1810 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 8, AUGUST 2004
Rational Harmonic Mode-Locking of Erbium-Doped
Fiber Laser at 40 GHz Using a Loss-Modulated
Fabry–Pérot Laser Diode
Gong-Ru Lin, Member, IEEE, Yung-Cheng Chang, Student Member, IEEE, and Jung-Rung Wu
Abstract—Rational harmonic mode-locking of an Er-bium-doped fiber ring laser (EDFL) at repetition frequency of 40 GHz is demonstrated by using a purely loss-modulated Fabry–Pérot laser diode (FPLD) at 1 GHz. The FPLD is neither lasing nor gain-switching, which requires a threshold modula-tion power of 18 dBm to initiate harmonic mode-locking of the EDFL. After chirp compensation, the nearly transform-limited pulsewidth and spectral linewidth of 3 ps and 1.3 nm are ob-tained at repetition frequency of 40 GHz, which corresponds to a time-bandwidth product of 0.31. The EDFL gradually evolves from harmonic mode-locking to injection-locking mode as the FPLD changes from loss-modulation to gain-switching mode by increasing its dc driving current.
Index Terms—Erbium-doped fiber laser (EDFL), Fabry–Pérot
laser diode (FPLD), harmonic mode-locking, injection-locking, loss modulation, rational harmonic mode-locking.
I. INTRODUCTION
S
HORT-PULSED erbium-doped fiber lasers (EDFLs) have been comprehensively investigated to generate high-bit-rate ( 10 GHz) optical carriers for versatile applications such as wavelength-division-multiplexing (WDM)/time-division-multiplexing transmission in fiber-optic communication networks, sampling and switching in pho-tonic network systems [1], [2]. Harmonic and rational harmonic active mode-locking schemes are currently the main technologies to meet these demands. Typical mode lockers to achieve loss modulation of the EDFL cavity are a Mach-Zehnder integrated-optic modulator [3], a semiconductor multiple quantum-well electroabsorption modulator [4], and a gain-switched Fabry–Pérot laser diode (FPLD) [5], etc. To overcome the difficulty of gain-depletion modulation in the EDFL with extremely long carrier lifetime ( 10 ms), versatile repetition frequency multiplication schemes have recently emerged, such as the fiber dispersion induced pulse split-ting [6], the intracavity fiber Fabry–Pérot filter (FFPF)-based high-repetitive pulse extraction [7], and the optical pulse-injec-tion-induced frequency multiplication [8], [9], etc. In particular, the harmonic mode-locking of EDFL can also be demonstrated using cross-gain-modulated semiconductor optical amplifierManuscript received July 25, 2003; revised April 27, 2004. This work was supported in part by the National Science Council under Grant NSC92-2215-E-009-028.
The authors are with the Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, R.O.C. (e-mail: grlin@fac-ulty.nctu.edu.tw).
Digital Object Identifier 10.1109/LPT.2004.831057
Fig. 1. Embodiment of an FPLD mode-locked EDFL. Amp: RF power amplifier. PD: high-speed photodetector. RFS: radio-frequency synthesizer. WDM: WDM coupler.
[10] or directly modulated laser diode [11]. In this work, the rational harmonic mode-locking of EDFL using a purely loss-modulated FPLD are investigated. The FPLD can be driving at either loss-modulation or gain-switching via the adjustment of dc current and radio-frequency (RF) power. The evolution of the EDFL lasing mechanism from mode-locking to injection-locking is observed as the FPLD changes from loss-modulation to gain-switching mode. The 40-GHz rational harmonic mode-locked EDFL pulse train generated using a loss-modulated FPLD at repetition frequency of 1 GHz is characterized.
II. PRINCIPLE ANDEXPERIMENTAL
Fig. 1 plots the setup of an EDFL rational harmonic mode-locked using a purely loss-modulated FPLD. An Erbium-doped fiber amplifier (EDFA) with maximum gain of 17 dB is con-necting with a 1560-nm FPLD via a 50% optical coupler (OC) to construct an EDFL ring cavity. A polarization controller (PC) before the FPLD is used to adjust the intensity of the injec-tion light. There is no direct feedback from the EDFA output to the EDFA input. A 35% OC is employed to monitor the EDFL output. The EDFL cavity length is 50 m, corresponding to a fun-damental cavity frequency of around 4.48 MHz. The FPLD exhibits a threshold current and a longitudinal mode spacing of 13 mA and 1.2 nm, respectively. To achieve pure loss-lation instead of lasing, the FPLD is un-dc-biased but modu-lated using an RF synthesizer (Rohde & Schwarz, SML01) in connection with a power amplifier, which generates RF power from 0 to 30 dBm. To achieve perfectly gain-switched lasing,
LIN et al.: RATIONAL HARMONIC MODE-LOCKING OF EDFL AT 40 GHz USING A LOSS-MODULATED FPLD 1811
Fig. 2. Evolution of EDFL from mode-locking to injection-locking with increasing dc driving current of FPLD.
the RF power and the dc bias level of the FPLD are adjusted to 24 dBm and 10 mA using a biased-Tee circuit. The EDFL output is monitored by a digital sampling oscilloscope [(DSO)
Agilent GHz], and by an
op-tical autocorrelator (Femtochrome, FR-103XL) with temporal resolution of 175 fs.
III. RESULTS ANDDISCUSSION
With either a large-signal modulated or a gain-switched FPLD, the EDFL was reported to be stably mode-locked or injection-locked [11], [13]. In contrast, the RF-modulated FPLD is not lasing in our case. The harmonic mode-locking of the EDFL is initiated as the modulation power of the FPLD exceeds 18 dBm, providing mode-locking pulsewidth and peak power of about 93 ps and 330 mW, respectively. The harmonic orders of the FPLD mode-locked EDFL is 228 (corresponding to frequency of 1.020 127 GHz). The perfect mode-locking is achieved by driving the FPLD at dc current of 1.5 mA and RF power of 21 dBm. However, the gain-switching of the FPLD starts up when the dc driving current of the FPLD increases, which leads to an injection-locked EDFL pulse train arising after the mode-locking one (see Fig. 2). The injection-locked pulsewidth are 26.4 ps, which exhibits a nearly identical pulse shape with that of the gain-switched FPLD (about 21 ps). The loss-modulation mechanism of the FPLD disappears and the mode-locking no longer exists after gain competing with the injection-locking process. Under the gain-switched FPLD seeding, the injection-locking is more pronounced than the mode-locking in the EDFL.
The rational harmonic mode-locking is achieved by detuning the modulation frequency of the FPLD from to , where is the fundamental mode frequency of the EDFL, and are the harmonic and rational harmonic mode-locking orders, respectively. The frequency-detuning changes the pulse repetition rate from to , which is exactly times the modulation frequency . In experiment, the modulation frequency is detuned by 113 kHz to obtain 40th-order rational harmonic mode-locking (cor-responding to and ). In comparison, the frequency multiplication of the gain-switched FPLD in EDFL is rarely hard to be obtained and maintained due to the narrower
Fig. 3. (a) and (c) Autocorrelation traces (solid-square curves with dashed fitting lines). (b) and (d) Output spectra (solid curves with dashed fitting lines) of the FPLD mode-locked EDFL pulses before and after dispersion compensation. (a) and (b) Measured before dispersion compensation. (c) and (d) Measured after chirp compensation.
Fig. 4. (a) EDFL pulsewidth as a function of inverse square root of pulse repetition frequency. (b) 40-GHz EDFL pulse trains without (upper) and with (lower) intracavity FFPF. (c) SSB phase noise spectra of the EDFL with FPLD at loss-modulation (trace i) and gain-switching (trace ii) modes. (d) Supermode noise spectra of EDFL with FPLD at loss-modulation (lower) and gain-switching (upper) modes.
locking bandwidth. The autocorrelated EDFL pulse exhibits a significant pedestal adjacent to the principle pulse, as shown in Fig. 3(a). The measured pulsewidth and linewidth are 13.6 ps and 1.25 nm, which correspond to a time-bandwidth product of 1.36. The elimination of such pulse shoulders relies on the chirp compensation by using dispersion compensated fiber. After propagating through a 32-m-long Corning DCF with dispersion constant of 81 ps/nm/km, a pedestal-free EDFL pulse shape with a deconvoluted pulsewidth of 3 ps is shown in Fig. 3(c). A nearly identical spectrum with 3-dB linewidth of 1.3 nm is plotted in Fig. 3(d). The time-bandwidth product of 0.31 is nearly the transform-limit of sech function. The autocorrelation-measured pulsewidth and peak power of rational harmonic mode-locked EDFL is reduced from 93 to 3 ps and from 330 to 140 mW, respectively, as the rational harmonic mode-locking order increases from 1 to 40 [see Fig. 4(a)], which is linearly proportional to the inverse square root of the repetition frequency, i.e., .
1812 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 8, AUGUST 2004
Originally, the peak amplitudes of adjacent EDFL pulses in rational harmonic mode-locking scheme are not equalized due to the lasing of mismatched lower-harmonic super-modes [see upper trace of Fig. 4(b)]. This has be improved by adding an intracavity high-finesse FFPF (MOI FFP-TF 1550-040M1000-6.5, Finesse Bandwidth MHz, Free Spectral Range GHz), as shown in lower trace of Fig. 4(b) [12]. The pulse train with equalized peak amplitudes is monitored by using DSO at four-time averaging mode. The average power fluctuation of the modified EDFL system is within 0.74%. Moreover, the single sideband (SSB) phase noise spectra and supermode noises of EDFL using the FPLD at loss-modulation and gain-switching modes are compared. The analysis reveals that the uncorrelated SSB phase noise ( 105 dBc/Hz at offset frequency 10 kHz from carrier) of the mode-locked EDFL with a loss-modulated FPLD is 3 dB higher than that with a gain-switched FPLD, as shown in Fig. 4(c). Since the uncorrelated phase noise is strongly correlated with the spontaneous emission noise of the FPLD-EDFL link, it is consequential that driving the FPLD at well below threshold inevitably introduces larger spontaneous emission noise. The FPLD mode-locked EDFL pulse has an root mean square timing jitter of 0.49 ps in the integral region from 10 Hz to 100 kHz, whereas the jitter of the FPLD injection-locked EDFL pulse is smaller (0.33 ps). Improving the phase noise performance of the FPLD mode-locked EDFL, thus, relies on increasing the driving current of the FPLD. However, such operation easily turns the EDFL from mode-locking to injection-locking since the FPLD is gain-switching at larger currents.
Fig. 4(d) compares the supermode noise spectra of the EDFL with the FPLD operating at different schemes. With the loss-modulated FPLD, the supermode noise was suppressed with a supermode suppression ratio (SMSR) of 37 dB, whereas, the SMSR of the EDFL injection-locked by a gain-switched FPLD is slightly lower (26.6 dB). Since the FPLD is playing not only a loss modulator but also an FFPF in the proposed configura-tion, which thus, bring the EDFL a comparable supermode noise suppression response with that of a similar mode-locked EDFL system using an electrooptic modulator and an FFPF. The use of the FPLD-based modulator essentially releases the require-ment of ultrahigh modulation bandwidth electrooptic or elec-troabsorption modulators, which further benefits from the ad-vantages such as the improvement in the extinction ratio of the pulses (due to the operation of the FPLD at loss-modulation mode). Although the modulation bandwidth of the FPLD is lim-ited at several gigahertz, high-repetition pulse train of EDFL can still be obtained by using rational harmonic mode-locking op-eration.
IV. CONCLUSION
We have demonstrated the rational harmonic mode-locking of the EDFL with repetition frequency up to 40 GHz by using an FPLD purely loss-modulated at 1 GHz. The FPLD is neither lasing nor gain-switching in contrast to conventional ap-proaches. The harmonic mode-locking threshold of the EDFL
is below 18 dBm. The highest rational harmonic mode-locking order of 40 is achieved by slightly detuning the FPLD modu-lation frequency of 113 kHz. The peak power and pulsewidth of rational harmonic mode-locked EDFL pulse after chirp compensation are decreased from 330 to 140 mW and from 93 to 3 ps, respectively, as the repetition frequency of pulse train increases up to 40 GHz. The adjacent peak amplitudes of the EDFL pulses are completely equalized using an FFPF. The FPLD mode-locked EDFL exhibits SSB phase noise and SMSR of 105 dBc/Hz (at offset frequency 10 kHz from carrier) and 37 dB, respectively. By increasing the dc driving current for the FPLD, the evolution between harmonic mode-locking and injection-locking mechanisms in the EDFL due to the change of the FPLD from loss-modulation mode to gain-switching mode has been investigated. The injection-locking becomes the dominant mechanism at larger FPLD currents due to the gain depletion effect, which eventually diminishes the rational harmonic mode-locking pulse train in the EDFL.
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