The static and dynamic characteristics of a wavelength-swept active mode locking (AML) fiber laser are presented in both the time-region and wavelength-region. This paper shows experimentally that the linewidth of a laser spectrum and the bandwidth of the sweeping wavelength are dependent directly on the length and dispersion of the fiber cavity as well as the modulation frequency and sweeping rate under the mode-locking condition. To achieve a narrower linewidth, a longer length and higher dispersion of the fiber cavity as well as a higher order mode locking condition are required simultaneously. For a broader bandwidth, a lower order of the mode locking condition is required using a lower modulation frequency. The dynamic sweeping performance is also analyzed experimentally to determine its applicability to optical coherence tomography imaging. It is shown that the maximum sweeping rate can be improved by the increased free spectral range from the shorter length of the fiber cavity. A reflective semiconductor optical amplifier (RSOA) was used to enhance the modulation and dispersion efficiency. Overall a triangular electrical signal can be used instead of the sinusoidal signal to sweep the lasing wavelength at a high sweeping rate due to the lack of mechanical restrictions in the wavelength sweeping mechanism.
©2011 Optical Society of America
Over the last decade, the wavelength-swept laser has been studied intensively because of the various sensing and telecommunication applications [1,2]. Conventional wavelength-swept lasers consist of a wide-band gain medium and a wavelength selecting filter in the cavity. The sweeping property of a laser is determined mainly by the mechanical-tuning characteristics of the filter, such as a fiber Fabry-Perot tunable filter (FFP-TF)  and a polygon scanning mirror . Recently, the Fourier domain mode locked (FDML) laser was developed for high speed optical coherence tomography (OCT) at a 370 kHz sweeping rate  and the polygon scanning mirror was used to implement high speed OCT imaging at a > 400 kHz sweeping rate . On the other hand, these types of wavelength-selecting filters have suffered from speed limitations, high cost, and bulk volume due to the mechanical operation to tune the center wavelength. Post calibration is also required when OCT fringe data is acquired non-uniformly in the frequency domain (k-space) due to the high speed sweeping of sinusolidal signal operation rather than linear signal operation . Some studies have reported k-domain linearization such as k-space resampling or k-linear wavelength swept laser without resampling .
To overcome the limits of the mechanically tunable wavelength selecting filter in the wavelength-swept laser, we have suggested an alternative tuning method that eliminates the mechanical tuning filter by replacing it with the electro-optic tuning of a lithium-niobate polarization controller . In addition, it was recently demonstrated that a wavelength-swept laser could also be realized by dispersion-tuning in a ring-type fiber laser configuration. Unlike mechanical-tuning-based swept lasers, the active mode locking (AML) fiber laser is based on an actively mode-locked pulse incorporating a dispersive medium within its cavity, in which the lasing wavelength tuning is obtained by continuously sweeping the electrical modulation frequency [7,8]. The successful application of wavelength-swept AML lasers to fiber optic sensing systems has been demonstrated  but to the best of the authors’ knowledge, there are no reports on its applicability to OCT imaging.
In this study, the static and dynamic characteristics of wavelength-swept AML fiber lasers will be presented in both the time-domain and wavelength-domain. The linewidth of the laser spectrum and bandwidth of sweeping wavelength are depend directly depending on the length and dispersion of the fiber cavity as well as the modulation frequency and sweeping rate for the mode-locking condition. For the feasibility of OCT imaging, the dynamic sweeping performance was also examined experimentally up to 700 kHz so that the maximum sweeping rate is determined by the free spectral range of the fiber cavity. Owing to the lack of mechanical restrictions in the wavelength sweeping mechanism, a triangular electrical signal can be used to sweep the lasing wavelength at a high sweeping rate.
2. Experiment setup
Figure 1 shows the experimental set-up of the constructed wavelength-swept AML fiber laser. The cavity consists of a reflective semiconductor optical amplifier (RSOA) (CIP SOA-R-OEC-1550) for gain, a circulator for the lasing cavity, a polarization controller (PC), a dispersion compensation fiber (DCF) for the high dispersive cavity, and an 50:50 output coupler. The reflection type gain medium in the fiber cavity is used to enhance the modulation and dispersion efficiency. The RSOA in this experiment has a 3 dB bandwidth of 40 nm and a 10 dB bandwidth of 90 nm under an applied current of 80 mA. Since the RSOA is a specially developed gain block used for ultra-high speed modulation over the GHz electrical bandwidth of direct RF modulation, it is supplied with a 7-pin butterfly/SMA package and the 50 Ω input impedance SMA port is matched to the RF signal .
Compared to conventional SOAs, the input light of the RSOA will pass through the active area and the dispersive fiber media twice. This means that the length of the cross gain modulation (XGM) effect doubles for the RSOA gain medium. Hence, the RSOA produces better modulation performance than the SOA . A previous study demonstrated that the WDM-PON transmitters and the fiber Bragg grating sensor interrogation systems benefit from using RSOAs with such a high speed RF modulation property [11,12].
The injection current to the RSOA was modulated directly using a sinusoidal signal from an RF signal generator (Agilent RF synthesizer, ESG D3000A) to obtain an active mode locking condition. An arbitrary waveform, such as a triangular electrical waveform, from a frequency sweeper (Tektronix function generator, AFG3252) was used to add the frequency variation to the RF synthesizer for lasing wavelength tuning. A 70-m-long DCF with a dispersion parameter of ~-90 ps/nm/km at λ = 1550 nm was inserted into the cavity to add a chromatic dispersion effect to the laser cavity for the wavelength tuning. As photons pass the DCF twice for one round-trip, the entire cavity length is approximately 144 m, involving the other fiber optic components.
The lasing wavelength, λm, of a stable short pulse train that meets the harmonic mode locking condition can be determined by modulating the injection current into the RSOA with an RF signal at a frequency, fm. The modulation frequency, fm, is an integer (N) times the free spectral range (FSR); fm = N·FSR. The integer time, N, is the order of mode-locking condition. Simply, the FSR0 can be expressed by the cavity length and the speed of light in the cavity, such as FSR0 = c0 / nL. When the fiber laser cavity has chromatic dispersion, the FSR becomes a function of the wavelength and so the active mode-locked wavelength, λm, can be varied linearly over the tuning range of FSR and its corresponding tuning range of fm . The tuning mechanism can be expressed simply as :7]. Equations (1) and (2) show that, if the modulation frequency of the RF signal is changed repeatedly, the wavelength of the AML laser will shift accordingly.
3. Static characterization
Figure 2(a) shows the time trace of the wavelength-swept AML laser. The time interval of the neighboring pulse corresponds to the modulation frequency, fm. Figure 2(b) shows the static output spectra of the laser in the approximately 400 MHz region of the modulation frequency. As fm was changed from 400.93 MHz to 401.83 MHz, the lasing wavelength, λm, shifted from 1501.1nm to 1584.7 nm, respectively. A tuning range of 83.6 nm was thereby obtained by Eq. (1) using the measured sensitivity parameter, S, of 92.9 nm/MHz. Since the FSR0 is approximately 1.4 MHz for a cavity length of 144 m, the next mode-locked lasing output appears at the same wavelength position as the modulation frequency, fm, increases for every 1.4 MHz increase with the next order, N. As shown in Fig. 2(b) the total bandwidth of the wavelength-swept AML laser can be determined simultaneously from the tuning range of FSR and gain range of SOA.
Figure 3(a) and 3(b) shows the experimental comparisons for the linewidth of the static AML laser and the bandwidth of wavelength-swept AML laser when the mode-locking order, N, is 280, 430, and 500, respectively. The linewidth of the laser output at the 1550 nm can be measured as 1.0 nm and 0.7nm when the N is 280 and 430, respectively. The linewidth was reduced further to 0.5 nm when the N was increased to 500. This means that the static linewidth is inversely proportional to the modulation frequency, fm, of the applied intensity modulated RF signal. Furthermore, at the same mode-locking order, the linewidth of the laser is also inversely proportional to the cavity length and dispersion parameter of DCF. For example, when N is 500, the linewidth becomes 1 nm, 0.7 nm and 0.5 nm for a DCF length of 50 m, 70 m and 100 m, respectively.
The bandwidth spectrum of the wavelength-swept AML laser was obtained at a sweeping rate, fs, of 100 Hz using the peak-hold mode of optical spectrum analyzer (OSA). As the N increased (higher fm), the tuning bandwidth, Δλm, became narrower. At N = 280, 430 and 500, the measured bandwidth was approximately 84 nm, 76 nm and 66 nm, respectively. As shown in Fig. 2(b) for N = 280, the fm of 400.93 MHz and 401.83 MHz correspond to the lasing at 1501.1 nm and 1584.7 nm, respectively. When N was changed to 500, the fm of 699.769 MHz and 700.849 MHz corresponded to lasing at 1516.7 nm and 1583.4 nm, respectively. Therefore, higher order of mode locking is useful for a narrower linewidth of a static AML laser, but is not helpful for achieving a broader bandwidth of the wavelength-swept AML laser.
4. Dynamic characterization
The dynamic sweeping performance was also analyzed experimentally to determine the maximum sweeping rate, fs, from the relationship with FSR of the fiber cavity. Figure 4(a) shows the output peak-hold mode spectra of the laser when the sweeping rate becomes 1, 10, 500 and 700 kHz, respectively. For a broader bandwidth condition of the wavelength-swept AML laser, N and the DCF length were set to 280 and 70 m, respectively, and thus the fm ranged from 400.93 MHz to 401.83 MHz. Since the monitored spectral intensity is related directly to the total integration time of the peak-hold mode of OSA, it appears that the intensity decreased with increasing sweeping rate due to the lower integration time of detector . However, the time domain tracing of the wavelength-swept AML laser in Fig. 4(b), which was measured with a InGaAs balanced photo-detector (New focus 1817-FC) with a 80 MHz bandwidth, a 3 ns rise time and an oscilloscope (Tektronics TDS3014B), showed a similar optical power level among the various sweeping rates of 1, 10, 100 and 500 kHz. This means that the instantaneous wavelength-swept lasing intensity is not affected seriously by the sweeping rate of the wavelength-swept AML laser.
The limitation of the maximum sweeping rate can be examined from this dynamic sweeping performance. When the sweeping rate, fs, reaches approximately half of the FSR, which is approximately 700 kHz in this experiment setup with a cavity length of 144 m, the triangular signal from the frequency sweeper does not act as a wavelength sweeper. Instead, as shown in Fig. 4(a), the spectrum of the lasing output is fixed to a certain wavelength of approximately 1545 nm. Under this lasing condition, the corresponding active mode-locking pulse becomes unstable in the time-trace monitoring. Since N is 280 and fm0 is 401.38 MHz, the value of FSR can be calculated to be 1.4335 MHz and the maximum sweeping rate becomes 716.75 kHz. It can be deduced that a shorter length of the laser cavity is preferred to achieve a higher sweeping rate of the wavelength-swept AML laser output because the FSR can be increased simply by decreasing the length of the cavity.
There some trade-off between the sweeping rate, instantaneous linewidth, and overall bandwidth. As summarized in the end of section 3, a lower order of mode locking is useful for a broader bandwidth of the wavelength-swept AML laser, but is not helpful for achieving a narrower linewidth of static laser output. Therefore, for a further narrowing of the linewidth under the given order of mode locking condition, it is important to increase the product of cavity length and dispersion parameter as much as possible . Considering a high sweeping rate owing to the shorter length of laser cavity, the best way to optimize the laser cavity is to achieve a high dispersion and short length simultaneously. Special types of dispersive components can be useful in the further design of laser cavity, such as high dispersion chirped fiber Bragg grating and high dispersion blazed grating pairs .
For the linewidth characterization of wavelength-swept AML laser according to the sweeping rate, this study measured the reflected signal of this laser source from three fiber Bragg grating (FBG) arrays through a three-port circulator . Each FBG has a center Bragg wavelength at 1539.97 nm, 1549.78 nm and 1563.44 nm, respectively, with 3 dB bandwidth of ~ 0.2 nm. With the different sweeping rate, three peaks of the reflecting signal were always obtained from FBG arrays on a time scale as shown in Fig. 5 . As sweeping rate increased, that the reflected signal become broader and overlapped each other. Since the monitored signal is a convolution integral of the spectral bandwidth of FBG and the linewidth of wavelength-swept AML laser, the relationship with linewidth broadening can be indirectly obtained using a deconvolution between the monitored signal and FBGs. The instantaneous linewidth of this laser source is ~ 1.5 nm at 1 kHz sweeping rate and it becomes approximately two times broader at 20 kHz sweeping rate. Further study for a quantitative characterization of the linewidth using various selections of FBGs is currently underway.
5. Interferometric characterization
The interferometric characteristics of wavelength-swept AML laser are presented for the application of OCT. Figure 6(a) shows an optical interferogram signal from a fiber interferometer at sweeping rates, fs, of 1, 10, 100 kHz and 500 kHz. As shown in the dynamic characterization in Fig. 4 and 5, the bandwidth of the sweeping wavelength becomes narrower and the linewidth of the laser spectrum becomes broader as the sweeping rate is higher. Therefore, at a high sweeping rate above few tens kHz, the interferogram has a relatively smaller duty cycle and temporal asynchronization. For a stable active mode-locking condition, the wavelength-swept operation of 1 kHz was at first used in axial direction tomography imaging. The interfered optical signal between the sample and the reference arms was detected using a balanced photo-detector (New Focus, 1817-FC) to remove the DC signal in the optical interferogram. The swept-source OCT system is basically a Mach–Zehnder interferometer composed of broadband 70/30 and 50/50 fiber couplers.
Figure 6(b) shows the measured point spread function (PSF) under various path length conditions. The signal to noise ratio (SNR) of the system at the position of ~ 110 μm from the path-matched depth was measured to be 46.1 dB . The sensitivity drops by 6 dB at a depth of ~ 700 μm, which corresponds to the theoretical imaging depth limitation determined from the instantaneous linewidth (~ 1.5 nm) of this laser source . The dashed line is fitted by Gaussian approximation of the peak data of PSF. Owing to the ~ 84 nm of the integrated bandwidth of the wavelength swept AML laser source, the theoretical axial resolution is calculated to be 12.4 μm. The measured axial resolution was 13.1 μm from the full-width at half-maximum of the PSF at a depth of ~ 110 μm from the path-matched position as shown in the inset of Fig. 6(b). A sample of cover glasses was prepared to demonstrate 2D imaging using the proposed wavelength-swept AML laser. As shown in Fig. 6(c), each layer of the three cover glasses can be distinguished clearly. For a practical OCT system using the proposed wavelength-swept AML laser, further investigations will be needed to modify for the deeper imaging, higher resolution and a higher sweeping rate because the narrower linewidth, broader bandwidth and larger FSR, respectively, can be implemented by proper design of the laser cavity with a shorter length and higher dispersion.
The characteristic of the wavelength-swept AML laser was demonstrated with reflective SOA for the effective modulation and reduced fiber length. Since the laser cavity does not have any mechanical-tuning-based wavelength-selecting filter, the triangular electrical signal can be used to sweep the lasing wavelength at a high sweeping rate. For a narrower linewidth, a longer length and higher dispersion of the fiber cavity as well as a higher order of mode locking condition are required simultaneously. For a broader bandwidth, a lower order of the mode locking condition is needed using a lower modulation frequency. The dynamic sweeping performance is strongly related to the FSR of the cavity, which is determined by the cavity length. The feasibility of the proposed novel laser source for OCT imaging was also demonstrated. Overall, further developments of the laser cavity with a shorter length and higher dispersion parameter are expected to improve the performance of OCT imaging and speed.
This research was supported by the World Class University program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology, South Korea (grant no. R31-2008-000-20004-0).
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