A spectrally clean kHz-linewidth single-polarization single-frequency distributed Bragg reflector Yb-doped phosphate fiber (YPF) laser at 1120 nm (> 1100 nm) for the first time is demonstrated. By enhancing the reflectivity of output fiber Bragg grating and optimizing the length of YPF to implement the effective ASE suppression and single-longitudinal-mode long-wavelength lasing, a stable output power of over 62 mW is achieved from a 31-mm-long highly YPF with a linewidth of 5.7 kHz. The signal to noise ratio of > 67 dB, the polarization extinction ratio of > 25 dB, and the relative intensity noise of < –150 dB/Hz for the frequencies above 10.0 MHz are obtained in such single-frequency fiber laser. This narrow linewidth fiber laser is an ideal laser source to generate the coherent single-frequency 560 nm light via frequency doubling for biochemical analysis application.
© 2016 Optical Society of America
Yb-doped single-frequency fiber lasers (SFFLs) have been drawn much attention for their outstanding performance in terms of narrow linewidth, low noise, good beam quality, and all-fiber format [1–7]. Although the emission spectrum range of Yb-doped fiber can extend to as long as 1200 nm [8–10], the operating wavelengths of reported SFFLs are all in the conventional region (980–1080 nm) [3, 11–13]. However, the Yb-doped SFFL above the wavelength of 1100 nm is also urgently desired and has not been reported so far. Among them, the typical representative of 1120 nm SFFL has several interesting potential applications requiring a narrow linewidth and/or linearly polarized output characteristics. Especially, it can be frequency-doubled compactly and efficiently at 560 nm to exhibit a several-fold improvement in the fluorochrome to autofluorescence ratios between phycoerythrin-labeled cells and unlabeled controls, which shows its ability to increase phycoerythrin and DsRed fluorescent protein detection sensitivity [14, 15]. Besides, it also can be used as a pump source for Tm-doped fiber laser by using strong narrow in-band absorption lines  and the 1178 nm Raman fiber laser or amplifier to further generate yellow light via frequency doubling [17–19].
Up to now, based on typical cavity length of several meters, several Yb-doped fiber lasers above 1100 nm have been demonstrated [20–23], but the discrimination of closely spaced longitudinal modes is difficult in such long-cavity fiber lasers. The short-cavity distributed Bragg reflector (DBR) structure is attractive approaches to realizing single-frequency lasing. For effective single-longitudinal-mode operation, the length of active fiber is limited to several centimeters, requiring that a high-gain coefficient can be provided. Due to the high solubility of rare-earth ions in phosphate glass, heavily Yb-doped phosphate fibers (YPFs) have been fabricated to develop successfully for short-cavity DBR SFFLs and high-gain fiber amplifiers shorter than 1083 nm [1–3, 24]. To further develop the Yb-doped SFFL at long-wavelength (> 1100 nm), the key issue is a sharp drop of the emission crosses-section at signal wavelength. There exist a strong gain competition with the amplified spontaneous emission (ASE) at short-wavelengths, which may results in the parasitic lasing and critical damage of the fiber components. Moreover, a short-cavity DBR configuration is necessary to enlarge the longitudinal mode spacing by shortening the cavity length. On the contrary, the length of active fiber should be chosen as long as possible in such cavity to enhance the pump absorption for guaranteeing the long-wavelength lasing and optical efficiency.
In this paper, a spectrally clean 1120 nm kHz-linewidth single-polarization DBR SFFL based on a 31-mm-long highly YPF is reported. More than 62 mW of single-polarization and single-longitudinal-mode laser output is obtained with a linewidth of 5.7 kHz. To our best knowledge, this is the first demonstration of the DBR Yb-doped SFFL, operating at long-wavelength (> 1100 nm).
2. Experimental setup
The experimental setup of 1120 nm single-polarization DBR SFFL is shown in Fig. 1. The DBR short-cavity fiber laser is constructed by cleaving high-reflection (> 99.9%) fiber Bragg grating (HR-FBG) and polarization-maintaining partial-reflection ( = 83.5% at signal wavelength) FBG (PM-FBG) very close to the grating area, and then directly splicing to a short length of self-developed heavily YPF. The 3 dB-bandwidth of the PM-FBG and HR-FBG are 0.05 nm and 0.35 nm, respectively. Because the PM-FBG is written in a single-mode PM fiber and the wavelength separation between the two reflection peaks, corresponding to two linear polarization states, is larger than the 0.2 nm bandwidth. This pair of FBGs is chosen so that only the reflection peak that corresponds to the slow axes of the PM-FBG would fall into the center of the HR-FBG, thus the single-polarization operation can be achieved. The highly YPF is drawn by using a rod-in-tube technique and 15.2 wt% Yb2O3 is doped uniformly in the core region. The peak absorption cross-section at 976 nm and emission cross-section at 1120 nm of the core glass are 1.33 × 10−20 cm2 and 0.02 × 10−20 cm2, respectively. The YPF is a 5.0/125 μm single-cladding fiber with a numerical aperture of 0.14. More details of the fiber properties can be found in [1, 2].
A 976 nm fiber-coupled laser diode (LD) with a maximum output power of approximately 360 mW is spliced to the pump port of a 980/1120 nm PM wavelength division multiplexer 1 (PM WDM 1). The common port of PM WDM 1 is then spliced to the HR FBG. The YPF is spliced directly with two FBGs. The common port of another PM WDM 2 is spliced to the PM FBG. The PM WDM 2 is used to separate the signal laser and the residual pump. In order to reduce the optical reflections, all spare ports of the PM WDMs are fusion spliced with angled physical contact (SC/APC)-type optical connectors. The laser cavity is directly mounted in a copper tube that is temperature-controlled by a cooling system with an accuracy of ± 0.1 °C.
3. Results and discussion
In general, the primary limitation on the laser emission at 1120 nm is the generation of ASE or parasitic lasing at shorter and higher-gain wavelengths, which can extract significant pump power. Although it may be removed by the output FBG with high reflectivity, this results in the reduction of optical efficiency. Therefore, the reflectivity is chosen should be balance the requirements for high conversion efficiency and effective ASE suppression. Due to the emission peak of YPF locates at approximately 1010 nm, the emission cross-section of which is about 20 times larger than that for 1120 nm; this significant difference would result in a strong ASE when the gain inside the cavity is sufficiently large.
In order to analyze the gain of one wavelength on the basis of the gain at two other wavelengths in the cavity with a pair of FBGs, numerical modeling is conducted based on the described simulation model [23, 25]. In the modeling, assuming the pump absorptions are 5, 10, and 15 dB. For different pump absorptions, Fig. 2(a) illustrates the simulation results of single-pass ASE gain at 1010 nm versus the reflectivity of output FBG. In fact, the FBG can suppress only 50 dB before spurious lasing occurs, and the gain of ASE should be < 25 dB. Theoretically speaking, under a certain gain of the ASE, higher pump absorption corresponds to the lower reflectivity of output FBG. Obviously, while the reflectivity of output FBG is smaller than 75% in the simulation, the single-pass gain at 1010 nm is higher than 25 dB, in which case, the ASE would rapidly increase. And then, the reflectivity of 55% (< 75%) with the YPF length of 40 mm is subsequently tested in the experiment. The experimental results of output spectra from the cavity sample are recorded by an optical spectrum analyzer (OSA) with a spectrum resolution of 0.1 nm, as plotted in the inset of Fig. 2(a). It is clear from the inset of Fig. 2(a) that the relative low reflectivity of output FBG is insufficient to suppress the ASE. As a result, the ASE and parasitic lasing at around 1010 nm appeared evidently at all the pump powers. However, in a real laser system, one should take into account the loss in the cavity. Thus, the reflectivity of approximately 80% is chosen for the PM-FBG in the experiment. Accordingly, the calculated single-pass gain of 1010 nm ASE is about < 7.5 dB, which is sufficiently safe for the laser emission at 1120 nm.
The length of YPF is optimized by using the described simulation model to solve the rate equation [26–28], providing the reflectivity of two FBGs and the launched pump power. Here, the reflectivity of the HR-FBG and PM-FBG are 99.9% and 80%, respectively. For the launched pump power of 200 mW, 500 mW, and 1000 mW, the achievable output powers in dependence of the length of YPF are calculated, the simulation results as shown in Fig. 2(b). The calculated optimal length of YPF is about 33 mm from the simulation. Under the condition of different pump powers, the calculated slope efficiencies are approximately 20%, 27%, and 29%, respectively. On the one hand, the practical pump absorption would be slightly weaker than the theoretical one. On the other hand, the length of YPF should be chosen as long as possible to enhance the pump absorption and thus assure the long-wavelength lasing and optical efficiency. Consequently, the YPF length of 37 mm that is longer than the optimal length is chosen in the experiment. By using the cut-back method from 37 mm so as to find the longest YPF length can be employed, which allows stable single-longitudinal-mode operating and yields efficient pump absorption simultaneously. For each of the cavity samples, the longitudinal mode characteristics are measured by a scanning Fabry–Perot interferometer (FPI) with a free spectral range (FSR) of 1.5 GHz and a finesse of 200. When the YPF is cut to 31 mm, stable single-longitudinal-mode output is produced. In this case, the effective cavity length is less than 35 mm, including the 31-mm-long YPF and half of the 5-mm-long PM-FBG (a reflection bandwidth of < 11.9 GHz). It gives a longitudinal mode spacing of >2.9 GHz. Therefore, a 31-mm-long highly YPF is selected for the SFFL used in the experimental measurement below.
The output power of 1120 nm single-frequency fiber laser as a function of the launched pump power and the absorbed pump power are measured for comparison, as shown in Fig. 3(a). The lasing threshold is around 40 mW. The maximum output power of 62 mW is obtained at the launched pump power of about 360 mW and the corresponding conversion efficiency is about 17.2% (the output power versus the launched pump power). Actually, there are some factors that contribute to loss, such as the insertion losses of the two PM WDMs, thus it is clear that the experiment is in very good agreement with the simulation result. Due to a part of pump is residual, and the efficiency can reach to be 19.7% if the residual pump is excluded. Therefore, the efficiency can be improved by further reducing the losses of optical components or increasing the launched pump power. Moreover, at the maximum output power, the fiber laser operates continuously for two hours and the output power instability of < ± 1% is observed.
The output spectrum of the fiber laser operating at 60 mW is recorded by an OSA with a spectrum resolution of 0.1 nm, as plotted in Fig. 3(b). It clearly shows that the signal-to-noise ratio (SNR) of this single-frequency fiber laser is higher than 67 dB and the laser spectrum centered at around 1120 nm. And there is no strong ASE or spurious lasing at short-wavelengths even at the maximum pump power. Furthermore, the single-frequency output of the fiber laser is confirmed by using a scanning FPI with a finesse of 200 and a resolution of 7.5 MHz as illustrated in the inset of Fig. 3(b). The FSR of the FPI is approximately 1.5 GHz as indicated in the figure. The absence of any peaks between the main resonances of the interferometer clearly indicates the operation on only one longitudinal mode. With the strict temperature control, the laser operated stably in a single-frequency regime without mode hopping and mode competition phenomena during 2 hours observation. Also, the polarization-extinction ratio (PER) of the fiber laser is measured by an optical polarization analyzer. The PER of > 25 dB sufficiently confirms single-polarization operation of this fiber laser.
The relative intensity noises (RINs) of the fiber laser at different pump powers are measured using an electrical spectrum analyzer with a resolution bandwidth of 1 kHz. Figure 4(a) shows the RINs in the frequency range of 0–50.0 MHz. It can be seen that the RIN spectra are dominated by peaks at the relaxation oscillation frequency in about 0.75 MHz with the RIN level of around –110 dB/Hz, which depending on the laser cavity layout and the pump current. While the frequencies from 2.0 to 50.0 MHz, the RIN of the fiber laser drops from –140 to < –150 dB/Hz, which approaches the shot noise limit of –154.5 dB/Hz.
To further investigate the noise characteristics of the fiber laser, the linewidth of fiber laser at the maximum output power is measured by a delayed self-heterodyne method with a 10-km-long fiber delay. The sweep time of the electrical spectrum analyzer is about 0.12 s with a resolution bandwidth of 100 Hz. Figure 4(b) shows the linewidth result of the measurement. The typical heterodyne signal is fit to a Lorentzian profile to estimate the spectral linewidth. It is 114 kHz with −20 dB from the peak, which indicates that the fiber laser possesses a linewidth of 5.7 kHz FWHM (full width at half maximum).
In summary, a spectrally clean 1120 nm kHz-linewidth single-polarization single-frequency DBR Yb-doped phosphate fiber laser is developed based on a 31-mm-long highly YPF, for the first time. By enhancing the reflectivity of output FBG and optimizing the length of YPF to realize the ASE suppression at short-wavelengths and single-longitudinal-mode operating at long-wavelength, an output power of more than 62 mW, a SNR of > 67 dB, and a laser linewidth of 5.7 kHz are obtained. A stable single-polarization laser output with a PER of >25 dB and a RIN of < –150 dB/Hz for the frequencies above 10.0 MHz are attained. The results show that the 1120 nm SFFL is an ideal laser system to generate the coherent single-frequency 560 nm light through frequency doubling for biochemical analysis application.
National Key Research and Development Program of China (2016YFB0402204), China State 863 Hi-tech Program (2014AA041902), NSFC (11674103, 61635004, 61535014, 51132004, and 51302086), the Fundamental Research Funds for Central Universities (2015ZP013 and 2015ZM091), China National Funds for Distinguished Young Scientists (61325024), Guangdong Natural Science Foundation (S2011030001349, S20120011380 and 2016A030310410), and the Science and Technology Project of Guangdong (2013B090500028, 2014B050505007, and 2016B090925004).
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