Fourier domain mode locked optoelectronic oscillator based on the deamplification of stimulated Brillouin scattering

Tengfei Hao; Jian Tang; Wei Li; Ninghua Zhu; Ming Li

doi:10.1364/OSAC.1.000408

1. Introduction

Frequency scanning microwave signals with a large time-bandwidth product (TBWP) are of primary importance in modern radar and communication systems. For example, in a radar system, to achieve a high range resolution while maintaining a large detection distance, the generated microwave signals should have a large TBWP [1,2]. In a frequency-hopping communication system, frequency scanning microwave signals with a high chirp rate and large bandwidth are desired to enhance the anti-reconnaissance and anti-jamming capabilities [3]. Frequency scanning microwave signals can be generated directly by electronic schemes such as voltage controlled oscillators (VCOs) [4] and arbitrary waveform generators (AWGs) [5], however suffers from limited bandwidth or complicated structure.

Numerous photonics-assisted methods have been proposed for frequency scanning microwave signals generation, such as direct space-to-time pulse shaping [6,7], temporal pulse shaping (TPS) [8,9] to spectral-shaping and wavelength-to-time mapping [10–12], thanks to the inherent advantages offered by photonics [13,14]. Although these photonics-based techniques are capable of generating microwave signals with high frequency and broad bandwidth, the TBWP is usually constrained to the limited temporal width, which can hardly meet the requirements of real world applications.

On the other hand, the optoelectronic oscillator (OEO) has been widely demonstrated to generate single frequency microwave signals with an ultra-low phase noise that does not degrade with the increasing of frequency [15–23]. A long fiber delay line is usually used in an OEO cavity to achieve a high quality-factor (Q-factor), which would ensure a low phase noise. However, a long fiber delay line would make the OEO to have a long mode building time. The frequency tuning speed is extremely low for a frequency tunable OEO, because new oscillation modes needs to build up repeatedly from the optical and electrical gain spectra at every new frequency position. Thus, continuous fast frequency scanning microwave signals can hardly be generated directly from a traditional OEO cavity. Recently, we proposed a scheme to generate fast frequency scanning microwave signals based on Fourier domain mode locking (FDML) technique to break the limitation of mode building time in an OEO [24]. Tunable linearly chirped microwave waveforms (LCMW) with continuous amplitude and phase are generated using the proposed OEO. The key element is a fast frequency scanning microwave photonics filter (MPF) based on a phase-shifted fiber Bragg grating (PS-FBG). The major drawback of this scheme is that the 3-dB bandwidth of the PS-FBG-based MPF is not uniform over the tuning range [25]. Since the side mode noises of the generated microwave signals are directly related to the bandwidth of the MPF, the phase noises of the fast frequency scanning signals vary with frequency, which degrades the performance of the FDML OEO. Besides, the frequency tuning range of the PS-FBG-based FDML OEO is limited by the total reflection bandwidth of the PS-FBG. A MPF can also be implemented based on the narrow-band filtering by stimulated Brillouin scattering (SBS) in an optical fiber [26–28]. SBS is the interaction between incident light and phonons within a fiber, which can generate narrowband gain and loss spectral areas [27]. The SBS-based MPF exhibit unique properties such as narrow-bandwidth, wideband tunability, and 3-dB bandwidth nearly independent of frequency [28]. In addition, the SBS effect has a relative low power threshold in fiber, thus is easy to achieve.

In this paper, we propose a new method to realize a fast frequency scanning FDML OEO based on the deamplification of SBS to overcome the limitation of PS-FBG-based scheme. By synchronizing the scanning period of the MPF to the cavity round-trip time, an OEO can be made to operate in the FDML regime. Scanning bandwidth and center frequency tunable LCMW are generated in the experiment, with a chirp rate up to 0.85 GHz/μs and a TBWP as high as 23,850. The proposed frequency scanning FDML OEO can be used in modern radar and communication systems.

2. Principle of the proposed FDML OEO

The schematic diagram of the proposed SBS-based FDML OEO is shown in Fig. 1

Fig. 1 Schematic illustration of the proposed FDML OEO based on the deamplification of SBS. The optical band-pass filter (OBPF) is used to suppress the passband response induced by the gain spectral area of SBS. Optical spectra of the nodes (a, b, and c) are also plotted. $λ_{c 1}$ to $λ_{cn}$ are the wavelength of the TLS during one frequency sweep, where n is an integer. $λ_{p}$ is the wavelength of the pump laser. TLS, tunable laser source; ISO: optical isolator; HNLF, high nonlinear fiber; PC: polarization controller; PD, photodetector.

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. The light wave at

λ_{p}

from a continuous wave (CW) pump laser is injected into a high nonlinear fiber (HNLF) via an optical circulator (OC) to stimulate the SBS effect. The SBS-induced deamplification forms an optical notch filter. A tunable laser source (TLS) is employed as the signal laser. The wavelength

λ_{c}

of the signal laser is smaller than that of the pump laser. The light wave from the signal laser is injected into the HNLF after passing through a phase modulator (PM). The

+ 1^{s t}

order sideband of the phase modulated signal is attenuated by the loss spectrum of the optical notch filter, which breaks the amplitude balance of the phase modulated sidebands. The output light wave from the port-3 of the OC is filtered by a flat-top optical band-pass filter (OBPF) to suppress the passband response induced by the gain spectral area of SBS. The unbalanced phase modulated light wave from the OBPF is sent to a photodetector (PD) after amplified by an optical gain medium. The overall operation is equivalent to a high Q-factor MPF, implemented based on phase modulation and phase-modulation to intensity-modulation (PM-IM) conversion. Compared with the MPF based on the amplification of SBS, the amplified spontaneous emission noise is avoided in our scheme. However, the phase shift fluctuations induced by the deamplification of SBS is converted to the additive phase noise after photo-detection, thus the output phase noise is degraded by the SBS effect [28]. An electrical signal with a frequency equals to the frequency difference between the TLS and the loss spectrum of the SBS effect is detected at the output of the PD. The output signal of the PD is amplified by an electrical gain medium and then is coupled by a 3-dB power splitter. Part of the amplified electrical signal is fed back to the PM to form a closed loop.

The passband of the MPF in the OEO cavity is determined by the wavelength difference between the TLS and the loss spectrum of the SBS effect. As shown in Fig. 1., by sweeping the lasing wavelength of the TLS, which is driven by a periodic saw-tooth signal, a frequency scanning MPF is achieved. It should be mentioned that there is no requirement in terms of tuning speed for the OBPF, since it is only used for suppressing the passband response induced by the gain spectral area of SBS. The corresponding optical spectra are illustrated in the lower part of Fig. 1. As shown in Fig. 2

Fig. 2 Dynamic frequency window in the FDML OEO cavity. The passband of the MPF changes in time.

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., a major difference between a FDML OEO and a traditional OEO is that a dynamic frequency window function (frequency window which changes in time), rather than a constant one, is applied in the proposed FDML OEO cavity. In the meantime, the period of the driving signal

T_{filter drive}

of MPF is synchronized with the round-trip time

T_{round - trip}

of the propagating signal in the OEO cavity in order to achieve FDML operation, i.e.

T_{round - trip} = n \times T_{filter drive},

where n is an integer. This produces a quasi-stationary operation where signal from one frequency sweep propagates through the FDML OEO cavity will returns to the MPF at the exact time when the passband of the filter is at the same spectral position. All longitudinal modes of an entire sweep in the OEO cavity are active simultaneously. As a result, the FDML OEO generates a sequence of frequency scanning microwave signals at the cavity repetition rate or a harmonic.

3. Results and discussions

A proof-of-concept experiment is carried out based on the scheme shown in Fig. 1. The TLS is a distributed feed-back (DFB) laser with a linewidth of 150 kHz. Wavelength tuning of the TLS is achieved by changing the driving current. The lasing wavelength of a DFB laser, also known as Bragg wavelength $(λ_{B})$ , can be expressed as:

λ_{B} = \frac{2 n_{e f f} \cdot Λ}{m},

where

n_{e f f}

is the effective refractive index,

Λ

is the grating period,

m

is the order of Bragg diffraction. The variation of driving current creates changes on the refractive index

n_{e f f}

of the laser cavity, thus leads to wavelength change. A laser diode controller and an electrical signal generator are used to drive the TLS. A narrow linewidth laser with a linewidth smaller than 5 kHz and a fixed wavelength is served as the pump. The pump power is about 11 dBm. A polarization controller (PC) is used to control the polarization state of the pump light wave. The PM has a 3-dB bandwidth of 20 GHz and a half-wave voltage of 7 V. A light wave converter with a 3-dB bandwidth of 15 GHz and a conversion gain of 300 V/W is used as the PD. An erbium-doped fiber amplifier (EDFA) and a low noise amplifier (LNA) are used to support enough gain for the oscillation signal. A 1-km HNLF is used to generate SBS.

First of all, the frequency response of the SBS-based MPF is measured. As shown in Fig. 3(a)

Fig. 3 (a) Setup for frequency response measurement of the MPF. (b) Measured frequency responses with center frequency tuned from 3 GHz to 13 GHz by changing the wavelength of the TLS.

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, the OEO loop is opened at the output of the PD. A vector network analyzer (VNA) is used to measure the frequency response. Fig. 3(b) shows the superimposed frequency responses with center frequency tuned from 3 GHz to 13 GHz by changing the wavelength of the TLS. Then the OEO loop is closed. A 188.7 kHz saw-tooth driving current is applied to the TLS in order to sweeping the wavelength. As mentioned above, the passband of the MPF in the OEO cavity equals to the frequency difference of the TLS and the loss spectrum of the SBS effect. Frequency scanning of the passband of the MPF is achieved by sweeping the wavelength of the TLS, since the wavelength of the pump laser is constant. While two independent lasers are involved in the proposed OEO, it is inevitable that output phase noise will be deteriorated by the lasers frequency noise. The period of the driving signal

T_{filter drive}

is equal to the roundtrip time

T_{round - trip}

, thus the OEO is operated at FDML regime. A sequence of frequency scanning microwave signals are generated at the output of FDML OEO when the loop gain exceeds the loss. Fig. 4

Fig. 4 The generated linearly chirped microwave waveform (LCMW) centered at 7.5 GHz with a scanning bandwidth of 1.2 GHz. (a) and (b) are the measured spectrum with different span.

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depicts the spectra of the generated LCMW centered at 7.5 GHz with a scanning bandwidth of 1.2 GHz. The spectra are measured by an electrical spectrum analyzer. The spectrum with a span of 15 GHz and resolution bandwidth (RBW) of 3 MHz is shown in Fig. 4(a). Some weak harmonics can be observed, which can be attribute to the harmonic components generated by the PM. Fig. 4(b) shows the measured spectrum with a smaller span. As can be seen, the mode spacing between two adjacent modes is 188.7 kHz, which is depend on the cavity length of the FDML OEO.

The time domain waveform is also measured by a high-speed digital phosphor oscilloscope with a sampling rate of 100 GS/s. The output waveform in about four scanning periods is shown in Fig. 5(a)

Fig. 5 (a) Measured time domain waveform of the generated LCMW centered at 7.5 GHz with a scanning bandwidth of 1.2 GHz. (b) The instantaneous frequency distribution. (c) The autocorrelation result. Inset: zoom-in view.

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. The inset shows the zoom-in view of the measured waveform. A continuous amplitude and phase can be observed, which indicates all longitudinal modes in the entire sweep is stored simultaneously in the OEO cavity. A small amplitude variation of the output waveform in one scanning period can also be observed, which is mainly caused by the power changes of the TLS when sweeping the wavelength by a saw-tooth driving current. Fig. 5(b) shows the instantaneous frequency distribution of the generated waveform, which is obtained by calculating the short-time Fourier transform (STFT) of the generated microwave waveform. As can be seen, the period of generated frequency-scanning microwave waveform is 5.3 μs, which agrees well to the period of the driving current. The frequency of the generated signal is almost linearly decreased within one period with a chirp rate of 0.23 GHz/μs. A slight up-scanning component can also be observed, which is mainly caused by the limited bandwidth of the laser diode controller (1 MHz for a sinusoidal driving current). Consider the bandwidth of 1.2 GHz, the TBWP of the output microwave waveform is as large as 6,360. Fig. 5(c) exhibits the compressed signal with inset shows the zoom-in view. The result is calculated by autocorrelation. The pulse width is 0.9 ns. Hence, a compression ratio of 5,889 is achieved.

By changing the amplitude variation range of the driving current, the scanning bandwidth of the generated microwave waveform can be tuned, as shown in Fig. 6(a)

Fig. 6 Reconfigurability of the proposed SBS-based FDML OEO. (a) Bandwidth tuning from 4.5 GHz to 0.55 GHz with a center frequency of 7.5 GHz. (b) Center frequency tuning from 1 GHz to 14.4 GHz with a scanning bandwidth of 1.2 GHz.

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. The achievable scanning bandwidth is as large as 4.5 GHz, corresponding to a TBWP as high as 23,850 and a chirp rate up to 0.85 GHz/μs. A short length of optical fiber is incorporated in the OEO cavity in order to achieve a fast tuning speed, however, at the same time the Q-factor of OEO loop is small, which will result in a relative large output phase noise. The center scanning frequency can also be tuned by changing the central sweeping wavelength of the TLS, which is obtained by alter the DC bias of the driving current. Fig. 6(b) shows the tuning of the central frequency from 1 GHz to 14.4 GHz with a frequency interval of 1.2 GHz. The amplitude variation of sidemodes for different center frequencies is mainly caused by the unsharp roll-off (5 dB/GHz) of the optical bandpass filter used in the experiment. The frequency tuning resolution of the proposed FDML OEO is determined by the minimum wavelength tuning step of the TLS, which is about 5 MHz in our experiment. The tuning range of the FDML OEO in the experiment is just limited by the bandwidth of the PD, which can be further extended.

Fig. 7

Fig. 7 Measured SSB phase noise of the microwave signal generated by the OEO.

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shows the single side-band (SSB) phase noise of the proposed OEO, which is measured under 10 GHz single frequency oscillation condition. A SSB phase noise of −101.1 dBc/Hz at 10 kHz offset frequency is obtained. There are some spurs after 100-kHz offset frequency, which corresponding to the side-modes defined by the cavity round-trip time. The phase noise can be further reduced, for example, by expanding the loop length to achieve a higher Q-factor.

4. Conclusion

In conclusion, we propose and experimentally demonstrate a novel reconfigurable fast frequency scanning FDML OEO based on the deamplification of SBS. LCMW are generated with a chirp rate up to 0.85 GHz/μs and a TBWP as high as 23,850. The scanning frequency can be tuned up to 15 GHz, which can be further extended, and the scanning range is as large as 4.5 GHz. The proposed SBS-based FDML OEO can have direct applications to radar imaging and communication systems.

Funding

National Natural Science Foundation of China (61535012, 61522509); Thousand Young Talent Program.

Acknowledgment

We thank Nuannuan Shi, Shuqian Sun, Xinyi Zhu and Hao Sun for comments and discussion.

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Fourier domain mode locked optoelectronic oscillator based on the deamplification of stimulated Brillouin scattering

Abstract

1. Introduction

2. Principle of the proposed FDML OEO

3. Results and discussions

4. Conclusion

Funding

Acknowledgment

References

Cited By

Figures (7)

Equations (2)

OSA Continuum