Jia Haur Wong

Tunable Multi-Tap Bandpass Microwave Photonic Filter Using a Windowed Fabry-Pérot Filter-Based Multi-Wavelength Tunable Laser

A center frequency-tunable multi-tap bandpass microwave photonic filter (MPF) is proposed and experimentally demonstrated. A specially designed multi-wavelength fiber ring laser, in which a windowed Fabry-Pérot (FP) filter is used as the wavelength selection and power control component, has been developed to serve as the optical source for the MPF. By adjusting the windowed FP filter, both the wavelength spacing and power profile of the multi-wavelength laser can be changed. The output of the optical source is phase modulated by a microwave signal. 25 km of single-mode fiber (SMF) is then used to act as a dispersive medium to introduce time delays between taps. Thus, a tunable bandpass response is obtained at the output of a high-speed photodetector (PD). In addition, the passband centered at DC is removed due to the use of phase modulation. The experimental results show that more than 45 wavelengths can be generated in the multi-wavelength ring laser. With the electronic tuning of the wavelength spacing, tuning of the passband center frequency of the MPF by 3 GHz is achieved.

Observation of timing jitter reduction induced by spectral filtering in a fiber laser mode locked with a carbon nanotube-based saturable absorber

A self-starting passively mode-locked fiber laser with a carbon nanotube-based saturable absorber and a fiber-based bandpass filter (BPF) is proposed. Incorporation of a BPF into the cavity leads to a great reduction of its timing jitter from 84.8 to 29.1 fs (10 Hz-3 MHz). This happens because the filtering effect can weaken the fluctuation of the central wavelength induced by the quantum noise, being one of the important contributions to timing jitter in the optical amplifying process.

Bidirectional passively mode-locked soliton fiber laser with a four-port circulator

We present an all-fiber bidirectional passively mode-locked soliton laser with what we believe is a novel cavity configuration. Using a four-port circulator, we incorporate two different semiconductor saturable absorber mirrors (SESAMs) into the laser cavity, which enables bidirectional mode locking. The laser allows the generation of two independent countercirculating mode-locked pulse trains, each with an individual fundamental repetition rate that can be adjusted by varying the SESAM pigtail length. Two countercirculating pulse trains with repetition rates of 21.3 and 15.2 MHz are obtained simultaneously. By controlling the intracavity loss imposed on these two pulse trains, either one of the two pulse trains can be switched on or off. The bidirectional operation with other repetition rates is also demonstrated.

Nonlinear coupling of relative intensity noise from pump to a fiber ring laser mode-locked with carbon nanotubes

Pump relative intensity noise (RIN) has been recognized as a major source of noise in mode-locked lasers. The coupling of RIN from the pump to the output of a passively mode-locked fiber laser (PMFL) is systematically investigated using a pump modulation technique. It is found that the linear RIN coupling ratio from pump to PMFL is decreased with an increase in modulation frequency and is independent of modulation power. Moreover, the nonlinear RIN coupling from pump to PMFL is clearly demonstrated with a square wave modulated pump. The nonlinear RIN coupling ratio is noise power dependent. An exponential decay model based on the view of gain modulation is proposed and explains well the behavior of the nonlinear coupling phenomena.

Photonic Time-Stretched Analog-to-Digital Converter Amenable to Continuous-Time Operation Based on Polarization Modulation With Balanced Detection Scheme

In this paper, we show analytically and experimentally that a polarization modulator which supports TE and TM modes of opposite phase modulation indexes can be utilized to reject dispersion-induced even-order distortions in a photonic Time-Stretched Analog-to-Digital Converter (TS-ADC). The output of the polarization modulator propagates through a single dispersive channel. This makes the present scheme amenable to continuous operation. Based on the virtual time gating principle, the continuous-time RF signal is time-stretched by a factor of 4 and segmented into four channels prior to digitization. For a single channel, differential operation is achieved by using a polarization beam-splitter that generates complementary pulses which are fed to a balanced detector. The differential operation helps to reject dispersion-induced even-order distortions and the balanced detection assists in the suppression of second-order distortion as well as improving the signal-to-noise ratio (SNR) by 6 dB. Using a 10 bit electronic ADC with a sampling rate of 2 GSamples/s, we demonstrate digitization of RF signals up to a frequency of 950 MHz and obtain ~ 3.56 effective number of bits (ENOB) with a single channel at ~ 31.6% of the electronic ADCs peak-to-peak full scale voltage. With adequate backend digitizing hardware, a four-channel continuous-time TS-ADC with a sampling rate of 8 GSamples/s can be realized to handle RF frequencies as high as 4 GHz.

Dissipative Soliton (12 nJ) From an All-Fiber Passively Mode-Locked Laser With Large Normal Dispersion

We experimentally demonstrate an all-fiber erbium-doped fiber laser operating in the large normal dispersion regime. The laser is mode-locked by use of nonlinear polarization rotation and generates dissipative solitons (DSs) with large positive chirp and steep spectral edges. Within the cavity, there is no additional spectral filter used for self-starting and stabilizing the mode-locking of the laser. The output DSs have a duration of 30.5 ps with pulse energies up to 12 nJ, which can be compressed externally to 240 fs.

Timing-jitter reduction of passively mode-locked fiber laser with a carbon nanotube saturable absorber by optimization of cavity loss

We investigate the relationship between timing jitter and cavity loss of a passively mode-locked fiber ring laser with a carbon nanotube as a saturable absorber. It is the first time that we experimentally demonstrated the reduction of timing jitter by properly increasing laser cavity loss. The lowest timing jitter is achieved when the cavity loss is optimized. Theoretical analysis is in agreement with the experimental observations on the effect of cavity loss for the reduction of timing jitter. Moreover, it is experimentally shown that, at an optimal value of cavity loss, the timing jitter is reduced significantly by 24%, while the relative intensity noise increased by 4% only.

An optically tunable wideband optoelectronic oscillator based on a bandpass microwave photonic filter

An optoelectronic oscillator (OEO) with wideband frequency tunability and stable output based on a bandpass microwave photonic filter (MPF) has been proposed and experimentally demonstrated. Realized by cascading a finite impulse response (FIR) filter and an infinite impulse response (IIR) filter together, the tunable bandpass MPF successfully replaces the narrowband electrical bandpass filter in a conventional single-loop OEO and serves as the oscillating frequency selector. The FIR filter is based on a tunable multi-wavelength laser and dispersion compensation fiber (DCF) while the IIR filter is simply based on an optical loop. Utilizing a long length of DCF as the dispersion medium for the FIR filter also provides a long delay line for the OEO feedback cavity and as a result, optical tuning over a wide frequency range can be achieved without sacrificing the quality of the generated signal. By tuning the wavelength spacing of the multi-wavelength laser, the oscillation frequency can be tuned from 6.88 GHz to 12.79 GHz with an average step-size of 0.128 GHz. The maximum frequency drift of the generated 10 GHz signal is observed to be 1.923 kHz over 1 hour and its phase noise reaches the -112 dBc/Hz limit of our measuring equipment at 10 kHz offset frequency.

Noise conversion from pump to the passively mode-locked fiber lasers at 1.5 μm

We characterize the noise conversion from the pump relative intensity noise (RIN) to the RIN and phase noise of passively mode-locked lasers at 1.5 μm. Two mode locking mechanisms, nonlinear polarization rotation (NPR) and semiconductor saturable absorber mirror (SESAM), are compared for noise conversion for the first time. It is found that the RIN and the phase noise of both types of lasers are dominated by the noise converted from the pump RIN and thus, can be predicted with the measured pump RIN and noise conversion ratios. The SESAM laser is found to show an excess noise conversion from the laser RIN to the laser phase noise due to the slow saturable absorber effect.

Wavelength-Tunable High-Energy All-Normal-Dispersion Yb-Doped Mode-Locked All-Fiber Laser With a HiBi Fiber Sagnac Loop Filter

We experimentally demonstrate a broadly wavelength-tunable high-energy Yb-doped mode-locked fiber laser without any dispersion compensation. Wide tunability and filtering effect are simultaneously obtained using a fiber Sagnac loop filter in an all-fiber laser mode-locked by a semiconductor saturable absorber mirror. The Sagnac loop filter is constructed with a segment of a high birefringence (HiBi) fiber, a 3-dB coupler, and a polarization controller. By optimizing the HiBi fiber length and adjusting the polarization controller within the loop, the wavelength of the output pulse with 3 nJ pulse energy could be continuously tuned over the range from 1032 to 1052 nm. The heating of the HiBi fiber also led to wavelength shifting of the output pulse with a sensitivity of approximately -0.3-nm°C.