All-optical, tunable, V- and W-band microwave generation using semiconductor lasers at period-one nonlinear dynamics with asymmetric mutual injection stabilization.

This study investigates an optically injected semiconductor laser operating at period-one nonlinear dynamics for all-optical microwave generation. A novel, to the best of our knowledge, all-optical stabilization scheme is proposed to greatly enhance the spectral purity of such generated microwaves, which sends a small fraction of the injected laser output back to the injecting laser, not the injected laser itself. Mutual injection with highly different injection power between the two lasers, i.e., highly asymmetric mutual injection, is thus formed. As a result, the microwave linewidth is reduced by up to at least 85 times, the phase noise variance is improved by up to at least 750 times, and a side-peak suppression ratio of more than 44 dB is achieved. Microwave generation that is tunable up to at least 110 GHz with a 3-dB linewidth down to below 2 kHz is realized.

High-entropy chaos generation using semiconductor lasers subject to intensity-modulated optical injection for certified physical random number generation.

This study investigates high-entropy chaos generation using a semiconductor laser subject to intensity-modulated optical injection for certified physical random number generation. Chaos with a continuous spectral profile that is not only widely distributed but also broadly flattened over a bandwidth of 33 GHz is generated. The former suggests that the chaos can be sampled at a high rate while keeping sufficient un-correlation between data samples, and the latter indicates that the chaos possesses high entropy, both of which enhance the generation rate of physical random numbers with guaranteed unpredictability. A minimum entropy value of 2.19 bits/sample is obtained without any post-processing and by excluding the contribution from measurement noise, suggesting that, to the least extent, the chaotic source can be used as a 2-bit physical random number generator at a rate of 160 Gbits/s.

Broadband chaotic microwave generation through destabilization of period-one nonlinear dynamics in semiconductor lasers for radar applications.

This Letter studies a photonic approach for chaotic microwave generation through destabilization of period-one (P1) nonlinear dynamics in a semiconductor laser subject to intensity-modulated (IM) optical injection. Chaos can be excited when the modulation sideband perturbation carried by the IM optical injection is a few gigahertz higher than the lower oscillation sideband of the P1 dynamics. As a result, chaotic microwaves with a spectral distribution of more than 50 GHz and a bandwidth of about 33 GHz are generated without any time-delay signature or modulation-induced peak. Such features provide the generated chaotic microwaves with preferable characteristics for radar applications, including high detection resolution, superior detection unambiguity, strong anti-jamming capability, and simultaneous multi-band detection.

V- and W-band microwave generation and modulation using semiconductor lasers at period-one nonlinear dynamics.

Microwave generation and modulation over the V- and W-bands are investigated using a semiconductor laser subject to both comb-like optical injection and direct modulation. The former not only excites period-one (P1) nonlinear dynamics for tunable microwave generation but also improves the stability and purity of such generated microwaves. The latter upconverts data onto the generated microwaves by superimposing the data effectively only onto the lower oscillation sideband of the P1 dynamics, which prevents the data from dispersion-induced degradation over fiber distribution. As a result, microwaves that are continuously tunable from 40 to 110 GHz with a 3-dB linewidth of less than 1 Hz and with phase noise better than -95dBc/Hz at 10-kHz offset are generated. A bit-error ratio better than the forward error correction limit, 3.8×10-3, is achieved for 12-Gb/s 16-quadrature amplitude modulation data after 25-km fiber distribution.

Frequency-modulated continuous-wave microwave generation using stabilized period-one nonlinear dynamics of semiconductor lasers.

Frequency-modulated continuous-wave (FMCW) microwave generation is studied using a semiconductor laser operating at stabilized period-one (P1) nonlinear dynamics when subject to comb-like (CL) optical injection. The phase locking established between the P1 dynamics and the CL optical injection not only improves the P1 oscillation stability considerably but also provides a mechanism to change the P1 oscillation frequency through varying the modulation frequency of the CL optical injection. As a result, a stable FMCW microwave at a central frequency of up to 40 GHz is generated with its frequency varying linearly, triangularly, or step-wisely over a range of 4 GHz during a repeated time period that can be reconfigured at least from 100 ns to 10 ms. This system is capable of operation up to at least 100 GHz.

Doppler-free coherent detection using period-one nonlinear dynamics of semiconductor lasers for OFDM-RoF links.

This study investigates coherent detection that is free from the Doppler frequency shift effect for orthogonal frequency division multiplexing radio-over-fiber (OFDM-RoF) links using period-one (P1) nonlinear dynamics of semiconductor lasers. Even under a dynamically time-varying Doppler frequency shift of up to 100 kHz, corresponding to a relative motion between a transmitter and a receiver with a velocity of more than 3850 km/h at 28 GHz, the microwave carrier of a received OFDM-RoF signal can be successfully regenerated instantaneously and uninterruptedly with its phase highly preserved through the P1 dynamics. No carrier frequency offset (CFO) due to the Doppler frequency shift effect happens if the regenerated microwave carrier is used as a microwave local oscillator for coherent detection of the received OFDM-RoF signal. As a result, a bit-error ratio of around 10-9 is achieved for coherent detection of a 28 GHz OFDM-RoF signal carrying 4 Gb/s 16-quadrature amplitude modulation data. Thus, no digital signal processing, either photonic or electronic, is required to compensate for such a CFO. This all-optical system is capable of operation up to at least 100 GHz.

Conversion from non-orthogonally to orthogonally polarized optical single-sideband modulation using optically injected semiconductor lasers.

This Letter investigates an optically injected semiconductor laser for conversion from non-orthogonally to orthogonally polarized optical single-sideband modulation. The underlying mechanism relies solely on nonlinear laser characteristics and, thus, only a typical semiconductor laser is required as the key conversion unit. This conversion can be achieved for a broadly tunable frequency range up to at least 65 GHz. After conversion, the microwave phase quality, including linewidth and phase noise, is mostly preserved, and simultaneous microwave amplification up to 23 dB is feasible.

Numerical study of ultrashort-optical-feedback-enhanced photonic microwave generation using optically injected semiconductor lasers at period-one nonlinear dynamics.

This study numerically investigates the enhancement of photonic microwave generation using an optically injected semiconductor laser operating at period-one (P1) nonlinear dynamics through ultrashort optical feedback. For the purpose of practical applications where system miniaturization is generally preferred, a feedback delay time that is one to two orders of magnitude shorter than the relaxation resonance period of a typical laser is emphasized. Various dynamical states that are more complicated than the P1 dynamics can be excited under a number of ultrashort optical feedback conditions. Within the range of the P1 dynamics, on one hand, the frequency of the P1 microwave oscillation can be greatly enhanced by up to more than three folds. Generally speaking, the microwave frequency enhances with the optical feedback power and phase, while it varies saw-wise with the optical feedback delay time. On the other hand, the purity of the P1 microwave oscillation can be highly improved by up to more than three orders of magnitude. In general, the microwave purity improves with the optical feedback power and delay time, while it only varies within an order of magnitude with the optical feedback phase. These results suggest that the ultrashort optical feedback provides the optically injected laser system with an extra degree of freedom to manipulate/improve the characteristics of the P1 microwave oscillation without changing the optical injection condition.

Photonic microwave time delay using slow- and fast-light effects in optically injected semiconductor lasers.

This study numerically and experimentally investigates a photonic approach for microwave time delay, which takes advantage of the redshift of the laser cavity resonance induced by external optical injection in a semiconductor laser. The strong enhancement around the redshifted cavity resonance not only amplifies the power, but also shifts the phase of the microwave signals carried by the optical injection. Such a microwave phase shift is approximately linear over a few gigahertz, leading to a constant microwave time delay over the frequency range. A different time delay can be achieved by simply adjusting the injection power or frequency. For the microwave frequencies up to 40 GHz investigated in this Letter, a continuously tunable range of more than 80 ps in time delay is achieved over an instantaneous bandwidth of approximately 7 GHz. The quality of the data carried by the microwave signals is mostly preserved after time delay. Thus, a bit-error ratio down to 10-9 at 2.5 Gb/s is achieved with a possible detection sensitivity improvement of 5 dB.

Photonic microwave carrier recovery using period-one nonlinear dynamics of semiconductor lasers for OFDM-RoF coherent detection.

This study investigates an all-optical scheme based on period-one (P1) nonlinear dynamics of semiconductor lasers, which regenerates the microwave carrier of an orthogonal frequency division multiplexing radio-over-fiber (OFDM-RoF) signal and uses it as a microwave local oscillator for coherent detection. Through the injection locking established between the OFDM-RoF signal and the P1 dynamics, frequency synchronization with highly preserved phase quality is inherently achieved between the recovered microwave carrier and the microwave carrier of the OFDM-RoF signal. A bit-error ratio down to 1.9×10-9 is achieved accordingly using the proposed scheme for coherent detection of a 32-GHz OFDM-RoF signal carrying 4 Gb/s 16-quadrature amplitude modulation data. No electronic microwave generators or electronic phase-locked loops are thus required. The proposed system can be operated up to at least 100 GHz and can be self-adapted to certain changes in the operating microwave frequency.

Radio-over-fiber DSB-to-SSB conversion using semiconductor lasers at stable locking dynamics.

In radio-over-fiber systems, optical single-sideband (SSB) modulation signals are preferred to optical double-sideband (DSB) modulation signals for fiber distribution in order to mitigate the microwave power fading effect. However, typically adopted modulation schemes generate DSB signals, making DSB-to-SSB conversion necessary before or after fiber distribution. This study investigates a semiconductor laser at stable locking dynamics for such conversion. The conversion relies solely on the nonlinear dynamical interaction between an input DSB signal and the laser. Only a typical semiconductor laser is therefore required as the key conversion unit, and no pump or probe signal is necessary. The conversion can be achieved for a broad tunable range of microwave frequency up to at least 60 GHz. In addition, the conversion can be carried out even when the microwave frequency, the power of the input DSB signal, or the frequency of the input DSB signal fluctuates over a wide range, leading to high adaptability and stability of the conversion system. After conversion, while the microwave phase quality, such as linewidth and phase noise, is mainly preserved, a bit-error ratio down to 10-9 is achieved for a data rate up to at least 8 Gb/s with a detection sensitivity improvement of more than 1.5 dB.

Photonic microwave stabilization for period-one nonlinear dynamics of semiconductor lasers using optical modulation sideband injection locking.

Photonic microwave generation using period-one nonlinear dynamics of semiconductor lasers suffers from poor spectral purity. A stabilization approach based on optical modulation sideband injection locking is investigated. An optical signal carrying a highly correlated modulation sideband comb simultaneously injection-locks the regeneration of the optical carrier and the lower oscillation sideband in the dynamics, establishing a phase-locking between the two spectral components. A linewidth of below 1 Hz is therefore achieved for microwave generation up to at least 40 GHz. Because of the frequency multiplication in yielding the comb-like optical signal, only an electronic microwave reference at the tenth subharmonic or higher of the generated microwave frequency is required.

High-power noise-like pulse generation using a 1.56-µm all-fiber laser system.

We demonstrated an all-fiber, high-power noise-like pulse laser system at the 1.56-µm wavelength. A low-power noise-like pulse train generated by a ring oscillator was amplified using a two-stage amplifier, where the performance of the second-stage amplifier determined the final output power level. The optical intensity in the second-stage amplifier was managed well to avoid not only the excessive spectral broadening induced by nonlinearities but also any damage to the device. On the other hand, the power conversion efficiency of the amplifier was optimized through proper control of its pump wavelength. The pump wavelength determines the pump absorption and therefore the power conversion efficiency of the gain fiber. Through this approach, the average power of the noise-like pulse train was amplified considerably to an output of 13.1 W, resulting in a power conversion efficiency of 36.1% and a pulse energy of 0.85 µJ. To the best of our knowledge, these amplified pulses have the highest average power and pulse energy for noise-like pulses in the 1.56-µm wavelength region. As a result, the net gain in the cascaded amplifier reached 30 dB. With peak and pedestal widths of 168 fs and 61.3 ps, respectively, for the amplified pulses, the pedestal-to-peak intensity ratio of the autocorrelation trace remains at the value of 0.5 required for truly noise-like pulses.

Supercontinuum generation in highly nonlinear fibers using amplified noise-like optical pulses.

Supercontinuum generation in a highly nonlinear fiber pumped by noise-like pulses from an erbium-doped fiber ring laser is investigated. To generate ultrabroad spectra, a fiber amplifier is used to boost the power launched into the highly nonlinear fiber. After amplification, not only the average power of the noise-like pulses is enhanced but the spectrum of the pulses is also broadened due to nonlinear effects in the fiber amplifier. This leads to a reduction of the peak duration in their autocorrelation trace, suggesting a similar extent of pulse compression; by contrast, the pedestal duration increases only slightly, suggesting that the noise-like characteristic is maintained. By controlling the pump power of the fiber amplifier, the compression ratio of the noise-like pulse duration can be adjusted. Due to the pulse compression, supercontinuum generation with a broader spectrum is therefore feasible at a given average power level of the noise-like pulses launched into the highly nonlinear fiber. As a result, supercontinuum generation with an optical spectrum spanning from 1208 to 2111 nm is achieved using a 1-m nonlinear fiber pumped by amplified noise-like pulses of 15.5 MHz repetition rate at an average power of 202 mW.

Optical feedback stabilization of photonic microwave generation using period-one nonlinear dynamics of semiconductor lasers.

Effects of optical feedback on period-one nonlinear dynamics of an optically injected semiconductor laser are numerically investigated. The optical feedback can suppress the period-one dynamics and excite other more complex dynamics if the feedback level is high except for extremely short feedback delay times. Within the range of the period-one dynamics, however, the optical feedback can stabilize the period-one dynamics in such a manner that significant reduction of microwave linewidth and phase noise is achieved, up to more than two orders of magnitude. A high feedback level and/or a long feedback delay time are generally preferred for such microwave stabilization. However, considerably enhanced microwave linewidth and phase noise happen periodically at certain feedback delay times, which is strongly related to the behavior of locking between the period-one microwave oscillation and the feedback loop modes. The extent of these enhancements reduces if the feedback level is high. While the microwave frequency only slightly changes with the feedback level, it red-shifts with the feedback delay time before an abrupt blue-shift occurs periodically. With the presence of the laser intrinsic noise, frequency jitters occur around the feedback delay times leading to the abrupt blue-shifts, ranging from the order of 0.1 GHz to the order of 1 GHz.

Photonic microwave amplification for radio-over-fiber links using period-one nonlinear dynamics of semiconductor lasers.

For radio-over-fiber links, microwave-modulated optical carriers with high optical modulation depth are preferred because high optical modulation depth allows generation of high microwave power after photodetection, leading to high detection sensitivity, long transmission distance, and large link gain. This study investigates the period-one nonlinear dynamics of semiconductor lasers for optical modulation depth improvement to achieve photonic microwave amplification through modulation sideband enhancement. In our scheme, only typical semiconductor lasers are required as the amplification unit. The amplification is achieved for a broad microwave range, from less than 25 GHz to more than 60 GHz, and for a wide gain range, from less than 10 dB to more than 30 dB. The microwave phase quality is mainly preserved while the microwave power is largely amplified, improving the signal-to-noise ratio up to at least 25 dB. The bit-error ratio at 1.25 Gbits/s is better than 10(-9), and a sensitivity improvement of up to at least 15 dB is feasible.

Optical double-sideband modulation to single-sideband modulation conversion using period-one nonlinear dynamics of semiconductor lasers for radio-over-fiber links.

To distribute microwaves over fibers, optical single-sideband (SSB) modulation signals are preferred to optical double-sideband (DSB) modulation signals. This study investigates an optically injected semiconductor laser at period-one nonlinear dynamics for optical DSB-to-SSB conversion. For the operating microwave frequencies up to 40 GHz investigated in this study, the proposed system regenerates or even enhances the microwave features of an optical DSB input while converting its optical feature into SSB with an intensity difference of at least 20 dB. The bit-error ratio at 622 Mb/s is down to 10(-9) with a sensitivity improvement of up to 3 dB. The proposed system can be self-adapted to certain changes in the operating microwave frequency and can operate stably under certain fluctuations in the input optical power and frequency.

Period-one oscillation for photonic microwave transmission using an optically injected semiconductor laser.

Optically injected semiconductor laser under periodone oscillation is investigated as a source for photonic microwave transmission over fiber. The period-one nonlinear dynamics of an optically injected laser is studied for the purpose of minimizing the microwave power penalty induced by chromatic dispersion. Over a large range of injection strengths and frequency detunings, we first obtain the mapping of the period-one oscillation characteristics, including the microwave frequency, the microwave power, and the single sideband (SSB) characteristics of the optical spectrum. By accounting for the fiber chromatic dispersion, we calculate its effect on the optical spectrum and the associated microwave power penalty. A mapping of the minimum microwave power deliverable after the maximum penalty is obtained. The system is shown to be least susceptible to the penalty when operated under strong injection with the frequency detuned above the Hopf bifurcation line. Microwave frequency beyond six times the relaxation resonance frequency can be effectively transmitted.

All-optical frequency conversion using nonlinear dynamics of semiconductor lasers.

When a semiconductor laser is subject to optical injection, it can enter the period-one dynamics through Hopf bifurcation. Under such nonlinear dynamics, equally and oppositely frequency-shifted optical signals from the injection emerge and are utilized for frequency conversion. Only a typical semiconductor laser is required as the conversion unit, where no pump or probe laser is necessary. The frequency shift can be continuously tuned by controlling the level or frequency of the injection. A bit-error ratio down to 10(-12) is observed with no or a slight power penalty for amplitude, frequency, and phase modulation at 2.5 Gbits/s, suggesting modulation format transparency of the system. Frequency down-, no-, and upconversion can be simultaneously achieved and individually selected, increasing the flexibility and reconfigurability of the system.

Radio-over-fiber AM-to-FM upconversion using an optically injected semiconductor laser.

A radio-over-fiber system uses light to carry a microwave subcarrier on optical fibers. The microwave is usually frequency modulated for wireless broadcasting. A conventional optical communication system usually operates at the baseband with amplitude modulation. The interface of the two systems thus needs an upconversion from the baseband to the microwave band with AM-to-FM transformation. An all-optical solution employing an optically injected semiconductor laser is investigated. The laser is operated in a dynamic state, where its intensity oscillates at a microwave frequency that varies with the injection strength. When the injection carries AM data, the microwave is frequency modulated accordingly. We demonstrate optical conversion from an OC-12 622-Mbps AM baseband signal to the corresponding FM microwave signal. The microwave is centered at 15.90 GHz. A bit-error rate of less than 10(-9) is measured.