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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 4, APRIL 2004
Pulsewidth-Variable Relaxation-Free Optical Millimeter-Wave Generation From a Semiconductor Fiber Ring Laser Young Min Jhon, Member, IEEE, Kyoung Sun Choi, Young Tae Byun, Jae Hun Kim, Seok Lee, Member, IEEE, and Dong Sun Seo, Member, IEEE
Abstract—Optical pulses of 10–50-GHz repetition frequencies have been generated by rational harmonically mode locking a semiconductor fiber ring laser using cross-gain modulation by 10-GHz external double optical pulses. The output is pulsewidth-variable from 6.4 to 14 ps, wavelength-tunable from 1530 to 1560 nm, and remains relaxation-free even though the injected external optical pulses have large relaxation oscillation noise. Index Terms—Millimeter wave, pulsewidth variable, relaxation free, ring laser, semiconductor fiber ring laser.
I. INTRODUCTION
H
IGH repetition rate ultrashort laser pulse sources are essential for ultrahigh-speed optical communication and all-optical signal processing. A mode-locked fiber ring laser (ML-FRL) is very attractive due to its ultrashort pulsewidth, low chirp, broad tunability, and high power. However, due to long cavity length and long upper level lifetime 8.4 ms of ML-FRL, relaxation oscillation is easily induced from perturbations like thermal fluctuations, mechanical vibrations, and especially modulator detuning, which require stabilization [1]. Previously, we have shown that a semiconductor-fiber ring laser (SFRL) using a semiconductor optical amplifier (SOA) as the gain medium is attractive since it preserves most of the merits of ML-FRL, but has no relaxation oscillation noise due to its short upper level lifetime [2]. Rational harmonic mode locking is a simple technique to multiply the repetition frequency of a harmonically mode-locked laser by detuning the modulation to obtain pulses with rational frequency , where and harmonic frequency are integers, and is the fundamental cavity frequency [3]. In our previous work, we have shown that 40-GHz laser pulses can be generated from a 10-GHz mode-locked laser [4]. In this letter, we report pulsewidth-variable relaxation-free wavelength-tunable optical millimeter-wave generation from
Manuscript received March 24, 2003; revised September 2, 2003. The work performed at Myongji University was supported by the Korea Science and Engineering Foundation under Grant R01-2000-000-00249-0 and the Integrated Photonics Technology Research Center (ERC, Inha University), the Ministry of Science and Technology, Korea. Y. M. Jhon, K. S. Choi, Y. T. Byun, J. H. Kim, and S. Lee are with the Photonics Research Center, Korea Institute of Science and Technology, Seoul 136-791, Korea (e-mail:
[email protected]). D. S. Seo is with the Department of Electronics, Myongji University, Kyonggi 449-728, Korea. Digital Object Identifier 10.1109/LPT.2004.823736
Fig. 1. Experimental setup for rational harmonic mode locking of SFRL using XGM by injecting double optical pulses from 10-GHz ML-FRL. (ODL: optical delay line, PC: polarization controller, OTF: optical tunable filter, DC: directional coupler).
a rational harmonically mode-locked SFRL using cross-gain modulation (XGM) by external double optical pulses, where the laser output is relaxation-free even though the injected external optical pulses have large relaxation oscillation noise. II. EXPERIMENT The schematic diagram of the experimental setup is shown in Fig. 1. A 10-GHz pulse train from a harmonically ML-FRL [5] is divided and combined after delaying one arm (ODL1) to obtain adjacent double pulses. The ML-FRL pulse had pulsewidth of 9 ps and energy of 70 fJ with average power of 1.5 dBm operating at 1551 nm. The double pulses are injected into the SFRL cavity to induce XGM inside the SOA, which mode locks the SFRL by periodic gain modulation. The gain saturation in the SOA is adjusted by changing the separation of the double pulses, which finally determines the SFRL pulsewidth. The SOA (Alcatel 1901) which was used as the gain medium of SFRL had a length of 1 mm, a recovery time of 1 ns, a maximum gain of 25 dB, and a saturation output power of 7.5 dBm. The output wavelength of the SFRL was tuned from 1530 to 1560 nm by an optical tunable filter (OTF1) with a 3-dB bandwidth of 1.2 nm. The cavity length of the SFRL was adjusted by an optical delay line (ODL2) to match the condition for rational harmonic mode locking. An optical isolator was used to ensure unidirectional laser operation. The total length of the SFRL cavity was 13 m. The output of the laser was observed by a 50-GHz sampling oscilloscope (Tektronix CSA 803 with Sampling Head SD-32)
1041-1135/04$20.00 © 2004 IEEE
JHON et al.: PULSEWIDTH-VARIABLE RELAXATION-FREE OPTICAL MILLIMETER-WAVE GENERATION
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Fig. 2. Output of SFRL for various cavity lengths to produce rational harmonically mode-locked pulses with repetition frequencies of (a) 20 GHz, (b) 30 GHz, (c) 40 GHz, and (5) 50 GHz (50 ps/div).
and an RF spectrum analyzer (Anritsu MS2688C, 40 GHz) combined with a 45-GHz photodetector (NewFocus 1014), and also by an optical spectrum analyzer (Ando AQ6315B, 0.05 nm). The pulsewidth of the laser pulse was measured by an autocorpulse shape. relator (Femtochrome FR-103XL) assuming III. RESULTS AND DISCUSSION The oscilloscope trace of the SFRL output at 1552 nm was observed while changing the cavity length by adjusting optical delay line ODL2 as shown in Fig. 2. Output pulses with repetition frequencies of 10 (not shown in the figure), 20, 30, 40, and 50 GHz were obtained according to the conditions for rational harmonic mode locking, which correspond to the first, second, third, fourth, and fifth rational harmonic, respectively. The output optical spectrum of the SFRL while tuning the optical tunable filter OTF1 is shown in Fig. 3, together with the spectrum of the external double pulses at 1551 nm. The output of the SFRL was tunable from 1530 to 1560 nm. All rational harmonics showed the same tuning characteristics. We observed the RF spectrum of the external optical pulses from the ML-FRL and the SFRL output to investigate the relaxation oscillation noise characteristics. Fig. 4(a) shows the RF spectrum of the external pulses from the ML-FRL when the
Fig. 3. Optical spectra of the injected ML-FRL external optical pulses (1551 nm) and of the SFRL output (1530–1560 nm) tuned by the intracavity filter OTF1.
modulation frequency of the lithium niobate intensity modulator (Sumitomo Cement T.MZH-1.5–10) was well tuned to the cavity length of the ML-FRL, and Fig. 4(c) shows the RF spectrum when it is slightly detuned for the 6-kHz inducing considerable relaxation oscillation noises. The corresponding RF spectra of the SFRL output at 30 GHz are shown in Fig. 4(b)
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Fig. 4. RF power spectra of external ML-FRL when (a) well-tuned and (c) detuned (the relaxation oscillation noise is increased). Note that the SFRL output remains relaxation-free (d) even when the ML-FRL is detuned (compared with the SFRL output when ML-FRL is (b) well-tuned); 10 dB/div., 20 kHz/div.
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 4, APRIL 2004
output pulsewidths were found to be larger than 9 ps and no pulse reduction was found with using double pulses. The delay between the double pulses is actually the separation of double pulses plus integral multiples of the spacing of the mode-locked pulses so that the delay is larger than the coherence length of the laser pulse. Therefore, the two pulses add up incoherently and no interference effect is observed. To reduce the pulsewidth further, the intracavity filter OTF1 with 1.2-nm bandwidth was replaced by a filter with 5-nm bandwidth, and the minimum pulsewidth at 20 GHz was reduced to be 5.7 ps. Similar pulsewidth-variable characteristics were found for all the harmonics for 10–50 GHz and the minimum pulsewidths obtained are shown in Fig. 5(b). Note that pulsewidth reduction is achieved for all the harmonics and the maximum pulsewidth reduction is found at 10 GHz from 9 ps input to 5.2 ps output. The time-bandwidth products were measured to be 0.32, 0.32, 0.40, and 0.39 for 10, 20, 30, and 40 GHz, respectively. Fig. 5(b) also shows the RF signal-to-noise ratio (limited up to 40 GHz by the bandwidth of the RF spectrum analyzer) and the background-to-peak ratio of the autocorrelation against the repetition rate. After a warm-up time of 1 h, the rational harmonically mode-locked laser pulses were maintained for 3 h for 10–40 GHz. However, for 50 GHz, there were only one or two modes oscillating making the output noisy and unstable even for short terms. This also explains the large background-to-peak ratio shown in Fig. 5(b). IV. CONCLUSION
Fig. 5. (a) SFRL output pulsewidth against separation of external double optical pulses. (b) Pulsewidth, RF signal-to-noise ratio, and background-to-peak ratio of autocorrelation against repetition rates.
and (d). Note that both spectra are nearly free of relaxation oscillation noise even though the noise in the ML-FRL has considerably increased. Such relaxation-free characteristics were observed for all rational harmonics 10–50 GHz over the entire tuning range of 1530–1560 nm. The pulsewidth of the SFRL output at 20 GHz was variable from 6.4 to 14 ps by changing the separation between the double optical pulses as shown in Fig. 5(a). Reducing the separation of the double pulses, the two pulses start to overlap for separation about double the pulsewidth 18 ps until they are actually combined to produce a single pulse with variable pulsewidth and pulse shape for separation shorter than the 9 ps . Minimum pulsewidth was obtained at a pulsewidth pulse separation of 5-8 ps. This implies that the speed of gain saturation given by the pulsewidth of the external pulse plays a crucial role in the gain dynamics of the SOA and it is not faster the better, but an optimized value exists. We tried pulses with pulsewidth of 4 ps for various energies, but the SFRL output pulsewidth was found to be larger than 8 ps. We also tried external single pulses with various energies, but the SFRL
We have demonstrated a relaxation-free SFRL rational harmonically mode-locked ring using XGM by external double optical pulses to generate millimeter-wave repetition frequency pulses of 10–50 GHz tunable from 1530 to 1560 nm. The pulsewidth is controlled by changing the separation the external double pulses. The output remains relaxation-free even though the injected external optical pulses have large relaxation oscillation noise. The technique is expected to be useful for a high repetition rate light source for millimeter-wave photonics and optical time-division multiplexing (OTDM) systems. It is also useful for an optical clock repetition-rate multiplier, providing a synchronized optical clock with various repetition-rates, especially when pulsewidth control, wavelength conversion, and noise reduction are required. REFERENCES [1] H. Takara, S. Kawanishi, and M. Saruwatari, “Stabilization of a modelocked Er-doped fiber laser by suppressing the relaxation oscillation frequency component,” Electron. Lett., vol. 31, pp. 292–293, 1995. [2] D. H. Kim, S. H. Kim, Y. M. Jhon, S. Y. Ko, J. C. Jo, and S. S. Choi, “Relaxation-free harmonically mode-locked semiconductor-fiber ring laser,” IEEE Photon. Technol. Lett., vol. 11, pp. 521–523, May 1999. [3] K. S. Choi, D. S. Seo, C. W. Jang, H. J. Ki, B. G. Kim, Y. M. Jhon, and S. Lee, “Wavelength tunable optical pulse generation at various repetition rates from a semiconductor fiber ring laser,” in Proc. IEEE/LEOS 2001, vol. 2, 2001, Paper ThH4, pp. 717–718. [4] Z. Ahmed and N. Onodera, “High repetition rate optical pulse generation by frequency multiplication in actively mode-locked fiber ring lasers,” Electron. Lett., vol. 32, pp. 55–57, 1996. [5] Y. M. Jhon, C. W. Jang, J. H. Kim, Y. T. Byun, and S. H. Kim, “Noise characterization of a 10 GHz harmonically mode-locked fiber ring laser stabilized by cavity length control,” in Proc. IEEE/LEOS 2002, vol. 1, 2002, Paper TuBB4, pp. 341–342.
