We propose and demonstrate a tunable single frequency fiber laser based on Fabry Pérot laser diode (FP-LD) injection locking. The single frequency operation principle is based on the fact that the output from a FP-LD injection locked by a multi-longitudinal-mode (MLM) light can have fewer longitudinal-modes number and narrower linewidth. By inserting a FP-LD in a fiber ring laser cavity, single frequency operation can be possibly achieved when stable laser oscillation established after many roundtrips through the FP-LD. Wavelength switchable single frequency lasing can be achieved by adjusting the tunable optical filter (TOF) in the cavity to coincide with different mode of the FP-LD. By adjustment of the drive current of the FP-LD, the lasing modes would shift and wavelength tunable operation can be obtained. In experiment, a wavelength tunable range of 32.4 nm has been obtained by adjustment of the drive current of the FP-LD and a tunable filter in the ring cavity. Each wavelength has a side-mode suppression ratio (SMSR) of at least 41 dB and a linewidth of about 13 kHz.
© 2013 OSA
Single frequency fiber laser has attracted much interest for its potential applications in optical communications , fiber sensing , and high-resolution spectroscopy  due to the advantage of its good coherence. Different techniques have been proposed to realize single frequency oscillation in fiber lasers [4–15]. These include the utilization of short cavity to increase the longitudinal-mode spacing such as distributed feedback (DFB) and distributed Bragg reflector (DBR) fiber lasers [7,8], the introduction of a segment of un-pumped erbium doped fiber (EDF) which operates as an auto-tracking ultra-narrow-band filter [9–11], the insertion of sub-ring to form compound cavity to increase the longitudinal-mode spacing [12,13], and the utilization of narrow gain profile of stimulated Brillouin scattering [14,15]. However, most of the approaches have difficulty in wide wavelength tuning.
C. H. YeH et al. ever reported a single frequency erbium-doped fiber laser (EDFL) by utilizing a Fabry Pérot laser diode (FP-LD) in Sagnac cavity . However, the necessary parameters for confirmation of single frequency operation of the EDFL such as radio frequency (RF) spectrum and output linewidth have not been measured and provided. Since the EDFL usually has a long cavity which would lead to a large number of densely spaced longitudinal modes, it is difficult to ensure stable single frequency operation even the FP-LD has narrowing effect.
In this paper, a widely wavelength tunable single frequency fiber laser is proposed and successfully demonstrated based on FP-LD injection locking. The FP-LD under injection locking can decrease the number of longitudinal-modes and effectively narrow the linewidth of the injected multi-longitudinal-mode (MLM) light at each roundtrip. As a result, single frequency operation can be possibly achieved when stable laser oscillation established after many roundtrips through the FP-LD in the ring cavity. A semiconductor optical amplifier (SOA) is used as the gain medium to further ensure the single frequency operation, since it can greatly shorten the laser cavity and consequently increase the longitudinal-mode spacing. As a result, single frequency operation has been achieved in the experiment. By adjustment of the drive current of the FP-LD, and the tunable optical filter (TOF) in the laser cavity, wavelength switchable and tunable operation has also been investigated.
2. Experimental setup and operation principle
Figure 1 shows the schematic configuration of the proposed single frequency fiber laser. It consists of an SOA, two isolators, a polarization controller (PC), a TOF, a FP-LD, and an output coupler with a splitting ratio of 1:9. The SOA provides the optical gain, which has a maximum gain of about 23 dB and a bandwidth of about 60 nm. Two isolators (ISO) are inserted respectively before and after the SOA to ensure unidirectional operation. The TOF can be tuned from 1530 nm to 1580 nm with a 3-dB bandwidth of 0.4 nm. The FP-LD is introduced into the cavity through a circulator (CIR). The PC is used to adjust the polarization state of the light inputting the SOA and ensure best gain status. The 10% output is divided into two branches by a 50/50 coupler: one is connected to an optical spectrum analyzer (OSA) with a resolution of 0.02 nm for optical spectrum measurement; the other part is detected at a high speed photo-detector (PD) with a 20 GHz bandwidth and is measured in the frequency domain using a 13 GHz electronic spectrum analyzer (ESA).
The principle of single frequency operation of the laser can be explained as following. When a MLM laser output is injected into a FP-LD, injection locking can be achieved by proper adjustment of the input power and wavelength . Figure 2(a) shows the optical spectra of the MLM injection, the FP-LD output before and after injection locking. Without optical injection, multiple cavity modes of the FP-LD were oscillated with a mode spacing of about 1.25 nm as shown in Fig. 2(a) (the middle curve). After the MLM injection (the upper curve in Fig. 2(a)), the cavity mode located at that wavelength was injection-locked and enhanced in intensity, while other modes were suppressed significantly as shown in Fig. 2(a) (the bottom curve). The corresponding RF spectra of the MLM injection and the FP-LD after injection locked by the MLM input are shown in Fig. 2(b). The RF spectra actually represent the envelopes of the beat frequency signals between the longitudinal modes in the detected signal, we can estimate the linewidth of the signal from the width of the envelope, since there would be no beat signal in RF spectra when there is no longitudinal-mode. It can be clearly seen from the figures that the FP-LD under injection locking can decrease the number of longitudinal-modes and effectively narrow the linewidth of the injection MLM light. When the FP-LD is inserted in the ring laser cavity, the narrowing effect can play an important role when stable laser oscillation established after many roundtrips through the FP-LD. As a result, single frequency operation can be possibly achieved.
Furthermore, by adjusting the TOF in the cavity to coincide with different mode of the FP-LD, wavelength switchable single frequency lasing can be possibly achieved, with 1.25nm hops corresponding to the modes of the FP LD. By adjustment of the drive current of the FP-LD, the lasing modes would shift and wavelength tunable operation can be obtained by simultaneously adjusting the TOF to coincide with the mode of the FP-LD. Combing these two effect, widely tunable single frequency lasing can be achieved in the proposed laser.
3. Results and discussions
Using the mechanism described above and to confirm the existence of single frequency operation of the laser, we conducted experiment with the ring laser cavity configuration shown in Fig. 1. First, wavelength switchable single frequency output was obtained by adjustment of the TOF to coincide with dominant lasing wavelength of the FP-LD when the drive current of the FP-LD is fixed. Figure 3 shows the spectra for the wavelength switched operation when the drive current is set at 80 mA. A switching range from 1554.89 nm to 1563.66 nm with a step of 1.25 nm has been achieved. The superimposed spectra traces in Fig. 3 demonstrate that the switchable single frequency fiber laser has an almost constant output power across the switching range. The SMSR is larger than 45 dB for all the wavelengths.
Then, the drive current of the FP-LD was adjusted for the purpose of broadening the wavelength tuning range, since the dominant output lasing wavelength of FP-LD will shift with the change of the drive current. By adjustment of both the TOF and the drive current of the FP-LD, tunable single frequency output has been achieved. Figure 4(a) shows some typical output spectra when the driving current of FP-LD is adjusted from 10 mA to 80 mA. It can be seen from the figure that a tuning range of 32.4 nm from 1531.26 nm to 1563.66 nm has been obtained. Some detailed spectra during the tuning process are depicted in Fig. 4(b).
In order to verify the single frequency operation of the laser, we measured the RF spectrum of the laser output using an ESA with a resolution bandwidth (RBW) of 300 kHz. Figure 5 shows the RF spectrum of the proposed fiber laser when lasing at 1545.13 nm. Since the estimated longitudinal mode spacing of the laser is to be about 20 MHz, the RF spectrum confirms that only single longitudinal mode exists within the cavity and the laser is indeed under single frequency operation.
A linewidth measurement of the single frequency fiber laser is implemented by using delayed self-heterodyne method. The configuration is shown in Fig. 6(a). The Mach-Zehnder interferometer (MZI) is composed of two 3 dB coupler. An acoustic optic modulator and a 20 km single mode fiber (SMF) are respectively inserted into the two arms, offering a frequency shift of 210 MHz and a delay of about 98 μs. The beating signal after the MZI is then detected by a PD and measured by an ESA. Figure 6(b) indicates a typical heterodyne RF spectrum measurement (the solid line), and the Lorentz fit of intensity is also shown (the dashed line). Assuming the laser spectrum to be Lorentzian-shaped, the full width at half maximum (FWHM) of the laser is estimated to be about 12.3 kHz, calculated from the −10 dB bandwidth of 74 kHz.
Figure 7 shows the linewidth and SMSR versus the lasing wavelengths of the proposed fiber laser. It can be seen from the figure that the SMSR maintained larger than 40 dB during the tuning process, while the laser linewidth in the whole wavelength tuning range is narrower than 13 kHz.
It should be noted that the tuning range is mainly limited by the tunable filter and the operation wavelength of the FP-LD. A wider tuning range may be obtained by using cascaded FP-LD. The short wavelength limitation of the filter also partly contributes to the lower SMSR of these wavelengths, as shown in Fig. 7. While for the limited wavelength-switchable range, it is because that the injection locking can only be achieved at the dominant lasing wavelength of the FP-LD that have relatively high power . Switchable single frequency lasing emitting can be obtained in other wavelength range by setting the FP-LD at another allowed driving current.
Another point we have to note is that the output power of the laser from 1531.26 nm to 1563.66 nm is increasing as shown in Fig. 4. This results from that these wavelengths were obtained with increase of the drive current of the FP-LD. Although the output power changed, the SMSR and the linewidth maintained a relevant stable value in the whole wavelength tuning range. We think the small fluctuation in SMSR and the linewidth may relate to the different degree of injection locking during the tuning process of the TOF and the FP-LD.
The 3-dB bandwidth of the BPF used in the experiment is fixed at 0.4 nm. The main function of the filter is to reject other modes except the desired mode of the FP-LD, but its 3-dB bandwidth should not be too large, otherwise, the narrowing effect of FP-LD would not be strong enough to ensure single frequency operation.
In the experiment, an SOA rather than many meters of erbium-doped fiber was used as the gain medium to further ensure the single frequency operation. Since an EDFL usually has a long cavity which would lead to a large number of densely spaced longitudinal modes, it is more difficult to ensure stable single frequency operation compared to a SOA-based fiber laser under the same mechanism induced by the FP-LD. Of course, without the FP-LD injection locking, single frequency operation can’t be achieved even with a greatly shortened laser cavity.
In a summary, we have proposed and experimentally demonstrated a switchable and tunable single frequency fiber laser based on FP-LD injection locking. The single frequency operation principle is based on the fact that the output from a FP-LD injection locked by a MLM light can have fewer longitudinal-modes number and narrower linewidth. A wavelength switchable range of from 1554.89 nm to 1563.66 nm with a step of ~1.25 nm has been achieved, by adjustment of the tunable filter. Furthermore, the proposed fiber laser can be tuned in a wide wavelength range from 1531.26 nm to 1563.66 nm, by tuning both the FP-LD and the TOF. A SMSR of > 41 dB and a line-width of < 13 kHz have been obtained over the whole wavelength tuning range.
This work was supported in part by the NSFC (No 61077030), the Research Fund for the Doctoral Program of Higher Education of China (No 20104401120009), Natural Science Foundation of Guangdong Province of China (No S2012010008850), and the Fundamental Research Funds for the Central Universities (21612201) in China.
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