An efficient single frequency fiber laser by using a newly-developed Er3+/Yb3+ co-doped single mode phosphate glass fiber with the net gain coefficient of 5.2 dB/cm and propagation loss coefficient of 0.04 dB/cm has been demonstrated. Over 300 mW stable continuous -wave single transverse and longitudinal mode seed lasering at 1.5 μm has been achieved from a 2.0 cm-long active fiber. The measured slope efficiency and the calculated quantum efficiency of laser emission are found to be 30.9% and 0.938 ± 0.081, respectively. It is found that the linewidth of the fiber laser is less than 2 kHz, and the measured relative intensity noise (RIN) is around −120 dB/Hz in the frequency range of 50 to 500 kHz.
©2010 Optical Society of America
Single frequency fiber laser has been the subject of intense research in the last two decades for applications, such as high resolution sensing, coherent telecommunication, optical frequency domain reflectometry, and as a seed laser for LIDAR [1–3]. Of these short resonance cavity configuration, such as distributed Bragg reflector (DBR), is beneficial to single frequency laser emission for mode-hop free, narrower linewidth, lower noise, and all in a compact all- fiber design [4–8]. Recently, Spiegelberg et al have reported DBR laser emission around 1550 nm in Er3+/Yb3+ co-doped phosphate glass fibers [7,8]. Single frequency laser with the output power of over 200 mW and the linewidth of < 2 kHz has been achieved from a 2-cm-length phosphate glass fiber by the authors. However, the effective length of the resonator is designed to be 5 cm, which easily leads to multi-longitude emissions. In order to select one longitudinal mode, the linear cavity should be shortened further or a composite fiber grating should be adopted. Shortening the resonance cavity will limit the laser output power and thus higher concentrations of rare-earth ions should be doped into the glass fiber core. Furthermore, the upconversion effects will be more serious with the increase of the concentrations of rare-earth ions , and a great deal of heat generated will decrease the quantum efficiency further. Therefore, developing the Er3+/Yb3+ co-doped phosphate glass fiber with high gain coefficient and low propagation loss and low heat accumulation are key points to achieve efficient single frequency lasers.
Recently, we have reported that a homemade 3.0 cm Er3+/Yb3+-codoped phosphate glass fiber could provide an internal gain up to 36 dB . Here we report a more efficient and compact single frequency fiber laser with high output power and narrow linewidth based on our newly-developed Er3+/Yb3+-codoped phosphate single mode glass fibers and the 3D short-cavity heat flow model.
2. Active fiber and single frequency fiber laser design
RE ions were doped uniformly in the core region with concentrations of 3.0mol% for Er3+, and 5.0mol% for Yb3+, respectively. The fluorescence lifetime of the 4 I 13/2-4 I 15/2 transition of Er3+ ions is 8.1 ms in a phosphate fiber 4 mm in length. The absorption and emission cross sections are 5.96 × 10−21 cm2, and 7.17 × 10−21 cm2 at 1534 nm, respectively. The refractive index of the core and cladding glass are measured to be 1.535 and 1.522 via a prism coupler (Metricon Model 2010) at 1310 nm, respectively. The phosphate glass fiber designed has a core diameter of 5.4 µm with a numerical aperture (NA) of 0.206 at 1.5 µm. The Er3+/Yb3+-codoped phosphate glass fiber was fabricated using a fiber-drawing tower (TDR-2, Japan) based on the rod-in-tube technique . The cross section of the phosphate glass fiber is detected via an amplified CCD viewer, as shown in the inset of Fig. 1 . The core-to-cladding offset is less than 0.4 µm. The mode-field diameter at 1550 nm is estimated to be 6.24 µm and the cut-off wavelength was calculated to be 1470 nm. The average propagation loss measured by the cut-back method is lower than 0.04 dB/cm at 1310nm, which is the lowest value reported in this kind of fiber [7,8,10–13]. The gain and noise figure characteristics of the Er3+/Yb3+-codoped phosphate glass fiber have been demonstrated, as shown in Fig. 1. A net gain per unit length of up to 5.2 dB/cm at 1535 nm was obtained from a 40-mm-length Er3+/Yb3+-codoped phosphate glass fiber, which is the highest gain coefficient reported in this kind of fiber [7,8,10–13]. The obtained noise figures of different signal wavelengths from 1525 nm to 1565 nm were less than 5.5 dB.
A laser cavity is established by one spectrally narrow band fiber Bragg grating (NB-FBG) and one dielectric mirror that is butt-coupled to the one end facet of a short piece of Er3+/Yb3+-codoped phosphate fiber, as shown in Fig. 2 . The NB-FBG with a 3-dB linewidth of 0.06 nm and a center-wavelength reflectivity 50.5% has been fabricated. The reflectivity ofthe dielectric mirror is larger than 99.5% at the signal wavelength of 1535 nm and smaller than 5% at the pump wavelength of 976 nm, which can diminish the pump light back to the pump laser diodes (LDs) and thus reduces the instability of the pump source. In order to improve the pump/signal coupling efficiency further, the NB-FBG had been irradiated in the Corning HI 1060 FLEX fiber with a mode-field diameter of 6.3µm at 1550 nm and 4.0µm at 976 nm. The NB-FBG was fused splicing with the 2-cm long phosphate fiber. The effective length of the resonator includes the 2.0 cm active fiber and a half of the 1.5 cm NB-FBG irradiated area. It is less than 3 cm, giving a longitudinal mode spacing of 3.4 GHz. The NB-FBG has a reflection bandwidth of less than 7.5 GHz, supporting only one longitudinal mode. The laser cavity was assembled into a copper tube, which was temperature-controlled by a cooling system with the resolution of 0.05°C. With a proper temperature control, the laser will operate in a single frequency without mode hop and mode competition phenomena. Two high power 976 nm FBG-stabilized pump lasers (PL1 and PL2) with orthogonal polarization output were combined through a polarization beam combiner (PBC). The pump lasers are coupled into the laser cavity through a 980/1550 nm WDM. The emission spectrum and the optical power of fiber laser is measured by an optical spectrum analyser (OSA, Anritsu MS9710C) and a power meter (PM, Ophir NovaII), respectively.
3. Single-frequency fiber laser performance
Figure 3 shows the laser output power at 1.5 μm from the Er3+/Yb3+-codoped phosphate glass fiber versus the pump power. The lasing threshold is around 80 mW. When the pump power is above the threshold, the laser output power is linearly enhanced with increasing the pump power. A maximum output power of 306 mW has been achieved from the 2.0 cm phosphate fiber at the pump power of 1072 mW, which is, to the best of our knowledge, the highest output power from this kind of fiber lasers reported to date. [7-12] The slope efficiency of the laser emission is measured to be 30.9% and the experimental quantum efficiency of the laser emission related to the absorbed pump power is estimated to be 58% since only 84% of the pump power is coupled into the phosphate fibre core due to the coupling loss, scattering, and pump leakage. It should be pointed out that the pump power illustrated in Fig. 3 is the nominal power before coupling into the WDM. No output power saturation phenomenon is observed, indicating that the output power will rise further with increasing the pump power. The center wavelength of laser emission spectrum of 1534.75 nm and the side mode suppression ratio (SMSR) of > 65 dB has been measured by the OSA. The transient fluctuations of the output power at 250 mW have been investigated as shown in the inset of Fig. 3. The output power fluctuations of < ± 0.18% of the average power were observed, which is caused by the small fluctuations in the pump laser power. Meanwhile, we have measured the long-term stability of the output power over 40 h. If the ambient temperature is held 23°C, the output power fluctuations were less than ± 0.5% over the entire period of time.
In order to assess the performance of Er3+/Yb3+ co-doped glass fiber and intend to further increase the laser output power, it is necessary to evaluate the quantum efficiency φ without and with laser action, the former is fluorescence quantum efficiency and the latter is defined as the fraction of emitted photons by the absorbed photons. Without laser action the fluorescence quantum efficiency (the ratio between its radiative and total rates) is given as φ = τ/τrad , and the value is gotten to be ~0.903. The fractional thermal loading η can be determined by the quantum efficiency φ as η = 1−φ (λex / <λem >) and the value is 0.431. With laser emission the quantum efficiency φ in fiber laser can be expressed as :16]. Figure 4 show the quantum efficiency in different pump power above the threshold value. The average quantum efficiency is found to be 0.938 ± 0.081 in our laser system, which is nearly the same as the Er3+ doped fiber laser reported by Barnes . The results show that an efficient energy-transfer exists in the Er3+/Yb3+ codoped phosphate glass fiber.
The single frequency operation was verified by a scanning Fabry–Pérot spectrum analyzer that had a free spectral range of 300 MHz and a finesse of 300. In order to further investigate the laser spectral characteristics, the linewidth of the fiber laser was measured by the self-homodyne method using a 48.8-km-fiber delay. Figure 5 shows the homodyne signalspectrum of the fiber laser measured by a radio frequency (RF) electrical spectrum analyzer (ESA, Aglient N9320A). It is 32 kHz with −20 dB from the peak, which indicates the laser linewidth is approximately 1.6 kHz FWHM. The rise at the zero frequency is caused by the RF spectrum analyzer. The fall at low frequencies below 2 kHz is caused by the low-frequency filter in the photoreceiver.
The relative intensity noise (RIN) of the fiber laser has been measured and is shown in Fig. 6 . The RIN at the low frequencies of < 50 kHz decreases from −86 dB/Hz to −120 dB/Hz with increasing the frequency and is stabilized at approximately −120 dB/Hz for frequencies above 50 kHz. The peak of RIN is observed at the several kHz, which is mainly caused by the ambient acoustics and vibration. The peak of the relaxation oscillation frequency of the fiber laser hasn’t been observed at the frequencies of < 500kHz.
In summary, we have demonstrated a 300 mW narrow linewidth fiber laser at 1.5 μm from an 2.0-cm short-length Er3+/Yb3+ heavily doped phosphate fiber. The fiber laser operates at a single frequency with the linewidth less than 2 kHz and the slope efficiency is 30.9%. The relative intensity noise (RIN) of the fiber laser is found to be −120 dB/Hz for frequencies above 50 kHz. The results indicate that the Er3+/Yb3+-codoped phosphate single mode glass fiber might be a promising candidate as an efficient narrow-linewidth single frequency fiber laser.
The authors would like to acknowledge support from the NSFC (Grant Nos. U0934001 and 60977060).
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