Flexible Ultra-Wide Electro-Optic Frequency Combs for a High-Capacity Tunable 5G+ Millimeter-Wave Frequency Synthesizer

Abstract

This paper presents a new scheme of a cost-effective tunable millimeter-wave (MMW) frequency synthesizer based on an ultra-wideband electro-optic frequency comb. The architecture for the quasi-tunable millimeter-wave frequency synthesizer mainly consists of a compact ultra-wide flat electro-optic frequency comb and a multi-tone frequency generator, which only includes a quantum dot mode-locked laser, a LiNbO${}_3$ dual-driving Mach–Zehnder modulator (DD-MZM) and Uni-traveling-carrier photodiode (UTC-PD). MMW signals generated with a quasi-tunable frequency are experimentally demonstrated. The difference in power is obtained for the different frequencies. The linewidth of the quasi-tunable frequency signals is less than 273 Hz. In addition, the single side band (SSB) phase noise of the 25, 37.5, 50 and 75 GHz is measured as −115, −106, −102 and −95 dBc/Hz at an offset of 1 kHz, respectively. The proposed frequency synthesizer has ultra-low phase noise, quasi-tunable frequency and simple structure. The research results of the frequency synthesizer are applied for 5G+ transmission with radio wave working at K-band and V-band. The flexible, compact and robust MMW frequency synthesizer is suitable for the future of ultra-high capacity 5G+ communication.

1. Introduction

The increased bandwidth demand for high capacity and ultra-low latency services of fifth generation mobile networks (5G) has promoted the exploration of high-capacity radio-over-fiber (ROF) system to accommodate a wide range of novel applications with diverse requirements, which will be prospective for 5G/5G+ wireless access [1], internet of things, disaster emergency communication and military applications. ROF possesses wide bandwidths provided by the millimeter-wave (MMW) frequency resource [2] and integrates the advantages of wireless and optical systems. MMW frequencies can provide orders of magnitude greater bandwidths between 30 and 300 GHz for ROF [3] for 5G/5G+ applications covering the frequency range of the K-band (18 to 26.5 GHz) and the V-band (50 to 75 GHz).

ROF systems are generally centralized radio access networks for 5G using the wavelength-division multiplexing (WDM) technique to offer ultra-wideband wireless delivery with low interference by combining data transmitted on different wavelengths based on many individual lasers, which are not intrinsically phase-locked. Benefiting from the superiority of stable frequency spacing and wideband phase coherence of the optical comb lines and optical frequency comb (OFC) has emerged as a potential substitute for hundreds of individual lasers in WDM systems [4], due to the reduced energy consumption and size of the 5G/5G+ transmitter. The generation of various frequencies within the comb lines also allows for high-fidelity frequency transfer to different frequency domains, such as the microwave, MMW and terahertz domains [5].

The latest advances in the MMW generation based on OFC mainly include micro resonator-based Kerr frequency combs [6], high nonlinearity fiber combs [7], electro-optic combs (EO-combs) [8] and mode-locked laser combs [9]. Due to their simple optical architecture and offering wide band and flexible tuning of the spectrum interval [10], electro-optic frequency combs based on external modulation have been revisited in recent years primarily in the context of arbitrary optical waveform generation, high bit-rate optical communication and MMW photonics synthesizers [11].

These generators based on electro-optic modulators (EOMs) have some advantages, including their frequency interval and tunable central-frequency, which are important for applications in telecommunication and spectroscopy [12]. In [13], a novel method to generate a MMW signal was proposed, which employed a uniform fiber Bragg grating based acousto-optic tunable filter (FBG-AOTF) and a DD-MZM with frequency multiplication factors of 2 and 4, paying for a large power penalty. To achieve higher frequency multiplication factors, external modulation techniques employ cascaded electro-optic modulator (EOM) to produce more harmonically related optical modes. In [14], a frequency 16-tupling technique was demonstrated using four cascaded MZMs.

With the proper adjustment of the DC bias, the amplitude and phases of the RF local oscillators, an MMW signal with a frequency of 16 times the input RF signals is demonstrated. Unfortunately this method is limited to big RF power and a big power penalty. According to the intrinsic photoelectric characteristics of the EOM, over-driving of conventional electro-optic modulators is provided for EO-comb generation [15].

However, optical comb lines suffer from inherent phase instabilities and big power deviations, leading to excess multiplication of MMW phase noise, which limits their usage in 5G+ transmission [16]. A range of EO-combs can be explored to generate directly or through upconversion of MMW signals [17] but suffer from high phase noise and are not as spectrally pure as photonic based sources [18,19].

Recently, MMW signal generation using a quantum dot (QD) laser has been greatly promoted, mainly due to the superior characteristics of nanostructure lasers constructed with progressive QD active regions [20,21,22,23,24]. Repetition rates of 29.8 and 55.6 GHz with pulse duration of 855 and 707 fs were obtained, respectively [21]. By regulating the gain current and the reversed-biased voltage, the generated signals with adjustable frequency and the minimum pulse width of 3.7 ps have been demonstrated, with an adjustable frequency range of 300 MHz [20].

A phase noise as low as 105 dBc/Hz at an offset frequency of 100 kHz was achieved by using a quantum dash mode-locked laser. In [25], they demonstrated intracavity MMW generation within THz quantum cascade lasers (QCLs) over the unprecedented range of 25 to 500 GHz. This will open up the possibility of compact, low noise MMW generation using mode-locked THz frequency combs. In [5], they launched a coherent comb with 8.9 GHz spacing with a span of almost 200 lines in the C-Band (1530 to 1565 nm), with only 20 dBm of microwave modulation power. Extremely spectrally pure microwaves at 32 GHz achieved a phase noise < −120 dBc/Hz, although the carrier frequency is limited to harmonics of the repetition frequency of the comb [26].

In [27], they proposed a scheme of MMW signal generation with a 20 nm bandwidth using a two section InAs/InP Quantum dot mode-locked laser (QDMLL), with an optical linewidth of less than 101 kHz. However, there are inadequate reports proving the MMW signal generated to have excellent low phase noise properties by these methods [16,28,29,30,31].

In this paper, we propose and experimentally demonstrate a novel tunable multi-tone MMW photonic generation based on ultra-wide electro-optic frequency combs for high-capacity 5G+ signal transmission. The architecture of the scheme mainly uses a flexible ultra-wide broadband electro-optic frequency comb generator, UTC-PD and tunable frequency filter. In particular, an electro-optic frequency comb is generated by only using a passive QDMLL and a DD-MZM, but not many optical devices, such as fiber Bragg grating (FBG), semiconductor optical amplifier (SOA) and erbium doped optical fiber amplifier (EDFA). Mode locking enhances the broadband spectrum, spectrum flatness, equal frequency spacing and low phase noise.

Compared to the reported MMW generators mentioned above [18,19,20], the proposed method is very flexible and compact, as it does not use polarization controllers, optical filters, SOA and phase-locked loop, which greatly improves the efficiency and robustness of the system.

2. Materials and Methods

2.1. EO-Combs with a DD-MZM

To generate wideband EO-combs with a DD-MZM, we apply an asymmetric drive condition. The electro-optic transfer function [16] of an DD-MZM is represented as a function of an applied modulation voltage $V\left(t\right)$, which is given by

$$T_{MZM}=\frac{P_{out}\left(t\right)}{P_{in}\left(t\right)}=\frac{IL}{2}\left[1+cos(\pi \frac{V\left(t\right)}{V_{\pi }}+{\varphi }_0)\right]$$

(1)

where $V_{\pi }$ is the voltage applied to generate a phase drift of $\pi$ between both arms as the half-wave voltage [28]. In practice, $V_{\pi }$ is diverse between DC and RF. $IL$ is the insert loss. The phase drift resulting from an internal path length differences between the two arms of the MZM is expressed as ${\varphi }_0$. Usually, as the other modulators have independent electrodes for RF and DC, the modulation voltage and the constant bias voltage $V_{bias}$ provided at the DC electrode are independent of each other. $V_{RF}\left(t\right)$ provided at the RF electrode is the time-varying modulation voltage. If the $V_{RF}\left(t\right)$ is supposed as a sinusoidal voltage, the modulation voltage $V\left(t\right)$ can be represented as

$$V\left(t\right)=V_{bias}+V_{RF}\left(t\right)=V_{bias}+V_{e}cos\left[{\omega }_{e}t+{\varphi }_e\left(t\right)\right]$$

(2)

where ${\varphi }_e\left(t\right)$ expresses a stochastic process that shows the phase fluctuations interpreted as a parasitic phase modulation with the RF signal. ${\omega }_e$ and V are, respectively, the angular frequency $({\omega }_e=2\pi {\varphi }_e)$ and the voltage amplitude of the RF driving signal.hus, the electro-optic transfer function [29] of the DD-MZM can be extended to

$$T_{MZM}=\frac{IL}{2}\left[1+cos\left(\pi \frac{V_{RF}\left(t\right)}{V_{\pi ,RF}}+\pi \frac{V_{bias}}{V_{\pi ,DC}}\right)+{\varphi }_0\right]$$

(3)

Supposing the optical carrier is linearly polarized and aligned to the electro-optic modulator’s polarization state axis, the electric field expression of input optical wave $E_{in}\left(t\right)$ is in scalar type as

$$E_{in}\left(t\right)=E_0\ast cos[{\omega }_{0}t+{\varphi }_0\left(t\right)]$$

(4)

where $\varphi$ is the phase of the light. If $\varphi =0$, the output power is the largest; and if $\varphi =\frac{\pi }{2}$, the light fields in the two branches are eliminated by interfering with each other. The output power is the smallest, which is zero in the ideal case. Now, assuming the input optical carrier is supported to the input port of the DD-MZM practically, the mathematical expression of the output optical signal modulated is given by

$$\begin{array}{cc}\hfill E_{out}\left(t\right)& =E_0\ast cos[{\omega }_{t}t+{\varphi }_0\left(t\right)]\hfill \\ \hfill & cos\left\{\frac{{\varphi }_{DC}}{2}+\frac{\pi }{2V_{\pi ,RF}}{V}_{e}cos[{\omega }_{e}t+{\varphi }_e\left(t\right)]\right\}\hfill \end{array}$$

(5)

where

$${\varphi }_{DC}=\pi \frac{V_{bias}}{V_{\pi ,DC}}+{\varphi }_0$$

(6)

Clearly, diverse spectral comb lines will be generated due to the nonlinearity of the electro-optic converse function, which determined on the static phase offset ${\varphi }_{DC}$ caused by the bias voltage and on the magnitude $V_e$ of the signal modulated.In order to well comprehend the modulated spectrum, the Bessel functions is used in mathematical Equation (5), which is given by

$$\begin{array}{cc}\hfill & E_{out}\left(t\right)=E_{0}cos\left(\frac{{\varphi }_{DC}}{2}\right)J_0\left(m\right)cos[{\omega }_{0}t+{\varphi }_0\left(t\right)]\hfill \\ \hfill & +E_{0}cos\left(\frac{{\varphi }_{DC}}{2}\right)\{\sum _{n=1}^{\infty }{J}_{2n}\left(m\right)\hfill \\ \hfill & \left[\begin{array}{c}cos[{\omega }_{0}t+{\varphi }_0\left(t\right)-2n({\omega }_{e}t+{\varphi }_e\left(t\right))+n\pi ]\\ +cos[{\omega }_{0}t+{\varphi }_0\left(t\right)+2n({\omega }_{e}t+{\varphi }_e\left(t\right))-n\pi ]\end{array}\right]\}\hfill \\ \hfill & +E_{0}sin\left(\frac{{\varphi }_{DC}}{2}\right)\{\sum _{n=1}^{\infty }{J}_{2n-1}\left(m\right)\hfill \\ \hfill & \left[\begin{array}{c}cos[{\omega }_{0}t+{\varphi }_0\left(t\right)-(2n-1)({\omega }_{e}t+{\varphi }_e\left(t\right))+n\pi -\frac{\pi }{2}]\\ +cos[{\omega }_{0}t+{\varphi }_0\left(t\right)+(2n-1)({\omega }_{e}t+{\varphi }_e\left(t\right))-n\pi +\frac{\pi }{2}]\end{array}\right]\}\hfill \end{array}$$

(7)

According to Equation (6) [28], the two RF signals with reversal phase drive the DD-MZM, i.e., $|V_1\left(t\right)|=|-V_2\left(t\right)|=V$; thus, $\Delta \varphi =2\pi V/V_{\pi }$, and a flat OFC can be generated.

2.2. Schematic Architecture

The schematic architecture of the tunable multi-tone 5G+ MMW frequency synthesizer based on the EO-combs is shown in Figure 1. It consists of QDMLL, a commercial LiNbO${}_3$ DD-MZM with the $V_{\pi }$ of $5.8$ V, an RF signal generator, an UTC-PD with a beating broadband of 110 GHz, a power divider and tunable frequency filter. The emission spectrum directly from the QDMLL is in the C-band wavelength range from 1520 nm to 1558 nm, using a GaAs/InP Fabry–Pérot laser cavity.

The output spectrum of the GaAs/InP QDMLL has many attractive useful characteristics [29], such as ultra-low relative intensity noise (RIN) (less than −160 dB/Hz), small optical linewidth (less than 158 KHz) and a large optical signal-to-noise ratio (OSNR) (up to 72 dB), etc. In the proposed architecture of tunable multi-tone MMW generators, the output spectrum of the QDMLL is modulated by the DD-MZM, which is driven by two opposite-phase sinusoidal wave RF signals.

The bias voltage is used to set the operating point of the DD-MZM, important for improving the flatness of the optical comb lines generated. Compared to the reported MMW generators, this scheme does not use polarization controllers, optical phase lock loops and optical filters. As a result, it is considered to be more compact and flexible, which makes it simple to generate tunable and low-phase noise MMW signals [32].

3. Results

3.1. Flexibility

The performance of the tunable MMW frequency synthesizer is researched by using high performance test instruments, such as complex optical spectrum analyzer (OSA), tunable RF filter, a 110 GHz UTC-PD and an electrical spectrum analyzer (ESA).The ultra-wide broadband electro-optic frequency combs are experimentally generated from a QDMLL modulated by a DD-MZM, shown in Figure 2. In experiments, a GaAs/InP QD Fabry–Pérot QDMLL is used to emit optical beams with a 75 GHz frequency spacing in the C-band.

The QD laser is mounted stably with the temperature of 18 ${}^{°}$C, with a threshold current of around 25 mA. The average output power of the QD laser is about 15 dBm, with the injection current fixed at 180 mA. A 11 nm bandwidth OFCs with 84, 54 and 28 comb lines at frequency spacing of 25, 37.5 and 75 GHz and 84 optical comb lines are experimentally created, respectively, shown in Figure 2a–c. A tailored OFC with frequency spacing of 25 GHz is generated from the DD-MZM driven by a sinusoidal RF signal (the total RF power is about 20 dBm, which is much lower than that in [12]).

By adjusting the bias voltage of the DD-MZM, the maximum power shift of 3.2 dB over 84 lines is created with an equal frequency spacing of 25 GHz for the wavelength range from 1536 to 1547 nm. Similarly, the maximum power shift of 3.8 and 5.2 dB over 58 and 28 modes are created with an equal frequency spacing of 37.5 and 75 GHz, respectively. Unlike other electro-optic frequency comb methods, such as methods proposed in [9,11], the ultra-wide broadband OFC generated by this scheme has greater flexibility.

The QD mode-locked laser used is passively mode locked laser, which has a relative intensity noise (RIN) of about 150–160 dB/Hz, which is much better than commercial QW DFB lasers. Compared with the reported MMW Optical Frequency synthesizer, this scheme has not used many optical devices, such as fiber Bragg grating filters, polarization controllers and optical phase lock loops. Therefore, this system structure is very simple and small sized, promoting the system robustness and stability.

3.2. Flatness

We measured and comparatively analyzed different frequency spacings of OFCs, as shown in Figure 3. The OSNR test was performed in the optical resolution bandwidth of 100 MHz. Figure 2, the number of flat comb lines is related to the maximum power shift with different frequency spacing. It is shown that the flatness of the 25 GHz OFC is better than the 37.5 and 75 GHz OFC by adjusting the modulation depth applied on the DD-MZM. The optical signal to noise ratio (OSNR) of the optical modes is also investigated, and the OSNR of the frequency spacings at 25, 37.5 and 75 GHz within the 10 dB bandwidth are shown in Figure 3. The results show that the OSNR at 25 and 37.5 GHz frequency spacing is significantly lower than that at 75 GHz frequency spacing.

Apparently, the comb lines generated are not flat in broadband, which is partially due to the characteristics of the DD-MZM. Restricted by the characteristics of commercial devices, the power deviation of the comb lines cannot be eliminated.

3.3. Stability

Tunable MMW signals are generated from ultra-wide broadband electro-optic frequency combs with different frequency spacing. The tunable MMW signal generation is based on the RF carrier created from two adjacent lines of the different OFCs. The K-band and V-band MMW signals generated based on EO-combs at different frequency intervals were measured and are shown in Figure 4a,b, respectively. The spectrum properties of the MMW signals generated were tested by the ESA with a 200 Hz resolution bandwidth (RBW).

In Figure 4, the powers of the frequency at 25, 37.5, 50 and 75 GHz are about −13.5, −9.5, −22 and −33 dBm, respectively. The power of MMW signal generated from 37.5 OFC is larger than that from 25 GHz OFC. The experiment shows linewidths of 250 Hz, 259 Hz, 265 Hz and 273 Hz is for the 25, 37.5, 50 and 75 GHz, respectively. The results prove that the OFC generated by the EOM and QD laser had excellent properties, such as great stability and ultra-low phase noise. These measurements show that the OFC generated by the QD laser has extremely low phase noise and high stability [16].

In addition, the phase noise is measured by a UTC-PD. We measure the single side-band (SSB) phase noise of the MMW generated by 25 and 37.5 GHz OFC as shown in Figure 5. The SSB phase noises of the 25, 37.5, 50 and 75 GHz are −115 dBc/Hz, −106 dBc/Hz, −102 dBc/Hz and −95 dBc/Hz at an offset of 1 kHz, respectively. It is clear that the SSB phase noise of the 25 GHz was the lowest compared to the other frequencies before the offset of 1 kHz, while the SSB phase noise of 75 GHz was the highest. The low phase noise of the MMW signals is due to the low power deviation and the low phase noise of the quantum dot laser. Beyond the offset of 100 kHz, the phase noise approaches a noise floor without any fluctuating peaks [20]. The experimental results show that the MMW frequency synthesizer based on this structure has high stability.

4. Conclusions

The OSNR was obtained with an optical resolution bandwidth of 100 MHz. We proposed a simple and flexible design scheme of a high capacity tunable 5G+ MMW synthesizer based on ultra-wideband electro-optical OFCs. The architecture is compact and integrated and mainly consists of a monolithic Fabry–Pérot QD laser and a conventional MZM and UTC-PD. The QD laser demonstrated excellent properties of equal frequency spacing, which greatly reduced the system complexity and device noise.

In addition, the comb lines produced by QD lasers were very flexible and tunable and were modulated by a DD-MZM. The EOM-OFCs involved an 11 nm spectrum with a 10 dB bandwidth. The frequency interval OFCs of 75, 37.5 and 25 GHz had 28, 58 and 84 flat spectral lines, respectively. The OSNR of the comb lines was more than 37 dB. The SSB phase noise of the MMW signals generated was −106 dBc/Hz and −115 dBc/Hz at 1 kHz offset for 37.5 GHz and 25 GHz. The results show that the linewidth of the MMW frequency synthesized based on the combs was less than 273 Hz.

The MMW signal was generated by the optical comb lines beating each other in the UTC-PD. The MMW signal frequency depended on the frequency interval of the comb lines, and the power was related to the power of the beating lines. Thus, the difference of power was obtained for the different frequencies, related to the beating frequency generation mechanism.

In this work, a flexible MMW frequency synthesizer was demonstrated. If the DD-MZM is adequately modulated by adequate power and appropriate bias voltage, tunable frequency synthesis from 7.5 to 75 GHz can be achieved by this scheme, involving from the K-band to V-band MMW. Experiments showed that the technology is simple in generating high-quality MMW and suitable for high-capacity 5G/5G+ communication transmission. Small size and ultra-wide broadband OFCs are extensively applied in 5G wireless access, satellite communications and intelligent wireless techniques.