A wavelength-switchable single-longitudinal-mode (SLM) dual-wavelength erbium-doped fiber laser (EDFL) incorporating a novel high-finesse ring filter is proposed and demonstrated. The ring filter consists of two optical couplers and a section of pumped erbium-doped fiber (EDF). Thanks to the gain generated by the EDF, the ring filter has spectral response with a high finesse. The incorporation of the ring filter leads to the suppression of undesirable modes in the dual-wavelength EDFL. An experiment is carried out. Two SLM wavelengths are generated. The side mode suppression ratio is greater than 50 dB. The wavelength spacing of the two wavelengths is tunable with a tuning step of ~10 GHz. A frequency switchable microwave signal from ~10 to ~40 GHz is thus generated by beating the two wavelengths at a photodetector (PD). The spectral width of the generated microwave signal is measured to be less than 5 kHz.
©2009 Optical Society of America
Photonic generation of microwave and millimeter-wave signals has been a topic of interest for last few decades, which can find numerous applications such as radio over fiber systems, antenna remoting, radar and modern instrumentation. Thanks to the advantageous features such as large tunable range, high output power and narrow linewidth, a dual-wavelength fiber laser is considered a good candidate for high-power and frequency-tunable microwave or millimeter-wave generation. Compared to other microwave generation techniques, such as optical injection locking  and optical phase lock loop , a dual-wavelength laser is particularly suitable for microwave generation since there is no need for a high-quality microwave reference source. The key challenge in developing a dual-wavelength fiber laser for microwave generation is to ensure that the two wavelengths are operating in single longitudinal mode (SLM). Compared to a semiconductor laser diode, a fiber ring laser usually has a long cavity, which would lead to the generation of an enormous number of densely spaced longitudinal modes around the central oscillating mode. Several techniques have been proposed to limit the number of longitudinal mode to be one for each wavelength. An important solution is to use a ultra-narrow band fiber Bragg grating (FBG) filter that is formed by incorporating an unpumped erbium-doped-fiber (EDF) based saturable absorber (SA) in a fiber loop mirror [3,4] or a standing-wave arm [5,6]. The key problem associated with the use of an unpumped EDF is the large loss (typically 3–6 dB/m), which would reduce greatly the slope efficiency of the laser. In addition, the power of the oscillating mode that injected into the unpumped EDF must be properly controlled to achieve a desired transmission function. If the power is too low, the FBG would be too weak to select the desired mode. On the other hand, if the power is too high, the 3-dB bandwidth of the generated FBG filter would be too broad, which would lead to the generation of multiple modes. Different from a regular FBG inscribed by ultraviolet illumination, the SA-based ultra-narrow band FBG is actually a self-tracking filter. If mode hopping occurs, the center frequency of the FBG filter will change correspondingly. No mechanism is available to pull the oscillating mode back to the original mode. As a result, the center frequencies of the dual-wavelength fiber laser would randomly shift in a certain range, leading to a reduced quality of the generated microwave signal. Other solutions include the use of a phase-shifted FBG [7,8], or a FBG-based Fabry-Perot filter (FPF) [9–11] in the laser cavity. The major limitation related to the use of a phase-shifted FBG or a FBG-based FPF is that the lasing wavelengths are not tunable or the tuning range is very small. Recently, a fiber laser having multiple-ring cavities with each cavity having a different free spectral range (FSR) was proposed [12–14]. Owing to the Vernier effect, the effective FSR of the entire laser cavity becomes the least common multiple of the multiple ring cavities. Mode suppression is thus achieved by carefully choosing the length of each ring. Compared to the approach using a self-tracking FBG filter that has an ultra-narrow passband, the approach using multiple-ring cavities has a much greater bandwidth or the finesse is low. Mode hopping can still be observed, especially when environmental conditions change.
In this paper, we propose and demonstrate a novel fiber laser that incorporates a high-finesse fiber ring filter. The ring filter consists of a section of EDF that is weakly pumped which would generate a small optical gain. Thanks to the optical gain, the ring filter would have a frequency response with a high finesse. A dual-wavelength erbium-doped fiber laser (EDFL) is then constructed incorporating the high finesse ring filter. The gain competition in the gain medium is suppressed by the polarization hole burning (PHB) effect . Two wavelengths with a side mode suppression ratio (SMSR) of more than 50 dB are generated. The wavelength spacing is tunable with a tuning step of ~10 GHz. By beating the two wavelengths at a photodetector (PD), a microwave signal with a frequency switchable from ~10 to ~40 GHz is generated.
The schematic diagram of the proposed SLM dual-wavelength EDFL is shown in Fig. 1(a). A section of EDF with a length of 8 meters (EDF1) is used as the gain medium. To ensure unidirectional operation, an optical isolator is placed before EDF1. A fiber Fabry-Perot interferometer (FFPI, Micron Optics) with an FSR of 10.2 GHz and a finesse of 4 is inserted to provide a coarse wavelength selection. To obtain dual-wavelength oscillation, the light wave is split into two channels by a 50-GHz WDM, with each channel having a polarization controller (PC) and a variable attenuator (ATT). The light waves from the two channels are then polarization multiplexed at a polarization beam combiner (PBC) to ensure that the two lasing wavelengths are orthogonally polarized, which would further introduce the PHB effect in the EDFs, and thus suppress the strong homogeneous line broadening and cross-gain saturation in the EDFs. The PBC is polarization-maintaining-fiber (PMF) pigtailed at the two input ports. The length of the PMF is about 0.5 meter, corresponding to a differential group delay (DGD) of 0.7 ps at 1550 nm. The PMFs and the PBC form two Lyot filters for the two channels. The inset in the Fig. 1(a) shows the wavelength selection for one channel. By simply adjusting the PC in that channel, the transmission spectrum of the Lyot filter is changed from the solid line to the dashed line . A switching of the oscillating wavelength in that channel is thus obtained. However, densely spaced longitudinal modes around the central oscillating mode will introduce multi-longitudinal-mode oscillating and mode hopping. To overcome the problem, a novel fiber ring loop is incorporated, which consists of a section of 0.6-meter EDF (EDF2) and a 20:80 optical coupler. EDF1 and EDF2 are co-pumped by a 980-nm laser diode (LD) via a 1550 nm/980 nm WDM coupler and a 3-dB optical coupler. The 3-dB optical coupler has a splitting ratio of 50:50 at 1550 nm and a splitting ratio of 10:90 at 980 nm, with 10% of the pump power being injected into a fiber ring loop. Due to the gain introduced by the weakly pumped EDF, the ring filter has a comb spectral response with a high finesse, which is used to effectively suppress the undesirable modes in the two transmission peaks. The laser output is monitored by an optical spectrum analyzer (OSA, Ando AQ-6317B) with a resolution of 0.01 nm, and the generated microwave signal is observed by an electrical spectrum analyzer (ESA, Agilent E4448A, 3 Hz-50 GHz).
The key device in the dual-wavelength fiber laser is the high-finesse ring filter, which is schematically depicted in Fig. 1(b). Mathematically, when two input light waves with the optical fields represented by E 1 and E 2 injected into the 2×2 optical coupler, the optical fields at the two output ports can be expressed as
where γ is the coupling factor. The optical field E 4 in the ring filter is then re-written as
E 4 is transmitted through the 1×2 optical coupler and EDF2 in the fiber ring, obtaining an effective gain (or loss) of g and a time delay of τ, and then sent to one of the input ports of the 2×2 optical coupler. If a steady state is achieved, we obtain
The corresponding transmission functions are
T 1 and T 2 correspond to two transmission functions of the comb filters with the output ports at O1 and O2, respectively. In the previously reported schemes [12,13], the output of the ring filter is from O1, thus the transmission function is T 1. Since there is no gain (no EDF) in the ring, i.e., g 1, we obtain from Eq. (7) T 1 1, which means that the fiber ring is almost an all-pass filter. To form a comb filter, one has to introduce a loss to the ring loop, making g<1. In that case, the finesse of the generated comb filter is very low (<2). To increase the finesse of the filter, in the proposed approach the output of the ring filter is from O2 and EDF2 is weakly pumped to make g>1. Correspondingly, T2 is the transmission function. The FSR and the 3-dB bandwidth of the proposed ring filter are given by
With parameters given by γ=0.5, g=1.2 and FSR=140 MHz, we obtain the simulated transmission spectrum of the ring filter, as shown in Fig. 2(a). The 3-dB bandwidth is calculated to be 7.3 MHz, giving a finesse of 19. In the experiment, the total cavity lengths without the ring filter for the two wavelengths are about 15.9 and 15.5 m, corresponding to two FSRs of 13 and 13.1 MHz. The 3-dB bandwidth of the ring filter is less than the FSRs of the cavities, indicating that the SLM condition is well satisfied. Figure 2(b) shows the impact of the effective gain on the finesse of the filter. From Fig. 2(b), we can see that the finesse is monotonically increasing for 0≤g≤1.2. In addition, a smaller γ means that a higher resonance power is fed back into the ring, resulting in a higher finesse.
3. Experimental results and discussion
An experiment based on the setup shown in Fig. 1(a) is performed. To verify the operation of the high finesse ring filter, we measure the RF spectra in the frequency range of -0.1~0.9 GHz when EDF2 is and is not pumped. When EDF2 is not pumped, a strong beat signal at 26 MHz is observed, as shown in Fig. 3(a). Since the FSRs of the laser cavities for the two wavelengths are ~13 MHz, a 13 MHz beat note is expected. However, no 13-MHz component is found in the experiment. It is believed that the 13 MHz beat note is suppressed due to the 0.6-m unpumped EDF in the ring filter. On the other hand, when a proper pump power is injected into the ring filter, the finesse of the filter is greatly increased, and an SLM operation is thus established, as shown in Fig. 3(b).
Figure 4(a) shows the optical spectra at the laser output. Two wavelengths at 1551.79 and 1551.95 nm with a wavelength spacing of 0.16 nm corresponding to a difference frequency of 20 GHz are observed. The SMSR for both wavelengths is ~51dB. It should be noted that the output port of the laser is placed just after EDF1 for high output power. The amplified spontaneous emission (ASE) from EDF1 would greatly reduce the SMSR. If the laser output port is placed before EDF1, an improved SMSR as high as 65 dB would be observed. The two wavelengths are heterodyned at the PD. As shown in Fig. 4(b), only one beat note at 20.6 GHz is observed, which demonstrates again that the dual-wavelength laser is operating at SLM. The inset of Fig. 4(b) gives a zoom-in view of the beat signal, displayed on the ESA with a span of 300 kHz and a resolution bandwidth of 4.7 kHz. We can estimate that the 3-dB bandwidth of the obtained electrical signal is less than 5 kHz. The stability of the system is also investigated. To do so, we let the system operate in a room environment for a period of 60 minutes. The optical spectra recorded at a 3-minunte interval are shown in Fig. 4(a). No significant fluctuations are observed. The wavelengths shift is less than 2 pm and the optical power fluctuation is smaller than 0.5 dB. The center frequency shift of the generated microwave signal is less than 10 MHz and the electrical power fluctuations are less than 1 dB. The frequency drift is mainly originated from the FFPI, which is very sensitive to temperature variations. Mode hopping is occasionally found during the 60-minute observation, but the separation of the modes is 140 MHz, corresponding to the FSR of the ring filter, which confirms that other modes close to the center wavelength are well suppressed. If the FSR of the ring filter is increased or a large-finesse FFPI is used, mode hopping should be completely eliminated.
The wavelength-switchability of the proposed fiber laser is also investigated. As shown in Fig. 5(a), the wavelength spacing of the two wavelengths is switched to be 0.08, 0.16, 0.25, 0.33 nm, with a microwave beat note at 10.31, 20.63, 30.95 and 41.30 GHz being observed on the ESA, respectively, as shown in Fig. 5(b).
A novel fiber laser incorporating a high finesse ring filter for SLM dual-wavelength operation was proposed and studied. The key device in the proposed fiber laser was the ring filter, which was designed to have a length of EDF that was weakly pumped to generate a gain. Based on our analysis, when the effective gain was 0≤g≤1.2, the finesse of the ring filter was monotonically increasing, leading to a better suppression of undesired modes. An experiment was performed. The results showed that a dual-wavelength operation with each wavelength having a single frequency was demonstrated. The SMSR was measured greater than 50 dB for both wavelengths. The wavelength spacing could be tuned with a tuning step of ~10 GHz. By beating the two wavelengths at a PD, a microwave signal with a switchable frequency from ~10 to ~40 GHz was generated.
This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) through its strategic project grants program.
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