Microwave Photonic PS-Pulse and 140 GHz RF Comb Generator

A microwave photonic RF comb generator is presented by combining an fs-pulse laser and a high-speed broadband photodetector module. The subsystem generates pulses with a FWHM of 5.8 ps and a flat RF comb up to 140 GHz. Furthermore, the capability of pulse distribution over fiber is investigated. The dispersion of the single mode fiber is managed by using an optical bandpass filter, because reducing the optical spectral width limits the pulse broadening. With this scheme, the electrical pulse shape remains constant up to 500 m of fiber transmission. For a demonstration with 1500 m of fiber, the optical bandpass filter dispersion management results in a tradeoff between minimum electrical pulse width and fiber length.


I. INTRODUCTION
M ICROWAVE photonics is a promising field with the ability to overcome various drawbacks of purely electronic microwave generation systems [1], [2]. Wide bandwidth, low phase noise, highly stable microwave signal generation, and low loss optical distribution are major advantages compared to purely electronic microwave generation [1], [2], [3]. Therefore, microwave photonics is a key enabler for research areas as next-generation RF sources, 6G communication systems with sub-THz frequencies in the F-band [4], [5], radio astronomy, and broadband microwave RADARs with high resolution [3], [6]. Additionally, the use of low-loss fibers for remote signal transport is another advantage of microwave photonics, e.g., applied in large aperture RADARs or fiber-based time synchronization [6].  Phase locking the input wavelengths, which generate the optical beat signal, improves the purity of a microwave photonics generated RF signal and reduces its phase noise [7]. A promising candidate for phase-locked optical wavelengths is an optical frequency comb (OFC) source [1], [2], [7]. In [8] a free-running mode-locked laser in combination with a modified uni-travelling carrier photodiode was used to generate a low phase noise 8 GHz signal in the X-band. In [9] two photonic integrated distributed feedback lasers (DFBs) were injection-locked to an OFC, to generate THz signals at 408 GHz for wireless transmission. By stabilizing the DFBs with the OFC, the linewidth and noise of the system were reduced.
In this work, a microwave photonic RF comb and ps-pulse generator is demonstrated. The generated RF comb lines range  from DC up to 140 GHz with 1 GHz spacing and the short pulses have 5.8 ps FWHM. The subsystem is investigated in terms of broadband RF comb generation and pulse distribution over fiber.

II. IMPLEMENTATION OF RF COMB GENERATOR
The microwave photonic RF comb generator, shown in Fig. 1, consists of two major components: a turnkey fs-pulse laser module [7] and a broadband photodetector module [10]. The fs-pulse laser has a 1 GHz repetition rate. Its ultra-low freerunning phase noise, measured with a phase noise analyzer and a photodetector, at 10 GHz carrier is below −60 dBc/Hz at a 100 Hz frequency offset and around −130 dBc/Hz at a 10 kHz frequency offset, while reaching below −150 dBc/Hz above 1 MHz frequency offset (Fig. 2). Fig. 3 left, shows the optical spectrum of the pulse of the fs-laser. The 3 dB optical bandwidth of the generated optical frequency comb is 12.9 nm. The corresponding autocorrelation trace is shown in Fig. 3 right. The optical pulse width (FWHM) is 508 fs. Hence, the laser will not limit the bandwidth of the photodetector module.
The second component is a broadband photodetector module with an estimated 3 dB-bandwidth of 145 GHz and a responsivity of 0.4 A/W at 1550 nm. It includes a 0.8 mm-RF connector as an RF interface for broadband operation. Fig. 4 left, shows the measured RF transmission response of the photodetector module up to 110 GHz and the simulated response up to 145 GHz. The transmission response was obtained by a simulation model, fitted to the measured transmission up to 110 GHz and the RF reflection s 22 up to 145 GHz [10]. Fig. 4 right, shows the measured and simulated RF reflection up to 145 GHz.
By combining these components into an optical subsystem, an electrical frequency comb with spectral line spacing equal to the repetition rate of the laser source is generated, ranging up to the photodetector cut-off frequency [1]. We use a variable optical attenuator (VOA) to control the optical power injected into the photodetector without changing the operation point of the fs-pulse laser. In addition, for maximum RF output power, a polarization controller aligns the polarization orientation into the photodetector to TE.

III. SUB-THZ FREQUENCY COMB AND PULSE GENERATION
Our photonic RF comb generator is characterized with various measurement techues to evaluate experimentally the potential performance for microwave applications. With a 100 GHz oscilloscope, the generated electrical pulses are recorded.   shows the time-domain measurement result of the pulses, measured for different average photocurrents of the photodetector, being controlled by the VOA. At higher average photocurrents, the pulse peak power deviates from a linear regression. This can be explained by photodetector saturation effects, leading to pulse broadening. At lower optical powers, the pulse width is approx. 5.8 ps, mainly limited by the bandwidth of oscilloscope. Fig. 6 shows the RF comb spectra by using a 110 GHz electrical spectrum analyzer (ESA). The measurement applies the same optical input power settings at the photodetector module as for the time-domain measurement. As a reference, the whole spectrum is shown for lowest average bias current setting of 10 μA. The resolution bandwidth is set to 20 kHz, and the video bandwidth is set to 20 Hz. One observation is the sharp drop at 50 GHz, concurrently with the drop in the noise floor of the ESA. This is a measurement artefact due to internal ESA frequency range switching. To proof this assumption, the optical frequency comb is measured with a different ESA with a lower electrical bandwidth (Fig. 7). As expected, the RF comb  spectrum is flat over the entire spectrum. For higher optical powers, the same effect as in the time-domain is visible: The photodetector starts to saturate, resulting in a power decrease for larger frequency components. Due to the pulsed input signal, the frequency response differs from Fig. 4 and corresponds to the previous discussed pulse behaviour.
To examine the behaviour of the photonic RF comb generator for even higher frequencies, a 145 GHz vector network analyzer (VNA) was applied. Because of the narrow linewidth of the generated frequencies and the frequency down conversion processes within the VNA, the measurement could not be extended over a spectral range larger than 500 MHz. Therefore, we exemplarily measured the spectral line at 140 GHz (Fig. 8), underlining the ability to generate an RF comb from 1 GHz to 140 GHz. Compared to the previous result, the intermediate frequency bandwidth (IFBW) was reduced to 10 Hz [11]. The free-running measured FWHM bandwidth of the generated RF line is below 130 Hz, which is within the accuracy of the measurement system.

IV. RF COMB AND PULSE DISTRIBUTION OVER FIBER
Another application is the pulse distribution over fiber. Therefore, the subsystem is further analyzed for different fiber lengths. One major challenge is the pulse broadening by dispersion. One solution for this problem would be the addition of dispersioncompensating fibers with the correct length into the optical path. An alternative approach is to limit the optical spectral width at the fiber input, also limiting the pulse broadening in the fiber [12, p. 65].
The output pulse width of an unchirped Gaussian pulse after transmission through a fiber with the length L can be modelled by the following equation [12, p. 61ff]: With a group velocity dispersion parameter β 2 of −20 ps 2 /km and two different fiber lengths of 500 m and 1500 m, the dependence of the output pulse width on the input pulse width is shown in Fig. 9. Two major observations can be made: First, the minimum achievable output pulse width depends on the fiber length (tradeoff between total dispersion and electrical bandwidth). Second, the input pulse width has to be adjusted to optimize the output pulse width. Since the width of an optical pulse depends on its spectral width, a tunable optical bandpass filter (OBPF) was added in front of the fiber for dispersion management by optimizing the spectral width. Fig. 10 shows the implementation for the pulse distribution over fiber experiments. As reference, Fig. 11 shows the unfiltered spectrum and the spectrum after filtering with a bandpass filter setting of 1.35 nm. Due to filtering, the total output power is decreased. The pulse distribution over fiber experiments were carried out for 500 m and 1500 m of fiber. Fig. 12 left, shows the pulse width for different OBPF settings after transmission through 500 m of fiber at a photocurrent of 150 μA. As expected, the pulse is blurred for larger optical spectral widths. The minimum pulse width at the end of the fiber can be achieved for 1.35 nm spectral width (equal to an electrical bandwidth of 168 GHz). The pulse width is the same as without fiber and OBPF. Thus, with the OBPF scheme, the pulse broadening by dispersion can be neutralized. For optical spectral width below this value, the pulse tends to degrade again, because the bandwidth of the generated OFC is reduced.
The time-domain results for a fiber length of 1500 m, shown in Fig. 12 right, have similarities with the result for 500 m of fiber. Nevertheless, two major differences are observable: The optimum output pulse width (FWHM 8.2 ps) is reached for 0.68 nm spectral width (equal to 84 GHz electrical bandwidth) of the OBPF. Therefore, the tradeoff expected from the theoretical considerations is experimentally approved.
The envelopes of the generated RF combs were measured with a lower RBW of 2 kHz. The results, shown for 500 m of fiber and for 1500 m in Fig. 13, show the expected behavior. For larger optical spectral widths, the envelopes have reduced optical power in the higher frequency range, partially below the noise floor of the ESA. Additionally, a sinc-function shape with decreased oscillation frequency for decreasing optical spectral widths is obtained. This can be explained as follows: When down-converting the optical comb in the photodetector, electrical comb lines are generated from combinations of all optical sub-combs. The dispersion changes the phase of each optical sub-comb, changing the relative phase between the generated microwave signals from different spectral locations [12, p. 59ff]. As a result, constructive and destructive interference for the generated RF signals at specific frequencies are obtained, resulting in an additional beating of the spectrum.
The flattest spectrum without a sinc-shape is achieved for 500 m of fiber at an optical spectral width of 1.35 nm (matching  the time-domain results). For 1500 m of fiber the flattest spectrum is obtained for an optical spectral width of 0.68 nm. As expected, this spectral width limits the electrical bandwidth of the photodetector, clarified by the similar shape of the electrical spectrum for transmission through 500 m and 1500 m of optical fiber. The limited bandwidth of the remaining spectrum leads to power decrease in the higher frequency range.

V. CONCLUSION AND OUTLOOK
We have demonstrated a photonic RF comb generator with 1 GHz comb line spacing up to 140 GHz. In the time-domain, the photonic RF comb generator generates electrical pulses with minimum FWHM of 5.8 ps and a repetition rate of 1 GHz. In the frequency-domain, the generated RF comb shows a flat response over the entire spectrum up to 110 GHz. The RF comb generator provides the capability for stable RF oscillator applications, when extracting single spectral carriers as exemplary demonstrated at a frequency of 140 GHz. Furthermore, the stable pulse distribution over fiber is investigated. Transmission of the pulse through 500 m of optical fiber is demonstrated without pulse broadening by using an off-the-shelf OBPF for dispersion management (FWHM pulse width 5.8 ps). For a longer fiber length of 1500 m, the pulse waveform is broadened (8.2 ps), because a tradeoff between fiber length and bandwidth is found. This matches the theoretical considerations.
The subsystem can be expanded with either electrical or optical components for sub-THz applications, like microwave photonic RADARs. To advance the implementation further, the OBPF can be replaced with other options, e.g., by using a pulse laser with narrower optical spectrum, improving the power efficiency of the subsystem. Alternatively, adding an arrayed waveguide grating (AWG) [13] chip into the subsystem, enables parallel synchronized sources with reduced optical spectrum.