We demonstrate a wide and fast wavelength-tunable mode-locked fiber laser based on tuning the mode-locking frequency. The laser is in a sigma-laser configuration, and a wideband semiconductor optical amplifier (SOA) at 1.3 µm wavelength region is used as a gain medium. Mode locking is achieved by direct modulation of the injection current to the SOA, and a dispersion compensation fiber (DCF) is used to provide desired intracavity dispersion. By tuning the modulation frequency, a wide tuning range over 100 nm is achieved. Lasing wavelength is measured to be linearly in proportion to the RF frequency applied to the SOA. The sweep rate over the entire wavelength range (100 nm) can be raised to be as high as 200 kHz.
© 2006 Optical Society of America
Wavelength-tunable lasers are versatile both in telecom and sensing applications. Many kinds of wavelength-tunable lasers, either laser diode (LD) based or fiber laser based, have been proposed so far. External cavity tunable LDs are commercially available , and erbium-doped fiber lasers have been demonstrated to have wide tuning bandwidth over 80nm . These lasers, however, require tunable filters, such as rotatable diffraction grating , polygonal mirror  or piezo-tunable Fabry-Perot filter . So far, most of these filters have mechanically moving parts, which limits the tuning speed. In some applications, such as fiber Bragg grating (FBG) sensors  and optical coherence tomography (OCT) , fast tuning speed and wide tunable range are desired.
In this paper, we propose and demonstrate a novel wide and fast wavelength-tunable fiber laser. It is a mode-locked fiber laser in the pulsed operation, and it does not require tunable filters. It is based on the change of the modulation frequency and chromatic dispersion in the laser cavity. We realize the fast tuning over 100 nm at the sweep rate as high as 200 kHz by sweeping the modulation frequency and using a semiconductor optical amplifier (SOA) as a gain medium.
The free-spectral range (FSR) of the laser cavity F is expressed as
where L is the cavity length, n is the refractive index in the cavity, and c is the speed of light in the vacuum. When the cavity has chromatic dispersion, the FSR is a function of the light wavelength λ. Denoting the FSR at a wavelength λ as F, and ignoring higher order dispersion, the wavelength λ and the FSR F have a relation,
where n is the refractive index at λ, and D is the dispersion parameter.
Active mode-locking is a technique to generate short pulse trains by applying a modulation to the laser cavity. For stable active mode locking, the modulation frequency f m to the cavity has to match with an integer (N) times of the FSR (=N×F), where N is the order of harmonic mode locking. That is, when we apply a modulation at f m to the dispersive cavity, the laser favors to operate at the wavelength λm to meet the harmonic mode-locking condition, expressed as,
where fm 0=NF. Thus the lasing wavelength can be tuned by changing the modulation frequency. This is sometimes called as dispersion tuning . It is found from Eq. (3) that the wavelength shift is more sensitive to the change of modulation frequency when L is large, and N and D are small.
Wavelength tuning range Δλm is primarily determined by lasing at the adjacent harmonic mode, (N-1)-th or (N+1)-th mode. It happens when the change of lasing frequency exceeds one FSR. From Eq. (3), Δλm is expressed as,
3. Experiment setup
The experimental setup of the proposed wavelength-tunable mode-locked fiber laser is shown in Fig. 1. The laser is in a sigma-laser configuration. An SOA (Covega BOA1017) having the 3 dB gain bandwidth of 70 nm is used as a gain medium in the laser cavity. Active mode-locking is realized by directly modulating the injection current to the SOA with the RF signal from a RF synthesizer. Direct modulation of the injection current instead of using an external modulator can reduce the intracavity loss and the cost, thus a wide tuning range is expected. Because the devices are pigtailed with polarization maintaining fibers (PMF) except for the dispersion compensation fiber (DCF), the DCF is placed between the polarization beam splitter (PBS) and the Faraday rotator mirror (FRM) to form the sigma-laser cavity. As the result, the intracavity polarization state is constant over entire wavelength band . A dispersion parameter of the DCF is -131.8 ps/nm/km at λ=1340 nm. Two isolators ensure unidirectional lasing of the laser cavity. 10 % of the light in the cavity is extracted from the 9:1 coupler as an output light.
4.1 Wavelength tunable range
We set the mode-locking frequency around 1 GHz, and test three different lengths of DCF (20 m, 35 m, 50 m). The bias injection current to the SOA is 160 mA and directly modulated by the RF current whose peak to peak is 280 mA. Figure 2 shows the change of lasing spectra in the case 20 m-long DCF is inserted as the mode-locking frequency is changed manually. Figure 3 is a plot of the output power as a function of the lasing wavelength. The lasing wavelength shifts toward the longer wavelength as the mode-locking frequency increases. It is linearly in proportion to the mode-locking frequency. Total tuning range over 105 nm is obtained, and the output power is about 3 dBm at the peak wavelength. And the 3 dB tunable bandwidth is about 70 nm as shown in Fig. 3. The output is pulsed at the repetition rate of the mode-locking frequency (around 1GHz), and typical pulse duration is as wide as 0.3ns because of highly dispersive cavity. Table.1 summarizes the free spectrum range (FSR), the tuning sensitivity and the tuning range for the each DCF length. The shorter the DCF length is, the higher the sensitivity is, because the dispersion in the cavity is in proportion to the DCF length L, as expected from Eq. (2). The tuning range is wider as the DCF length is shorter. In the case of 20 m-long DCF, tuning range as wide as 105 nm was obtained. It is because the harmonic order N and the dispersion is smaller as the DCF length is shorter, as expected from Eq. (3). In theory, tuning range should be equal to (FSR)×(tuning sensitivity), whereas the actual tuning range is limited to be about half of it. When the tuning of mode-locking frequency exceeds half of the FSR, two wavelengths begin to be lased. Figure 4 is a plot of the lasing wavelength as a function of mode-locking frequency when the 35 m-long DCF is inserted. The frequency range for one-wavelength lasing finds to be 1.3 MHz in spite of the FSR of 2.36 MHz, and at other frequency range two different wavelengths are lased simultaneously. We attribute it to the rational mode-locking .
In the rational mode-locking operation, at the mode-locking frequency F expressed as
a pulse train is generated at a repetition frequency of m×F. Here, F 1 is a FSR of the cavity, and m is an integer number greater than one. Multiwavelength oscillation occurs at a frequency which satisfies both the normal mode-locking condition at one wavelength and the rational one at another wavelength. The rational mode-locking is more easily caused at lowerharmonics of m, and we found that only 2nd harmonic rational mode-locking occur in the experiment. Figure 5 shows the optical spectra and mode-locked pulses when both mode-locking conditions are satisfied. Two wavelengths are lased and we find that repetition rates of the mode-locked pulses are different, one is 1 ns and the other is 500 ps.
Thus, we found that the tunable range of the mode-locking frequency is half of the FSR due to the rational mode-locking. We have to design the laser so that the FSR is more than twice of the desired tuning range.
It is confirmed in both theory and experiment that the tuning range is wider with the shorter DCF. However, when the DCF is too short, an instantaneous spectral bandwidth of the lasing spectrum becomes broad due to a small dispersion. Equation (2) shows that decrease in dispersion in the cavity increases the tuning sensitivity, which means that the mode-locking is too weak to define a lasing wavelength. Figure 6 shows instantaneous lasing spectra at λ=1330 nm with different lengths of DCF. The full-width half maximum (FWHM) of the lasing spectra are around 0.15–0.2nm for all cases, whereas the sideband is reduced and the spectral purity is enhanced as the DCF is longer.
4.2 Wavelength tuning rate
The lasing wavelength is swept by modulating the mode-locking frequency. A pattern of a RF modulation signal is shown in Fig. 7. A triangular waveform signal is applied as a modulation signal to sweep the lasing wavelength linearly. In the up-scan range where the frequency changes lower to higher, the lasing wavelength shifts toward longer wavelength, whereas in the down-scan range where the frequency higher to lower, the lasing wavelength shifts toward shorter wavelength. One period of a sweep corresponds to the area covered by a square in Fig. 7.
Figure 8 shows the results when the mode-locking frequency is modulated so as to cover the range where only one wavelength is lased when a 20 m-long DCF is inserted. Optical spectra (left) are measured using an optical spectrum analyzer at the peak-hold mode, and temporal waveforms are measured using an oscilloscope. The laser can be operated at a different sweep rate, 1 kHz, 50 kHz, 100 kHz, 200 kHz. The laser is periodically swept over 105 nm range at a sweep rate as high as 1 kHz. As the sweep rate increases, both spectrum and temporal waveform are almost unchanged until a sweep rate as high as 50 kHz, but the tuning range gradually decreases at higher sweep rate. Tuning range of about 100 nm is achieved at a sweep rate as high as 100 kHz. A decrease in the tuning range at a high sweep rate is due to a decrease in the number of pulse roundtrips. At an edge of the SOA gain profile, a pulse can not be formed during the roundtrips in the cavity at a high sweep rate. Figure 9 shows a result when 35 m-long DCF is inserted. Tuning range of about 70 nm is achieved at a sweep rate as high as 1 kHz. As the sweep rate increases, tuning range was also gradually decreased and the temporal waveform changed. The sweep rate at which the tuning range begins to decrease is lower than that with the 20 m-long DCF because the total cavity length is longer. A decrease in the tuning range at the high sweep rate is also due to a decrease in the number of pulse roundtrips.
4.3 Tuning linearity
A lasing wavelength is in proportion to a mode-locking frequency as shown in section 4.2. A lasing wavelength can be swept linearly by applying the modulation waveform in which modulation frequency changes linearly. We measure a tuning linearity of the laser under the sweep operation. It is measured by using a fiber Fabry-Perot interferometer (FFPI). Experimental setup is shown in Fig. 10. By measuring a temporal response of each peak of a FFPI, we can measure a tuning linearity of the laser.
A characteristic of the FFPI is as follows, FSR is 99.9 GHz, 3 dB bandwidth is 13.48 GHz, and finesse is 7.41. We measure a tuning linearity of the laser at a sweep rate of 100 Hz which is a resolution limit rate of the oscilloscope. Temporal waveform is shown in Fig. 11(a), and calculated linearity is shown in Fig. 11(b). Temporal waveform has a little noise because the bandwidth of the laser is not so narrow that each temporal peak contains a element of multiple spectral peaks of the FFPI. Each periodic temporal peak means each transmission peak of the FFPI. The laser can tune the lasing wavelength linearly under a sweep operation at a sweep rate of 100 Hz.
We demonstrated a wide and fast wavelength tunable mode-locked fiber laser using dispersion-tuning technique. By using an SOA as a gain medium and inserting a DCF for increasing a dispersion in the cavity, tuning range of over 100 nm was achieved. By modulating the mode-locking frequency, the laser could be swept over 100 nm at a sweep rate as high as 200 kHz. Although the tunable range of mode-locking frequency is limited due to the rational mode-locking, we confirmed that a wider tuning range is obtained by using a shorter DCF. However there is a trade-off between the tuning range and the instantaneous spectral bandwidth.
The sweep rate of our proposed dispersion-tuned mode-locked laser is limited only by the cavity length. It has a potential of faster sweep by using a dispersive element which can provide a higher dispersion with a short length. The proposed laser has a wide tuning range and a high sweep rate, and we expect that it is applicable as a light source for OCT, fiber sensors and so on.
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