An optical frequency comb consists of evenly spaced spectral lines, which allow high-precision frequency metrology and are also used in a wide range of applications [1]. Interest in microresonator-based Kerr frequency combs has increased in recent years because of their compactness and high repetition rate [2]. Kerr combs also have the advantages of high coherence [3,4] and broadband coverage [4–6]; therefore, they could potentially be applied in optical communications, optical signal processing, and spectroscopy [7–11]. Of interest are Kerr soliton combs [4,12–15], which result from the balance between anomalous dispersion and Kerr nonlinearity, and they have been demonstrated to cover an octave [5,6]. Recently, based on a soliton comb with a spacing of 100 GHz, a 30.1 Tbit/s wavelength-division multiplexing transmission experiment covering the full C and L bands was demonstrated [9].

However, Kerr comb spacing depends on the circumference of the microresonator [2], and it is a challenge to change the comb spacing significantly after device fabrication. The number of comb lines within the desired wavelength bands is therefore limited when the comb line spacing is quite large (e.g., $>200\mathrm{GHz}$). This limited number could limit the applications of Kerr comb, such as in coherent communications and spectroscopy. One way to increase the number of comb lines is to generate a second comb from another microresonator and to interleave the two combs [9]. However, these two combs are not coherent to each other. Another way is to redesign and fabricate a larger microresonator with smaller comb spacing (e.g., 25 GHz ITU spacing). Such a large microresonator would require a much higher pump power because of the larger mode volume [16], and the conversion efficiency from the pump to the comb is relatively lower [17]. Moreover, this approach is neither cost effective nor tunable. Overcoming this challenge in a flexible and tunable manner presents an interesting problem.

Electro-optical (EO) modulation can also be used to generate comb lines [18]. Such EO-generated combs have been previously inserted into Kerr combs to measure the repetition rate of the Kerr comb [4,19] or to enable the referencing of a Kerr comb spacing of $>100\mathrm{GHz}$ to a microwave frequency standard [20]. Carrying this insertion idea one step further could result in an effective way to increase the number of lines in the Kerr comb, which could then overcome the above mentioned challenge and be potentially advantageous for many applications. In addition, further demonstrating the tunable insertion of multiple lines into a Kerr frequency comb would be interesting.

In this Letter, we experimentally demonstrate the flexibility of inserting multiple EO comb lines into a Kerr soliton comb via EO modulation. Different numbers of lines can be inserted by changing the modulation frequency. When measured, the linewidth of the newly generated EO comb is comparable to that of the Kerr comb. As an example, we also show the advantage of EO comb insertion in the application of coherent communications. The combination of Kerr and EO combs is demonstrated in a coherent communication system when one Kerr and five EO lines are encoded with 10 Gbaud quadrature phase-shift-keyed (QPSK) signals. Thus, the number of data channels is increased, and the spectral efficiency of the communication system could be potentially improved.

Figure 1(a) shows the conceptual diagram of the EO line insertion into a Kerr frequency comb that is used for data transmission. When a Kerr comb is sent into an optical modulator with a modulation frequency of $f_e$, each Kerr comb line generates several EO sidebands spaced $f_e$ apart. Thus, the comb at the output of the modulator comprises a combination of a Kerr comb and an EO comb. The comb spacing in the newly formed comb is equal to $f_e$, which is determined by the radio frequency (RF) modulation frequency. Ideally, an evenly distributed channel spectrum for optical communications can be formed when the Kerr comb spacing is an integer multiple of the modulation frequency $f_e$, and a flexible number of EO comb lines can be inserted into the Kerr comb when the RF modulation frequency $f_e$ is changed.

 

Fig. 1. (a) Conceptual diagram of an EO comb insertion into a Kerr comb using EO modulation. Each EO sideband is generated from an adjacent Kerr comb line. Both the Kerr and EO combs are then modulated with data. (b) Experimental setup for inserting the EO comb lines into a Kerr frequency comb and for the data modulation. Kerr comb is generated from a silicon nitride microresonator. ECDL, external-cavity diode laser; AFG, arbitrary function generator; EDFA, erbium-doped fiber amplifier; PC, polarization controller; FBG, fiber Bragg grating; TOF, tunable optical filter; IM, intensity modulator; PM, phase modulator; PS, RF phase shifter; ESA, electronic spectrum analyzer; I/Q, in-phase/quadrature; and QPSK, quadrature phase-shift-keyed.

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Figure 1(b) shows the experimental setup for inserting EO lines into a Kerr comb by using EO modulation. A continuous-wave light is amplified by a high-power erbium-doped fiber amplifier and coupled into a silicon nitride microresonator to generate a Kerr frequency comb. An arbitrary function generator controls the laser wavelength, and a soliton comb can be generated when the wavelength is swept from shorter to longer wavelengths, stopping at a step region of the resonance [15,21]. The pump wavelength is 1555.9 nm, and the measured loaded $Q$-factor of the pump mode is $1.3\times {10}^6$. The waveguide in the microresonator has a height of 900 nm and a width of 1500 nm. The coupling loss is around 3 dB per facet. The high pump power at the output of the microresonator is suppressed by a fiber Bragg grating. A tunable optical filter is used to select several Kerr comb lines, and these Kerr lines are then sent into cascaded intensity and phase modulators, which are used to generate a flat EO comb [18]. By changing the RF modulation frequency, a flexible number of EO sidebands are generated around the Kerr comb lines. The newly formed comb splits into two paths: one is used to measure the comb spacing, and the other is used to send the selected comb lines into a transmitter. The comb line is modulated with 10 Gbaud QPSK signals and demodulated by a coherent receiver.

Figure 2(a) shows the spectrum of a soliton comb that is generated in the microresonator. The spectral envelope exhibits a ${\mathrm{sech}}^2$ shape with a 3 dB bandwidth of 5 THz. The repetition rate of the soliton comb is measured using the EO down-conversion technique [4,19]. An RF signal of 36 GHz drives both EO modulators, which generate EO sidebands. The difference between the two adjacent Kerr comb lines is reduced to 48.002 GHz, based on the detection of the spacing between their second EO sidebands by a high-speed photodetector (PD). Thus, the Kerr comb spacing is measured as 192.0020 GHz [see Fig. 2(b)]. Note that, in order to synchronize the Kerr comb and the EO comb, the Kerr comb spacing can be phase-locked to an RF reference [4,22] which also drives the EO modulators. The inset of Fig. 2(a) shows the five Kerr comb lines, which are of almost equal power and which are selected for later use in EO comb insertion.

 

Fig. 2. (a) Optical spectrum of a Kerr frequency comb in a single-soliton state (resolution bandwidth $[\mathrm{RBW}]=0.1\mathrm{nm}$). The inset shows the five selected Kerr comb lines in the 1543–1551 nm range. (b) Repetition rate beat note of the soliton comb in (a) ($\mathrm{RBW}=1\mathrm{kHz}$).

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Figure 3 shows the comb spectra at the output of the cascaded modulators (left) and the corresponding RF beat notes of the comb lines (right) when the RF modulation frequency $f_e$ is changed to 1/7, 1/8, and 1/9 of the original Kerr comb spacing. The RF phase shifter and the bias voltage of the intensity modulator are optimized to flatten the newly generated EO lines. Figure 3(c) shows that when the RF modulation frequency is set as 21.3336 GHz, eight EO peaks are generated between two adjacent Kerr lines in the optical spectrum analyzer, indicating that the newly generated comb lines could be used as carriers for data modulation. Note that for high-order EO sidebands between two Kerr lines, two closely spaced EO sidebands of relatively low power are generated from the left and right Kerr lines [23]. Therefore, we only investigate the EO lines close to the Kerr lines and encode them with high-order modulation format signals. The power of high-order EO sidebands could be improved by increasing the RF signal power and the newly formed comb would become more flattened.

 

Fig. 3. Optical spectra of combined Kerr and EO combs after EO modulation when the modulation frequency $f_e$ is (a) 1/7, (b) 1/8, and (c) 1/9 of the Kerr comb spacing ($\mathrm{RBW}=0.1\mathrm{nm}$). The corresponding RF beat notes are shown on the right ($\mathrm{RBW}=1\mathrm{kHz}$).

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To study the effect of EO modulation on the Kerr comb and also demonstrate that the newly inserted EO comb lines are suitable for data modulation, we measure the linewidths of both the Kerr and EO comb lines on the basis of the delayed self-heterodyne method [24]. Figure 4(a) shows the linewidth measurement setup for the Kerr and EO comb lines. The selected comb line is amplified before being divided into two paths. One path contains an acoustic optical modulator (AOM), which is used to shift the frequency of the comb line by 590 MHz. The other path contains a 20.3 km fiber delay, which is longer than the coherence length of the input comb line. Then, the combined beam is sent to a PD after being amplified, and the RF spectrum of the beat note is recorded by a digital phosphor oscilloscope. Figure 4(b) shows the linewidth measurements for different comb lines. The linewidth of the generated EO comb line falls in the range of 20–30 kHz, which is comparable to that of the Kerr comb line. In addition, the measured linewidth of the Kerr line remains nearly the same after EO modulation [see Fig. 4(c)].

 

Fig. 4. (a) Experimental setup for the linewidth measurement based on the delayed self-heterodyne method. AOM, acoustic optical modulator; and DPO, digital phosphor oscilloscope. (b) Measured linewidths versus different wavelengths. Black triangle dots correspond to the Kerr comb lines without EO modulation. The linewidth measurements for the Kerr comb line of 1547.24 nm, with and without EO modulation, are shown in (c).

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To further demonstrate that this insertion technique provides high-quality communication-grade optical lines, we select one Kerr comb line and several surrounding EO lines and modulate them with data (Fig. 5). Multiple comb lines in the 1547.9–1549.5 nm range are filtered (see the lines in the gray-dashed boxes of Fig. 3) with different comb spacing in the case of different modulation frequencies. Figure 5(a) shows the spectrum of multiple data channels in the coherent receiver when those comb lines are encoded with 10 Gbaud QPSK signals. The error vector magnitude (EVM) performance in Fig. 5(b) shows that these data channels are error free, demonstrating that the newly inserted EO lines are suitable for data transmission. Thus, we are able to increase the channel number to six with only one channel from a Kerr comb line. The EVM difference between these data channels can be explained by the optical signal-to-noise ratio (OSNR) difference in the receiver. In addition, we also show the case when the Kerr comb line at 1548.76 nm has not been EO modulated and is directly modulated with 10 Gbaud QPSK. We see that the signal power of the corresponding data channel is higher than those of data channels in the case of EO modulation, and the noise level is lower. The resulting higher OSNR makes the EVM lower and the signal quality better [see Fig. 5(b)]. This phenomenon can be attributed to the following reason. In the case of EO comb line insertion, EO modulators can bring loss to the input Kerr comb, and the amplified spontaneous emission noise level of each comb line may increase when it is amplified. The amplified signal power is also limited. Thus, the OSNR of the corresponding data channel would then decrease, and the signal quality could be affected.

 

Fig. 5. (a) Optical spectra of the channels in the coherent receiver when multiple comb lines in the gray-dashed boxes of Fig. 3 are modulated with 10 Gbaud QPSK data ($\mathrm{RBW}=0.1\mathrm{nm}$). The spectrum of the channel when the Kerr comb line at 1548.76 nm is directly modulated with data is also shown. (b) Measured EVMs versus different comb lines for different modulation frequencies of the modulator. As a comparison, the EVM of the Kerr comb line directly modulated with the data is also shown.

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In conclusion, the number of available comb lines can be increased when the Kerr comb is input into EO modulators, which proves useful in potentially improving the spectral efficiency of optical communications systems. A coherent communication experiment confirms the high coherence of Kerr and EO combs when six comb lines are encoded with 10 Gbaud QPSK signals. We also find that EO modulation can bring loss to the Kerr comb and affect the signal quality when the comb is modulated with data. Thus, using EO modulation to increase the number of comb lines is preferred for cases in which many data channels are needed and OSNR requirements are not too stringent. Furthermore, EO comb insertion could be beneficial for other applications. For example, the increased number of comb lines could improve the number of taps in a tapped-delay-line [25] used for all-optical signal processing.