High-repetition-rate mode-locked Er-doped fiber lasers by harmonic injection locking

Triggering of different pulsed regimes in fiber cavity laser by a waveguide electro-optic switch

A novel practical method for electronic triggering of essentially different pulsed regimes in fiber cavity lasers is introduced. The method relies on electronic control of complementary transmission characteristics of a fiber-coupled LiNbO₃ waveguide electro-optic switch (WEOS) which plays the role of the variable output coupler in a fiber cavity. The method was studied using a testbed laser configuration comprised of a semiconductor optical amplifier (SOA) and an all-fiber cavity. Modulation of the WEOS-based output coupling in the fast gain recovery configuration allowed not only high-quality mode locking and harmonic mode-locking at certain pulse repetition rates determined by the cavity round trip time, but it also allowed nanosecond pulsed output of the same quality to be yielded by cavity dumping at widely and continuously tunable repetition rate (ranging from kHz to MHz). Thus, WEOS-based electronically variable output coupling allows uniquely high flexibility for lasing regimes and characteristics within a single all-fiber cavity configuration.

Highly tunable repetition-rate multiplication of mode-locked lasers using all-fibre harmonic injection locking

Higher repetition-rate optical pulse trains have been desired for various applications such as high-bit-rate optical communication, photonic analogue-to-digital conversion, and multi-photon imaging. Generation of multi GHz and higher repetition-rate optical pulse trains directly from mode-locked oscillators is often challenging. As an alternative, harmonic injection locking can be applied for extra-cavity repetition-rate multiplication (RRM). Here we have investigated the operation conditions and achievable performances of all-fibre, highly tunable harmonic injection locking-based pulse RRM. We show that, with slight tuning of slave laser length, highly tunable RRM is possible from a multiplication factor of 2 to >100. The resulting maximum SMSR is 41 dB when multiplied by a factor of two. We further characterize the noise properties of the multiplied signal in terms of phase noise and relative intensity noise. The resulting absolute rms timing jitter of the multiplied signal is in the range of 20 fs to 60 fs (10 kHz–1MHz) for different multiplication factors. With its high tunability, simple and robust all-fibre implementation, and low excess noise, the demonstrated RRM system may find diverse applications in microwave photonics, optical communications, photonic analogue-to-digital conversion, and clock distribution networks.

Centimeter-scale Yb-free heavily Er-doped silica fiber laser

The laser behavior of a centimeter-scale Er³⁺/Al³⁺ codoped silica fiber with core numerical aperture and core diameter of 0.15 and 8 μm, respectively, is reported. The core glass was prepared by the sol-gel method combined with high-temperature sintering; it contained Er³⁺ ion concentration as high as 1.32×10²⁰  ions/cm³ and an Al/Er mole ratio of 10. The high doping homogeneity of Er³⁺ ions in the fiber core was confirmed by an electron probe microanalyzer element scanning, long Er³⁺:  I_13/24 emission lifetime of 11.4 ms, and low refractive index fluctuation of fiber core (±1×10^-4). The signal gain of fibers with 4.6 cm, 10 cm, and 16 cm lengths was tested in the 1500-1620 nm range. A gain of 7 dB is achieved at 1560 nm in a 16-cm-long EDF under 230 mW pump power. Aimed for CO₂ sensor application, the laser behavior of Er/Al codoped fiber was tested at 1572 nm. A slope efficiency of 12% and an output power of 15 mW were achieved in a 10-cm-long fiber under 166 mW absorbed pump power. The newly developed silica fiber is promising for use in high-repetition-rate lasers.

Tenth-order rational-harmonic frequency multiplication and detuning of optical pulse injection-locked erbium-doped fiber laser

The jitter and frequency-detuning dynamics of a 10-GHz rational-harmonic frequency-multiplied pulse train generated from an erbium-doped fiber laser (EDFL) is studied. The EDFL is self-feedback seeded and optically injection locked by a gain-switched laser diode (GSLD) with a pulse width and an average power of 17.6 ps and 0.2 mW, respectively, at a repetition frequency of 1 GHz. The repetition frequency of the optical pulse train can be tenth-order multiplied by a slight detuning of the repetition frequency of the GSLD to match the rational-harmonic injection-locked condition of the EDFL. As the repetition frequency is multiplied from 1 to 10 GHz, the peak power, the pulse width, and the frequency-detuning bandwidth of the injection-locked EDFL pulses decrease from 1.2 to 0.3 W, from 40 to 21 ps, and from 40 to 9 kHz, respectively. The timing jitter of the injection-locked EDFL repeated at 1 GHz remains unchanged (< 0.5 ps) within the detuning bandwidth, which inevitably increases to 1.2 ps after tenth-order multiplication.

Synchronized two-color operation of a passively mode-locked erbium-doped fiber laser by dual injection locking

The recently introduced harmonic injection locking is a method for generating pulse trains at high repetition rates from passively mode-locked lasers. We report the simultaneous injection locking of two spectral bands in an erbium-doped fiber laser by injection of two spectrally distinct and temporally synchronized pulse trains. The injection-locked laser simultaneously produced pulses at wavelengths of 1.53 and 1.55 microm, each at a 7.5-GHz repetition rate and with a pulse width of 10 ps. We compared the experimental results with those of a passively mode-locked laser.

Clock multiplication in a singly resonant fiber parametric oscillator

Soliton trains can be parametrically generated in fiber loops wherever an incoming soliton train interacts with a cw pump in the nonlinear loop. The phase-conjugated signal may either copy or multiply the incoming repetition rate, depending on whether the corresponding cavity harmonic is exactly matched or differs by an amount equal to an integer fraction of the longitudinal mode spacing. It is also shown that the phase relationship between adjacent solitons in the multiplicative conditions continuously evolves on propagation. This effect limits the range of potential application of multiplicative loops.