We report a novel multi-wavelength laser source based on hybrid gain medium, with a semiconductor optical amplifier and erbium doped fiber amplifier, and a double-ring structure. More than 60 lines with more than 50 dB signal-to-noise ratio (SNR) have been achieved in our setup. The wavelength interval between the wavelengths is ~0.32nm. The 3dB spectral width of each lasing line is ~ 0.019nm. The laser can be tuned from ~1526 nm to ~1562 nm by adjusting the loss in the cavity.
© 2009 Optical Society of America
Multiwavelength fiber lasers have been widely studied for their potential applications in optical communications, wavelength-division-multiplexing (WDM) communication systems, optical fiber sensors, and, optical instrument testing. The erbium doped fiber has been used as the gain medium for the lasers. However, since erbium doped fiber (EDF) is a homogenous gain medium, the fiber lasers based on EDF often suffer from strong mode competition and unstable multi-wavelength lasing at room temperature. There have been a range of approaches used to solve this problem which includes cooling EDF to liquid-nitrogen temperature , using inhomogeneous gain medium in the laser cavity (such as semiconductor optical amplifier(SOA) instead of EDF [2-4]), utilizing four wave mixing effect[5-8] or inhomogeneous loss mechanism by using highly-nonlinear fiber, and adding a frequency shifter or phase modulator[10-12] and so on. In References 3 and 4, researchers used two cascaded SOAs in the cavity to increase the bandwidth and the average power. However, since SOA has a relatively high noise figure, the signal to spontaneous noise ratio (SNR) was usually not high in these papers (~30dB), which may be an issue in some future applications. In Reference 5, simultaneous lasing at more than 70 wavelengths was achieved by using a highly nonlinear fiber combined with a Fabry-Perot filter. The SNR of the laser in this paper was ~44dB. However, nonlinear optical effects in that paper required high power to reduce the cross gain saturation in EDF which may not be easily obtained.
In our work, we present a stable room-temperature multiwavelength laser. The setup based on hybrid gain medium and a double-ring spectrum reshaping structure is used to improve the mode suppression ratio and decrease the spectral linewidth of the output. More than 60 simultaneous lasing lines have been achieved with high SNR (more than 50 dB). The linewidth of the lasing lines is about 0.019 nm with wavelength spacing of 0.32 nm.
2. Principle of operation
The diagram of the experimental setup of the proposed double-ring system is shown in Fig. 1 EDFA1 is used as the gain medium with the small gain of ~ 30dB and the saturation power of 23dBm. The SOA is used in the cavity to suppress the mode competition. In this work, the SOA is driven by a current of 300mA at a temperature of 20 C. This produces a small signal gain of 23.5dB with 10μW input signal at 1550nm. Optical circulator 1(OC1) transfers the light from the left ring to the right ring. DI is a delay interferometer with 25 ps delay between the two arms. It has a comb like transmission spectrum. EDFA2 in the right ring compensates the optical loss due to DI. Optical circulator 2(OC2) transfers the light back to DI. Polarization controller (PC) is used to optimize the polarization state in the cavity. An optical spectrum analyzer is used to measure the optical spectrum of the laser using the 10% output port of the optical coupler.
The DI which is fabricated using Si-SiO2 waveguide technology has two arms with a 25 ps time delay between the two arms. The light is split into two beams, and, at the output of DI, two beams of optical signals with the phase difference (corresponding to 25 ps delay) recombine and interfere with each other. The optical intensity at the DI output can be given by:
where Iup and Ilow are intensity of two beams travelling along the two arms of the DI, t is the travel time delay between the two arms; c is the speed of light in vacuum and λ is the wavelength of the optical signal. Figure 2(a) shows the transmission spectrum of a DI (delayed interferometer). The DI acts as a comb filter which makes the wavelengths that match the peaks of the comb oscillate when it is in a laser cavity. The wavelength spacing between two adjacent transmission peaks is 0.32 nm for t = 25 ps. Figure 2(b) shows the transmission spectrum of a round trip (two trips) through the same DI (as in Fig. 1). The linewidth of the peaks is reduced for the two trip transmission (equivalent to two cascaded DIs). Multiple transmission through identical DIs will further reduce the linewidth of the transmission peaks. Figure 3 shows the linewidth as a function of number of cascaded DIs. With increasing number of DIs, the linewidth decreases monotonically, the largest decrease takes place when the number of DIs varies from 1 to 2, the change from 1 to 2 appears as an inflexion point in Fig. 3 primarily because the discrete set of numbers (number of DIs) for the x-axis.
3. Experimental results and analysis
In the experiment, the current in SOA is 300 mA. The EDFA1 can provide gain as high as 23dBm. The power of the EDFAs can be adjusted by changing the pump power coupled into the EDF. The SOA has been used to suppress the mode competition in EDF. Figure 4 (a) shows the measured transmission spectrum of DI used in the experiment. The extinction ratio is about 15dB. To illustrate the properties of this double-ring structure, we disconnect the port 3 of OC1 and the input port of EDFA1, Figure 4 (b) shows the spectrum seen from port 3 of OC1. Note that the transmission spectrum shape is modified as expected. The extinction ratio is increased to ~22dB. It shows that two identical DIs are better in suppressing spontaneous noise. The increase in intensity of the transmission spectrum as the wavelength increases in Fig. 4 is due to the slight increase in gain of EDFA in the spectral range shown. The spectral dependence of gain is not considered in the model calculation shown in Fig. 2. The DI is relatively insensitive to power, temperature and polarization changes. When the pump power to EDFA2 is increased, there is no change in the shape of spectrum. For changes in input polarization the transmissions peaks move by < 0.02 nm.
Figure 5 illustrates the stable multiwavelength lasing achieved by appropriately adjusting the polarization controller. Up to 60 simultaneously lasing lines are obtained within the 3-dB bandwidth at room temperature with a wavelength spacing of 0.32 nm, which coincides with the transmission profile of the DI filter. The SNR is measured at ~ 50dB. The hybrid gain medium effectively improves the SNR  and supports stable multiwavelength lasing operation, and, the double-ring structure reshapes the transmission spectrum and generates lasing lines with narrow linewdith. The measured linewidth is ~ 0.019nm. The SNR obtained in this paper (50 dB) is better than that in Ref. 13 (42 dB) which also uses a hybrid gain medium. The improvement in this paper in the SNR is due to the use of DI and double-ring structure.
We have investigated the stability of the ring laser. Figure 6 shows the results of repeated scanning every 10 min. The peak power variation is less than 1 dBm. The well packaged DI is important in eliminating the influence of thermal and power fluctuation in the ring. The double-ring structure generates lasing lines with narrow linewidth which improves the stability of the output power because fewer longitudinal modes can oscillate.
Figure 7 shows the wavelengths are shifted to shorter wavelengths (for the same power) by increasing the loss in the ring. This is done by inserting an intensity attenuator before the SOA. The shift is due to higher carrier density in the SOA. The wavelength can be tuned from ~1526 nm to ~1562 nm. During the tuning (with higher loss), the number of lasing wavelengths is decreased.
In conclusion, we have experimentally demonstrated a stable multi-wavelength fiber ring laser based on cascaded hybrid gain mediums, with a semiconductor optical amplifier and erbium doped fiber amplifier, and a double-ring structure. The homogeneous line broadening in EDF is suppressed effectively and the SNR of the multiwavelength output is higher than 50 dB with lasing linewidth ~0.019nm. The laser is very stable with the peak power fluctuation less than 1dB. The wavelength can be tuned in a range from ~1526 nm to ~1562 nm.
References and Links
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