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We demonstrate a regeneratively mode-locked optical pulse source at about 10 GHz using an optoelectronic oscillator constructed with an electro-absorption modulator integrated distributed feedback laser diode. The 10 GHz RF component is derived from the interaction between the pump wave and the backscattered, frequency-downshifted Stokes wave resulted from stimulated Brillouin scattering in an optical fiber. The component serves as a modulation source for the 1556 nm laser diode without the need for any electrical or optical RF filter to perform the frequency extraction. Dispersion-compensated fiber, dispersion-shifted fiber, and standard single-mode fiber have been used respectively to generate optical pulses at variable repetition rates.
©2005 Optical Society of America
1. Introduction
Picosecond optical pulse source at high repetition rates is a key component for applications in time-division multiplexing communications, optical sampling, and optical switching. A common pulse generation technique depends on active mode locking of an erbium doped fiber ring laser with a lithium niobate intensity modulator [1,2]. In particular, regenerative mode-locking [3]–[5] is a convenient method to obtain a stable pulse train at a high repetition rate. Compared to the conventional active mode-locking method, this scheme does not require any external source for modulation and is proved to be more robust against fluctuations in ambient temperature. The modulation frequency is automatically regulated to ensure the ideal zero-detuning condition in all instances. To realize regenerative mode locking, a desired frequency at a higher harmonic of the cavity beat-note needs to be extracted using a narrow-band RF filter [6,7] or an intra-cavity Fabry-Perot filter (FPF) [8]. The control loop is completed by feedback of this RF clock to the intra-cavity modulator in a regenerative manner.
In this work, we present a new method to generate self-starting [9,10] picosecond pulses at high repetition rates using stimulated Brillouin scattering (SBS) [11] in an optical fiber. SBS is a nonlinear process that generates a Stokes wave down-shifted from the frequency of the incident pump wave by an amount determined by the material properties of the nonlinear medium. At an incident wavelength near 1550 nm, the Stokes shift in an optical fiber is about 10 GHz. By applying SBS, the radio frequency component can be generated by the laser itself instead of depending on an additional high-finesse electrical or optical RF filter for the extraction. The components are initially generated by amplified continuous wave (cw) output from an electro-absorption modulator integrated distributed-feedback laser diode module (EML). They are fed back to the electro-absorption modulator section of the EML to produce regeneratively mode-locked optical pulses at around 10 GHz.
2. Concept and theory
Stimulated Brillouin scattering (SBS) [12] is a nonlinear process that leads to the generation of a Stokes wave and occurs commonly in an optical fiber. The frequency of the wave is downshifted from that of the incident light by an amount determined by the properties of the medium. The pump field generates an acoustic wave through the process of electrostriction, which thus modulates the refractive index of the medium and result in a pump-induced index grating. Consequently, the pump light is scattered through Bragg refraction. The SBS occurs in the backward direction with the Brillouin shift given by
where n is the modal index at the pump wavelength λp and νA is the acoustic velocity. In this work, we use three different types of fibers to perform the SBS. The fibers are dispersion compensating fiber (DCF), dispersion shifted fiber (DSF), and standard single mode fiber (SMF). Stokes waves are generated from these 3 fibers with a cw optical pump at 1556.0nm. The waves are characterized using a 40-GHz RF spectrum analyzer and the results are shown in Fig. 1. The RF peaks generated from the DCF, DSF, and SMF are located at 9.76 GHz, 10.49 GHz and 10.71 GHz, respectively. The difference in the position of gain peaks for the fibers is attributed to the different structures and doping levels of germanium inside the fiber cores.
Fig. 1. Brillouin-gain spectra of the three different types of fibers at a pump wavelength of 1556.0 nm.
3. Experiment
Fig. 2. Experimental setup on the self-starting optical pulse source. DCF: Dispersion compensating fiber; DSF: dispersion shifted fiber; EML: Electro-absorption modulator integrated distributed-feedback laser diode module; EAM: Electro-absorption modulator; EDFA: Erbium doped fiber amplifier; ISO: isolator; PC: Polarization controller; PS: Power splitter; SMF: Standard single mode fiber.
Our experimental setup is illustrated in Fig. 2. A cw pump light at 1556.0nm is generated from an EML that is biased at 100mA. The light is launched into an erbium doped fiber amplifier (EDFA) through a 70:30 coupler. The power of the pump light is adjusted to be above the SBS threshold by tuning the EDFA gain. Part of the pump light is backscattered by acoustic noise in the optical fiber and forms the Stokes waves. The Stokes waves propagate in the opposite direction and interfere with the pump light. Thus, acoustic waves are generated which in turn stimulate more Brillouin scattering. Therefore, the effect of SBS will increase with the amount of pump light. The output is detected with a 10 Gb/s p-i-n receiver. Since the maximum input power of the receiver is limited, the backscattered light is diminished by an optical attenuator before entering the receiver. The electrical signal is then fed to a preamplifier and a power amplifier. The amplified signal is directed to the electro-absorption modulator (EAM) section to modulate the output of the EML. 30% of the laser output is coupled out and is characterized using an optical spectrum analyzer of 0.01-nm resolution and a 32-GHz photodetector connected to a digital sampling oscilloscope.
4. Results and discussion
Fig. 3. a) Output pulse trains generated using 1-km DCF (top), 12-km DSF (middle), and 2.6 km SMF (bottom) (b) the corresponding optical spectra for the output pulse trains.
By feeding the RF component back to the EAM section of the EML, optical pulses are generated at a repetition rate that is determined by the amount of frequency downshift. Since the frequency is different for different types of fibers, the repetition rate can be changed by changing the fiber medium for SBS. With the same configuration, we have employed DCF, DSF, and SMF to generate self-starting optical pulses at different repetition frequencies. Fig. 3(a) shows three output pulse trains generated with 1-km DCF (top), 12-km DSF (middle), and 2.6-km SMF (bottom), respectively. In the case of DCF, a pulse train at 9.76 GHz is generated. With the use of DSF and SMF, pulses at 10.49 and 10.71 GHz are produced respectively. The corresponding optical spectra are shown in Fig. 3(b). The spectra are symmetric with over 20-dB modulation of the ~0.08-nm-spaced peaks corresponding to ~10-GHz pulse rate. The peak wavelengths of the three outputs are ~1556.1 nm. A signal-to-background suppression ratio as high as 56 dB is obtained.
In order to verify that the outputs are modulated by the frequency components resulted from the interaction between the pump and Stokes waves, the pulse trains are characterized again using the 40 GHz RF spectrum analyzer. Fig. 4(a) shows the RF spectra obtained from the mixing of the pump and the Stokes waves generated by 1-km DCF (top), 12-km DSF (middle), and 2.6-km SMF (bottom), respectively. The spectra are the same as those shown in Fig. 1. The RF spectra of the outputs are depicted in Fig. 4(b). The input and output spectra show that the repetition rates of the outputs are equal to the RF frequencies of the input components, verifying that the EML is modulated by the feedback input. The results also show that by feeding the RF components into the EAM section, higher harmonics will be developed through optical pulse generation.
Fig. 4. a) RF spectra showing the mixing of the pump and the Stokes waves to generate frequency components at 9.76 GHz (top), 10.49 GHz (middle), and 10.71 GHz. (b) The RF spectra of the corresponding output pulse trains
We also compare the characteristics between the input and the fundamental frequency component of the output. Fig. 5(a) shows the spectrum of the 9.76-GHz input electrical component generated with the DCF. The spectrum is measured at a resolution bandwidth of 300 kHz and a spanning range of 500 MHz. Fig. 5(b) depicts the fundamental frequency component of the output. By comparing the RF spectra in Fig. 5(a) and 5(b), it is clear that the peak frequencies are the same and that the linewidth of the output is much narrower. Hence, it can be concluded that the optical to electronic feedback loop has successfully locked the output of the laser diode.
Fig. 5. The spectrum of (a) the input signal and (b) the fundamental frequency of the output signal measured at a resolution bandwidth of 300 kHz.
Fig. 6. The peak power and the width of the output pulses generated with (a) DSF, (b) DCF, and (c) SMF.
The variations of the peak powers and widths of the optical pulses generated with the three different types of fibers are plotted in Fig. 6. The power varies from 0.86 to 0.92 mW and the pulse width varies slightly between 32.4 and 34.4 ps. The results show that our approach is capable of generating similar quality of optical pulses at different repetition rates simply by selecting the proper fiber medium.
5. Conclusions
We have successfully demonstrated a new method to generate picosecond pulses at about 10 GHz using stimulated Brillouin scattering (SBS) in an optical fiber. The EML, together with an optical-to-electrical feedback loop, serves as an optoelectronic oscillator and lock the RF spectral peak generated by SBS. The approach also offers the capability to generate optical pulses at different frequencies by suitable choice of the transmission fiber.
Acknowledgments
The work described in this paper is supported by the Research Grants Council of the Hong Kong Special Administrative Region (CUHK 4369/02E and 4220/00E).
References and links
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