Robust high-power, single frequency operation of semiconductor external cavity lasers is demonstrated in a novel extended two-level doped-fiber absorbing system. Outstanding wavelength and line-width stability is observed in long external cavities.
©2003 Optical Society of America
External cavity lasers are of great interest for a variety of applications in telecommunications, sensing, tunable lasers, spectroscopy etc. There have been many publications on the subject: low chirp single-frequency operation under direct modulation , high-speed dense WDM applications , plug-in-operation in which the oscillation wavelength is selected by an external fiber Bragg grating , 10Gb/s transmission , 20Gb/s , and more recently with a piece of erbium doped fiber in the cavity demonstrating narrow-line-width operation . In the latter publication, it was shown that a long piece of erbium doped fiber with a narrow-bandwidth fiber Bragg grating reflector, stabilized the line-width of a semiconductor external cavity at a wavelength of 1.53 microns. This result showed that additional feedback from the spatial-hole burnt absorbing grating induced by the intra-cavity standing-wave provides a mechanism for extending the grating length, thereby reducing the line-width as expected from the Schawlow-Townes formula. No details were given in the paper, other than the fact that lightly absorbing erbium-doped fiber was used as the intra-cavity element. In this paper, we describe a novel high-power semiconductor laser system with strongly absorbing rare-earth doped fiber and a high reflection fiber Bragg grating in the external cavity, demonstrating superb line-width and single-frequency stability from threshold to output power in excess of 300mW. The unusual result of our experiment is the high-power single-frequency output in the self-pumped regime.
2. Experimental Results
A high-power 980nm packaged pigtailed pump lasers (SDL Inc.) producing over 300mW maximum at 500mA drive current is used in our experiments. The laser threshold current is ~10 mA and the slope efficiency is 0.6 W/A. Figure 1 shows the spectrum of the free running laser without external feedback measured for different currents. The spectrum is wide and uncontrolled with several lasing modes apparent at all currents above threshold. The shift in the central lasing wavelength is ~15nm for a change in the drive current of over 200 mA. We note that the spectrum changes dramatically and it is highly structured, as is normal for these types of Fabry-Perot resonator lasers. The large number of lasing modes is a consequence of the long high-power semiconductor resonator.
With a simple fiber Bragg grating (1mm long with 3% reflectivity) spliced approximately 84cm from the package of the laser, the spectrum narrows from the effects of coherence collapse, and this is shown in Fig. 2. The locking ability of the laser in this condition is good, but it fundamentally operates in the multimode regime, since there are many intra-cavity modes in the long external fiber cavity. Under some conditions, however it has been reported  that the grating-locked pump-laser can operate in both the coherence collapse (multimode) and in the coherent (single-mode) state, chaotically fluctuating between two regimes. The side-mode suppression ratio in the coherence collapse regime is very poor, at best between 2-3 dB, deteriorating at higher operating currents (200mA).
For single frequency operation of this laser, either an intra-cavity frequency selective element has to be introduced or the bandwidth of the grating has to be narrow enough to perform the single longitudinal mode selection. The former method is simple, but does not guarantee smooth single frequency performance of the laser over large values of the drive current. The latter method only works if an external cavity is short, since the grating bandwidth and hence its selectivity is inversely proportional to the grating length. Ideally, a weak diffraction grating distributed over the length of the cavity would have strong frequency selection, as in the case of fiber lasers. In the work described by Loh et al. , using a semiconductor laser with an erbium-doped fiber, the power output was low and single longitudinal mode operation required precise polarization control of the signal reflected from an external fiber grating. In our experiments, the laser, the output power range, and doped-fiber combinations is completely different.
In subsequent experiments, the fiber grating at 980nm was removed from the external cavity and a 63cm long piece of Yb doped fiber spliced to the 27 cm laser pigtail. Beyond the Yb doped fiber, a fiber grating, FBG1 reflecting at the wavelength of Yb absorption peak was written in spliced in hydrogen-loaded flexcore fiber. The semiconductor laser was driven at ~350 mA during the fiber grating writing process and the laser output spectrum was constantly monitored using an optical spectrum analyzer. The fiber grating writing process was terminated at the point when laser spectrum became pure single frequency. The schematics of the configuration are shown in the inset in Fig. 3. The details of the gratings and the Yb doped fiber are given in Table 1.
Two cavities were fabricated for these experiments, each with similar Bragg grating, but different absorption and length of Yb doped fiber, as described in Table 1.
For the first cavity design (SDL1), the threshold of an external cavity laser was measured to be ~5mA and the output spectrum versus the drive current from threshold to 500mA is shown in Fig. 3. The laser operates narrow line up to an output power of 150mW. What is also significant is the absence of semiconductor chip Fabry-Perot and external cavity modes, maintaining a side-modes suppression of at least -10dB for the entire range of drive currents. This clean, side-mode free operation is unique for this type of homogeneously broadened doped-fiber laser system. Contrast these results with the ones shown in Fig. 2.
The un-pumped, intra-cavity loss measured at a wavelength of 976nm, is ~95%. In this self-pumped system, the Yb doped fiber reaches transparency at the threshold of 5mA and the output-power steadily climbs to 300mW at 500mA (slope efficiency of 0.6W/A).
The measured light-current characteristics are shown in Fig. 4. The light-current curve for this laser is highly linear, and is attributed to the long cavity and therefore a closely spaced external cavity mode spectrum. The results for the higher absorption cavity (SDL-1) are also shown in Fig. 5. For this cavity, the L-I characteristics are highly non-linear, indicating several mode-hop regions. Even so, detailed measurements show the laser operates single frequency from threshold to 100mA. External cavity SDL-2 operates single frequency over the entire drive current range, as measured using a Burleigh wave meter (resolution ±0.8pm), and also suffers from mode hops of much smaller frequency. Figure 6 shows the change in the absolute wavelength as a function of the drive current. The wavelength excursions at points of mode-hops are a maximum of 0.05nm, reducing with increased drive current to < 0.01nm. This laser demonstrates superb wavelength stability, with the wavelength remaining almost constant at high currents.
The high-power, single-frequency results of experiments are interesting. The two-level Yb doped fiber absorbs and emits a photon, so that it behaves as a loss-less fiber. The reflection from the grating sets up a standing wave in the external cavity. Spatial hole-burning in the doped fiber causes a modulation of the population density, forming a permittivity grating. This helps to stabilize the operation of the laser, since the length of the doped fiber grating defines the full-width at half-maximum (FWHM) bandwidth of ~3pm (~400MHz), commensurate with the formation of a weak grating . It is worth pointing out that this laser operated single frequency without complex external stabilization of the cavity; the fiber was disturbed during the measurements, but the laser did not show any change in the operating wavelength. This is surprising, since there are large phase-fluctuations induced by handling the fiber.
The thermal effects in the semiconductor chip dominate narrow line-width operation of the laser. The lasing frequency changes slowly tracing the profile of the external Bragg grating reflector, until the net gain in the cavity drops below unity for the lasing mode. As expected, at this point the lasing wavelength jumps to an adjacent mode with higher gain.
The frequency sweep with current is expected from the thermal changes in the cavity of the semiconductor, and slight changes in the phase from thermal effects in the fiber . What is intriguing is the fact that twisting the fiber has little influence on the laser, since polarization effects are expected to vary the output power wildly. The high power output clearly helps with the narrowing of the line-width, owing to the inverse power dependence; however, in such a long cavity, it is impossible to have stable single-frequency operation. Thus the strong influence of the two-level doped fiber is clear from the measurements, indicating a complex behavior. We performed measurements of long-term operating wavelength stability for the free-running laser SDL2, having no temperature stabilization of the external resonator. The instant wavelength readings from the wavelength meter were collected every 60 seconds. The laser maintained single mode operation with at least 10dB side mode suppression at any time, and the operating wavelength slowly deviated at ~±1.5 pm/hr, following the changes in the laboratory temperature. The absence of low frequency laser amplitude fluctuations  allows us to conclude that the device was operating in stable single frequency mode. The robustness of the laser and detailed laser line width measurements are under study and the results will be reported elsewhere.
It has been demonstrated that a high-power semiconductor laser can be operated in the single-frequency regime over large drive currents and output powers. Single frequency output power of 300mW has been measured, with only a few mode hops over a drive current variation from 0 – 500mA. More importantly, the long cavity single-frequency operation is robust against external influences like handling the fiber and laser can operate without polarization controller in external cavity. The novel behavior of this cavity is under further investigation.
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