Narrowing Linewidth of Wavelength-Swept Active Mode Locking Laser Based on Cross Gain Modulation

: We demonstrate a novel narrow-linewidth conﬁguration of a wavelength-swept active mode locking (AML) ﬁber laser. The frequency response of the modulation depth of a semiconductor optical ampliﬁer can be improved in a higher modulation frequency region by adapting the cross gain modulation (XGM) conﬁguration, compared to that of the conventional direct gain modulation (DGM) conﬁguration. As a su ﬃ cient modulation depth is implemented for an AML at higher modulation frequencies at around gigahertz order, it results in a narrower linewidth of lasing output. For the same modulation frequency of 1361.25 MHz, the linewidth of 0.25 nm with DGM becomes narrower up to 0.113 nm with XGM, which corresponds to an improved point spread function with 1.41 mm of 6-dB roll-o ﬀ . and G.H.K. obtained and analyzed the data; S.J.P., G.H.K., H.D.L., M.J., and C.-S.K. discussed the experimental results; S.J.P., G.H.K., H.D.L., M.J., and C.-S.K. contributed to writing and reviewing the paper.

It has been experimentally investigated that there is a tendency for reducing linewidth of the wavelength-swept AML lasers at higher modulation frequencies [15,16]. As a theoretical approach to this issue, a discrete model of wavelength-swept AML laser has been proposed recently to show that the AML matching conditions with both higher modulation frequency and higher dispersion cavity are helpful to induce a narrower spectral linewidth [17,18]. However, under the direct gain modulation (DGM) configuration used for most wavelength-swept AML laser schemes, this linewidth narrowing approach by increasing the modulation frequency is mostly limited below the region of a few hundred MHz. It has been experimentally shown that the lasing output cannot be built from the amplified spontaneous emission (ASE) spectra due to the modulation response degradation in a higher modulation frequency region. Because the carrier lifetime of an SOA is several hundreds of picoseconds, the modulation response of SOA is dramatically decreased at high frequencies around GHz range in a conventional DGM method wavelength-swept AML laser [20,21].
Meanwhile, it has been studied that the cross gain modulation (XGM) method can be used to modulate the optical gain by injecting intensity-modulated optical pump light into the SOA, instead of the conventional current-modulated electric pump injection into the SOA under the DGM configuration [22][23][24][25][26]. It is also known that the XGM can improve the frequency response of the modulation depth of the SOA by reducing the lifetime in the cavity [22,23]. In this study, we investigate the static and dynamic characteristics of the novel wavelength-swept laser by adapting the XGM method into the AML condition for the first time, as far as we know.

Experiments and Results
Before constructing a laser cavity, we tried to confirm experimentally that the XGM works better than the DGM in a high-frequency modulation condition. Figure 1a,b show the experimental setup to measure the modulation depth of the SOA in the DGM and XGM configurations, respectively. The modulation depth of the SOA (M SOA = Ao/Ap) is newly defined as the ratio of the amplitude of the output electric signal modulation (Ao) compared to the amplitude of the pump electric signal modulation (Ap). The electrical output power from the light passing through a SOA (InPhenix, IPSAD1501) with C-band gain was measured using a 1-GHz bandwidth photodetector and an 8-GHz bandwidth electric spectrum analyzer (ESA) (Advantest, R3267). The amplified spontaneous emission (ASE) of another SOA (InPhenix, IPSAD1509) with a central wavelength of 1552.59 nm and power of 0.13 mW was used at the input port of the SOA. The modulation amplitude of the pump electric signal was set to 14 dBm and the initial frequency increased from 100 MHz to avoid the damage to the SOA due to low modulation frequency. In the DGM configuration in Figure 1a, the injected current of the SOA is directly modulated as a sinusoidal signal from a radio frequency signal generator (RF-SG) (Anritsu, 68369A/NV). A bias-T circuit was used to combine a DC current and an RF modulation signal prior to the position of the SOA. In the XGM configuration of Figure 1b, an external-cavity Littrow laser with a central wavelength of 1541.53 nm, a linewidth of~100 kHz, and an output power of 1.38 mW was employed as the optical pump light into a series made up of a Mach-Zehnder modulator (MZM), a circulator, and a SOA. The modulation depth of the optical pump light was controlled with the modulation depth of the pump electric signal from the RF-SG. The direction of the optical pump light from the circulator (CIR1) port 1 to port 2 was opposite to the optical amplification signal light in the SOA gain part.
For both configurations shown in Figure 1a,b, it was observed that the modulation depth of the SOA decreased with increasing modulation frequency in a higher region over 100 MHz; however, the decreasing trends of modulation depth of the SOA were different for the modulation frequencies for both the configurations. Thus, the relative modulation depth of the SOA (M SOA /M i ) is defined as the ratio of the modulation depth of the SOA at each modulation frequency compared to that at 0 MHz.  Figure 1c shows the comparison of the relative frequency response of the modulation depth of the SOA for both the DGM and XGM cases. It is clearly shown that the frequency response of the modulation depth of the SOA decreases as the modulation frequency increases in both the DGM and XGM. The 3-dB points appeared at 462.962 and 930.509 MHz in the DGM and XGM, respectively. It means that the frequency response of the modulation depth of the SOA was enhanced by approximately two-fold by using the XGM. Figure 2 shows the experimental set-up of the wavelength-swept AML laser based on the XGM configuration. The cavity consists of a SOA (InPhenix, IPSAD1501) acting as a gain medium, two optical circulators, three polarization controllers for optimizing the polarization states of the light propagating through the single mode optical fiber, an optical isolator to block the pump light to prevent undesired lasing, and a chirped fiber Bragg grating (CFBG) (TeraXion) acting as a dispersion medium (15 ps/nm at 1.5 µm) and an output coupler (reflectivity of~60%). A 12.5 mW external-cavity Littrow laser was used as the optical pump light. The optical pump light was modulated by the MZM (iXblue, MX-LN-20) using a sinusoidal signal from the RF-SG. The intensity-modulated optical pump light was close to 4.1 mW due to the high loss of the MZM (−3.5 dB) at the 1.2-GHz modulation frequency. To achieve the high modulation depth of the SOA for the mode locking condition, a booster optical amplifier (BOA) (Aeon, Models CL) was inserted to increase the optical power of the intensity-modulated optical pump light at port 2 of the CIR2 up to 11 mW because it is well known that the extinction ratio of an SOA is improved by increasing the power of the pump light [22,23]. However, the power of the pump light needed to be controlled properly. In the case of the optical pump light being too strong, the gain of the SOA in the ring cavity cannot be fully increased to generate the laser output. The intensity-modulated optical pump light is injected through the circulator port 1 of CIR1 into the SOA in the direction opposite to the lasing signal. This injected pump light leads gain modulation of the SOA by depleting the gain of SOA. Therefore, the modulation depth of the SOA can be controlled via the modulation depth of the injected light. Moreover, the injected pump light reduces the carrier lifetime in the cavity due to stimulated recombination; high modulation frequency can be implemented. In this setup, an ASE of 0.2 mW comes out from the SOA in the cavity and acts as a signal light in the XGM configuration. The wavelength of the optical pump light is another important parameter for a higher extinction ratio of the XGM. In general, the extinction ratio is maximized when the wavelength of the optical pump light is at the peak of the gain spectrum [21][22][23][24] and the peak of the gain spectrum is red-shifted when strong optical pump light is injected into the SOA due to the depletion of carriers [24,27]. However, setting the pump light at the center of gain spectrum causes excessive undesired lasing of optical pump light, which decreases the output power at the desired wavelength and the SNR. Therefore, the optical wavelength of the pump light was set to 1590.9 nm at the side of the gain spectrum and the measured extinction ratio was 7.5 dB.
A laser output with a short pulse train occurs when the modulation frequency matches the AML condition as follows [9]: where f m is the modulation frequency applied to the SOA, N is the order of harmonic mode locking. The free spectral range of the laser cavity, FSR, is expressed as [9]: where L is the cavity length, n eff is the effective refractive index in the fiber ring cavity, and c is the speed of light in vacuum. The zero-order FSR, FSR 0 , is driven from the cavity length and the speed of light in the fiber cavity [28,29]. However, since each wavelength has different AML conditions due to the dispersion medium of the CFBG, the deviation in the lasing wavelength, ∆λ m , is expressed as a function of the modulation frequency, f m [9]; where ∆f m = f m -f m0 , f m0 is the central mode-locking frequency, D total is the dispersion parameter. Figure 3a,b are examples of the spectral peak shape of static wavelength-swept AML laser at different mode-locking orders under the DGM and XGM configuration, respectively. For the central wavelength of 1574.1 nm and a cavity length of 3.752 m, FSR 0 is given as 54.45 MHz. Thus, the mode-locking orders, N, of 13, 17, 21, 25 correspond to the modulation frequencies, f m , of 707.85, 925.65, 1143.45, 1361.25 MHz, respectively. We compared the linewidths of the XGM and DGM by applying the same RF signals with a different mode-locking order into the SOA directly or into the MZM for the same main cavity with the same FSR 0 . In the DGM condition with the mode-locking frequencies higher than 707.85 nm for the 13th mode-locking order, it was observed that the linewidth tends to become broader as the frequency increases. However, the mode locking does not occur properly in higher modulation frequency ranges because the relative frequency response of the modulation depth of the SOA deteriorates in the high mode-locking frequencies, as shown in Figure 1c. On the contrary, in the case of the XGM, the linewidth gradually decreased as the modulation frequency increased over 707.85 MHz. The linewidth, defined as a 3-dB spectral width, was 0.119 nm for the XGM, but it was 1.291 nm for DGM, at a modulation frequency of 1361.25 MHz of the 25th mode-locking order. Inset of Figure 3b shows the relationship between the linewidth and modulation frequency under the XGM condition. Thus, we can experimentally confirm that the enhanced frequency response of the modulation depth of the SOA by using the XGM was indeed helpful to achieve a narrower linewidth of the AML laser output.  The sensitivity was 3.64 nm/MHz. Undesired lasing at 1590.9 nm was also observed, which was caused by the optical pump light reflected from the SOA. The intensity difference between the signal light and optical pump light was around 18 dB. The linewidth and intensity of the laser varied depending on the wavelength, but there was no significant difference within the tuning range. The signal to noise ratio (SNR) was measured over 35 dB in most of tuning region. As shown in Figure 4b, the average linewidth of the laser was 0.112 nm and the narrowest linewidth was 0.086 nm at the central wavelength of 1559.7 nm for the modulation frequency of 847.11 MHz. Figure 4b also shows the linearity of the wavelength-swept AML laser based on the XGM with the R-squared value of 0.99997. To measure the dynamic characteristics of the XGM, the spectrum was measured while sweeping the modulation frequency. For high linearity and a high duty cycle, a negative saw-tooth waveform from an arbitrary function generator (AFG) (Tektronix, AFG3022C) was externally applied to a frequency modulation (FM) port of the RF-SG to sweep the modulation frequency. Figure 5a shows the peak-hold spectra at a sweeping rate at 100 kHz, and the 6-dB tuning bandwidth was measured to be 41.25 nm; this result is in good agreement with the 3-dB optical bandwidth of the SOA, which was~45 nm. Figure 5b shows the temporal series of the laser output, measured with 350 MHz photodetector (PD) (Thorlabs, PDB430C-AC). The dynamic characteristics were evaluated based on a 6-dB roll-off of the point spread function (PSF), which was acquired through the Fourier transform of an interference signal by passing through a fiber-type Mach-Zehnder interferometer (MZI). Figure 5c shows the measured sweeping trace of a sawtooth wave drive function at 20 kHz. This 6-dB PSF parameter using the XGM was measured as 1.41 mm.

Conclusions
We demonstrated a narrower linewidth wavelength-swept AML laser based on the XGM configuration. The 3-dB point of the frequency response of SOA was improved approximately two-fold in XGM compared to that of DGM. The experimental results show that the linewidth is narrowed with increasing the mode locking frequency based on XGM. Due to enhanced frequency response, the wavelength-swept laser based on XGM successfully worked at 1361.25 MHz modulation frequency and was measured as 0.113 nm, which is much narrower than that of DGM with 0.25 nm. The proposed laser also showed properties of high linearity, high SNR, and sweep rate of 100 kHz, in addition to the narrower linewidth. This study proposes a novel method that can overcome one of the disadvantages, the broad linewidth, of wavelength-swept AML lasers for practical applications.