This paper presents the experimental studies on the all-fiber frequency shifted feedback erbium laser. Impact of the gain section profile on laser operation is shown. Two methods of wavelength tuning are presented. We obtained 8.3 and 10 nm tuning range in electrical and manual tuning respectively. Pulse duration can be also controlled and changed from 2 to 5 ps. The shortest pulses duration we have obtained was 1.7 ps.
©2009 Optical Society of America
During last decades mode-locked lasers have became primary and irreplaceable sources of ultrashort pulses and optical frequency combs. Erbium doped fiber lasers are especially interesting and have been widely studied in last 20 years. They have numerous advantages (e.g. wide gain bandwidth, narrow linewidth, reliability) and have found applications in many fields from metrology to telecommunication. Frequency Shifted Feedback (FSF) is one of the technique that can be used to obtain mode-locking in this type of lasers. Such sources are especially interesting for device characterization, metrology, optical reflectometry, frequency measurements, optical ranging, imaging and medical diagnosis. A complete description of FSF lasers can be found in . FSF mode-locking can be obtained in Yb fiber lasers [2–4] and in Er fiber lasers as well [5–7]. This technique is also very effective when widely tunable operation is required . Ryu et al. have presented the injection seeding of the FSF optical frequency comb (OFC) source with the external cavity diode laser . This technique was previously demonstrated in case of multi-wavelength sources  and Q-switched lasers . Ryu et al. have shown that using acousto-optical modulator (AOM) 1.8 THz wide OFC can be generated and the frequency of its modes can be controlled by the external laser injected into the cavity. Tunability and repetition rate increasing up to 150 GHz was also presented in .
Stellpflug et al. has presented an extensive paper devoted to the numerical simulation of titanium-sapphire and neodymium-based FSF lasers . Erbium fiber laser requires however more complicated model. Huge advantage of EDF is the possibility of controlling its gain profile. Typical EDF has the gain bandwidth from 1525 nm to 1565 nm (telecommunication C-band) with maximum around 1530 nm. Gain profile can be however easily modified and shifted towards longer wavelengths. This so called L-band EDFA bases on secondary pumping phenomena in un-pumped active fiber and can be constructed simply by increasing the Er fiber length or by using two separate EDF stages .
In this paper we present the impact of the gain profile on FSF laser operation. We show how different gain section configurations affect the operation wavelength of the laser. We present two methods of wavelength tuning – electrical tuning with different RF power applied to the AOM and manual tuning using tunable Bragg grating. We also show that in certain configurations RF power can be used to control the pulse duration.
2. Laser configuration
Configuration of the frequency shifted feedback all-fiber laser is shown in Fig. 1. It consists of gain section (erbium doped fiber EDF pumped with 980nm laser diode using WDM coupler), optical isolator (it guarantees unidirectional operation), output coupler and acousto-optical modulator (AOM). Modulator down-shifts the frequency of the input light by 40 MHz and is driven using RF generator which can provide up to 21dBm output power. The total length of the cavity is approximately 5 meters.
Because our laser is an all-fiber configuration it is easier to adjust the modulation frequency to the length of the laser than vice versa. Figure 2 shows the AOM attenuation characteristics. Although modulator was design to be driven with 40 MHz signal, we can shift the frequency by 2–3 MHz without introducing large losses (Fig. 2 – left). Also the RF power can be changed however this increases AOM attenuation significantly (Fig. 2 – right). As we will show, this intracavity losses controlled by the RF power applied to the AOM can be used to tune the laser wavelength and to control the pulse duration.
In order to investigate the impact of the gain medium profile and laser setup on the laser operation we designed four laser configurations (Table 1). They differ in type of the erbium fiber used, length of the EDF and in amount of power that is out-coupled. We have used three different gain sections (EDF 1, EDF 2 and EDF 3). They were all pumped with 200 mW laser diode at 980 nm. Their gain profiles are shown in Fig. 3 (measured with tunable laser source, 3 mW input power). EDF 1 is build with typical active fiber with medium Er concentration. It is characterized by the 30 dB/m attenuation at 1530 nm. The length of the fiber we have used is 2.5 meters. Gain profile is very flat with small peak around 1530 nm. EDF 2 and EDF 3 were constructed using highly doped active fiber (attenuation 80 dB/m at 1530 nm). They have different length which has impact on their gain profile. EDF 2 has flat gain characteristic with peak value at 1560 nm. EDF 3 has longer Er fiber so the whole pump power is absorbed and secondary pumping effect can be observed. That is why gain at 1530 nm is reduced and the gain peak is shifted beyond the 1560 nm.
3. Experimental results
3.1 Tuning with the RF power
We have started our experiments with the configuration #1. Output spectrum for different RF power is presented in Fig. 4. Even though the AOM shifts down the frequency of the input light laser operates around 1530 nm. One reason is the gain profile that has its peak there. Also the AOM that tries to move the frequency comb to the longer wavelengths simultaneously slightly favors shorter wavelengths with its attenuation characteristics (see Fig. 2, attenuation vs. wavelength). Due to high out-coupling ratio (50 percent) average output power was 26 mW (for RF power 21dBm). By changing the RF power from 21 to 9 dBm we could narrow the bandwidth of the optical frequency comb which affects the duration of the generated pulses. In our autocorrelator setup we were able to introduce only 10 ps delay. This was however enough to observe the changes in the pulse duration even though autocorrelation signal was weak and noisy. For RF power = 21 dBm pulse duration was 2 ps and it increased to approximately 5 ps for RF power = 9 dBm. In the same time center wavelength was almost unchanged.
In second configuration we have used the same gain section as in the first one. We have only changed the output coupler (now only 10 percent of the power was out-coupled). This had the impact on the average output power (which was now only 4 mW) and also on operation wavelength as is shown in Fig. 5. For high RF powers (low AOM attenuation) laser generated frequency comb around 1560 nm. For RF power = 16 dBm (AOM attenuation is approximately 8–9 dB) we observed wavelength switching (also described in [5,12]). This interesting phenomena can be easily understand when we look for the gain profile. Even though the peak gain is at 1530 nm AOM shifts the laser operation wavelength. However, if we introduce to much intracavity losses (with to small RF power and/or with to high out-coupling ratio) amplification at 1560 nm is to small to support the lasing in this region. That is why laser start to operate around 1530 nm where the amplification is still sufficient.
In the configuration #1, when 50:50 output coupler was used, losses inside the cavity were to large to observe the lasing in 1560 nm region even for RF power = 21 dBm. In second configuration we could obtain OFC at longer wavelengths by reducing the intracavity losses with only 90:10 output coupler. The operation wavelength (1560 or 1530 nm) could be changed by using the RF power. Similar situation was obtained in configuration #3. Output spectra are shown in Fig. 6. The gain profile in this case has peak at 1560 nm however when it is combined with the AOM wavelength dependent attenuation it gives result similar to the previous configuration. This time however the wavelength switching occurs when AOM attenuation is higher than 14 dB (RF power = 9 dBm).
This situation changes in case of fourth configuration when EDF 3 is used as an active section. As we shown in Fig. 3 gain bandwidth for 1.2 m long highly doped Er fiber is significantly shifted towards longer wavelengths and the gain in 1530 nm region is strongly suppressed. This affects the central wavelength of the generated OFC which is in 1560 nm region even for very high AOM attenuation (>20dB). This is shown in Fig. 7.
The central wavelength of the laser shifts from 1567.5 nm to 1559.2 nm when the RF power is changed from 21 dBm to 0 dBm. Simultaneously 3dB bandwidth changes from 2.1 nm to 1.1 nm. The shortest pulses were obtained for maximal RF power. Interferometric autocorrelation is shown on Fig. 8. Pulse duration is 1.7 ps (assuming Gaussian shape). Figure 8 shows also how central wavelength and average output power depends on RF power.
3.2 Tuning with the fiber Bragg grating
Tuning method presented above has several advantages. It does not need any additional components inside the laser cavity and both pulse duration and central wavelength can be controlled in relatively wide range. However, the main disadvantage is the problem of controlling only one parameter without changing another (e.g. when tuning the central wavelength in fourth configuration we simultaneously change output power). If we are interested in tuning the wavelength without affecting the output power and pulse duration the configuration with tunable Bragg grating would be suitable. In our experiments the optical circulator and manually tunable fiber Bragg grating were inserted into the cavity (between isolator and output coupler in configuration #2). Cavity wass approximately 10 m long. TBG has bandwidth of 1 nm and tuning range from 1546 to 1556 nm. When modulation frequency (RF power = 21 dBm) was adjusted to match the 2nd harmonic of the cavity round trip mode-locking was obtained. Output spectra are shown in Fig. 9. Average output power (1 mW) and 3dB bandwidth (0.2 nm) did not change within the whole TBG tuning range. Pulses were too long to measure their duration with our autocorrelator however 0.2 nm bandwidth corresponds to approximately 20-25 ps pulses.
In this paper we described our experiments with FSF all-fiber erbium laser. Laser generates pulses as short as 1.7 ps with 40 MHz repetition frequency. Simple turn-key configuration does not require any polarization adjustment. We investigated the impact of the gain section profile on the laser operation. We have shown that with proper design of the gain section wavelength tuning of the laser can be obtained by simply changing the RF power applied to the AOM. This method can also be used to control the pulse duration. Continuous wavelength tuning over more than 8 nm was obtained. We have also presented a configuration with tunable fiber Bragg grating that allowed us to tune the laser in 10 nm range (limited only by the grating properties).
The work presented in this paper was supported by the Polish Ministry of Science and Higher Education grant N515 029 32/2079.
References and links
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