Selective Optical Feedback Research Articles

Modulation Properties of an Extended Cavity Diode Laser and Dynamic Mode Splitting

We have found a fine structure in the frequency dependence of modulation efficiency of a diode laser (DL) with a strong selective optical feedback. The FM response of an extended cavity diode laser (ECDL) to the microwave current modulation has two peaks whose frequencies are close to the intermode frequency of the laser longitudinal modes. The frequency splitting of the peaks depends on the power of laser radiation and on the difference between the frequency of operating ECDL mode and the frequency of the closest DL mode. We have observed similar splitting in AM noise spectra with the same type of dependencies on power and aforementioned frequency detuning. The peculiarity of the AM noise spectrum has been observed and explained previously by nonlinear interaction of fields in the active region of a DL that results in the frequency splitting of the modes. This dynamic splitting of the laser cavity modes explains the specific features of the modulation characteristics.

Inducing tunable single-mode operation of Fabry-Perot semiconductor laser coupled with very-short cavity with selective optical feedback

We report on achieving tuned single mode operation of a multimode semiconductor laser by coupling it to a very-short external cavity with selective feedback. We show that the induced uniform feedback light from the very-short external cavity can induce single-mode oscillation over a broad range of optical feedback. Then by applying selective external to the optimum settings of the single-mode oscillation, we predict tuning of the single-mode output over a wavelength range of ~ 20 nm by detuning the peak wavelength of optical feedback from the central wavelength of the modal gain. The study is based on modeling of the dynamics of multimode semiconductor laser subject to selective optical feedback. The model takes into account mechanisms of both self- and cross-modal gain suppression, and the study is concerned with the very-short external cavity that corresponds to stable continuous-wave (CW) operation.

Harmonic distortion dependent on optical feedback, temperature and injection current in a vertical cavity surface emitting laser

In this paper, selective optical feedback is used to investigate the nonlinearity behaviours of a vertical cavity surface emitting laser (VCSEL) with the modulation signal. A single mode VCSEL with both parallel and orthogonal optical feedback (OF) signals modulated at 1 MHz frequency over a range of modulation depth is investigated. We also investigate the nonlinear characteristics of the orthogonal polarization modes XP and YP of the VCSEL by changing the injection current and temperature. The results show an enhancement in the harmonic distortions (HDs) of both XP and YP modes with parallel OF, and the total suppression of HDs with orthogonal OF. We show that for the VCSEL with orthogonal OF, the second and third harmonic components of the XP and YP modes decrease and reach the noise floor level of the output power spectrum. Additionally, peaks of second and third harmonic components change radically when varying the bias current and temperature. The results reveal that orthogonal OF can be employed as a new tool to improve the linear dynamic range and to control the nonlinear characteristics of the VCSEL, thus making these devices a promising optical source in present and future optical communication applications.

Optical-feedback cavity-enhanced absorption spectroscopy with a quantum cascade laser

Optical-feedback cavity-enhanced absorption spectroscopy is demonstrated in the mid-IR by using a quantum cascade laser (emitting at 4.46 μm). The laser linewidth reduction and frequency locking by selective optical feedback from the resonant cavity field turns out to be particularly advantageous in this spectral range: It allows strong cavity transmission, which compensates for low light sensitivity, especially when using room-temperature detectors. We obtain a noise equivalent absorption coefficient of 3 × 10(-9)/cm for 1 s averaging of spectra composed by 100 independent points. At 4.46 μm, this yields a detection limit of 35 parts in 10(12) by volume for N(2)O at 50 mbar, corresponding to 4 × 10(7) molecules/cm(3), or still to 1 fmol in the sample volume.

Tunable terahertz quantum cascade lasers with external gratings

We demonstrate a frequency tunable external cavity terahertz quantum cascade laser using an abutted antireflection-coated silicon lens to reduce facet reflection and as a beam-forming element, with an external grating providing frequency selective optical feedback. Angle tuning of the grating allows a single longitudinal mode of the laser ridge to be selected, resulting in discontinuous tuning over a 165 GHz range around a center frequency of 4.4 THz. Another device exhibited 145 GHz of total tuning with 9 GHz of continuous tuning near the longitudinal modes of the laser.

Coherence function control of Quantum Dot Superluminescent Light Emitting Diodes by frequency selective optical feedback

Low coherent light interferometry requires broad bandwidth light sources to achieve high axial resolution. Here, Superluminescent Light Emitting Diodes (SLDs) utilizing Quantum Dot (QD) gain materials are promising devices as they unify large spectral bandwidths with sufficient power at desired emission wavelengths. However, frequently a dip occurs in the optical spectrum that translates into high side lobes in the coherence function thereby reducing axial resolution and image quality. We apply the experimental technique of frequency selective feedback to shape the optical spectrum of the QD-SLD, hence optimizing the coherence properties. For well-selected feedback parameters, a strong reduction of the parasitic side lobes by a factor of 3.5 was achieved accompanied by a power increase of 40% and an improvement of 10% in the coherence length. The experimental results are in excellent agreement with simulations that even indicate potential for further optimizations.

Spatially Filtered Feedback for Mode Control in Vertical-Cavity Surface-Emitting Lasers

Transverse mode control of a vertical-cavity surface-emitting laser (VCSEL) is achieved by spatially filtered optical feedback. The spatial filter consisted of a pinhole, either 42 ?m or 81 ?m, in the 5.8 × magnified VCSEL image plane of a confocal lens arrangement. The pinhole was translated to map out the voltage drop across the VCSEL from spatially-selective feedback. For low injection currents, spatial filtered feedback enabled single fundamental mode operation due to higher-order mode suppression by appropriately locating the pinhole. For higher injection currents with five laser modes, spatial selection of feedback enabled control of the relative mode intensities and wavelengths. The maximum increase in mode intensity achieved was 1.8 × for the 4 ?m pinhole and 2.4 × for the 81 ?m pinhole. The maximum frequency shift achieved was - 2.7 GHz and - 7.1 GHz with the 42 ? and the 81 ?m pinholes. The spatially selective optical feedback produced long-lived changes to the VCSEL even after the feedback was blocked-results for two separate trials where the recovery time was 38 minutes are presented (although results varied for different bias and feedback conditions).

Polarized optical feedback from an extremely short external cavity for controlling and stabilizing the polarization of vertical cavity surface emitting lasers

The authors present experimental results showing that polarization selective optical feedback from an extremely short external cavity induces switching between the two fundamental transverse modes with orthogonal linear polarization of the otherwise (without optical feedback) polarization stable vertical cavity surface emitting laser. Moreover, when properly selecting the external cavity length within a subwavelength range the polarization state of the emitted light can be chosen in either of the two fundamental transverse modes and stabilized against injection current variations. The numerical results reported are in good agreement with experiments.

Fast, low-noise, mode-by-mode, cavity-enhanced absorption spectroscopy by diode-laser self-locking

A new technique of cavity enhanced absorption spectroscopy is described. Molecular absorption spectra are obtained by recording the transmission maxima of the successive TEMoo resonances of a high-finesse optical cavity when a Distributed Feedback Diode Laser is tuned across them. A noisy cavity output is usually observed in such a measurement since the resonances are spectrally narrower than the laser. We show that a folded (V-shaped) cavity can be used to obtain selective optical feedback from the intracavity field which builds up at resonance. This induces laser linewidth reduction and frequency locking. The linewidth narrowing eliminates the noisy cavity output, and allows measuring the maximum mode transmissions accurately. The frequency locking permits the laser to scan stepwise through the successive cavity modes. Frequency tuning is thus tightly optimized for cavity mode injection. Our setup for this technique of Optical-Feedback Cavity-Enhanced Absorption Spectroscopy (OF-CEAS) includes a 50 cm folded cavity with finesse ∼20 000 (ringdown time ∼20 μs) and allows recording spectra of up to 200 cavity modes (2 cm−1) using 100 ms laser scans. We obtain a noise equivalent absorption coefficient of ∼5×10−10 cm−1 for 1 s averaging over scans, with a dynamic range of four orders of magnitude.

Low-Frequency Fluctuations in Vertical-Cavity Surface-Emitting Lasers With Polarization Selective Feedback: Experiment and Theory

We present theory and measurements on a vertical-cavity surface-emitting laser (VCSEL) biased close to its solitary threshold and subject to polarization selective external optical feedback. The system shows low-frequency fluctuations (LFF) in the selected polarization mode (PM). Below solitary laser threshold, the orthogonal PM remains silent, while it only responds after a dropout event in the main mode above threshold. Our calculations show good agreement with the measurements and identify a type of synchronization between the low-frequency dynamics of the two PMs.

Multimode semiconductor laser with selective optical feedback

We study a multimode semiconductor laser subject to moderate selective optical feedback. The steady state of the laser is destabilized by a Hopf bifurcation and exhibits a period-doubling route to chaos. We also show the existence of a heteroclinic connection between a saddle node and an unstable focus that can be associated with experimentally observed multimode low-frequency fluctuations. This heteroclinic connection coexists with a chaotic attractor resulting from the period-doubling process.

Generation of amplitude-squeezed light from 1550 nm distributed feedback semiconductor laser under wavelength selective optical feedback conditions

Excess noise that degrades the squeezing of a 1550 nm distributed feedback semiconductor laser is identified using a Mach Zehnder interferometer. This excess noise is reduced using wavelength selective optical feedback. Amplitude-squeezed states then are generated at room temperature.

1.3-μm Pr-doped In-Ga-based fluoride fiber amplifiers pumped by grating-fiber-pigtailed 0.98-μm-band laser diodes with a detuned wavelength of 1 μm

At most efficient pump wavelength, a praseodymium-doped In-Ga-based fluoride fiber is directly pumped by four 0.98-/spl mu/m-band laser diodes. These lasing wavelengths are detuned from 0.98 to 1 /spl mu/m by external selective optical feedback from fiber grating reflectors. Maximum signal output power of +13.5 dBm is obtained at 1.296 /spl mu/m. Four-wavelength multiplexed signals at 1.296-1.311 /spl mu/m are amplified with a deviation of gain less than 1.9 dB. By using the amplifier as a power booster, data of 2.5 Gb/s is successfully transmitted more than 100 km.

A 980-nm band laser diode pump source with a detuned wavelength of 1000 nm for praseodymium-doped fiber amplifiers

By using practical high-power 980-nm band laser diodes, efficient direct pumping for a praseodymium-doped fiber amplifier (PDFA) is achieved. The lasing wavelength is detuned from 980 to 1000 nm by selective optical feedback from a fiber grating.

Chemistry in Lasers. IV. The Relationship between Spectral Line Narrowing, Translational Degrees of Freedom, and Diffusion of Laser Active Molecules

Abstract Some of the specific interactions between the radiative ensemble and the optical resonant cavity are examined in relation to the time-oriented sequence of molecular states previously developed. Changes in emission line width by several orders of magnitude due to spectral condensation upon selective optical feedback indicate that the usual translational freedom of laser active molecules is greatly reduced. Following the Einstein diffusion model, the reduced translational freedom corresponds to lesser values of the diffusion coefficient D, leading to anomalous concentration gradients in the cavity.