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An ultrafast optical frequency sweeping technique for narrow linewidth lasers is reported. This technique exploits the large frequency modulation bandwidth of a wideband voltage controlled oscillator (VCO) and a high speed electro-optic dual parallel Mach–Zehnder modulator (DPMZM) which works on the state of carrier suppressed single sideband modulation(CS-SSB). Optical frequency sweeping of a narrow linewidth fiber laser with 3.85 GHz sweeping range and 80GHz/μs tuning speed is demonstrated, which is an extremely high tuning speed for frequency sweeping of narrow linewidth lasers. In addition, injection locking technique is adopted to improve the sweeper’s low optical power output and small side-mode suppression ratio (SMSR).
© 2015 Optical Society of America
1. Introduction
Fast optical frequency sweeping techniques have been adopted for various applications such as atomic and molecular physics, ultrafine material spectroscopy structure detection [1], optical frequency domain reflectometry (OFDR) [2], optical frequency domain imaging (OFDI) [3] and so on. A well-known tunable technology for optical frequency sweeping relies on cavity reconfiguration via mechanical, optical or electrical means to achieve wavelength tuning, but the linewidth is invariably sacrificed for swift cavity reconfiguration because of the contradiction of tuning speed and narrow linewidth for intra-cavity tuning [4], [5]. The other technology involves applying acousto-optic modulator (AOM) or electro-optic modulator (EOM) out of the cavity of the laser for frequency shifting [6–11], and the frequency sweeping is achieved by varying the radio frequency (RF) signal on the modulator. This method can achieve ultrafast optical frequency tuning speed and wideband optical frequency sweeping range without sacrificing the linewidth of the laser.
Both AOMs and EOMs have the practical advantage of readily accomplishing optical frequency sweeping by adjusting the frequency of RF signal fed to their RF ports. AOMs are commonly used in occasions of small tuning range such as 10-100 MHz level shifts [6], [7]. Frequency tuning over large ranges is possible using broadband EOMs. Serrodyne optical frequency shifting that uses EOMs and sawtooth or sine wave signal generators is described in the references [8], [9], and 1.5 and 3 GHz frequency shifting range is achieved. Kawanishi et-al [10,11] reported an optical frequency sweeping technique using a computer controlled arbitrary waveform generator (AWG) multiplied by a wideband electric frequency multiplier ( × 32) and a single sideband modulator and obtained 6.4GHz sweeping range in just 0.5μs. But the tuning speed is limited by the sweep time of the linear RF signal generated by commercially available AWG. This fast optical frequency sweeping technique exhibits the disadvantages of comparatively low side-mode suppression ratio (SMSR) and low conversion efficiency. The conversion efficiency is defined by with maximum efficiency , at which point the SMSR is [9].
In this manuscript, an optical frequency sweeping technique with ultrafast frequency tuning speed for narrow linewidth lasers is demonstrated by employing a wideband voltage controlled oscillator (VCO) with large frequency modulation (FM) bandwidth and dual parallel Mach–Zehnder modulator (DPMZM). An optical frequency sweeping range of 3.85 GHz is achieved with a tuning speed of 80GHz/μs, which to the best of our knowledge, is the fastest optical tuning speed for narrow linewidth laser. In addition, injection locking technique is adopted to improve the SMSR and amplify the low output optical power of the frequency sweeper caused by conversion loss.
The experimental setup used for the generation of ultrafast optical frequency sweeping with injection locking is presented in Sec. 2. The sweeper’s performance is presented in Sec. 3, and Sec. 4 is the conclusion.
2. Experiment setup
The experimental setup used to demonstrate the fast optical frequency sweeping with injection locking is shown in Fig. 1. A home-made 1550.31 nm distributed feedback (DFB) fiber laser is used as the master laser which has 1.2 mw output power and 12 KHz linewidth. A DFB diode laser (RSLS-1550) which has no internal optical isolator is used as the injection locking slave laser and has 1 mW output power.
Fig. 1 Schematic of the fast optical frequency sweeping using injection locking with dual parallel Mach–Zehnder modulator driven by VCO; VCO: voltage controlled oscillator; BPF: band pass filter; Det: detector; DFB laser: distributed feedback laser; DAC: digital to analog converter
The signal used to drive the DPMZM (LN86) is generated from a VCO which is driven by a digital signal processor (DSP) controlled digital to analog (D/A) converter who has fast sawtooth voltage output capability to change the VCO’s output frequency. The output of the DPMZM is injected into the slave laser through a circulator. A 50 cm length difference Mach-Zehnder (MZ) interferometer is used to characterize the tuning range and speed of the fast optical frequency sweeper.
3. Fast optical frequency sweeping performance
In order to get the tuning performance of the VCO, Fig. 2 (a) shows the measurement setup of the FM bandwidth of the VCO. Sinusoidal voltage signal from AWG is input to the VCO. The DPMZM works on the state of single sideband modulation, and its output is connected with a 50 cm fiber length difference interferometer which is acting as a frequency discriminator. An Optilab 12 GHz bandwidth photodetector (LR-12-A) and oscilloscope is used to acquire the amplitude of the beating signal which is a reflector of the tuning range of the VCO. Because of the limited bandwidth of the VCO, the amplitude of the beating signal is varied when the frequency of the sinusoidal signal is changed. As such, then we can obtain the FM magnitude response. The measured FM magnitude response of the VCO (HMC-C029) we used is shown in Fig. 2 (b), which shows that the FM bandwidth is approximately 56MHz which is a high value compared to the commercial AWG. The inset in Fig. 2 (b) shows the voltage-frequency tuning characteristic of the VCO.
Fig. 2 (a) VCO FM magnitude response measurement setup; (b) FM magnitude response of the VCO; AWG: arbitrary waveform generator; VCO: voltage controlled oscillator; PD: photo-detector; DPMZM: dual parallel Mach-Zehnder modulator
The offset voltage, amplitude and frequency of the sawtooth signal are 5 V, 10 V and 2 MHz respectively and the symmetry is 10%, so the rising edge is 50ns. This is the fastest rising edge we can achieve right now which is limited by the speed of the DSP and the D/A converter. This fast rising edge signal is applied to the VCO and the frequency is tuned from 4.7 to 8.55GHz. The electrical band pass filter (BPF) (4.5-8.6 GHz) is used to filter out the undesired frequency caused by the high order harmonics of the VCO output. The DPMZM works at the carrier suppressed single sideband modulation (CS-SSB) status set as follows: the bias points of the two sub-MZ arms of the modulator are at null point, and the main-MZ arm works on linear point, and the broadband RF signal output of VCO applies to the 90° hybrid coupler (Pulsar QS-8), whose output is delivered to the two RF ports of DPMZM.
The optical output from the DPMZM is sampled by an optical spectrum analyzer using peak hold sweep mode and is shown in Fig. 3. The RF signal fed to the 90° hybrid coupler is a continuous frequency-swept signal generated by VCO. Figure 3 shows that the suppression ratio of undesired components is 15 dB. The sub-peak on the left side is caused by the phase error of the broadband 90° hybrid coupler (the maximum phase error is 8° for 4-16 GHz band) which results in the generation of undesired −1st sideband. The output power is about −20dBm caused by the low conversion efficiency and large insertion loss of the DPMZM.
To improve the low SMSR and low power output of the frequency shifted optical signal, the output of the DPMZM is then injection locked to a DFB semiconductor laser through an optical circulator.
In order to confirm the injection lock state, the linewidth of the slave laser in free running state and lock state are measured as shown in Fig. 4, from which the linewidth of the slave laser in free-running state is approximately 400 kHz. The linewidth of the injection locked slave laser is about 12 kHz, which is the same as the master fiber laser (shown in the insets of Fig. 4 (b)). The sideband at about 159 and 161 MHz is caused by the relaxation oscillation of the master fiber laser where its relaxation oscillation frequency is about 1 MHz.
According to the injection locking theory, the slave laser may lose lock when the sweeping range is too large. In order to avoid losing lock, a pre-compensation current is applied to the slave laser. The pre-compensation signal is a continuous voltage signal generated by another channel of the DSP board, and is fed to the slave DFB laser’s current driver. The frequency and shape of the pre-compensation signal is the same as that of the signal to the VCO’s input. As shown in Fig. 5, the red circles and black squares are respectively the maximum and minimum compensation current values of the slave laser remained in injection locked state at different input voltage to the VCO. The blue triangle is the average. The red solid line is the linear fit of the blue triangle points. It is the pre-compensation current that should be applied to the slave DFB laser for continuous injection locking.
The optical output from the injection locked slave laser with pre-compensation is shown in Fig. 6. The diagram shows that side-mode suppression ratio is approximately 9dB higher than the no optical injection locking condition as shown in Fig. 3. Furthermore, the optical power output from the injection locked slave laser is 0 dBm which is 20 dB higher than that obtained directly from DPMZM without injection locking. A higher output power salve laser can have more power output.
The fast optical frequency sweeping output of the slave laser from the circulator port 3 is then input to a 50 cm-length difference interferometer measurement setup. The interfering signal is detected by an Optilab 12 GHz bandwidth photodetector (LR-12-A) and sampled by an oscilloscope. Figure 7 shows the output of the interferometer acquired by the oscilloscope .
Based on the length difference of MZ interferometer and the time domain interference signals acquired by the oscilloscope, the optical frequency variation and frequency tuning speed can be obtained by the Hilbert transform [12] as shown in Fig. 8. From the figure, we can see that the optical sweeper reaches a sweeping range of 3.85 GHz, and the optical frequency tuning speed is approximately 80GHz/μs, which is the fastest optical frequency tuning speed for narrow linewidth lasers as far as we know. The tuning linearity is a little nonlinear because of the non-perfect linear tuning character of the VCO and the D/A output. The frequency tuning speed can be higher when a faster changed sawtooth signal is applied. It is ultimately limited by the FM bandwidth of the VCO and can be maximumly achieved to .The frequency sweeping range and tuning speed also can be much broader and faster by applying high-order microwave frequency multiplier on the VCO output to achieve several tens of GHz frequency sweeping range [11].
Fig. 8 Optical frequency variation and frequency tuning speed of high speed optical frequency sweeping
4. Conclusion
By employing a wideband VCO with large FM bandwidth, an ultrafast optical frequency sweeping technique for narrow linewidth lasers is demonstrated. Ultrafast optical frequency tuning speed of approximately 80 GHz/μs is demonstrated. To the best of our knowledge, this speed is the fastest optical frequency tuning speed for narrow linewidth laser that had been reported. The optical frequency sweeping range of the fast optical frequency sweeper is 3.85 GHz. In addition, injection locking technique is adopted to enlarge SMSR and output power of the frequency sweeper. This method is expected to be helpful for real-time spectrum analysis with high resolution both in time and spectral domains, as well as for improving the performance of many optical frequency domain measurement techniques.
Acknowledgements
This work was supported in part by the National Natural Science Foundation of China (No. 61108028, 61178031, 61405218) and Shanghai Natural Science Foundation under grant 14ZR1445100).
References and links
1. T. Shioda, T. Yamamoto, T. Sugimoto, Y. Tanaka, K. Higuma, and T. Kurokawa, “1 MHz-resolution spectroscopy based on light frequency sweeping using a single-sideband optical modulator,” Jpn. J. Appl. Phys. 46(6A6A No. 6A), 3626–3629 (2007). [CrossRef]
2. B. Soller, D. Gifford, M. Wolfe, and M. Froggatt, “High resolution optical frequency domain reflectometry for characterization of components and assemblies,” Opt. Express 13(2), 666–674 (2005). [CrossRef] [PubMed]
3. S. Yun, G. Tearney, J. de Boer, N. Iftimia, and B. Bouma, “High-speed optical frequency-domain imaging,” Opt. Express 11(22), 2953–2963 (2003). [CrossRef] [PubMed]
4. L. A. Coldren, G. A. Fish, Y. Akulova, J. S. Barton, L. Johansson, and C. W. Coldren, “Tunable semiconductor lasers: a tutorial,” J. Lightwave Technol. 22(1), 193–202 (2004). [CrossRef]
5. B. P.-P. Kuo and S. Radic, “Fast wideband source tuning by extra-cavity parametric process,” Opt. Express 18(19), 19930–19940 (2010). [CrossRef] [PubMed]
6. Y. Wang, Q. Qiu, S. Shi, J. Su, Y. Liao, and C. Xiong, “High-precision optical phase-locking based on wideband acousto-optical frequency shifting,” Chin. Opt. Lett. 12, 1671–7694 (2014).
7. J. Thom, G. Wilpers, E. Riis, and A. G. Sinclair, “Accurate and agile digital control of optical phase, amplitude and frequency for coherent atomic manipulation of atomic systems,” Opt. Express 21(16), 18712–18723 (2013). [CrossRef] [PubMed]
8. R. Houtz, C. Chan, and H. Müller, “Wideband, efficient optical serrodyne frequency shifting with a phase modulator and a nonlinear transmission Line,” Opt. Express 17(21), 19235–19240 (2009). [CrossRef] [PubMed]
9. D. M. S. Johnson, J. M. Hogan, S. W. Chiow, and M. A. Kasevich, “Broadband optical serrodyne frequency shifting,” Opt. Lett. 35(5), 745–747 (2010). [CrossRef] [PubMed]
10. T. Kawanishi, T. Sakamoto, and M. Izutsu, “Fast optical frequency sweep for ultra-fine real-time spectral domain measurement,” Electron. Lett. 42(17), 999 (2006). [CrossRef]
11. T. Kawanishi, T. Sakamoto and M. Izutsu, “Optical frequency sweep technique using single sideband modulation,” ECOC 2005, We4.P.62, (2005). [CrossRef]
12. T.-J. Ahn and D. Y. Kim, “Analysis of nonlinear frequency sweep in high-speed tunable laser sources using a self-homodyne measurement and Hilbert transformation,” Appl. Opt. 46(13), 2394–2400 (2007). [CrossRef] [PubMed]
