Narrow-linewidth laser source with precision frequency tunability for distributed optical sensing applications Fang Weia, Bin Lua,b, Yulong Caoa,b, Zhengqing Pana, Dijun Chena, Qing Yea, Haiwen Cai*a, Ronghui Qua, and Hao Zhaoc a Shanghai Key Laboratory of All Solid-state Laser and Applied Techniques, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai, China 201800; b University of Chinese Academy of Sciences, Beijing, China 100049; cShanghai Bandweaver Technologies Co. Ltd., Shanghai, China 201203
ABSTRACT We demonstrate a narrow-linewidth laser source for high spatial resolution distributed optical sensing by utilizing the high-order modulation sidebands injection locking. A pair of phase-locked lasers with arbitrary frequency offset from 5 GHz to 50 GHz is generated. Meanwhile, a linearized frequency sweep covering range of 15 GHz in 6 ms with frequency errors of 240 kHz from linearity is also achieved using the same scheme, the instantaneous linewidth of the frequency-swept laser is measured to be ~2.5 kHz. Keywords: distributed sensing, BOTDA/BOTDR, OFDR, phase locked narrow linewidth laser source, linearized frequency swept.
1. INTRODUCTION Laser sources with high spectral purity and precise frequency control are crucial tools in distributed optical sensing applications. For typical applications, such as distributed fiber optic temperature and strain sensing based on Brillouin interaction (Brillouin optical time domain analysis, BOTDA) or Brillouin scattering (Brillouin optical time domain reflectometer, BOTDR), a pair of phased locked narrow-linewidth laser sources with frequency offset close to the local Brillouin frequency around 12 GHz is required. In BOTDA, these laser sources can act as contrapropagating pump and probe lasers to generate acoustic wave with maximum gain, furthermore, the phase locked pump and probe lasers will greatly narrow the linewidth of the Brillouin spectrum and enlarge the coherent length of the Brillouin scattering process1. Meanwhile, these laser sources can also be used in BOTDR, and the frequency offset LO laser allows to transfer the coherent heterodyne detection signal to radio frequency (RF) band which can reduce the excess noise and the cost of detection processing circuits2. Usually, the required laser sources are implemented using Brillouin laser and optical phase lock loop. Other applications, such as optical frequency domain reflectometry (OFDR), benefit from having the laser’s frequency linear change in time with a broad frequency sweeping span in an extremely controlled manner. For instance, the theoretical spatial resolution, R, for an OFDR system depends inversely on the optical frequency sweeping span, B, through the relation R=c/2nB, where c and n are the light velocity in the vacuum and the refractive index respectively3, 4. Meanwhile, the tuning nonlinearity and lack of coherence may degrade the reflectometry performance. However, for tunable laser source the linewidth will be invariably sacrificed to get agile frequency tuning performance, and usual direct laser frequency sweeps show inherent non-linearity in the frequency modulation response versus frequency control factors such as injection current, temperature, or mechanical displacement, especially at high speeds and large sweep ranges. Simultaneous achievement of narrow linewidth operation and fast precise tunability is challenge to conventional laser source. The required laser frequency control can be achieved by modulated sideband of an ultra-stable laser through an acoustooptic modulator (AOM) or an electro-optic modulator (EOM), and optical frequency tuning can be accomplished by *
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24th International Conference on Optical Fibre Sensors, edited by Hypolito José Kalinowski, José Luís Fabris, Wojtek J. Bock, Proc. of SPIE Vol. 9634, 96346V · © 2015 SPIE CCC code: 0277-786X/15/$18 · doi: 10.1117/12.2194489 Proc. of SPIE Vol. 9634 96346V-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/08/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx
precise adjusting the radio frequency (RF) of the device driver electronics5, but the tuning range and the tuning speed are limited by the electronic bottleneck. An alternative method is to establish phase-locking link between a tunable singlemode laser and an individual comb line of optical frequency comb with desired frequency/phase offset using OPLL2. In this work, we propose and experimentally demonstrate a scheme for generating a pair of phase locked high-coherence laser sources with precision tunable frequency offset based on high-order modulation-sideband injection locking, meanwhile the frequency sweep rate and range are simultaneously multiplied by the order of the sideband. A pair of phase-locked lasers with frequency offset from 5 GHz to 50 GHz and a linearized frequency sweep covering range of 15 GHz in 6 ms with frequency errors of 240 kHz from linearity are achieved respectively using this scheme. The proposed scheme has potential applications in distributed optical sensing mentioned above.
2. PRINCIPLE AND EXPERIMENTAL SETUPS The frequency swept principle is visually illustrated in Figure 1. A comb of multi-orders sidebands from narrowlinewidth master laser is generated through electro-optical modulator driven by high-purity radio frequency (RF) signal, the sidebands and the carrier are mutually coherent but with frequency offset from each other by multiples of the RF signal ω , as depicted in Figure 1 (a). The nth sideband can seed the slave laser for injection-locking to maintain phase locking between the master and slave laser with frequency offset of n ω , and the injection-locked diode laser acts as a frequency filter and an amplifier of nth modulation-sideband, allowing to transfer the modulation sideband’s special characters, such as the narrow linewidth and precision frequency tunability to the slave laser. Furthermore, the frequency of the sidebands can be linearly tuned by linear changing the frequency of the driven RF signal, and a frequency shift of the RF driven signal Δω will introduce a frequency shift of n Δω to the nth sideband. A current compensation to the slave diode laser is implemented to maintain stable optical injection locking to the nth sideband. In this way, the frequency of the slave laser can be swept linearly by n Δω using the nth sideband, as depicted in Figure 1 (b). z!asrsun
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Figure 1. schematic illustration of the spectrum for the frequency swept of modulation sidebands and injection locked slave laser. 100 MHz Reference
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Figure 2. Schematic diagram of linearized frequency swept laser source based on modulation-sideband injection-locking.
The configuration of the experiment setup used in this frequency swept laser source is shown in Figure 2. The master laser is planar external cavity low noise laser (RIO ORIONTM laser module) with the typical linewidth less than 3 kHz and the output power of 10 mW. The slave laser is single-mode DFB-type butterfly-packaged diode laser. An intensity electro-optical modulator with bandwidth up to 12 GHz is used to produce the multiple sidebands, and it is biased at the null point to suppress the carrier for eliminating its impact on injection locking. For the demonstration experiments, the EOM is driven by wideband voltage controlled oscillator (VCO, Hittite, HMC-C029) covering 5.0~10.0 GHz with high output power and low single side band (SSB) phase noise, and as a side benefit, the output RF signal can be tuned continuously by analog tune voltage. Additional RF amplifier is contributed yielding broader comb of sidebands. It is obvious that the spectral purity and the tuning performance are limited by the RF driven signal to the EOM, a fractionalN frequency synthesizer is utilized to assistant VCO for generating direct linearized modulated or arbitrary value output
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RF signal with low phase noise, which has demonstrated pure RF signal generation spanning from 5 GHz to 10 GHz with hertz frequency resolution and neat tunability. Meanwhile, the VCO tune voltage from the LPF can be justified by the variable gain amplifier and expected to synchronously compensate the slave laser current, making sure the frequency of the slave laser sweeps in lock step with the targeted sideband.
3. EXPERIMENTAL RESULTS AND DISCUSSIONS The slave diode laser is injected directly by a small part of the modulation-sidebands. Coarse longitudinal mode match is accomplished by thermal tuning of the slave laser, and further fine match is achieved by adjusting the drive current, transverse mode matching is guaranteed by polarization maintaining fiber. To prevent instability in the slave laser caused by injecting too much injected power especially in the case of high order locking, the injected power must be reduced by a variable optical attenuator (VOA), and the injected ratio between the injected sideband power at the desired order and the slave laser free-running output power should be adjusted for optimal locking. Optical spectra of the master laser and the slave laser injection-locked at 2nd order sideband with the RF signal set at 6 GHz by frequency synthesizer are measured by the optical spectrum analyzer (OSA), as shown in Figure 3 (a), the SMSR of the injection locked slave laser is better than 30 dB. To verify the coherence of the master laser and slave laser, the two lasers are interfered using an optical coupler, and the beat signal is detected by an amplified PIN photodiode receiver module and then sent into the spectrum analyzer (SA, Agilent, E4405B, 9 kHz~13.2 GHz). The beat spectrum, which is acquired over 5 MHz centered on 12 GHz, with a resolution bandwidth (RBW) of 3 kHz, is shown in Figure 3 (b), indicating that the master laser and slave laser are a pair of phased locked lasers with frequency offset of 12 GHz, the sidelobe peaks offset by 1 MHz from the central frequency, which corresponds to the bandwidth of the LPF, arise from the PLL electronic signal coupling into the RF signal amplifier-modulator chain. The slave laser injection locked at higher order (up to 5th order) is also experimentally demonstrated, so two phase locked lasers with arbitrary frequency offset from 5~50 GHz can be acquired using tunable RF signal and proper order sideband injection lock. These laser sources are available for the application of BOTDR/BOTDA. 20
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Figure 3 (a) Experimentally measured optical spectra of the master laser and the slave laser injection-locked at 2nd order sideband with the RF signal set at 6 GHz; (b) the electrical spectrum of the beat signal between the master laser and the slave laser.
Besides the phase locked lasers with desired frequency offset, the injection locked slave laser can also be used in generating broadband linearized frequency sweep. By using the injection locked slave laser at 5th order sideband, the slaved laser is controlled swept with equal frequency stepping. The VCO tune voltage from the loop filter is added to the current control of the slaver laser. The gain of the amplifier is adjusted to exactly compensate the current of slave laser, making sure it keeps locked to the 5th order sideband, and then the slave laser is guided linearized swept through frequency synthesizer. The frequency sweep can be derived from a fiber asymmetric Mach-Zehnder interferometer using Hilbert transformation, the linearity and the range of the frequency sweep is assessed6. The length of the fiber delay line used here is ~ 25 m. The frequency synthesizer are set to sweep from 5.5 GHz to 8.5 GHz with the step of 10 kHz in 6 ms. The instantaneous optical frequency as a function of time and the residual errors from a linear fit are shown in Figure 4 (a). The optical frequency of our laser is observed to remain linear sweep to within a standard deviation of 240 kHz throughout a 15 GHz chirp in 6 ms from Figure 4(a). The tuning rate and the coherence of the tuning laser can be evaluated by the linewidth measurement equipment based on the fiber delayed self-heterodyne interferometric technique with frequency shift of 200 MHz generated by AOM. The linwidth of the electric spectrum can demonstrate the spectral purity and the tuning linearity of the chirped laser, and the tuning rate is acquired by the central frequency deviation. The
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measurement results are shown in Figure 4 (b), showing the 20 dB linewidth is 50 kHz, which corresponds to the 3 dB Lorentz linewidth of ~2.5 kHz. The spectrum shapes of the tuned lasers have a frequency shift at the spectrum center compared to the frequency of AOM (the frequency shift is measured to be 150.27 MHz), which is in accordance with the tuning rate of the 2500 GHz/s. The experimental results show the injection locked slave laser is tuned linearly with high optical spectral purity. This scheme is suitable for OFDR system, and 0.67-cm spatial resolution can be obtained with the frequency sweeping span of 15 GHz in theory.
Figure 4 (a) The instantaneous optical frequency changes as a function of time and the residual errors from a linear fit; (b) linewidth of the laser with linearized swept- frequency.
4. CONCLUSION By using the combination of EOM driven high-order sideband-injection-locking and synchronous current compensation, phase locked lasers with arbitrary frequency difference from 5~50 GHz or linearized frequency swept laser with sweeping band of 15 GHz has been achieved. The proposed laser sources are promising for applications in distributed optical sensing applications.
ACKNOWLEDGMENTS The work is supported by the National Natural Science Foundation of China (Grant No.61405227, No.61405218), the National High Technology Research and Development Program of China (Grant No.2012AA041203), the Science and Technology Commission of Shanghai Municipality (Grant Nos.11DZ1140202 and 13XD1425400), Shanghai Natural Science Foundation (14ZR1445100).
REFERENCES [1] Li, Y., Bao, X., Snoddy, J., and Chen, L., “Brillouin spectrum narrowing in high extinction ratio nanosecond pulse from phase locked DFB lasers,” Proc. SPIE 7386, 73861F-73861F-73868 (2009). [2] Kupershmidt, V. and Adams, F., “Optical Phase Lock Loop (OPLL) with tunable frequency offset for distributed optical sensing applications,” Proc. SPIE 7677, 76770O-76770O-76775 (2010). [3] Xu, D., Du, J., Fan, X., Liu, Q., and He, Z., “10-Times Broadened Fast Optical Frequency Sweeping for High Spatial Resolution OFDR,” Optical Fiber Communication Conference, W3D.2 (2014). [4] Xu, D., Du, J., Fan, X., Liu, Q., and He, Z., “High spatial resolution OFDR based on broadened optical frequency sweeping by four-wave-mixing,” Proc. SPIE 9157, 91576J-91576J-91574 (2014). [5] Biesheuvel, J., Noom, D. W. E., Salumbides, E. J., Sheridan, K. T., Ubachs, W., and Koelemeij, J. C. J., “Widely tunable laser frequency offset lock with 30 GHz range and 5 THz offset,” Opt. Express 21, 1400814016 (2013). [6] Wei, F., Lu, B., Wang, J., Xu, D., Pan, Z., Chen, D., Cai, H., and Qu, R., “Precision and broadband frequency swept laser source based on high-order modulation-sideband injection-locking,” Opt. Express 23, 4970-4980 (2015).
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