shown in Fig. 5(a) that the wavelength of YRFL can be continuously tuned from 1055 nm to 1080 nm and the -3 dB bandwidths of the YRFLs are kept as 0.2 nm in the entire tuning range. Thanks to the narrowband tunable filter used in our experiment, the bandwidth of the YRFL is comparable to the QPM bandwidth of the 10 mm-long PPLN (0.21 nm), which is essential for the efficient frequency doubling. The output powers against the tuning wavelengths measured after polarizer is shown in Fig. 5(b). The result shows the output powers of different wavelengths are in the range of 2.75-2.95W, with a fluctuation < 8%.
Fig. 5. (a) The spectra of the tunable YRFL. (b) The output power of YRFL after polarizer against tuning wavelength.
To realize the tunable green random laser, the temperature of the PPLN needs to be adjusted to fulfill the QPM condition at the specific YRFL wavelength. Figure 6(a) depicts the spectra of the tunable green random laser by tuning the wavelength of YRFL and the temperature of PPLN simultaneously. The theoretical bandwidth of the frequency-doubled green laser is 0.1 nm in accordance with the bandwidth of 0.2 nm at fundamental wavelength, which cannot be measured accurately by using the spectrometer (Ocean Optics, Maya2000) with relatively low spectral resolution. Nevertheless, the continuous tunable range of green random laser from 529.9 nm to 537.3 nm is clearly shown in Fig. 6(a) by tuning the YRFL wavelength and increasing the temperature of PPLN from 40 °C to 200°C. Figure 6 (b) shows the green laser output power versus the polarized input power of YRFL. 110 mW of the green radiation is obtained at 2.8 W of the input 1064.82 nm YRFL. The variation of green random laser power and conversion efficiency at maximum YRFL power across the tuning range is shown in Fig. 6(c). As evident, the frequency doubling output powers are in the range of 106-125 mW, with the conversion efficiency fluctuating around 4%. The reason for the lower frequency doubling efficiency of tunable YRFL than that of the filtered YRFL in the spectral center with an efficiency of 6% is mainly attributed to the broader -3 dB bandwidth, which results in the lower effective optical power within the PPLN acceptance bandwidth.
Fig. 6. (a) The spectra of the tunable green random laser. (b) Output power of green laser versus the input power of 1064.82 nm YRFL. (c) Variation of green random laser power and conversion efficiency across the tuning range, at maximum YRFL power.
5. Conclusions
In summary, we make a thorough investigation to tailor the efficiency and spectrum of green random laser generated by frequency doubling of YRFL. We experimentally demonstrate a new route to enhance the efficiency of green radiation generation via frequency doubling by tailoring the temporal dynamics and statistics of the filtered YRFL radiation. By fixing the filtering radiation wavelength at 1064.5 nm and tuning the central wavelength of YRFL, the results show that, compared to the filtered radiation in the center of the spectrum, the efficiency of frequency doubling can be nearly doubled by utilizing the filtered ytterbium-doped random fiber lasing in the wings of the spectrum. We also realize the frequency doubling of a tunable YRFL and generate the continuously tunable green random laser in the range of 529.9 nm to 537.3 nm with >100 mW output power. It is to be addressed that although in this work the manipulation of efficiency and spectrum are demonstrated individually, the system can be simply modified to combine the efficiency enhancement and wavelength tuning by using another tunable filter instead of the FBG based filter used in this work. Therefore, this work provides an effective way to generate high efficiency and wavelength tunable visible random laser, which can extend the applications of RFLs to visible regions, such as temporal ghost imaging, speckle-free imaging and sensing.
Funding
National Natural Science Foundation of China (62005186, 62075144); Fundamental Research Funds for the Central Universities (YJ201979, YJ201982).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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