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Single-Sideband Modulation Based on an Injection-Locked DFB Laser

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Single-Sideband Modulation Based on an Injection-Locked DFB Laser 462 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 22, NO. 7, APRIL 1, 2010 Single-Sideband Modulation Based on an Injection-Locked DFB Laser in Radio-Over-Fiber Systems Cheng Hong, Cheng Zhang, Mingjin Li, Lixin Zhu, Li Li, Weiwei Hu, Anshi Xu, and Zhangyuan Chen, Member, IEEE Abstract—We report an experimental demonstration of optical [5]. Stimulated Brillouin scattering effect in fiber can be intro- single-sideband (SSB) modulation in 60-GHz radio-over-fiber duced to amplify only one modulation sideband [6]. In [7], SSB systems based on an injection-locked distributed-feedback (DFB) modulation is realized by separating the two heterodyning op- laser. Two heterodyning optical modes with 60-GHz spacing tical modes, modulating only one mode, and combining them are injected into the DFB laser and one of them is used to lock the DFB laser. When the DFB laser is directly modulated, the again with an optical coupler. By changing the relative state modulation index of the locked mode is 15 dB larger than that of polarization between two heterodyning modes, SSB modu- of the unlocked one. The 2.5-Gb/s data transmission at 60 GHz lation could also be realized using a polarization-dependent op- is successfully achieved over 50-km standard single-mode fiber tical phase modulator without separating the two heterodyning using the proposed SSB scheme. modes [8]. Other methods for SSB modulation are investigated Index Terms—Distributed-feedback (DFB) laser, injec- too, such as using a nested Mach–Zehnder modulator (MZM) tion-locking, radio-over-fiber (RoF), single-sideband (SSB) [9], or vestigial sideband filtering in combination with optical modulation. carrier suppression [10]. Recently, a novel approach was in- troduced to generate SSB modulation using injection-locked semiconductor lasers [11], [12], in which the longer wavelength I. INTRODUCTION modulation sideband is resonantly amplified by the injection- ILLIMETER-WAVE (mm-wave) radio-over-fiber locked lasers cavity mode. (RoF) technology has been intensively studied in re- In this letter, we propose a novel approach to realize SSB centM years for its applications in future cellular networks and modulation in mm-wave RoF systems based on an injection- indoor wireless access [1]–[4]. RoF technologies can make locked distributed-feedback (DFB) laser. Unlike [11] and [12], the wireless access network more flexible by using a fiber in our scheme, the DFB laser is used as a wavelength-selective to link the central office (CO) and remote base station (BS). modulator and its frequency response need not to be very high. This can simplify the BS for most of its functions, such as One of the two injected heterodyning modes is used to lock the signal processing, can be moved to the CO. The 60-GHz band DFB laser. When the DFB laser is directly modulated by the mm-wave has gained considerable interest because it can meet baseband data, the locked mode is strongly modulated while the demand of the higher data rate in wireless access, and avoid the other mode is almost not affected. More than 15-dB mod- the fierce frequency band competition for its large bandwidth ulation index difference between the two heterodyning modes and short reach in atmosphere. is achieved in our experiment. Data transmission of 2.5-Gb/s on However, in mm-wave RoF systems, the chromatic disper- a 60-GHz carrier over 50-km SMF is also successfully demon- sion of the standard single-mode fiber (SSMF) will result in se- strated using the proposed SSB modulation. vere power fading of the received radio signal, especially for the double-sideband (DSB) modulation format. Many single-side- II. PRINCIPLE AND EXPERIMENTAL SETUP band (SSB) modulation techniques have been proposed to solve Fig. 1 illustrates the proposed scheme of the SSB modula- this problem [5]–[12]. An optical filter such as a fiber grating tion based on an injection-locked DFB laser. When two het- can be utilized to filter out one undesired modulation sideband erodyning modes with 60-GHz spacing are injected into the slave DFB laser, only one mode is tuned to lock the slave laser. The stimulated emission contributes mainly to the locked mode. Manuscript received September 29, 2009; revised December 14, 2009; The output power of the locked mode is deeply affected by the accepted January 08, 2010. First published February 02, 2010; current version published March 05, 2010. This work was supported by the National High driving current of the slave laser. However, since the unlocked Technology Research and Development Program of China (863 Program) mode is 60 GHz away from the cavity mode, it is slightly mod- under Grant 2006AA01Z261 and by the National Natural Science Foundation ulated. Therefore, the modulation index of the locked mode is of China (NSFC) under Grant 60736003. The authors are with the State Key Laboratory of Advanced Optical Com- significantly higher than that of the unlocked one. munication Systems and Networks, Department of Electronics, Peking Uni- The experimental setup is shown in Fig. 2. The optical car- versity, Beijing 100871, China (e-mail: [email protected]; zhangch. rier suppression modulation is used to generate the two hetero- [email protected]; [email protected]; [email protected]; Lili_15@pku. dyning modes with 60-GHz spacing. It is realized by driving an edu.cn; [email protected]; [email protected]; [email protected]). Color versions of one or more of the figures in this letter are available online MZM biased at with a 30-GHz reference signal. An optical at http://ieeexplore.ieee.org. notch filter is used to select the two first-order sidebands whose Digital Object Identifier 10.1109/LPT.2010.2040983 frequency spacing is 60 GHz and reject the undesired optical 1041-1135/$26.00 © 2010 IEEE Authorized licensed use limited to: Peking University. Downloaded on April 02,2010 at 05:29:46 EDT from IEEE Xplore. Restrictions apply. HONG et al.: SSB MODULATION BASED ON AN INJECTION-LOCKED DFB LASER IN RoF SYSTEMS 463 Fig. 1. Optical spectra illustrating SSB modulation based on an injec- tion-locked DFB laser. (a) Free-running laser mode without modulation. (b) Two heterodyning modes used for injection-locking. (c) Injection-locked laser without modulation. (d) Injection-locked laser modulated by baseband data. Fig. 3. Measured optical spectrum of SSB modulation (frequency detuning a PXS GHz; total injection power aSdBm; 2 dBm for each injection mode). Fig. 2. Experimental setup for SSB modulation and transmission. (PD: photo- diode. PC: polarization controller. OC: optical circulator. FM: frequency mul- tiplier. LPF: low-pass filter.) carrier. An erbium-doped fiber amplifier (EDFA) and an atten- Fig. 4. Measured modulation index ratio between the lock and unlock mode uator are introduced to adjust the optical power injected into a versus (a) injection power with fixed frequency detuning of 3.7 GHz and 2.5 GHz; (b) frequency detuning with fixed total injection power at 5 and 2.5-Gb/s-grade slave DFB laser. A polarization controller (PC) 8 dBm. is used to align the polarization state of input light with that of the DFB laser. One of the two heterodyning modes is tuned to lock the DFB laser. The DFB laser has a threshold current carrier which is 60-GHz away from the cavity mode suffers of 14 mA and is biased at 25 mA. A 2.5-Gb/s, pseu- more loss. We introduce the modulation index ratio dorandom bit sequence, nonreturn-to-zero (NRZ) data from a to measure the asymmetry of the proposed SSB modulation, in pattern generator is used to directly modulate the DFB laser. which and represent the modulation indexes of the The output from the DFB laser is launched into 50-km SMF locked and unlocked modes, respectively. and the loss of the fiber link is compensated by an EDFA. After The modulation index ratio can be controlled by varying the transmission, the signal is detected by a photodiode with 3-dB injection power and frequency detuning (frequency dif- bandwidth of 70 GHz. The received 60-GHz subcarrier signal ference between the injection-locked mode and the free-run- is down-converted to the baseband using a 60-GHz mixer and ning slave DFB laser, ). Fig. 4(a) a local oscillator (LO). The recovered 2.5-Gb/s baseband signal shows the relationship between the ratio and the in- is used to measure the eye diagram and bit-error-rate (BER). jection optical power within the locking range in our scheme. We can see that higher can be achieved with lower III. RESULTS AND DISCUSSION injection power. Fig. 4(b) shows the relationship between the We modulate the slave DFB laser with a single-tone ratio and the injection frequency detuning. Higher radio-frequency (RF) signal to evaluate the effect of SSB can be gotten at the negative frequency detuning. The modulation in our scheme. Fig. 3 shows the optical spec- slave laser can be recognized as an amplifier with its cavity trum of the injection-locked DFB laser under RF modulation mode red-shifted here [13], [14]. The cavity mode is closer to the (modulation frequency GHz, RF power dBm). The locked mode with lower injection power and negative frequency injection-locked mode is strongly modulated whereas the other detuning, and the modulation sidebands of the injection-locked one is very weakly modulated. The power difference of the mode are more enhanced, while the unlocked mode is out of the two heterodyning modes is also shown in Fig. 3. The unlocked gain range of the cavity mode. Authorized licensed use limited to: Peking University. Downloaded on April 02,2010 at 05:29:46 EDT from IEEE Xplore.
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