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Optical frequency comb generation using two cascaded intensity modulators

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2014 J. Opt. 16 035401

(http://iopscience.iop.org/2040-8986/16/3/035401)

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Journal of Optics

J. Opt. 16 (2014) 035401 (4pp) doi:10.1088/2040-8978/16/3/035401

Optical frequency comb generation usingtwo cascaded intensity modulatorsLei Shang, Aijun Wen and Guibin Lin

State Key Laboratory on Integrated Services Networks, Xidian University, Xi’an 710071,People’s Republic of China

E-mail: lshang@xidian.edu.cn

Received 14 November 2013, revised 18 December 2013Accepted for publication 2 January 2014Published 13 February 2014

AbstractA simple scheme based on two cascaded intensity modulators to generate a flat opticalfrequency comb (OFC) is proposed and experimentally demonstrated. By setting the twocontrollable parameters of the intensity modulation (RF amplitude and DC bias), five comblines with a comb flatness within 0.6 dB are generated. When cascaded with another intensitymodulator, 25 comb lines with a spectral power variation of less than 0.9 dB are obtained. Thescheme, with a high unwanted-mode suppression ratio 13.1 dB and very low RF signals0.58Vπ , is relatively simple, cost-efficient and adjustable, with the frequency interval of theOFC varying with the microwave frequency applied to the modulators.

Keywords: optical frequency comb (OFC), intensity modulator, optical communications

1. Introduction

Optical frequency combs (OFC) have enabled revolutionaryprogress in optical synthesis and metrology [1–7]. Moreover,optical combs are also of great interest in other applications,such as dense wavelength division multiplexing, optical or-thogonal frequency division multiplexing, ultrashort opticalpulse generation and arbitrary waveform generation [8–12].Many schemes have been proposed for OFC generation. OFCgeneration based on mode locked lasers can generate fre-quency combs with a fixed phase relationship, stable operation,adjustable wavelength, and precise comb spacing; however,this approach is relatively complicated and expensive [13, 14].OFC generation by externally modulating a single laser sourcewith microwave signals has proved to be very economical. Thismethod has many advantages, including a simple configura-tion, adjustable wavelength, and precise comb spacing. Severalmethods have been reported using Mach–Zehnder modulatorsand phase modulators [15–20]. In [15], nine lines within a2 dB power variation were obtained by two cascaded intensitymodulators. In [16], a dual-parallel Mach–Zehnder modulatorwas employed to generate an optical frequency comb with ninesidebands within a 2 dB power variation. Specially tailoredradio frequency waveforms were used to improve the flatness,

where 38 comb lines within a 1 dB spectral variation wereobtained in [17]. OFC with 15 lines within a 1 dB powervariation or 17 lines within a 3 dB power variation weregenerated with cascaded IM and PM in [18]. In this scheme,the number of comb lines is in direct proportion to the phasemodulation index. But the RF signal applied to the PM wasabout 3Vπ . A scheme using one intensity modulator andtwo phase modulators directly with a sinusoidal waveform togenerate an optical frequency comb is reported in [19]. A totalof 29 comb lines with a spectral power variation less than1.5 dB at 10 GHz were obtained. However, three modulatorsmust be employed, making the cost increase, and the appliedRF signals were greater than 2.5Vπ . Recently 25 comb lineswith a less than 1 dB power variation were obtained by cas-caded polarization modulators in [20]. However, a polarizationmodulator is expensive compared to an intensity modulator.

In this paper we use two cascaded intensity modulators toobtain an ultraflat optical frequency comb. A total of 25 comblines with a spectral power variation less than 1 dB and anunwanted-mode suppression ratio of more than 13.2 dB areobtained. This scheme requires very low RF signal levels,of about 0.58Vπ . The proposed OFC generator can findapplications in channelized receivers, microwave photonicsfilters, high-capacity optical communication systems, andmodern instrumentation.

2040-8978/14/035401+04$33.00 1 c© 2014 IOP Publishing Ltd Printed in the UK

J. Opt. 16 (2014) 035401 L Shang et al

Figure 1. Schematic diagram of the proposed optical frequencycomb generator based on cascaded intensity modulators. LD: laserdiode. IM: intensity modulator. RF: radio frequency. DC: dc powersupply. OSA: optical spectrum analyzer.

2. Numerical analysis

The schematic diagram of the proposed optical frequencycomb generator is shown in figure 1. It consists of a laserdiode (LD), two intensity modulators (IM1, IM2), two RFsources (RF1, RF2) and two DC biases (DC1, DC2). Assumethat the field of the optical source is defined as Ein(t) =E∗0 cos(ωt), where E0 denotes the amplitude of the opticalfield and ω is the angular frequency of the optical carrier.The electrical RF driving signal sent into the first MZM1 isV1(t)= V ∗RF sin(ω1t), where VRF andω1 are the amplitude andthe angular frequency of the input electrical signal. Therefore

the optical field at the output of IM1 is given by

Eout1(t)=Ein

2

+∞∑n=−∞

[Jn(m)ejnwt+jϕ

+ Jn(−m)ejnwt−jϕ](1)

where Jn( ) denotes the nth-order Bessel function of thefirst kind, m = π∗VRF/Vπ is the RF modulation index andϕ = π∗VDC/Vπ is the phase shift caused by the DC voltage.We can get the expressions for the carriers,

E0 =E0

2J0(m)(ejϕ

+ e−jϕ) cos(ωt)

E±1 =±E0

2J1(m)(ejϕ

− e−jϕ) cos [(ω±ωm)t]

E±2 =E0

2J2(m)(ejϕ

+ e−jϕ) cos [(ω± 2ωm)t] . . . .

(2)

If we let |E0| = |E±1| = |E±2|, we can get

J0(m)= J2(m)

J0(m)(ejϕ+ e−jϕ)= J1(m)(ejϕ

− e−jϕ).(3)

To satisfy equation (3), m = 1.84 and ϕ = 0.5. So, by preciselyadjusting the two parameters, namely the RF amplitude VRFand the DC bias VDC, five flat spectral lines can be generated.When cascaded with another intensity modulator, the optical

Figure 2. Simulated output spectrum under optimized conditions: (a) the spectral output of IM1, (b) an enlarged drawing of (a), (c) spectraloutput of IM2 and (d) an enlarged drawing of (c).

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J. Opt. 16 (2014) 035401 L Shang et al

signal with five spectral lines is sent to IM2, which is driven bya RF signal with a frequency that is one fifth that of RF1. Basedon the same principle as described above, each spectral linecan generate another five spectral lines in IM2, giving a totalof 25 lines in the frequency comb with two cascaded intensitymodulators. To ensure the phase correlation between the 25spectral lines, RF1 can be generated by applying a frequencymultiplier to RF2.

Computer simulations with the Virtual Photonics Inc.(VPI) software package have been performed to study theperformance of our proposed optical frequency comb genera-tion technique. A CW laser with a linewidth of 10 MHz at afrequency of 193.1 THz is sent to the cascaded intensity modu-lators. The intensity modulators have the same half-voltage of3.5 V and the same extinction ratio of 30 dB. The frequency andamplitude of RF1 are 20 GHz and 2.05 V, while the frequencyand amplitude of RF2 are 4 GHz and 2.05 V. The DC voltageapplied to IM1 and IM2 is 0.55 V.

We can get five flat spectral lines with a spacing of 20 GHzfrom a single IM. The power variation is within 0.1 dB andthe unwanted-mode suppression ratio is 15.4 dB, as shown infigures 2(a) and (b). When cascaded with another IM, 25 comblines with a flatness of less than 0.43 dB are obtained, as shownin figures 2(c) and (d). The unwanted-mode suppression ratiois 14.1 dB.

The flatness of our frequency comb is determined by theRF modulation index m and the DC bias. The variation of theflatness with the RF modulation index m and the phase shiftϕ caused by the DC bias are shown in figures 3(a) and (b)respectively. We can see there is an optimum value for theRF modulation index m and the DC bias to obtain an ultraflatOFC. The flatness of the OFC is defined as the difference of themaximum power and the minimum power of the comb lines.From figure 3, we can see that when the RF modulation indexm is about 1.84 and the phase shift ϕ caused by the DC biasis about 0.5, the flatness reaches a minumum. So an ultraflatOFC can be obtained, as shown in figure 2.

3. Experiments

We experimentally demonstrated the scheme shown in figure 1.In the experiment, the center wavelength of the CW laseris 1550 nm, and the output power is 10 dBm (YokogawaAQ2200-136). The half-wave voltages of the two intensitymodulators are both 3.5 V and the 3 dB band-width is 25 GHz(Sumitomo). The frequency of the RF1 signal is set to be20 GHz and the frequency of the RF2 signal is set to be 4 GHz.A RF power amplifier is used to amplify the RF2 signal. Theoptical spectra are monitored by an optical spectrum analyzerwith a resolution of 0.01 nm (Advantest Q8384). By adjustingthe amplitude of RF1 and the DC1, an OFC with five linescan be obtained, as shown in figure 4(a). From figure 4(a),it can be seen that the five generated tone carriers have a0.6 dB flatness and a 14.8 dB unwanted-mode suppressionratio. Figure 4(b) shows the optical spectrum of the generatedoptical frequency comb with 25 lines, as generated by the twocascaded intensity modulators. The flatness is about 0.9 dBand the unwanted-mode suppression ratio is 13.2 dB, which is

Figure 3. Relationship between the flatness of the OFC and the RFmodulation index (a) and the phase shift ϕ caused by the DCbias (b).

higher than the result reported in [20]. The experimental resultsagree well with the theoretical analysis and VPI simulationsas given above.

4. Conclusion

In conclusion, we have theoretically and experimentallydemonstrated a scheme using two cascaded intensity modula-tors to generate an OFC. Simply by adjusting the RF powersand the DC biases to two intensity modulators, 25 comb lineswith a flatness of 0.9 dB and unwanted-mode suppressionratio of 13.2 dB are obtained. Moreover, this scheme requiresvery low RF signals, of about 0.58Vπ . This method for OFCgeneration is simple and adjustable, because the frequencyspacing is determined by the RF signal of the modulator.

Acknowledgments

This work was supported by the National Basic ResearchProgram of China (973 Program: 2010CB328300), 111 project(under grant: B08038), the Fundamental Research Fundsfor the Central Universities (K5051301010), the NationalNatural Science Foundation of China (under grant: 61072070,

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J. Opt. 16 (2014) 035401 L Shang et al

Figure 4. Optical spectra of (a) the five-line frequency comb with aspacing of 20 GHz generated by a single IM and (b) the 25-linefrequency comb with a spacing of 4 GHz generated by two cascadedIMs.

61301179), and the Specialized Research Fund for theDoctoral Program of Higher Education (under grant:20110203110011).

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