Fundamental Repetition Frequency Research Articles

Active and Passive Mode-Locked Fiber Lasers for High-Speed High-Resolution Photonic Analog-to-Digital Conversion

We present a complete set of mode-locked fiber lasers designed for photonic analog-to-digital conversion. Design, simulation, fabrication, and characterization of Er-doped fiber lasers are carried out through optimizing their performance to suit the requirements of high-speed and high-resolution photonic-assisted analog-to-digital converters. The required properties are achieved by active mode-locking on the basis of amplitude modulation of the cavity losses through a Mach-Zehnder modulator, and passive mode-locking on the basis of fast semiconductor saturable absorbers and intracavity polarizing fibers. The use of an intracavity polarizing fiber in combination with a fast saturable absorber allows dynamic control of the operation regimes through polarization control. Besides these oscillators designed with ring cavity structure, a short linear cavity with fundamental repetition frequency at the order of 1 GHz is studied, and an effective continuous tunability of the repetition frequency is obtained by an electromechanical transducer. Pulsed repetition rates up to 20 GHz, pulsewidths down to 500 fs, and time jitters below 180 fs are obtained. Fiber lasers based on standard and commercially available telecom devices are demonstrated to be a low-cost, compact, and versatile optical source for photonic-assisted analog-to-digital conversion.

Passive harmonic mode-locking in Er-doped fiber laser based on graphene saturable absorber with repetition rates scalable to 2.22 GHz

Passive harmonic-mode locking of erbium-doped fiber laser with atomic multilayer graphene is presented. The laser could operate at several harmonics (from 2nd to 21st) of the fundamental repetition frequency of the ring resonator (106 MHz). The highest achieved repetition rate was 2.22 GHz (which corresponds to the 21st harmonic) with sub-picosecond pulse durations and over 40 dB of the supermode noise suppression. The saturable absorber was formed by multilayer graphene, mechanically exfoliated from pure graphite block through Scotch-tape and deposited on the fiber ferrule.

Fundamental and harmonic mode-locking in erbium-doped fiber laser based on graphene saturable absorber

Graphene-based passively mode-locked erbium doped fiber laser is presented. Multilayer graphene was obtained by mechanical exfoliation of pure graphitic block and deposited on the fiber ferrule. The mode-locking performance was investigated under various laser pumping conditions. The laser could operate at fundamental repetition rate of 16.34MHz with 844fs pulse duration and 30mW average output power. Also harmonic-mode locking of the laser is demonstrated. 294 and 800MHz repetition rates were obtained (corresponding to the 18th and 49th harmonic of the fundamental repetition frequency, respectively) with nearly transform-limited pulses.

10 GHz passive harmonic mode-locking in Er–Yb double-clad fiber laser

In this paper we report passive harmonic mode-locking of an Erbium/Ytterbium-doped double-clad fiber laser with repetition rates up to 10 GHz. The modes synchronization was obtained using Nonlinear Polarization Rotation mechanism. The stable operation at 634th harmonic of the fundamental (15.8 MHz) repetition frequency was achieved. Soliton pulses with the duration of less than 900 fs and 300 mW average output power were obtained.

High-Repetition-Frequency Low-Noise Fiber Ring Lasers Mode-Locked With Carbon Nanotubes

We demonstrate high-repetition-frequency low-noise passively mode-locked erbium-doped fiber ring lasers using single-walled carbon nanotubes as saturable absorbers. Pulses with a fundamental pulse repetition frequency of 200 MHz and 124-fs width are generated from a ring laser cavity. By substantially simplifying the cavity with a component that integrates multiple functions into a single device, the fundamental pulse repetition frequency could be increased to 447 MHz with pulsewidths of 270 fs.

Experimental investigation of self-starting operation in a F8L based on a symmetrical NOLM

We experimentally analyze the self-starting operation of a figure-eight mode-locked fiber laser. The design is based on a power-balanced nonlinear optical loop mirror (NOLM) with highly twisted low-birefringence fiber and a quarter-wave (QW) retarder in the loop. The NOLM operates by nonlinear polarization rotation. Self-starting mode-locking requires a careful adjustment of the NOLM low-power transmission, which is easily realized with our setup by adjusting the angle of the QW retarder. The laser is capable of generating ∼20ps pulses at the fundamental repetition frequency of 0.78MHz.

A passively mode-locked fiber laser at 1.54 μm with a fundamental repetition frequency reaching 2 GHz

We demonstrate a fundamentally mode-locked fiber laser with a repetition frequency in excess of 2 GHz at a central wavelength of 1.535 mum. Co-doped ytterbium-erbium fiber provides the gain medium for the laser, affording high gain per unit length, while a semiconductor saturable absorber mirror (SAM) provides the pulse shaping mechanism in a standing wave cavity. Results are shown confirming cw mode-locking for 1 GHz and 2 GHz repetition frequency systems. The response of the frequency comb output to pump power variations is shown to follow a single pole response. The timing jitter of a 540MHz repetition-rate laser has been suppressed to below 100 fs through phase-lead compensated feedback to the pump power. Alternatively, a single comb line of a 850MHz repetition-rate laser has been phase-locked to a narrow linewidth cw laser with an in-loop phase jitter of 0.06 rad(2). The laser design is compatible with low-noise oscillator applications.

Mode-locked performance of hybrid soliton pulse source utilizing fiber grating external cavity lasers

Mode-locking characteristic of hybrid soliton pulse source (HSPS) utilizing linearly chirped raised-cosine flat top apodized fiber Bragg grating (FBG) is investigated by using coupled-mode equations. It is found that the fundamental repetition frequency range of HSPS is significantly extended by using linearly chirped raised-cosine flat top apodized FBG instead of linearly chirped Gaussian apodized FBG. The range of repetition frequencies over which proper mode-locking is obtained is 2–3.3GHz with linearly chirped raised-cosine flat top apodized grating whereas this range is 2.1–2.95GHz with linearly chirped Gaussian apodized grating.

80~200 GHz erbium doped fibre laser using a rational harmonic mode-locking technique

Pulse trains with repetition rates as high as 80~200 GHz were obtained at 1.55 µm, using rational harmonic mode-locking with an erbium doped fibre laser. A fundamental repetition frequency (fosc) of 40 GHz was first generated by applying a 40 GHz modulation frequency to an LN intensity modulator in the fibre cavity, in which the longitudinal mode spacing (Δf) was 1.5 MHz. By rationally detuning the modulation frequency by Δf/n, the authors succeeded in obtaining a repetitive oscillation of (nfosc+Δf). The maximum frequency was 200 GHz, at which n corresponds to 5.

Locking range of a hybrid mode-locked monolithic DBR semiconductor laser at millimeter-wave frequencies

We present the first investigation of the detuning characteristics at 33 GHz of a hybrid mode-locked monolithic distributed Bragg reflector semiconductor laser. Hybrid modelocking is achieved by applying a radio-frequency (RF) signal, at a frequency corresponding to the fundamental laser repetition frequency, to the saturable absorber of the passively mode-locked laser. Measurements show a 3-dB locking range greater than 40 MHz with an applied external RF signal power of 0 dBm. At this power level, the laser beat signal exhibited phase-noise less than -78 dBc/Hz and -93 dBc/Hz at 10 kHz and 5 MHz offsets, respectively.

Which ear has the asynchronous signal? Interaural effects

Pairs of repeated noise patterns were presented to each ear: one pair with the same fundamental repetition frequency, and one pair with a slightly different fundamental frequency. The task of the listener was to identify the ear with frequency asynchrony. Under conditions where the detection of frequency asynchrony of complex sounds is nearly perfect, the discrimination of which ear exhibits the asynchrony is near chance. Several lines of evidence suggest that the latter difficulty is related to binaural processing of frequency asynchrony across the two ears. Evidence: (1) the accuracy of detection of binaural frequency asynchrony rivals that of monaural frequency asynchrony; (2) the introduction of interaural intensity differences between the two ears improves which-ear discrimination, and reduces binaural asynchrony discrimination; (3) under conditions of equivalent monaural asynchrony, which-ear discrimination reflects differences in binaural asynchrony.

LASER-INDUCED MICROWAVE SOUND BY SURFACE HEATING

Microwave sound of discrete frequencies has been generated by the irradiation of a solid with the uniformly spaced, ultrashort optical pulses produced by a mode-locked Nd:glass laser. The sound, rich in harmonic content, has its fundamental frequency fixed by the repetition frequency of the laser pulses. Believed to be thermally generated in the optical skin depth where the energy is absorbed, the sound has been detected at room temperature up to 2 GHz, the tenth harmonic of the fundamental repetition frequency. Such high harmonic content indicates that the acoustic pulses have rise times significantly less than 1 nsec and demonstrates the feasibility of utilizing ultrashort laser pulses to produce ultrashort acoustic pulses.