Supermode Noise Research Articles

Supermode-noise suppression using a nonlinear Fabry–Pérot filter in a harmonically mode-locked fiber ring laser

A simple efficient method for stabilizing a harmonically mode-locked fiber ring laser is proposed. In this method, a linear optical filter and a nonlinear Fabry–Pérot filter in which the refractive index is optical intensity dependent are located in the laser cavity. The linear filter is used to select a fixed lasing wavelength, and the Fabry–Pérot filter introduces a negative all-optical feedback mechanism that is able to suppress pulse-to-pulse amplitude fluctuations in the laser cavity. The scheme was experimentally demonstrated using a fiber Bragg grating as the linear filter and a laser diode biased below threshold as the nonlinear Fabry–Pérot, and stable harmonically mode-locked pulses with a supermode noise suppression ratio >55 dB were obtained.

Characterization of the noise and correlations in harmonically mode-locked lasers

In a harmonically mode-locked laser multiple optical pulses propagate inside the laser cavity. The noise in different pulses inside the laser cavity is in general correlated. Information regarding the sign and magnitude of the noise correlations is contained in the distribution of the spectral weight among the supermode noise peaks that appear in the pulse energy and timing noise spectral densities. We show that the supermode noise spectrum obtained experimentally by measurement of the photodetector current noise spectral density can be used to determine the correlations in the energy and the timing noise of different pulses in the laser cavity. We also present simple models for the timing noise in harmonically mode-locked lasers that demonstrate the relationship between the noise correlations and the supermode noise peaks.

A stable smoothly wavelength-tunable picosecond pulse generator

Smooth wavelength tuning over 10 nm has been realized in a dispersion-tuned harmonically mode-locked fiber ring laser. Supermode noise is suppressed by 13 dB by adding a semiconductor optical amplifier (SOA) into the cavity and the suppression improves as the SOA gain increases. 5.3-ps pulses are obtained at a repetition rate of 10 GHz. A simple numerical model demonstrates the effects of the intracavity dispersion and the SOA on the pulse characteristics.

Ultralow noise and supermode suppression in an actively mode-locked external-cavity semiconductor diode ring laser

We report what is to our knowledge the lowest phase and amplitude noise characteristics achieved to date in a 10-GHz pulse train produced by the active harmonic mode locking of an external-cavity semiconductor diode laser. Supermode noise has also been suppressed below -140 dBc/Hz by use of a high-finesse fiber Fabry-Perot etalon as an intracavity filter. Novel noise sideband measurements that extend to the Nyquist offset frequency suggest a significant advantage in using harmonic (rather than fundamental) mode locking to produce ultralow-noise pulse trains, owing to the relationship between the noise roll-off frequency and the fundamental cavity frequency.

Supermode noise of harmonically mode-locked erbium fiber lasers with composite cavity

The supermode noise of a harmonically mode-locked erbium-doped fiber laser with composite cavity is investigated both theoretically and experimentally. We propose a simple model based on the transfer function of the composite cavity. From this model, the frequencies of the cavity modes and their frequency-dependent losses are determined. A comprehensive experimental study of the composite cavity laser is carried out, covering a wide range of path-length differences between both arms of the fiber interferometer that form the composite cavity. Experimental results are in good agreement with our model. In particular, the path length difference determines periodic frequency windows within which supermodes of the composite cavity are observed. Outside these windows, supermodes are not detectable. Our results show quite remarkably that, although the number of supermodes is reduced with respect to the simple cavity, no measurable reduction of the overall supermode noise is obtained, contrary to what has recently been previously suggested.

Pulse repetition frequency multiplication via intracavity optical filtering in AM mode-locked fiber ring lasers

We demonstrate pulse repetition frequency multiplication in AM mode-locked fiber ring lasers using optical filtering realized via an intracavity fiber Fabry-Perot filter (FFP) and show that the generated optical pulses are highly stable in amplitude noise and timing jitter. A 3.477-GHz optical pulse train is generated using a modulation signal of 869.284 MHz, a fourth subharmonic multiple of the 3.48-GHz free spectral range of FFP. The generated optical pulses exhibit a high degree of pulse stability in terms of a large suppression of supermode noise, a low amplitude noise of 0.93 %, and a timing jitter of 1.2 ps.

Experimental study of supermode noise of harmonically mode-locked erbium-doped fibre lasers with composite cavity

An experimental study of supermode noise of harmonically actively mode-locked erbium-doped fibre lasers in the composite-cavity configuration is reported. Our study shows that the number of supermodes is reduced, as expected. However, this reduction is by far less efficient than what was previously reported. In particular, we found in measured RF spectra that supermodes appear in wide periodic frequency windows. Our most significant result is that, contrary to what has been previously reported, we did not measure in the composite-cavity laser any significant reduction of the overall supermode noise in comparison with the standard single-cavity configuration.

Active modelocking of miniaturefibre Fabry-Perot laser in ring cavity

Active modelocking at 10 GHz of a 10 mm-long fibre Fabry-Perot laser in a ring cavity is demonstrated. Very stable pulse trains without supermode noise were obtained. The pulsewidth was ~10 ps, which could be shortened to 1.6 ps by insertion of highly-nonlinear fibre.

Technique to generate equal amplitude, higher-orderopticalpulses in rational harmonically modelocked fibre ring lasers

A novel technique of pulse amplitude equalisation in higher-order optical pulses in a rational harmonically modelocked fibre ring laser is proposed and demonstrated. The combination of intra-cavity optical filtering, and even-order modulation sideband generation using nonlinear characteristics of a Mach-Zehnder intensity modulator can effectively eliminate pulse amplitude instability at higher-order rational harmonic modelocking. The generated optical pulse trains exhibit equal amplitudes with a high degree of spectral purity in terms of a large suppression of supermode noise, low amplitude noise, phase noise, and timing jitter.

Active mode locking of tunable multi-wavelength fiber ring laser

We report an active mode-locked tunable multiple-wavelength fiber laser constructed using a unidirectional ring cavity combined with an amplitude modulator and a single sampled fiber Bragg grating (SFBG). Simultaneous generation of up to six-wavelength picosecond pulses with identical repetition rate and a wavelength spacing of about 1.6 nm (200 GHz) is demonstrated. The spectral width and side-mode suppression ratio are 0.01 nm and 40 dB for all the wavelengths. The average output power is 4.5 dBm and the supermode noise suppression is more than 40 dB. The equal-step wavelength tuning over a range of 1.6 nm for all the wavelengths has also been achieved by applying tension to the SFBG.

A 40-GHz 850-fs regeneratively FM mode-locked polarization-maintaining erbium fiber ring laser

An 850-fs pulse train at 40 GHz was directly generated from a regeneratively FM mode-locked fiber laser. The soliton effect was used to generate a short pulse and to suppress the supermode noise. The laser was locked to an external signal by a phase-locked loop (PLL) technique and the wavelength was tunable between 1540 and 1555 mn.

High-repetition-rate optical pulse generation using a rational harmonic mode-locked fiber laser

Rational harmonic mode locking takes place in an actively mode-locked fiber laser when the modulation frequency f/sub m/=(n+1/p)f/sub c/, where n and p are both integers and f/sub c/ is the inverse of the cavity round-trip time, the 22nd order of rational harmonic mode locking has been observed when f/sub m//spl ap/1 GHz. An optical pulse train with a repetition rate of 40 GHz has been obtained using a modulation frequency f/sub m/=10 GHz. The theory of rational harmonic mode locking has also been developed. The stability of the mode-locked pulses is improved considerably when a semiconductor optical amplifier is incorporated into the fiber laser cavity. The supermode noise in the RF spectrum of a mode-locked laser is removed for a certain range of current in the semiconductor optical amplifier.

Wavelength-tunable actively mode-locked erbium-doped fiber ring laser using a distributed feedback semiconductor laser as mode locker and tunable filter

A wavelength-tunable actively mode-locked erbium fiber ring laser was demonstrated using a distributed feedback semiconductor laser as an intensity mode locker and a tunable optical filter. Very stable optical pulse trains at gigabit repetition rates were generated using harmonica mode locking. The supermode noise was suppressed to 60 dB below the signal level and the root-mean-square timing jitter (0.45 kHz–1 MHz) was found to be about 1% of the pulse duration. A continuous wavelength tuning range of 1.8 nm was achieved by changing the semiconductor laser temperature from 11.4 to 30 °C.

Novel method to simultaneously compress pulses and suppress supermode noise in actively mode-locked fiber ring laser

Nonlinear polarization rotation is used to compress pulses or suppress supermode noise in mode-locked fiber lasers at different situations. In this letter, we propose a novel method to simultaneously realize the above two effects by exploiting nonlinear polarization rotation in actively mode-locked fiber ring laser at a special state. A simple model derived by theoretical analysis is given to explain this phenomenon. We have also investigated the different transmission situations of nonlinear polarization rotation in the 10-GHz actively mode-locked fiber ring laser. Experimental observations agree well with theoretical predictions.

A Polarization-Maintaining and Dispersion-Managed 10-GHz Mode-Locked Erbium Fiber Ring Laser Providing Both sech2- and Gaussian-Shaped Pulses

We have constructed a polarization-maintaining and dispersion-managed 10-GHz regeneratively mode-locked Erbium fiber ring laser which generates highly stable and nearly transform-limited 3.5- to 7-ps sech2pulses at 1560 nm. By forcing the wavelength to a narrow-band ripple in the transfer function of the laser in-line filter, highly stable 15-ps Gaussian-like pulses were also obtained. We have studied in detail the temporal and spectral shapes of the pulses and confirmed a good agreement with the corresponding theoretical fits for transform-limited sech2and Gaussian pulses, respectively. The pulses have a timing jitter below 0.2 ps and the highest reported supermode noise suppression of 90 dB. This remarkably high supermode noise suppression is likely a result of the dispersion-management in the laser cavity. The wavelength is tunable within 1530–1570 nm. The laser has excellent long-term stability and is very robust to thermal and mechanical disturbances.

Low-threshold 115-GHz continuous-wave modulational-instability erbium-doped fiber laser

A pulse train with a repetition rate of 115GHz was obtained at 1.55 microm from a continuous-wave modulational-instability erbium-doped fiber laser. This laser has a Fabry-Perot filter whose free spectral range is set at 115GHz, corresponding to the modulational-instability gain peak. This filter enables us to initiate modulational instability easily with a low threshold and produce a stable, continuous-wave pulse train. The stability of the pulse train is greatly improved by suppression of supermode noise with a combination of self-phase modulation and a narrow-band optical filter.

Supermode noise suppression in a harmonically modelockedfibre laser by selfphase modulation and spectral filtering

Harmonically modelocked fibre lasers suffer from supermode noise because the slow gain relaxation time cannot equalise the individual pulse energies. The authors show that the combination of selfphase modulation and spectral filtering provides a fast intensity dependent loss in the cavity, which equalises the pulse energies and completely suppresses supermode competition.