Wavelength-stepped, actively mode-locked fiber laser based on wavelength-division-multiplexed optical delay lines
Introduction
Lasers with time varying wavelength output are of great interest for their applications to optical imaging [1], testing for optical components [2], spectroscopy [3] and sensing [4], [5]. One version of such lasers produces discrete wavelength steps that can provide simpler data acquisition electronics for high speed applications [5], [6] compared to the continuously wavelength swept counterparts [7], [8]. For the wavelength stepped lasers in [5] and [6], the laser cavities were very long (a few to tens of km) to accommodate long optical pulse duration or large dispersion requirement. The long fiber length requires careful management of dispersion in the fiber.
We note that a wavelength stepped laser with much shorter cavity length is possible by adopting short mode-locked pulses and the temporal–spectral-multiplexing (TSM) as originally demonstrated in [9]. In the report, two mode-locked pulses with closely spaced wavelength steps (0.7 nm) were produced using erbium doped fiber (EDF) as gain medium and fiber Bragg gratings (FBG) as digitized dispersion elements. The two pulses with different wavelengths were enforced to go through the amplifier at different times. This is to reduce the gain competition normally expected in a homogeneously broadened gain medium such as EDF in room temperature [9], [10], [11]. The separated pulses were recombined at the location of an intensity modulator used for mode locking. The separation and recombination of the optical pulses in time domain was realized by a matching pair of FBG’s. Simultaneous lasing of two wavelengths was observed without significant gain competition. This result was unexpected since the temporal and spectral separation of the two pulses were much shorter than the gain recovery time and homogeneous linewidth of EDF, respectively. Also the potential impact of the gain competition on the noise characteristics of the laser output has not been addressed that would be critical for the application of such wavelength stepped lasers. Very recently, another mode-locked laser configuration with short cavity length was reported using a semiconductor optical amplifier and a very long chirped fiber Bragg grating [12].
In this paper we propose and demonstrate a novel configuration for the wavelength stepped mode-locked laser with short cavity length based on standard wavelength division multiplexers (WDM) used for communications. This approach provides additional advantages of individual control of optical parameters for each wavelength component such as polarization, insertion loss and dispersion. If needed, individual control of the timing, intensity and polarization of each pulses becomes possible. Since WDM’s with a large channel number are commercially available in the optical communication wavelength bands (1300 to 1600 nm), this approach can potentially be used for various applications requiring few to 100’s of wavelength channels. The cavity lengths using this approach can be less than several tens of meters. For the experimental demonstration of the operating principle of the proposed configuration, we built a wavelength stepped fiber laser with an EDF and WDM’s. Simultaneous lasing of three wavelengths was observed. We also measured the noise characteristics of the different wavelength components that have not been previously addressed. Based on the findings we suggest future improvements for practical use of such lasers.
Section snippets
Operating principle
Fig. 1 (a) shows the functional schematic of the proposed construction of the wavelength stepped mode-locked laser. The fiber laser cavity consists of an optical amplifier, two pairs of dense wavelength division multiplexers (DWDM’s), an intensity modulator and output couplers. The lasing wavelengths are selected by the -channel DWDM’s. Each DWDM pair splits signals at different wavelengths into separate fibers having different lengths and recombines them into a single fiber as shown in Fig.
Experimental setup
The experimental setup is shown in Fig. 2. For the optical amplifier, a 3 m-long EDF (Fibercore M-12) pumped by a laser diode at 980 nm was used. The pump power coupled to EDF was fixed at 208 mW. Although an amplifier with fast enough gain recovery such as semiconductor optical amplifier would provide better suppression of gain competition, we used an EDF amplifier currently available in our laboratory. Also the use of an EDF amplifier allows the analysis of the effect of gain competition and
Single wavelength mode-locked operation at optimum modulation frequencies
In order to ensure that the laser properly operates at each wavelength channels, we carried out experiments for mode-locking at single wavelength at a time. Fig. 4 shows the characteristics of single wavelength mode-locked pulse trains for each channel. In order to achieve the best mode-locking condition in this setup, the modulation frequency was optimized and the PC was adjusted to select one wavelength with optimum pulse shape. The laser output was monitored by an optical spectrum analyzer
Noise characteristics
We characterized the amplitude and the phase noise of each channel by measuring the noise spectrum of the laser using the technique used in [18], [19], [20]. The measurement is based on the comparison between the RF spectra around the first harmonic and higher harmonic peaks. For each channel, single sideband noise over frequency offset of 100 Hz–500 kHz from the center of the peaks was measured to estimate the root mean square (rms) fluctuations of amplitude and timing. Because the RF spectrum
Conclusion and discussion
In conclusion, we proposed and experimentally demonstrated a novel design for an actively mode-locked wavelength-stepped fiber laser. A mode-locked pulse train of three different wavelength steps with 0.8 nm spacing around the center wavelength of 1555 nm have been successfully produced from a 31 m-long cavity with a repetition rate of 6.24 MHz. The demonstrated design uses two pairs of commercially available DWDM’s that allows easy migration to high number of wavelength steps. The laser allows
Acknowledgment
The authors would like to acknowledge helpful discussions from Hee Su Park at Korea Research Institute of Standards and Science (KRISS).
Funding
This work has been supported by the Brain Korea 21 PLUS Project of Korea Government .
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