Semiconductor laser diode with weak optical feedback: self-coupling effects on P-I characteristics

We calculate the field power spectrum of a Semiconductor Laser in a finite time window in presence of weak feedback and compare it with numerical simulations. The asymmetry of the side peaks, their frequency shift and the linewidth reduction are characterized. We consider the limit of very weak feedback, in which only side peaks associated with relaxation oscillations modified by the feedback appear in the spectrum.

Compact, efficient, diode-laser-based blue light sources are useful for many applications including optical storage and laser printing. Blue light can be generated by frequency doubling the output of reliable, high-power 860-nm lasers in potassium niobate. The Second Harmonic Generation (SHG) conversion efficiency can be substantially increased by placing the nonlinear crystal inside of a resonator in order to increase the infrared intensity in the crystal.1,2,3,4 The build-up of a high infrared intensity and efficient generation of blue output requires precise frequency matching of the diode laser and the SHG resonator. Previously, electronic locking1 and weak optical feedback techniques2,3,4 have been used to maintain this frequency matching. Electronic locking has the disadvantage of requiring an optical isolator to prevent destabilization of the laser frequency by light scattered from the resonator. Weak optical feedback has the disadvantage of requiring electronic control of the phase of the optical feedback as well as precise control of the feedback amplitude.3,4 We describe the operation of an antireflection-coated GaAlAs diode laser in an extended laser cavity that contains the SHG resonator as an intracavity element. The use of an antireflection coating on the laser and strong optical feedback is intended to suppress coupled-cavity effects and produce stable operation of the laser at a single frequency that is resonant with the SHG resonator. With the extended cavity we have obtained blue output powers of up to 12.5 milliwatts.

The quantum mechanical theory of semiconductor lasers based on Langevin equations1,2 is modified to include the effects of weak optical feedback with dispersive loss.3

The linewidth narrowing of a semiconductor laser due to weak optical feedback is analysed, taking into account both phase condition and threshold change for the feedback-induced modes. The achievable linewidth reduction lies in between two limiting cases, 1/(1 + X √(1 + α2))2 and 1/(1 + X)2, where α and X are the linewidth enhancement factor and the feedback parameter, respectively.

It is shown theoretically that semiconductor lasers become bistable with weak optical feedback and that the intensity noise is increased by transitions between the bistable states due to steady-state fluctuations. The bistability occurs in a narrow range of the feedback magnitude, depending on the phase of feedback light at the semiconductor facet. This is in good agreement with observed noise increase at low frequencies caused by external feedback.

We report the first experimental observation of the route to chaos for semiconductor lasers with weak optical feedback. For increasing feedback level, the laser undergoes a quasiperiodic route interrupted by frequency locking, which is also predicted by a comprehensive numerical analysis. Experiments further demonstrate the occurrence of quantum-noise-induced transitions between two attractors with different relaxation oscillation frequencies. The attractors are created by two subsequent Hopf bifurcations of the same external cavity mode.

The transient dynamics of a single-mode semiconductor laser subjected to both weak optical feedback and light injection is studied. Even at detuning values for which injection locking is not effective, turn-on jitter and frequency jitter can still be strongly reduced by adjusting either the length of the external cavity or the detuning.

We analyse the stability of the steady-state solutions of the compound system formed by a single-longitudinal-mode semiconductor laser and an external reflector. Of the large number of these external cavity modes (ECMs) created in pairs of modes and antimodes by moderate feedback, the antimodes are always unstable, while the modes may be stable or unstable when created. The ECMs that have a large positive frequency shift with respect to the emission frequency of the solitary laser are unstable when created. In contrast, the ECMs that have a large, negative frequency shift are stable on creation and remain stable over a relatively large feedback range. For sufficiently large feedback an ECM that is created stable gives way to a time-dependent solution (limit cycle or torus) that is localized in phase space around the ECM; several time-dependent attractors may coexist. Approximating the dynamical equations, taking into account terms up to second order, we obtain relations for the amplitudes of the oscillations of the laser variables for these attractors. The external cavity length and the injection current are key parameters which determine these amplitudes. Our approximations are in good agreement with numerical simulations, and hold even far from the bifurcation points where these attractors originate.

Numerical analysis of weak optical positive feedback (OPF)controlling chaos is studied in a semiconductor laser. The physical model ofcontrolling chaos produced via modulating the current of semiconductor laseris presented under the condition of OPF. We find the physical mechanism thatthe nonlinear gain coefficient and linewidth enhancement factor of the laserare affected by OPF so that the dynamical behaviour of the system can beefficiently controlled. Chaos is controlled into a single-periodic state, adual-periodic state, a tri-periodic state, a quadr-periodic state, apenta-periodic state, and the laser emitting powers are increased by OPF insimulations. Lastly, another chaos-control method with modulating theamplitude of the feedback light is presented and numerically simulated tocontrol chaotic laser into multi-periodic states.

We present a simple method for narrowing the intrinsic Lorentzian linewidth of a commercial ultraviolet grating extended-cavity diode laser (TOPTICA DL Pro) using weak optical feedback from a long external cavity. We achieve a suppression in frequency noise spectral density of 20 dB measured at frequencies around 1 MHz, corresponding to the narrowing of the intrinsic Lorentzian linewidth from 200 kHz to 2 kHz. Provided additional active low-frequency noise suppression and long-term drift compensation, the system is suitable for experiments requiring a tunable ultraviolet laser with narrow linewidth and low high-frequency noise, such as precision spectroscopy, optical clocks, and quantum information science experiments.

We present an experimental observation of the output responses of a Nd:YAG microchip laser with an anisotropic external cavity under weak optical feedback. The feedback mirror is stationary during the experiments. A pair of acousto-optic modulators is used to produce a frequency shift in the feedback light with respect to the initial light. The laser output is a beat signal with 40 kHz modulation frequency and is separated into two orthogonal directions by a Wollaston prism. Phase differences between the two intensity curves are observed as the laser works in two orthogonal modes, and vary with the external birefringence element and the pump power. Theoretical analyses are given, and the simulated results are consistent with the experimental phenomena.

Except some special cases the color printing is usually performed by overprinting a series of four plates in yellow (Y) magenta (M) cyan (C) and black (BK) ink. Process Color Scanner is the device that reads the fullcolor on ginal to be reproduced it into four color components (Y and BK) converts the totalvalue into the dot sizes provides the output on the photosensitive materials such as monochrome film on monochrome paper. The optical dot formation of our Process Colon Scanner SG series is performed as follows: 1. Ten light spots that may be independently switched (ON and OFF) at high speed are formed in line on a photosensitive material. 2. Such photosensitive material wrapped around the surface of a notating drum forms the dots on its surface according to the light spots switched by the signals from the dot generator as the drum notates. (See Fig. 1) Conventionally the gas laser (HeNe on Ar) has been used as the light source where a beam from a single laser tube is splitted into ten beams by the use of a beam splitter and an external modulator is provided to each laser beam to control its switching. As the semiconductor laser has the features such as high speed self-modulability and compact size we have developed new exposure system with ten semi conductor laser elements to replace conventional system consisting of a gas a beam splitter and ten external modulators. (See Fig. 2) 1. CONSTRUCTION OF THE SYSTEM 1. 1. Light emitting section The semiconductor laser emitting the light of 780 nm wave length is employed for the matching with photosensitive materials currently available. And as this type of semiconductor laser is also used in compact disc system etc. it offers stable performance and good availability. 1. 2. Method to control the beam Unlike the gas laser beam with an excellent directivity the semi- 68 / SPIE Vol. 1254 Optical Hard Copy and Printing Systems (1990)© (1990) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Coherence collapse is a dramatic spectral broadening that occurs in single-mode semiconductor lasers subject to weak (10-3–10-4) optical feedback well above threshold.1 It is a particularly vexing and elusive phenomenon because it mimics the properties of deterministic chaos and noise-driven instabilities. Here we describe what we believe to be the first experimental observation of coherence collapse using phase-conjugate external feedback. A GaAs/AlGaAs index guided laser diode (Hitachi HLP-1400) is coupled by a variable attenuator—consisting of two polarizers and a half-wave plate—to a passive phase conjugate mirror (PPCM) that uses a barium titanate crystal.2 An optical isolator is placed in the ring to make the PPCM undirectional. This avoids deleterious effects caused by counterpropagating beams. The line broadening was observed with a 16.5-GHz Fabry-Perot interferometer. External cavity modes were seen in the rf intensity noise spectra. This PPCM could produce reflectivites up to 15% at the laser facet. For these experiments, the reflectivity was varied from zero up to 2 × 10-3. At the lower reflectivites (10-4) the initially well-behaved laser experiences undamped relaxation oscillations which beat with the external cavity modes in the nonlinear laser medium to produce the broadband (~10 GHz) single-mode spectra characteristic of coherence collapse.

This article highlights the important roles of the non-radiative recombination (NRR) rate and optical feedback (OFB) in improving the dynamics and frequency noise (FN) of laser diodes (LDs). We categorize the dynamics of lasers based on bifurcation diagrams of photon numbers. Our findings show that a lower NRR rate stabilizes the laser output and promotes continuous wave mode. In regions with strong OFB, reducing the NRR rate shifts the operation from chaotic to stable modes, aligning the FN with the characteristics of solitary lasers. In the chaotic region, the FN increases by more than twelve orders of magnitude compared to the solitary laser noise level. In contrast, under weak OFB, as the NRR rate increases, low-frequency FN rises significantly. Under very strong OFB, FN drops dramatically to levels four orders of magnitude lower than those observed in solitary lasers as the injection current increases. By carefully engineering the NRR rate and manipulating the OFB strength, we can effectively stabilize LD performance and advance laser technology.

On the theory of a single-mode laser with weak optical feedback