hv photonics

Communication Generation of Broadband Optical Frequency Comb Based on a Gain-Switching 1550 nm Vertical-Cavity Surface-Emitting under Optical Injection

Huiping Ren 1, Li Fan 1,2, Na Liu 1, Zhengmao Wu 1 and Guangqiong Xia 1,*

1 School of Physical Science and Technology, Southwest University, Chongqing 400715, China; [email protected] (H.R.); [email protected] (L.F.); [email protected] (N.L.); [email protected] (Z.W.) 2 School of Electronic and Information Engineering, Southwest University, Chongqing 400715, China * Correspondence: [email protected]

 Received: 24 September 2020; Accepted: 22 October 2020; Published: 23 October 2020 

Abstract: In this work, broadband optical frequency comb (OFC) generation by a gain-switching vertical-cavity surface-emitting laser (VCSEL) subject to optical injection is investigated experimentally. During implementing the experiment, a 1550 nm VCSEL under a large signal current modulation is driven into the gain-switching state with a broad noisy spectrum. By further introducing an optical injection, a high performance OFC can be produced. The experimental results demonstrate that the power and wavelength of the injection light seriously affect the performance of the produced OFC. Under proper optical injection parameters, two sub-combs originating from two orthogonal polarization components of the VCSEL can splice into a broadband total-OFC. By selecting optimized operation parameters, a high quality total-OFC can be acquired, with stable comb lines, high coherence, wide bandwidth of 70.0 GHz (56.0 GHz) within 10 dB (3 dB) amplitude variation and low single sideband phase noise at the fundamental frequency below 120.6 dBc/Hz @ 10 kHz. − Keywords: vertical-cavity surface-emitting laser; gain-switching; optical injection; optical frequency comb

1. Introduction Optical frequency comb (OFC) is composed of a series of equally spaced discrete comb lines, and these discrete comb lines corresponding to different frequency components can be simultaneously provided in a frequency band [1]. OFCs with good stability, high degree of coherence, as well as high repetition rates have been extensively applied in , metrology, optical communication, THz generation, optical arbitrary waveform generation, and other fields [1–14]. Many approaches for generating OFCs have been proposed and investigated, such as mode-locking, external modulation, and gain-switching of semiconductor [15–23]. Mode-locked lasers can generate an OFC with broad bandwidth, but the line spacing of the OFC is difficult to tune since it is determined by the reciprocal of the round trip time for light propagating in the cavity. By using external modulation of continuous wave light, an OFC with adjustable comb line spacing can be obtained. However, in order to improve the flatness of the comb, multiple external modulators are required, which results in higher cost and insertion loss [24,25]. The approach for generating OFCs based on gain-switching semiconductor lasers has attracted much attention owe to the following advantages. On one hand, such an approach is easy to implement due to relatively simple experimental facility. On the other hand, the comb line spacing of the generated OFC is easily adjusted by varying the modulation frequency [26–29].

Photonics 2020, 7, 95; doi:10.3390/photonics7040095 www.mdpi.com/journal/photonics Photonics 2020, 7, 95 2 of 12

Compared with edge emitting semiconductor lasers, vertical cavity surface-emitting lasers (VCSELs) have some unique advantages such as low threshold current, reduced manufacturing costs, easy to integrate, high fiber coupling efficiency, etc. [30,31]. Recently, some great progresses have been made in the OFC generation based on VCSELs. Prior et al. experimentally and theoretically confirmed that a gain-switching 1550 nm VCSEL could generate a broadband total-OFC composed by two orthogonal polarization sub-combs [32,33]. To further extend the optical span of the total-OFC, the research group proposed that an optical injection is introduced into the gain-switching VCSEL, and the results showed that an OFC with enhanced optical span can be obtained by carefully adjusting the polarization of optical injection [34,35]. At present, the research on generating broadband OFC by optical injection of gain-switching VCSEL mainly focuses on the influence of the polarization direction of the injection light on the comb performance, while the dependence of comb quality on the injection parameters has not been reported in detail. In this work, based on a gain-switching 1550 nm VCSEL under optical injection, we investigate experimentally the generation of broadband OFC, and the influences of injection power and injection light wavelength on the bandwidth, the number of comb lines, and signal to noise ratio (SNR) are focused on. A broadband OFC with 70.0 GHz (56.0 GHz) bandwidth within 10 dB (3 dB) power variation can be obtained through selecting appropriate injection parameters.

2. Experimental Setup Figure1 shows the experimental setup for generating optical frequency comb (OFC). A commercial 1550 nm VCSEL (Raycan) is used in this experiment, and its current and temperature are controlled by a high accuracy and low noise current-temperature controller (ILX-Lightwave, LDC-3724C). A continuous light output from a tunable laser (TL, Santec TSL-710) is injected into the VCSEL after passing through a variable attenuator (VA1), a polarization controller (PC1), a 20/80 fiber coupler (FC1), an optical circulator (OC) successively. The injection power Pi can be adjusted by controlling VA1 and be monitored by an optical power meter (PM). PC1 is employed to control the polarization of the injection light. A frequency synthesizer (MFS, Agilent E8257C) is used to generate a sinusoidal radio frequency (RF) signal. The output of the VCSEL passes through OC, an erbium doped fiber amplifier (EDFA), and then is divided into two parts by another 50/50 fiber coupler (FC2). A half of the FC2 output is employed to measure X-polarized OFC (named as X-OFC) and Y-polarized OFC (named as Y-OFC) via PC2 and a polarization beam splitter (PBS), and the other half is used to observe the total-OFC composed by X-OFC and Y-OFC. In the measurement system, an optical spectrum analyzer (OSA, Aragon Photonics BOSA lite +, 20 MHz resolution) is adopted to measure optical spectrum. Power spectrum and single sideband (SSB) phase noise are recorded by an electrical spectrum analyzer (ESA, R&S FSW, 67 GHz bandwidth) after being converted into an electric signal by a photodetector (PD1, U2T-XPDV2150R, 50 GHz bandwidth), and the time series is monitored by a digital oscilloscope (OSC, Agilent X91604A, 16 GHz bandwidth) after being converted into an electric signal by another photodetector (PD2, New Focus 1544B, 12 GHz bandwidth). Considering that the used ESA and OSC possess different bandwidths, two PDs with different bandwidths are adopted for ESA and OSC, respectively. During the experiment, the temperature of the 1550 nm VCSEL is stabilized at 20.47 ◦C. Photonics 2020, 7, 95 3 of 12

Photonics 2020, 10, x FOR PEER REVIEW 3 of 12

MFS

DC Bias Tee Measurement System VCSEL

50 Total-OFC OSA VA1 PC1 FC1 EDFA FC2 80 TL PC2 PBS X-OFC PD1 ESA Y-OFC 20 OC 50

PM PD2 DSO

FigureFigure 1. Schematic ofof the the experimental experimental setup. setup. TL: TL: tunable tunable laser; laser; VCSEL: VCSEL: vertical-cavity vertical-cavity surface-emitting surface- emittinglaser; VA: laser; variable VA: variable attenuator; attenuator; PC: polarization PC: polarization controller; controller; FC: fiber FC: coupler; fiber coupler; PM: power PM: power meter; meter;OC: optical OC: circulator;optical circulator; MFS: microwave MFS: microwave frequency frequency synthesizer; synthesizer; DC: direct current;DC: direct EDFA: current; erbium EDFA: doped erbiumfiber amplifier; doped fiber PBS: amplifier; polarization PBS: beampolarization splitter; beam OSA: splitter; optical OSA: spectrum optical analyzer; spectrum PD: analyzer; ; PD: photodiode;ESA: electrical ESA: spectrum electrical analyzer; spectrum DSO: analyzer; digital DSO: storage digital oscilloscope. storage oscilloscope.

3.3. Results Results and and Discussion Discussion First,First, we we characterize characterize the the basic basic properties properties of th ofe the1550 1550 nm nmVCSEL VCSEL used used in this in work this workunder under free- running.free-running. To obtain To obtain polarization-resolved polarization-resolved power-current power-current (P-I) curve, (P-I) wecurve, inspect we inspectthe spectrum the spectrum output fromoutput one from port one of the port PBS of theduring PBS adjusting during adjusting PC2. Normally, PC2. Normally, there are there two are peaks two can peaks be observed can be observed in the spectrum.in the spectrum. One peak One locating peak locating at shorter at wavelength shorter wavelength corresponds corresponds to Y polarization to Y polarization component component (Y-PC), and(Y-PC), the other and the peak other locating peak locatingat longer at wavelength longer wavelength corresponds corresponds to X polarization to X polarization component component (X-PC). For(X-PC). PC2 Foris set PC2 at a is specific set at adirection, specific direction,the power the of X-PC power achieves of X-PC its achieves minimum. itsminimum. Under this Undercondition, this thecondition, output thefrom output the port from of thethe portPBS ofis thethe PBSY-PC. is theCorrespondingly, Y-PC. Correspondingly, the output the from output the fromother the port other of theport PBS of theis the PBS X-PC. is the Figure X-PC. Figure2a me2asuresa measures the polarization-resolved the polarization-resolved P-I curve.P-I curve. As presented As presented in this in this diagram, the threshold current I of the VCSEL is about 1.6 mA. For I I < 8.0 mA, Y-PC is diagram, the threshold current Ith of theth VCSEL is about 1.6 mA. For Ith ≤ I < 8th.0≤ mA, Y-PC is always lasingalways whereas lasing whereasX-PC is suppressed, X-PC is suppressed, where the where Y-PCthe and Y-PC X-PC and are X-PCcharacterized are characterized by the solid by line the solidand dashedline and line, dashed respectively. line, respectively. Figure 2b Figure records2b the records corresponding the corresponding optical spectr opticalum spectrum of the VCSEL of the VCSELbiased atbiased 5.0 mA at 5.0(about mA 3.13 (about Ith). 3.13It canIth be). Itobserved can be observed that the dominant that the dominant Y-PC oscillates Y-PC oscillates at 1551.97 at nm 1551.97 and the nm suppressedand the suppressed X-PC is located X-PC is at located 1552.23 at nm, 1552.23 and nm,the wavelength and the wavelength (frequency) (frequency) interval between interval betweenthe two polarizationthe two polarization components components is approximate is approximate 0.26 nm 0.26 (32.38 nm (32.38 GHz). GHz). By Bymeasuring measuring the the noise noise spectral spectral distribution,distribution, the the relaxation relaxation oscillation oscillation frequency frequency ( (FFr)r) of of the the VCSEL VCSEL can can be be obtained obtained [36,37]. [36,37]. Figure Figure 2c2c showsshows the the variation variation of of FFrr of of the the VCSEL VCSEL with with the the bias bias current. current. Obviously, Obviously, the the relaxation relaxation oscillation oscillation frequencyfrequency exhibits exhibits a nonlinearnonlinear variedvaried trend trend with with the the increase increase of of bias bias current, current, which which is similar is similar with with that thatreported reported in Reference in Reference [38]. In[38]. the In following the following experiments, experiments, the bias the current bias iscurrent fixed at is 5.0 fixed mA. at Under 5.0 mA. this Undercondition, this condition, the free-running the free-running 1550 nm VCSEL 1550 nm has VCSE a relaxationL has a relaxation oscillation oscillation frequency frequency of about 2.63 of about GHz. 2.63 GHz.

Figure 2. Experimentally measured (a) polarization-resolved P-I curve, (b) optical spectrum of the free-running 1550 nm VCSEL biased at 5.0 mA, (c) Fr-I curve.

Photonics 2020, 10, x FOR PEER REVIEW 3 of 12

MFS

DC Bias Tee Measurement System VCSEL

50 Total-OFC OSA VA1 PC1 FC1 EDFA FC2 80 TL PC2 PBS X-OFC PD1 ESA Y-OFC 20 OC 50

PM PD2 DSO

Figure 1. Schematic of the experimental setup. TL: tunable laser; VCSEL: vertical-cavity surface- emitting laser; VA: variable attenuator; PC: polarization controller; FC: fiber coupler; PM: power meter; OC: optical circulator; MFS: microwave frequency synthesizer; DC: direct current; EDFA: erbium doped fiber amplifier; PBS: polarization beam splitter; OSA: optical spectrum analyzer; PD: photodiode; ESA: electrical spectrum analyzer; DSO: digital storage oscilloscope.

3. Results and Discussion First, we characterize the basic properties of the 1550 nm VCSEL used in this work under free- running. To obtain polarization-resolved power-current (P-I) curve, we inspect the spectrum output from one port of the PBS during adjusting PC2. Normally, there are two peaks can be observed in the spectrum. One peak locating at shorter wavelength corresponds to Y polarization component (Y-PC), and the other peak locating at longer wavelength corresponds to X polarization component (X-PC). For PC2 is set at a specific direction, the power of X-PC achieves its minimum. Under this condition, the output from the port of the PBS is the Y-PC. Correspondingly, the output from the other port of the PBS is the X-PC. Figure 2a measures the polarization-resolved P-I curve. As presented in this diagram, the threshold current Ith of the VCSEL is about 1.6 mA. For Ith ≤ I < 8.0 mA, Y-PC is always lasing whereas X-PC is suppressed, where the Y-PC and X-PC are characterized by the solid line and dashed line, respectively. Figure 2b records the corresponding optical spectrum of the VCSEL biased at 5.0 mA (about 3.13 Ith). It can be observed that the dominant Y-PC oscillates at 1551.97 nm and the suppressed X-PC is located at 1552.23 nm, and the wavelength (frequency) interval between the two polarization components is approximate 0.26 nm (32.38 GHz). By measuring the noise spectral distribution, the relaxation oscillation frequency (Fr) of the VCSEL can be obtained [36,37]. Figure 2c shows the variation of Fr of the VCSEL with the bias current. Obviously, the relaxation oscillation frequency exhibits a nonlinear varied trend with the increase of bias current, which is similar with that reported in Reference [38]. In the following experiments, the bias current is fixed at 5.0 mA. PhotonicsUnder this2020 condition,, 7, 95 the free-running 1550 nm VCSEL has a relaxation oscillation frequency of about4 of 12 2.63 GHz.

Figure 2. Experimentally measured ( a) polarization-resolved P--II curve, ( b) optical spectrum of the free-running 15501550 nmnm VCSELVCSEL biasedbiased atat 5.05.0 mA,mA, ((cc)) FFr-r-II curve.curve.

Next, we will investigate the output characteristics of the VCSEL under only current modulation. Figure3 displays the time series, optical spectra, and power spectra of the laser under current modulation with modulation frequency fm = 2.8 GHz and modulation power Pm = 5.0 dBm (a), − 7.0 dBm (b), 13.0 dBm (c), and 17.0 dBm (d), respectively. For Pm = 5.0 dBm (Figure3(a1–a3)), − an approximate sinusoid time series with a period of T (=1/ fm) is observed, and the corresponding optical spectrum includes a few comb lines since the laser is not driven into a gain-switching state due to relatively weak modulation power. However, the optical spectrum possesses a high signal to noise ratio (SNR, about 65 dB). SNR, defined as the difference between the maximum power level of the output optical spectrum and the noise pedestal, is an important index to characterize the OFC quality. As shown in Figure3(a3), the strongest peak of the power spectrum locates at 2.8 GHz, which is equal to the modulation frequency fm. For Pm = 7.0 dBm (Figure3(b1–b3)), the time series presents a regular pulse train with a period of 357.1 ps, which is equal to the reciprocal of modulation frequency fm (= 2.8 GHz), and the optical spectrum is broadened and contains much more comb lines (Figure3(b2)). As a result, the laser is driven into a gain-switching with regular pulse state. Correspondingly, much more higher-order harmonics can be observed in the power spectrum. For Pm = 13.0 dBm (Figure3(c1–c3)), the periodic waveform with two peak intensities can be clearly observed in the time series, more comb lines with a frequency interval of fm/2 emerge in the optical spectrum, and suharmonic components are observed in the power spectrum. All these features indicate that the VCSEL operates at a period-2 state [39], which means that the period of the output pulse doubles. When Pm is increased to 17.0 dBm (Figure3(d1–d3)), the time series shows random intensity pulsing. The optical spectrum presents elevated and broadened noise spectrum without discernible comb lines. In the power spectrum, although the discrete harmonics remain clearly discernible, the level of continuous noise is significantly stronger than the noise floor. Under this case, the relatively strong spontaneous emission noise makes the frequency chirping noisier, and then the comb lines in optical spectrum are lost. Meanwhile, optical pulses switch-off to a level at which the spontaneous emission noise dominates the evolution [40]. The similar state has also been observed in a gain-switching of discrete mode lasers or distributed feedback lasers [22,26]. The width of noise spectrum of gain-switching 1550 nm VCSEL is related to the frequency interval between the two polarization components of the free-running VCSEL [35]. Additionally, through comparing Figures2b and3, one can see that the optical spectrum exists in a red-shift after adopting current modulation due to the heating effect resulted by the current modulation. Above results show that it is difficult to generate an OFC with broad bandwidth based on a 1550 nm VCSEL under only current modulation. However, after further introducing optical injection into the gain-switching VCSEL operating at the state shown in Figure3d, it is possible to generate a high quality OFC. In the following investigations, we fix fm at 2.8 GHz and Pm at 17.0 dBm. Photonics 2020, 7, 95 5 of 12

Figure 3. Time series (first column), optical spectra (second column), and power spectra (third column) of a current modulated 1550 nm VCSEL at I = 5.0 mA and fm = 2.8 GHz under different values of Pm, where (a) Pm = 5.0 dBm, (b) Pm = 7.0 dBm, (c) Pm = 13.0 dBm, and (d) Pm = 17.0 dBm. The gray curves − in the power spectra denote the noise floor.

Then, an external optical injection is further introduced into the VCSEL to obtain high quality OFC. As reported in References [34,35], the performance of the OFC generated in VCSELs under optical injection are dependent on the polarization of the injection light. Our experimental results show that for the current modulated 1550 nm VCSEL operating at the state shown in Figure3d, the optimized polarization of the injection light is in the intermediate between Y-PC and X-PC. Therefore, in the whole experiment, the polarization direction of the injection light is fixed at the intermediate between Y-PC and X-PC. Figure4 gives the optical spectra of total-OFC, Y-OFC, and X-OFC output from the gain-switching VCSEL under optical injection with injection light wavelength λi = 1553.4843 nm and injection power Pi = 266.80 µW. As shown in Figure4a, the total-OFC displays a flat and broadband spectrum structure, and the envelope of the optical spectrum is similar with that of the optical spectrum obtained under only current modulation (as shown in Figure3(d2)). The bandwidth of the total-OFC is about 70.0 GHz (56.0 GHz) within 10 dB (3 dB) amplitude variation corresponding to 26 (21) comb lines. Here, the OFC bandwidth is defined as the continuous frequency range within which the 1

Photonics 2020, 7, 95 6 of 12 difference among the powers of comb lines is less than 10 dB (3 dB). Via PBS, the optical spectrum distributions of two sub-combs named as Y-OFC and X-OFC can be measured as shown in Figure4b,c, respectively. As shown in Figure4b, Y-OFC locates at short wavelength region since it is devoted by Y-PC, and the 10 dB bandwidth is about 33.6 GHz (13 comb lines). The X-OFC (as shown in Figure4c) originating from X-PC, locates at long wavelength region, and the 10 dB bandwidth is about 30.8 GHz (12 comb lines). As a result, almost same number of comb lines is getting for X-OFC and Y-OFC. The reason for that is as follows: the X-PC and Y-PC possess comparable power under strong current Photonics 2020, 10, x FOR PEER REVIEW 6 of 12 modulation (as shown in Figure3(d2)), and meanwhile the optical powers injected into X-PC and X-PCY-PC areand identical Y-PC are since identical both since the polarization both the polariza and thetion wavelength and the wavelength of the injection of the lightinjection are nearlylight are in nearlythe intermediate in the intermediate between Y-PCbetween and Y-PC X-PC. and From X-PC. the threeFrom diagrams,the three diagrams, it can be seen it can that be theseen total-OFC that the total-OFCoriginating originating from splicing from two splicing sub-combs, two sub-comb possessess, possesses better performance. better performance. As a result, As wea result, will only we willanalyze only the analyze performance the performance of the total-OFC of the total-OFC in the following. in the following. Optical Power (dBm) Power Optical

Figure 4. ExperimentallyExperimentally measured measured ( (aa)) total-OFC, total-OFC, ( (bb)) Y-OFC, Y-OFC, ( c) X-OFC X-OFC output output from from a a gain-switching gain-switching

VCSEL under optical injection with injection light wavelength λλii = 1553.48431553.4843 nm nm and and injection power

Pi == 266.80266.80 μµW.W.

Besides wide bandwidth and good amplitude flat flatness,ness, a high quality OFC should also possess high spectral coherence. To To evaluate the coherence of the generated total-OFC, the detailed power spectrum and SSB phase noise of total-OFC is meas measuredured with an ESA, where the SSB phase noise is obtained via a toolbox of “Phase Noise” in the ESA. Here, the electrical beat tone signal locating at obtained via a toolbox of “Phase Noise” in the ESA. Here, the electrical beat tone signal locating at fm (=2.8f m (= GHz)2.8 GHz) is chosen is chosen for a for representative. a representative. As shown As shown in Figure in Figure 5a, SNR5a, SNR is about is about 66 dB, 66 and dB, andthe 3 the dB linewidth3 dB linewidth is less is than less than 1 Hz 1 Hzlimited limited by bythe the minimum minimum resolution resolution of ofthe the ESA. ESA. Figure Figure 5b5b presents thethe measured SSB phase noise, and the phase noise is about 120.6 dBc/Hz at 10 kHz offset frequency, measured SSB phase noise, and the phase noise is about −−120.6 dBc/Hz at 10 kHz offset frequency, which means that the comb lines are stable and highly coherent. coherent. Actually, Actually, high mutual coherence can also be observedobserved betweenbetween two two lines lines belong belong to to di ffdifferenterent polarization polarization sub-combs sub-combs due due to the to factthe fact that that two twopolarization polarization modes modes in a VCSELin a VCSEL are anti-correlated are anti-correlated [32]. [32].

Photonics 2020, 7, 95 7 of 12 Photonics 2020, 10, x FOR PEER REVIEW 7 of 12

FigureFigure 5. ExperimentallyExperimentally measured measured ( (aa)) detailed detailed power power spectrum spectrum and and ( (bb)) SSB SSB phase phase noise noise centered centered at at 2.82.8 GHz GHz under under a resolution bandwidth of 1 Hz.

AboveAbove experimental experimental results results have have demonstrated demonstrated th thatat a a high high quality quality OFC OFC can can be be generated generated by by a a gain-switchinggain-switching VCSEL VCSEL subject subject to to optical injection with given λi = 1553.48431553.4843 nm nm and and Pii == 266.80266.80 μµW.W . InIn the the following, following, we we will will investigate investigate the the performance performance of total-OFC total-OFC under different different Pii values. Figure Figure 66 showsshows the the optical optical spectra spectra output output from from the the gain-switching gain-switching VCSEL VCSEL under under optical optical injection injection with with a a fixed fixed injectioninjection light light wavelength wavelength λiλ =i 1553.4843= 1553.4843 nm nmand and different different injection injection power power Pi. ForP iP.i = For 9.12P iμ=W9.12 (FigureµW 6a),(Figure some6a), comb some lines comb with lines uneven with unevenamplitude amplitude and small and SNR small appear SNR in appear the optical in the spectrum, optical spectrum, and the total-OFCand the total-OFC bandwidth bandwidth is about is28.0 about GHz 28.0 corresponding GHz corresponding to 11 comb to 11 lines. comb For lines. Pi = For27.20P iμ=W27.20 (FigureµW 6b)(Figure and6 67.84b) and μ 67.84W (FigureµW (Figure6c), a relatively6c), a relatively strong strongoptical optical injection injection significantly significantly suppresses suppresses the noise the levelnoise and level enhances and enhances SNR of SNR the of comb thecomb lines, lines,and the and total-OFC the total-OFC has a has bandwidth a bandwidth about about 33.6 33.6GHz GHz (13 comb(13 comb lines) lines) for forFigure Figure 6b6 andb and 39.2 39.2 GHz GHz (15 (15 comb comb lines) lines) for for Figure Figure 66c,c, respectively.respectively. WhenWhen PPii isis increasedincreased to to 266.80 266.80 μµWW (Figure (Figure 66d),d), thethe noisenoise pedestalpedestal isis furtherfurther decreased,decreased, andand thethe properproper opticaloptical injectioninjection equalizes equalizes the the power power of of X-OFC X-OFC and and Y-OFC. Y-OFC. Under Under this this case, case, as as shown shown in in Figure Figure 66d,d, aa flatflat total-OFCtotal-OFC with with 70.0 70.0 GHz GHz bandwidth bandwidth (26 comb comb lines) lines) is is generated. generated. Further Further increasing increasing Pii toto 1422.40 1422.40 μµWW (Figure(Figure6 6e),e), a a strong strong injection injection power power is beneficial is beneficial for suppressing for suppressing noise, noise, but it alsobut degradesit also degrades the flatness the flatnessof total-OFC. of total-OFC. As a result, As a theresult, bandwidth the bandwidth decreases decreases to 30.8 to GHz 30.8 (12 GHz comb (12 comb lines). lines). In particular, In particular, for a fortoo a strong too strong optical optical injection injection of Pi =of2868.00 Pi = 2868.00µW, thereμW, there are only are twoonly comb two comb lines includedlines included into the into 10 the dB 10bandwidth, dB bandwidth, and the and bandwidth the bandwidth decreases decreases to 2.8 GHz.to 2.8 GHz. Figure7 shows the bandwidth variation of total-OFC with the injection power Pi under 10 10 10 λi = 1553.4843(a) P = 9.12 nm μW. As shownBW = 28.0 in GHz this diagram,(b) P = 27.20 with μW theBW increase = 33.6 GHz of injection (c) P = 67.84 power μW PiBW, the = 39.2 bandwidth GHz 0 i 0 i 0 i of OFC firstly increases very sharply to a level aboutλi 70 GHz, after maintainingλi the level witnin a -10 λi -10 -10 range of injection power, and finally experiences10 dB a downward trend. For10 dB101.72 µW P 1284.80 µW, -20 10 dB -20 -20 ≤ i ≤ the bandwidths-30 of the total-OFCs are-30 beyond 60.0 GHz. Therefore,-30 for a given λi = 1553.4843 nm, a broadband-40 OFC can be generated-40 under 101.72 µW P 1284.80-40 µW. ≤ i ≤ Finally,-50 we will analyze the influence-50 of different injection light-50 wavelength on the performance of the-60 total-OFC. Figure8 displays the-60 optical spectra output from-60 the gain-switching VCSEL under -70 -70 -70 optical1553 injection 1553.2 with 1553.4P 1553.6i = 266.80 1553.8µ 1554W1553 anddi 1553.2fferent 1553.4λi 1553.6. For 1553.8λi = 1553.1544 1554 1553 1553.2 nm (Figure 1553.4 1553.68a), 1553.8 the injection 1554 10 10 10 light wavelength(d) P = 266.80 isμW outsideBW = 70.0 the GHz noise pedestal(e) P = 1422.40 of μW gain-switchingBW = 30.8 GHz VCSEL.(f) P = 2868.00 Under μW thisBW circumstance, = 2.8 GHz 0 i λi 0 i λi 0 i λi the optical spectrum is composed of the optical spectrum without optical injection (Figure10 dB3d) and the -10 -10 10 dB -10 regenerated10 dB injection light together with its modulated sideband. For λi = 1553.2911 nm (Figure8b), -20 -20 -20 the injection-30 light wavelength is gradually-30 near to the Y-PC of the 1550-30 nm VCSEL. Under this condition, the noise-40 pedestal is suppressed obviously,-40 and meanwhile the Y-PC-40 is motivated to generate Y-OFC with 36.4-50 GHz bandwidth (including-50 14 comb lines). For λi = 1553.4038-50 nm (Figure8c), X-OFC is also stimulated-60 while Y-OFC is still the primary-60 component of the total-OFC,-60 the total-OFC bandwidth is -70 -70 -70 about 28.01553 GHz 1553.2 (11comb 1553.4 1553.6 lines). 1553.8 For 1554 λ1553i = 1553.4843 1553.2 1553.4 nm 1553.6 (Figure 1553.88 1554d),1553 the injection 1553.2 1553.4 light 1553.6 wavelength 1553.8 1554 is almost in the middle of X-PC and Y-PC. UnderWavelength this case, (nm) the intensity of X-OFC is similar with that of Y-OFC,Figure and 6. then Experimentally a broadband measured total-OFC total-OFC with a output bandwidth from ofa gain-switching about 70.0 GHz VCSEL (26 comb under lines) optical can be injection with λi = 1553.4843 nm and different values of Pi, where (a) Pi = 9.12 μW, (b) Pi = 27.20 μW, (c) Pi = 67.84 μW (d) Pi =266.80 μW, (e) Pi = 1422.40 μW, and (f) Pi =2868.00 μW.

Photonics 2020, 10, x FOR PEER REVIEW 7 of 12

Figure 5. Experimentally measured (a) detailed power spectrum and (b) SSB phase noise centered at 2.8 GHz under a resolution bandwidth of 1 Hz.

Above experimental results have demonstrated that a high quality OFC can be generated by a gain-switching VCSEL subject to optical injection with given λi = 1553.4843 nm and Pi = 266.80 μW. In the following, we will investigate the performance of total-OFC under different Pi values. Figure 6 shows the optical spectra output from the gain-switching VCSEL under optical injection with a fixed injection light wavelength λi = 1553.4843 nm and different injection power Pi. For Pi = 9.12 μW (Figure 6a), some comb lines with uneven amplitude and small SNR appear in the optical spectrum, and the total-OFC bandwidth is about 28.0 GHz corresponding to 11 comb lines. For Pi = 27.20 μW (Figure 6b) and 67.84 μW (Figure 6c), a relatively strong optical injection significantly suppresses the noise level and enhances SNR of the comb lines, and the total-OFC has a bandwidth about 33.6 GHz (13

Photonicscomb lines)2020, 7 ,for 95 Figure 6b and 39.2 GHz (15 comb lines) for Figure 6c, respectively. When8 P ofi 12is increased to 266.80 μW (Figure 6d), the noise pedestal is further decreased, and the proper optical injection equalizes the power of X-OFC and Y-OFC. Under this case, as shown in Figure 6d, a flat generated.total-OFC with For λ70.0i = 1553.5648GHz bandwidth nm (Figure (26 comb8e),the lines) injection is generated. light wavelength Further increasing is slightly Pi to nearer 1422.40 to theμW X-PC(Figure component, 6e), a strong which injection results inpower the increase is beneficial of the powerfor suppressing of X-OFC andnoise, the but decrease it also of degrades the power the of Y-OFC.flatness The of total-OFC. total-OFC As has a aresult, 28.0 GHz the bandwidth bandwidth decreases (11comb lines).to 30.8For GHzλi (12is increased comb lines). to 1553.7097 In particular, nm (Figurefor a too8f), strong the injection optical lightinjection wavelength of Pi = 2868.00 is far away μW, fromthere theare Y-PC,only two and comb therefore lines only included the X-OFC into the is motivated.10 dB bandwidth, The bandwidth and the bandwidth of the total-OFC decreases is about to 2.8 33.6 GHz. GHz (13 comb lines).

10 10 10 (a) P = 9.12 μW BW = 28.0 GHz (b) P = 27.20 μWBW = 33.6 GHz (c) P = 67.84 μW BW = 39.2 GHz i i i 0 0 λi 0 λi -10 λi -10 -10 10 dB 10 dB -20 10 dB -20 -20

-30 -30 -30

-40 -40 -40

-50 -50 -50

-60 -60 -60

-70 -70 -70 1553 1553.2 1553.4 1553.6 1553.8 1554 1553 1553.2 1553.4 1553.6 1553.8 1554 1553 1553.2 1553.4 1553.6 1553.8 1554

10 10 10 (d) P = 266.80 μW BW = 70.0 GHz (e) P = 1422.40 μW BW = 30.8 GHz (f) P = 2868.00 μW BW = 2.8 GHz 0 i λi 0 i λi 0 i λi 10 dB -10 -10 10 dB -10 10 dB -20 -20 -20

-30 -30 -30

-40 -40 -40

-50 -50 -50

Photonics-60 2020, 10, x FOR PEER REVIEW -60 -60 8 of 12

-70 -70 -70 1553 1553.2 1553.4 1553.6 1553.8 1554 1553 1553.2 1553.4 1553.6 1553.8 1554 1553 1553.2 1553.4 1553.6 1553.8 1554 Figure 7 shows the bandwidth variation of total-OFC with the injection power Pi under λi = Wavelength (nm) 1553.4843 nm. As shown in this diagram, with the increase of injection power Pi, the bandwidth of OFC firstlyFigureFigure increases 6.6. ExperimentallyExperimentally very sharply measured to a level total-OFC about output 70 GHz, from after aa gain-switchinggain-switching maintaining VCSELtheVCSEL level underunder witnin opticaloptical a range injection with λi = 1553.4843 nm and different values of Pi, where (a) Pi = 9.12 μW, (b) Pi = 27.20 μW, of injectioninjection power, with λ andi = 1553.4843 finally experi nm andences different a downward values of P trend.i, where For (a) 101.72Pi = 9.12 μWµW,≤ P (bi )≤P 1284.80i = 27.20 μµW,W, the (c) Pi = 67.84 μµW (d) Pi =266.80 μµW, (e) Pi = 1422.40 μµW, and (f) Pi =2868.00 μµW. bandwidths(c) Pi = 67.84of theW, total-OFCs (d) Pi =266.80 are W,beyo (e)ndPi =60.01422.40 GHz.W, Therefore, and (f) Pi = for2868.00 a givenW. λi = 1553.4843 nm, a broadband OFC can be generated under 101.72 μW≤ Pi ≤ 1284.80 μW.

80

60

40

20

0 0 500 1000 1500 2000 2500 3000 Injection Power (μW)

FigureFigure 7. 7. EvolutionEvolution of ofthe the total-OFC total-OFC bandwidth bandwidth as a as function a function of the of injection the injection power power Pi underPi under λi = 1553.4843λi = 1553.4843 nm. nm.

Finally,Figure9 we shows will theanalyze variation the influence trend of the of total-OFCdifferent injection bandwidth light with wavelength the injection on the light performance wavelength ofunder the total-OFC.Pi = 266.80 FigureµW. Here, 8 displays the frequency the optical step sp ofectra the injection output from light the is set gain-switching as 2 GHz. For VCSELλi < 1553.2428 under opticalnm or injectionλi > 1553.8547 with Pinm, = 266.80 the injectionμW and different light is outside λi. For λ thei = 1553.1544 noise pedestal nm (Figure as shown 8a), the in Figureinjection8a. Under this case, the gain-switching VCSEL cannot be locked to generate OFC. For 1553.2428 nm light wavelength is outside the noise pedestal of gain-switching VCSEL. Under this circumstance, the≤ λ 1553.4683 nm, compared with X-PC the injection light wavelength is nearer to Y-PC, Y-OFC is opticali ≤ spectrum is composed of the optical spectrum without optical injection (Figure 3d) and the regeneratedstimulated and injection dominant light while together the X-OFCwith its is modulated suppressed sideband. or subsidiary. For λ Thei = 1553.2911 OFC bandwidth nm (Figure fluctuates 8b), thebetween injection 28.0 light GHz wavelength and 36.4 GHz. is gradually For 1553.4683 near nmto the< λ Y-i

Photonics 2020, 10, x FOR PEER REVIEW 9 of 12

10 10 10 (a) λ = 1553.1544 nm BW = 0 GHz (b) λ = 1553.2911 nm BW = 36.4 GHz (c) λ = 1553.4038 nm BW = 28.0 GHz 0 i 0 i 0 i λi λi -10 λi -10 -10 10 dB 10 dB -20 10 dB -20 -20 -30 -30 -30 Photonics 2020, 7, 95 9 of 12 -40 -40 -40

-50 -50 -50

-60 -60 -60

70.0 GHz.-70 For 1553.5004 nm λi 1553.8547-70 nm, the injection light-70 wavelength is nearer to X-PC. 1553 1553.2 1553.4 1553.6 1553.8≤ ≤ 1554 1553 1553.2 1553.4 1553.6 1553.8 1554 1553 1553.2 1553.4 1553.6 1553.8 1554 As a result, the X-OFC is dominant while the Y-OFC is subsidiary or suppressive, and the bandwidth 10 10 10 of total-OFC(d) λ is= within1553.4843 nm theBW range = 70.0 GHz of 28.0 GHz–42.0(e) λ = 1553.5648 GHz.nm BW Additionally,= 28.0 GHz (f) λ it= 1553.7097 should nm beBW pointed = 33.6 GHz out that 0 i 0 i λi 0 i λi λi although-10 the results in Figure9 are obtained-10 under 2 GHz frequency10 dB -10 step of the injection10 light,dB the 10 dB samePhotonics variation 2020-20 , 10, x trend FOR PEER can beREVIEW obtained -20 for a finer frequency step. -20 9 of 12 -30 -30 -30

-40 -40 -40 10 10 10 -50 -50 -50 (c) λ = 1553.4038 nm BW = 28.0 GHz (a) λ = 1553.1544 nm BW = 0 GHz (b) λ = 1553.2911 nm BW = 36.4 GHz i 0 i 0 i 0 λi -60 -60 λi -60 -10 λi -10 -10 10 dB -70 -70 10 dB -70 -201553 1553.210 dB 1553.4 1553.6 1553.8 1554-201553 1553.2 1553.4 1553.6 1553.8 1554-201553 1553.2 1553.4 1553.6 1553.8 1554 Wavelength (nm) -30 -30 -30 -40 -40 -40

Figure-50 8. Experimentally measured-50 total-OFC output from the gain-switching-50 VCSEL under optical injection-60 with fixed Pi = 266.80 μW-60 and different values of λi, where-60 (a) λi = 1553.1544 nm, (b) λi =

-70 -70 i i -70 i 1553.29111553 1553.2 nm, ( 1553.4c) 1553.4038 1553.6 1553.8 nm, ( 1554d) λ1553= 1553.4843 1553.2 1553.4 nm, 1553.6 (e) λ 1553.8 = 1553.5648 1554 1553 nm, 1553.2 and 1553.4 (f) λ 1553.6=1553.7097 1553.8 nm. 1554

10 10 10 (d) λ = 1553.4843 nm BW = 70.0 GHz (e) λ = 1553.5648 nm BW = 28.0 GHz (f) λ = 1553.7097 nm BW = 33.6 GHz Figure0 9i shows the variation trend0 i of the total-OFCλi bandwidth0 i with the injection light λi λi wavelength-10 under Pi = 266.80 μW. Here,-10 the frequency step 10of dB the-10 injection light is set as10 2 dB GHz. For 10 dB λi < 1553.2428-20 nm or λi > 1553.8547 nm,-20 the injection light is outside-20 the noise pedestal as shown in Figure -308a. Under this case, the gain-switching-30 VCSEL cannot -30be locked to generate OFC. For -40 -40 -40 i 1553.2428-50 nm ≤ λ ≤ 1553.4683 nm, compared-50 with X-PC the injection-50 light wavelength is nearer to Y-

PC, Y-OFC-60 is stimulated and dominant-60 while the X-OFC is suppressed-60 or subsidiary. The OFC bandwidth-70 fluctuates between 28.0 GHz-70 and 36.4 GHz. For 1553.4683-70 nm < λi < 1553.5004 nm, Y-OFC 1553 1553.2 1553.4 1553.6 1553.8 1554 1553 1553.2 1553.4 1553.6 1553.8 1554 1553 1553.2 1553.4 1553.6 1553.8 1554 and X-OFC have almost similar power, and twoWavelength sub-combs (nm) link up to form a total-OFC with broad bandwidth about 70.0 GHz. For 1553.5004 nm ≤ λi ≤ 1553.8547 nm, the injection light wavelength is nearerFigure to X-PC. 8. 8. ExperimentallyExperimentally As a result, measured the measured X-OFC total-OFC is total-OFC dominant output output whilefrom fromthe the gain-switching theY-OFC gain-switching is subsidiary VCSEL VCSEL under or suppressive, optical under optical injection with fixed P = 266.80 µW and different values of λ , where (a) λ = 1553.1544 and theinjection bandwidth with fixed of total-OFCPi = 266.80i μisW within and different the rang valuese of 28.0of λ iGHz–42.0, where (ai) GHz.λi = 1553.1544 Additionally,i nm, (b )it λ shouldi = nm, (b) λ = 1553.2911 nm, (c) 1553.4038 nm, (d) λ = 1553.4843 nm, (e) λ = 1553.5648 nm, and be pointed1553.2911 out inm, that (c )although 1553.4038 thenm, results(d) λi = 1553.4843in Figure nm, 9 arei (e) obtainedλi = 1553.5648 under nm, 2 and GHzi (f) frequency λi =1553.7097 step nm. of the (f) λ = 553.7097 nm. injectioni light, the same variation trend can be obtained for a finer frequency step. Figure 9 shows the variation trend of the total-OFC bandwidth with the injection light wavelength under Pi = 266.80 μW. Here, the frequency step of the injection light is set as 2 GHz. For λi < 1553.2428 nm or λi > 1553.8547 nm, the injection light is outside the noise pedestal as shown in Figure 8a. Under this case, the gain-switching VCSEL cannot be locked to generate OFC. For 1553.2428 nm ≤ λi ≤ 1553.4683 nm, compared with X-PC the injection light wavelength is nearer to Y- PC, Y-OFC is stimulated and dominant while the X-OFC is suppressed or subsidiary. The OFC bandwidth fluctuates between 28.0 GHz and 36.4 GHz. For 1553.4683 nm < λi < 1553.5004 nm, Y-OFC

and X-OFC have almost similar(GHz) Bandwidth power, and two sub-combs link up to form a total-OFC with broad bandwidth about 70.0 GHz. For 1553.5004 nm ≤ λi ≤ 1553.8547 nm, the injection light wavelength is nearer to X-PC. As a result, the X-OFC is dominant while the Y-OFC is subsidiary or suppressive, and the bandwidth of total-OFC is within the range of 28.0 GHz–42.0 GHz. Additionally, it should be pointed out that although the results in Figure 9 are obtained under 2 GHz frequency step of the injection light, the same variation trend can be obtained for a finer frequency step. FigureFigure 9.9. Evolution of the total-OFC bandwidth as a functionfunction of the injection light wavelength forfor thethe gain-switchinggain-switching VCSELVCSEL atat injectioninjection powerpower PPii = 266.80266.80 μµW.

4.4. Conclusions InIn summary, summary, we we experimentally experimentally investigated investigated the the broadband broadband OFC OFC generation generation based based on a ongain- a gain-switchingswitching VCSEL VCSEL subject subject to optical to optical injection, injection, and the andperformance the performance of the generated of the generated OFC are analyzed. OFC are analyzed. Firstly, a large modulated current with modulation frequency fm = 2.8 GHz and modulation Bandwidth (GHz) Bandwidth power Pm = 17.0 dBm is adopted to drive a 1550 nm VCSEL into the gain-switching state with a broad noisy spectrum. Next, an injection light is further introduced for generating high quality OFC. The experimental results show that for a given injection light wavelength λi = 1553.4843 nm,

Figure 9. Evolution of the total-OFC bandwidth as a function of the injection light wavelength for the

gain-switching VCSEL at injection power Pi = 266.80 μW.

4. Conclusions In summary, we experimentally investigated the broadband OFC generation based on a gain- switching VCSEL subject to optical injection, and the performance of the generated OFC are analyzed.

Photonics 2020, 7, 95 10 of 12 the bandwidth of total-OFC (constituted by X-OFC and Y-OFC) firstly increases very sharply, then maintains at a relatively high level, and finally experiences a downward trend with the increase of injection power Pi. For a given injection power Pi = 266.80 µW, when λi is closer to Y-PC of the 1550 nm VCSEL, the Y-OFC is the major contributor to the total-OFC. On the contrary, when λi is closer to X-PC of the 1550 nm VCSEL, the X-OFC becomes the major contributor to the total-OFC. In particularly, if λi is set at the middle of two polarization modes, X-OFC and Y-OFC are almost equalized. As a result, the bandwidth of the generated OFC is enhanced obviously. Under optimized operation parameters, we experimentally acquire a total-OFC with a bandwidth of 70.0 GHz (56.0 GHz) within 10 dB (3 dB) amplitude variation, whose single sideband phase noise at the fundamental frequency is below 120.6 dBc/Hz @ 10 kHz. − Author Contributions: H.R. was responsible for performing the experiment, analyzing the results, and writing of the paper. L.F. was responsible for performing part of the experiment and revising the manuscript. N.L. was responsible for recording experimental data. Z.W. and G.X. were responsible for the discussion of the results and reviewing/editing/proof-reading of the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported in part by the National Natural Science Foundation of China under Grant 61775184 and Grant 61875167, and in part by the Fundamental Research Funds for the Central Universities of China under Grant SWU120017. Conflicts of Interest: The authors declare no conflict of interest.

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