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An all-optical generation of a millimeter wave carrier from a multiwavelength Brillouin-erbium fiber laser is presented. Four-channel output with spacing of about 21.5 GHz is generated from the fiber laser by controlling the gain in the cavity. A dual-wavelength signal with spacing correspondent to six orders of Brillouin frequency shift is obtained by suppressing the two channels at the middle. Heterodyning these signals at the high-speed photodetector produces a millimeter wave carrier at 64.17 GHz. Temperature dependence characteristic of Brillouin frequency shift realize the flexibility of generated millimeter wave frequency to be tuned at 6.6 MHz/ °C.
©2012 Optical Society of America
1. Introduction
Spectrum congestion in the microwave band and the millimeter wave hunger for bandwidth due to the rapid growth of information technology demands fuelled the interest in the development and implementation of hybrid radio over fiber systems. The utilization of millimeter wave frequencies enables the design of compact and low-cost wireless millimeter wave communications front-ends which can offer convenient terminal mobility and high capacity channels. It is an attractive candidate for short-distance sensors and indoor communications based on pico-cell zone around 60 GHz owing to high atmospheric losses [1]. Recently, a lot of research had been carried out to develop millimeter wave generation and transport techniques. These include the optical generation of low phase noise wireless signals and their transport overcoming the chromatic dispersion in fiber.
Microwave or millimeter wave signals can be generated by heterodyning two optical signals with a wavelength spacing corresponding to the desired microwave or millimeter wave signal [2]. The beating frequency appears at the output of the photodetector (PD) is equal to the spectral spacing between the two wavelengths. High frequency can be generated as long as the PD bandwidth is not a constraint. For high spectral purity and absolute frequency stability, the phase fluctuations of the two lasers need to be correlated by optical injection locking or optical phase-locked loop as reported in [3, 4]. Basically, these two methods require feedback control together with the employment of local oscillators.
As an alternative to the aforementioned methods, highly coherent beat signal is attainable by deriving two light beams from a same gain medium or two related gain media. The laser source should have either a single wavelength with dual longitudinal modes or two wavelengths operating in single longitudinal mode for each wavelength [5].
Stimulated Brillouin scattering attracted much attention in the development of fiber lasers and signal processing due to its existence at low threshold power. The narrow bandwidth of Brillouin gain spectrum was exploited in the sideband selective amplification. The generation of Brillouin Stokes signals with frequency downshifted is implemented in the development of multiple-channel-output devices. These two characteristics fueled the interest of the generation of millimeter wave based on SBS in optical fiber. Several works on the generation of microwave or millimeter wave carriers based on SBS in optical fibers had been reported. The generation of millimeter wave signal based on selective sideband Brillouin amplification induced by stimulated Brillouin scattering in the optical fiber had been demonstrated [6–9]. However, oscillators are needed to generate harmonic sidebands. The pump lasers to generate SBS for sideband amplification also need to be tuned precisely to coincide with the signals to be amplified due to the narrow Brillouin amplification bandwidth.
Simple and interesting methods to generate microwave or millimeter wave carrier based on cascading Brillouin effect in the fibers were reported in [10–14]. The beating of Brillouin pump and its Brillouin Stokes signal produces RF carrier which the frequency is equal to the multiples of Brillouin frequency shift. The generation of 11 GHz microwave signal with 4 MHz linewidth from a SMF-28 fiber cavity was demonstrated in [10]. Then, Shen et al. replaced the SMF-28 fiber with a photonic crystal fiber as the Brillouin gain medium, microwave signals at 9.788 GHz and 19.579 GHz were generated by mixing the first-order Stokes wave and the pump wave, and mixing the second-order Stokes wave and the pump wave, respectively [11]. The linewidth of the two signals are 13.5MHz and 8.5 MHz, respectively. These ideas have greatly reduced the cost and the complexity of the system since the electrical signal generator is not needed. However, high pump power at hundreds of milli-Watts is needed for the excitation of desired Stokes signal. However, the employment of photonic crystal fiber to lower down the threshold adds to the overall system cost. On the other hand, generation of RF signal based on SBS was reported in [12]. The structure is simple; nevertheless, several spools of fiber need to be cascaded to generate higher order Stokes signals for the generation of higher frequency.
Channel filtering from multiwavelength fiber laser is also a possible approach to produce a dual-wavelength output. Brillouin-erbium fiber laser (BEFL) which exploits the Brillouin gain of the fiber and erbium gain in the erbium-doped fiber (EDF) is efficient in generating multiple channel lasing signals. In the operation of BEFL, homogeneous Brillouin gain with the EDF gain allows lasing action to occur in a single longitudinal mode without intrinsic mode competition given a stable environment [15]. With proper design of the cavity, multiple lasing wavelengths with a constant spacing can be produced through the cascaded SBS process in the optical fiber [16]. The spacing of about 10-12 GHz is commonly known in standard optical fibers [17–20].
The generation of microwave signal by channel filtering from a multiwavelength BEFL was proposed in [19]. In order to apply channel filtering techniques, the filter must have a steep guard band to remove the unwanted signal. This can be a paramount challenge to any optical filters to have a 20-dB suppression within 0.08 nm spacing. The wavelength spacing can be widened by isolating its odd- and even-order Stokes signals propagation in a double-Brillouin-frequency shifter (DBFS) [21]. By integrating the DBFS in a laser cavity, a multiwavelength BEFL with 21 GHz spacing has been demonstrated [22].
In this paper, we demonstrate a 64 GHz millimeter wave carrier generated from a multiwavelength BEFL for the first time to the best of our knowledge. A filtered dual-wavelength signal with spacing correspondent to six orders of Brillouin frequency shift is obtained and is heterodyned at the high-speed PD to produce a millimeter wave carrier at 64.17 GHz. Based on the proposed method, the use of high frequency generator can be eliminated as compared with other microwave generation approaches such as optical injection locking, optical phase-lock loop and external modulation. The generation of higher order Stokes signals can be easily achieved in a single spool of fiber and lower power in contrast to the works reported in [10–14].
2. Experimental setup
Figure 1 shows the architecture of millimeter wave carrier generation from a BEFL. The design and operation principle of the proposed BEFL was published in [22]. An external cavity tunable laser source with 1 MHz laser linewidth is employed as the Brillouin pump (BP) for the operation of the BEFL. It is amplified by the EDF gain block before being injected to the DBFS. The gain block is formed by a section of 21.5 m EDF pumped by a 1480 nm laser diode (LD). A wavelength selective coupler (WSC) is used to multiplex the input signal and the 1480 nm LD into the EDF. The DBFS consists of a fiber based 4-port circulator and a spool of optical fiber as the Brillouin gain medium. In our experiment, a spool of 6.7 km SMF-28 fiber with 0.22 dB/km attenuation, the effective group index of 1.4682 at 1550 nm and the nonlinear coefficient of 1.1 W−1km−1 is utilized. Output signal at the port-4 of the circulator is frequency shifted as much as 2vB (double Brillouin frequency) from the input signal injected from port-1 to port-2. In other words, the output signal is the product of second-order Stokes signal from the input signal. The operating principle of the DBFS was reported in [21]. The frequency-shifted signal is redirected to the cavity through a 90/10 directional coupler (DC). Ten percent of the signal is deviated to the output of the BEFL while ninety percent is amplified by the gain block before propagating to the DBFS to form a round-trip oscillation. The cascaded SBS process is controlled by adjusting the erbium gain via the amount of 1480 nm pump power supplied into the EDF. In order to have a dual-wavelength output with about 60 GHz frequency spacing, four lasers must be produced from the BEFL. In our experiment, this condition is achieved at 4 mW Brillouin pump power and 60 mW 1480 nm pump power.
In order to select two lasing wavelengths only, a dedicated filter is essentially required to suppress the unwanted laser lines. To achieve this, a filter block is constructed that comprises of two isolators (Iso1 and Iso2) and two fiber Bragg gratings (FBG1 and FBG2) as illustrated in Fig. 1. The transmission spectra of the two FBGs and the filter block are shown in Fig. 2(a) . The center wavelengths are 1557.143 and 1557.091 nm for FBG1 and FBG2, respectively.For the filter block characterization, the center wavelength is 1557.132 nm with 0.367 nm spectral width at 20 dB suppression level as illustrated in Fig. 2(b). Optical isolators are placed before each FBG to suppress the reflected signals from propagating back to the laser cavity. Figure 2(b) depicts the overlapping spectra between four-wavelength output of the BEFL and the filter block transmission. The operating wavelength of BEFL is determined by the Brillouin pump wavelength that initiates the lasing. It is tuned to 1556.870 nm so that the desired rejection band is accurately matched with the FBG reflection band. As a result, the lasing wavelengths are at 1556.870 nm, 1557.042 nm, 1557.214 nm and 1557.386 nm with spacing of about 0.172 nm. After the filtering mechanism, there are only two lasers separated by 0.516 nm. These lasers are then amplified to higher peak powers of −0.76 and −0.20 dBm by an erbium-doped fiber amplifier (EDFA) to be detected by a high speed photodetector as shown in Fig. 2(c). The noise floor within the FBG reflection band is higher after amplification owing to the presence of amplified spontaneous emission (ASE) from the EDFA itself. A photodetector from u2t (model no.: XPDV2120R) with 50 GHz 3-dB bandwidth is used for the detection. However, it can operate at higher frequencies up to 60 GHz with lower performance. The radio frequency (RF) spectrum is measured by implementing an Agilent 8564EC electrical spectrum analyzer. An external harmonic mixer is adopted for high frequency measurement to extend the measurable frequency from 40 GHz to high frequency up to 110 GHz.
Fig. 2 (a) Transmission spectra of FBG1, FBG2 and filter block, (b) optical spectrum at the output of the multiwavelength fiber laser and the filter block transmission spectrum, and (c) filtered dual-wavelength output before and after the EDFA.
3. Results and discussions
Generated millimeter wave beat signal at 64.1667 GHz is measured with 100 MHz span and 300 kHz resolution bandwidth as illustrated in Fig. 3 . The millimeter wave 3-dB linewidth is observed around 4.8 MHz. The utilization of laser with narrower linewidth would help in narrowing the generated carrier linewidth since the spectral purity of beat signal in heterodyning method is determined by the linewidth of the lasers used [3].
Signal stability is investigated by scanning the RF spectrum for every ten minutes. The RF power and frequency stabilities is recorded as in Fig. 4 . It is found that the frequency drifts between 64.1670 GHz and 64.1665 GHz with 0.0004% maximum deviation from the average. The carrier frequency can be further stabilized by placing the fiber spool in a temperature controller since the Brillouin shift is sensitive to temperature variation [23]. RF power constancy is important in the implementation of oscillator. The measured average peak power is at −52.87 dBm and its peak power fluctuation is ± 0.37 dB. Fluctuations in the millimeter wave power can only be reduced with the enhancement of laser stability. The measured power is low due to the bandwidth limitation of the photodetector. The 10 dB cable loss also contributed to the low RF power.
The Brillouin frequency shift, vB has strong strain and temperature dependence characteristic [24, 25]. It has been shown to vary linearly with strain, ε, and temperature, T, given by [24]
where Cvε and CvT are strain and temperature coefficients respectively. These characteristics can be exploited in the proposed system for the tunability of the generated frequency. Since the beating frequency is equal to the frequency offset of the filtered dual-wavelength output from the multiwavelength fiber laser, divergence in the Brillouin shift will be transferred to the generated frequency. In order to have a thermally-tunable frequency, the fiber is wound in a spool and placed in a temperature controller to minimize the strain effect, hence limiting the control parameter to temperature only. Temperature is tuned from 31°C to 51 °C with 2 °C steps and the generated frequencies are recorded. Generated frequencies are measured every ten minutes and ten measurements are taken for each temperature. The frequencies are averaged and its relationship with the temperature variation is studied. Figure 5 shows the linear relationship between the generated frequencies and temperature. The measurement data fit very well with the regression line (R-Squared = 0.9999) with 6.6 MHz/ °C. Temperature can be set higher to achieve wider frequency tuning as long as the plastic fiber spool can stand the heat.4. Conclusion
The implementation of multiwavelength BEFL in the generation of millimeter wave by heterodyning two filtered channels from a multiwavelength is experimentally proven. The wider frequency spacing, which is equal to two order of Brillouin frequency shift, realizes the channel filtering from BEFL. Millimeter wave at ~64 GHz is generated from the proposed method and it can be tuned at 6.6 MHz/ °C by controlling the temperature of the Brillouin gain medium. The generation of millimeter wave can be enhanced by the utilization of narrow linewidth laser for the initiation of multiwavelength generation. Supplementary control mechanism in the design of fiber laser cavity would stabilize the beat signal and hence produce a high quality millimeter wave signal for its implementation in radio over fiber communication systems.
Acknowledgments
This work is partly supported by the Universiti Putra Malaysia under Research University Grant Scheme 05-01-09-0783RU and the Ministry of Higher Education High Impact Research #A000007-50001, the Ministry of Science, Technology and Innovation (National Science Fellowship and MOSTI/BGM/R&D/19(3)), and Telekom Research and Development Sendirian Berhad.
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