Ultra-flat Optical Frequency Comb Generation Research Articles

Proposed Scheme for Ultra-Flat Optical Frequency Comb Generation Based on Dual-Drive Mach–Zehnder Modulators and Bidirectional Recirculating Frequency Shifting in Single Loop

Recirculating frequency shifting has attracted much attention for its advantages in the generation of the flexible and high-quality optical frequency comb. A new scheme of ultra-flat optical frequency comb generation system based on single-loop bidirectional recirculating frequency shift is proposed and studied in this paper. The generation system employs two pairs of dual-drive Mach–Zehnder modulators and several polarization devices. Compared with the method of single-loop unidirectional recirculation frequency shift, under the same cycles, the number of comb lines generated by the proposed method is doubled, and the generated optical frequency combs have less noise accumulation and better flatness. The theoretical model is established, and the proposed scheme is verified by software simulation. A 111-line optical frequency comb with the spacing of 12.5 GHz, the flatness of 0.76 dB, and the optical signal-to-noise ratio of 27.39 dB was obtained by adopting the proposed scheme.

Ultra-flat optical frequency comb generation based on a polarization modulator and a Butterworth band-stop filter.

A new, to the best of our knowledge, scheme for the generation of ultra-flat optical frequency combs (OFCs) is proposed, which mainly uses a polarization modulator (PolM) and a first-order Butterworth band-stop filter. The PolM is driven by a digital periodic square-wave signal, and the output of the PolM is an uneven OFC. The function of the Butterworth band-stop filter is to obtain flat and stable OFCs by changing its bandwidth. The comb spacing of the OFCs can be adjusted by changing the period of the square-wave signal. The theoretical model for the scheme is also established. The simulation results show that the generator constructed according to the proposed scheme can produce 64, 128, and 256 comb lines with 200 MHz comb spacing and with spectral flatness lower than 0.3 dB. The comb spacing can be further increased by using the square-wave signal at GHz rates, and the generated OFCs exhibit good frequency tunability.

Ultra-flat optical frequency comb generation based on amplitude modulator and Gaussian band-stop filter

A new scheme to generate ultra-flat optical frequency comb (OFC) with tunable frequency spacing is proposed and demonstrated based on an amplitude modulator driven by a digital periodic square-wave signal. A Gaussian band-stop filter is used to adjust flatness of the comb lines that generated from the amplitude modulator. The frequency spacing is adjusted by changing the period of the square-wave signal. The theoretical analysis model of the scheme is established and the relationship between the modulation index of the amplitude modulator and the flatness and power of OFC is studied. Through experiment simulations, it is shown that high-quality OFCs with the number of 32, 64, 128 comb lines, the tunable frequency spacing, and the flatness of 0.3 dB can be achieved.

Ultra-flat optical frequency comb generation based on phase modulation with simple digital driving signal

A new scheme to generate flat and stable optical frequency combs (OFCs) with phase modulator and Gaussian band-stop filter is proposed. A periodic square-wave signal controlled with a bit sequence is used to drive the phase modulator to get an uneven OFC, and then a Gaussian band-stop filter is used to obtain an ultra-flat OFC. The comb spacing and flatness of the OFC can be adjusted by changing the period of the square-wave signal and the bandwidth of the Gaussian band-stop filter, respectively. The theoretical model of the proposed scheme is established, and the simulation results show that high quality OFCs with flatness lower than 0.5 dB and hundreds of spectral lines can be generated.

Power efficient ultraflat optical frequency comb generation by cascading modulators

A power efficient optical frequency comb (OFC) is highly desired for applications with limited power supply, e.g., microwave photonics applications in moving vehicles. An ultraflat OFC generation is demonstrated with 0.19-dB flatness, 15 comb lines, and 18.3-dBm total RF power consumption. The scheme is based on spectral convolution. A five-line comb with 0.06-dB flatness and 4.0-GHz spacing is first generated in an intensity modulator. Then the signal is injected to a phase-modulator driven by ×5 frequency of 20.0 GHz to obtain the ultraflat 15-line comb with 0.19-dB flatness. The comb spacing is adjustable by controlling the input frequency of microwave source within the bandwidth of phase-modulation. Our work demonstrates a power efficient approach to obtain ultraflat comb and may benefit microwave photonics applications requiring low-power consumption.

Tunable ultraflat optical frequency comb generator based on optoelectronic oscillator using dual-parallel Mach–Zehnder modulator

A tunable ultraflat optical frequency comb generator based on the optoelectronic oscillator (OEO) using a dual-parallel Mach–Zehnder modulator (DPMZM) is proposed and experimentally demonstrated. By incorporating a tunable DPMZM-OEO, five comb lines were generated with frequency spacing from 5 to 12 GHz under a wide central wavelength tuning from 1530 to 1560 nm, and a comb flatness of 0.3 dB is obtained. The corresponding signal generated by the DPMZM-OEO is also measured, and the phase noise of the frequency tunable signals is as low as −125 dBc/Hz at 10-kHz frequency offset.

Optical frequency comb generation scheme using a novel Mach–Zehnder modulator with four arms

We propose a simple and cost-effective ultra-flat optical frequency comb generator based on a novel Mach–Zehnder modulator which employs a 1×4 multimode interference coupler and four optical phase-modulator waveguides. A 25-tone optical frequency comb with flatness within 0.2 dB and an unwanted-mode suppression ratio of 27 dB is obtained. This approach may provide a new generation of combs that enable planar integration and may permit a direct link from the radio frequency to optical domain on a chip.

A flexible and ultra-flat optical frequency comb generator using a parallel Mach–Zehnder modulator with a single DC bias

A flat optical frequency comb generator utilizing a single dual parallel Mach–Zehnder modulator (DPMZM) is detailed investigated both in theory and experiment. In the scheme, the two sub-modulators of DPMZM are biased at the maximum and minimum transmission points respectively. By adjusting the RF signals power, two optical frequency combs with even harmonics or odd harmonics can be generated. So at the end of DPMZM, 5 and 7-line flat OFCs with a side-comb suppression ratio (SCSR) higher than 18dB are theoretically and experimentally generated. The optical frequency comb generator scheme is relatively simple and valuable due to only one bias voltage may be controlled.

Tunable and ultraflat optical frequency comb generator based on cascaded intensity modulators

An optical frequency comb generator based on two-cascade intensity modulators is proposed and experimentally demonstrated. By carefully adjusting direct current biases and drive amplitudes of radio frequency signals of the two intensity modulators, the combs consisting of 3, 5, 9, 15, or 25 lines with the relative amplitude flatness within 1 dB can be generated. The scheme is relatively simple and adjustable, where the frequency spacing varies with microwave frequency applied on modulators.

Frequency chirp linearization for ultraflat optical frequency comb generation based on group velocity dispersion

We propose to insert group velocity dispersion between cascaded phase and amplitude modulation for ultraflat optical frequency comb (OFC) generation. With the dispersion, the sinusoidally varied chirp of the continuous wave light induced by phase modulation becomes linear within a relatively wide time interval. This is useful to improve the flatness of the generated OFC by directly cascaded phase and amplitude modulation. Simulation shows a flat comb of 37 tones within 0.88dB power variation when the modulation index of the phase modulation reaches 20. An ultraflat comb generator with 10GHz frequency spacing is also demonstrated. The flatness of the 15 tones around the center wavelength has been improved to 0.98dB.

Widely wavelength-tunable ultra-flat frequency comb generation using conventional dual-drive Mach-Zehnder modulator

Wavelength-tunable ultra-flat optical frequency comb generation is demonstrated using only a conventional Mach-Zehnder modulator that is asymmetrically dual-driven by large-amplitude sinusoidal signals with different amplitudes. A 10 GHz-spaced frequency comb with a 10 dB bandwidth of 230 GHz was experimentally generated. In addition, 50 nm widely wavelength tunable operation was achieved.