Spectral linewidth reduction in semiconductor lasers by an external cavity with weak optical feedback

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.

Semiconductor lasers are very different from other lasers because refraction variation can't be avoided when the gain is changed. Refraction variation can be introduced the theory of semiconductor laser by a dimensional parameter. This parameter is called linewidth enhancement factor (LEF). The value of LEF is very important for many aspects of laser behavior. The LEF characterizes the linewidth broadening and chirp due to fluctuation in the carrier density. A simple method to measure the linewidth enhancement factor of laser diodes is presented in this paper. The method uses the self-mixing effect at a weak feedback level. An optical beam is reflected and injected into the laser diode cavity by an external target, and is then mixed with the light inside the cavity, causing variations of the optical output power. The waveform of the optical power is determined by the feedback factor C and the LEF. A theoretical formula to compute LEF is proposed for the case when the feedback level C is smaller than 1. The experimental results show this method is feasible and simple when a laser diode operates at single longitudinal mode.

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 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.

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

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.

Characterization procedures of different types of semiconductor lasers in a typical transmission line are investigated, with particular attention paid to unavoidable low-level optical feedback. The asymmetric response between front and rear facets is compared with the symmetric response of the solitary device.

In this paper, Cassidy's method is presented and is applied to measure the linewidth enhancement factor (LEF) of a 370 GHz self-pulsating Fabry-Perot laser. The importance of the LEF is addressed, the method is detailed, and the device under test is presented. It exhibits a self-modulation at 370 GHz even though it is DC biased. The measured LEFs vary according to the wavelength, values of 4 have been measured at threshold current, which is typical in the case of multi-quantum well lasers. In this paper we propose to present measurement of the linewidth enhancement factor (LEF) in semiconductor laser based, 370 GHz source generator. First we explain the origin of LEF in semiconductor material, then we present several techniques to measure it. We present a computational simulation using modelled ASE spectra, and in part 4, we detail our device and experimental set-up and provide some results. The linewidth enhancement factor (LEF) or alpha parameter is a distinguishing feature of semiconductor devices. Semiconductor devices generally have a larger linewidth than other solid state lasers. This broadened linewidth leads to effects such as mode instability, filementation in broad area devices and frequency chirping under modulation. This last effect is of particular interest, especially in the field of optical communications. It can be suppressed if the LEF is reduced. Therefore in the design of semiconductor lasers or semiconductor amplifiers (SOA), a number of factors have to be taken into consideration, but one of the most important of these is the linewidth enhancement factor. For instance, when the gain of an SOA is affected by the injection of a pulsed source, refractive index changes caused by changes in carrier density will lead to an instantaneous variation of the frequency. This effect is exacerbated as the gain is saturated.

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.

GaAs-based quantum dot (QD) semiconductor lasers at 1300 nm exhibit a number of benefits for use in metropolitan and local area networks. Low modulation-induced chirp attributed to small linewidth enhancement factor (LEF) in QDs overcomes some limitations in a directly-modulated laser compared to quantum well counterparts. Along with the temperature-insensitive low threshold current, these QD lasers demonstrate considerable prospects towards Peltier-free direct-modulation operation. In this work, we experimentally investigate the temperature-dependence of the LEF as a function of injection current and wavelength in state-of-the-art 1300 nm In(Ga)As/GaAs QD lasers grown by molecular beam epitaxy. These structures demonstrate a large state separation and a comparatively small inhomogeneous linewidth. Gain is measured using the Hakki-Paoli method and carrier related mode shift is extracted by correcting for thermal effects. Previous reports have described LEF values of 1-5 over smaller temperature ranges (20-85°C) than the GPON commercial specifications in 1300 nm In(Ga)As/GaAs QD structures. Here, at temperature range of -10 to 85°C extending beyond GPON commercial window, it is shown that even at higher temperatures where contribution from the excited state transition is expected to increase the LEF, the measured LEF remains essentially zero. This is ascribed to the large gain from the ground state transition. Such a low LEF at elevated temperatures bodes well for the use of such structures in applications where feedback insensitivity may be important. By analyzing individual modes as a function of their individual lasing thresholds, we also describe an interplay of mode position and lasing linewidth.

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.

The effective linewidth enhancement factor αeff and the spectral linewidth Δυ are reported for the first time for a partly gain-coupled distributed feedback laser with a gain corrugation in a strained multiple quantum-well active region emitting at 1.55 μm wavelength. The αeff value is estimated to be as low as 1 and is independent of the laser cavity length L, while a decrease in the linewidth-power product ΔυPo with L has been observed, with a measured linewidth of 2 MHz at 10 mW optical output power for 300-μm-long devices. The experimental results demonstrate that a small αeff is obtained due to the strain and the gain coupling effects, while further reduction of the spectral linewidth with L is the result of longitudinal index coupling effects.

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.

As an active research field, the self-mixing interferometry (SMI) based on semiconductor lasers (SLs) is a highly promising and emerging technique for non-contact sensing and parameter measurement of SLs. The basic structure of an SMI system consists of an SL, a lens and an external target. When a portion of reflected light from the target travels back to the laser cavity, a new lasing field is built up leading to both amplitude and phase modulations. The modulated output power is called a self-mixing signal which carries the information of both the target and SL's feature parameters. Alpha factor, also known as linewidth enhancement factor, is one of the most important SL's feature parameters. It characterizes the characteristics of SLs, such as the linewidth, the chirp, the injection lock range and the dynamic performances. This paper presents a new method for retrieving alpha factor of SLs by making use of a self-mixing interference (SMI) waveform. According to the well-known Lang-Kobayashi (L-K) theory, the SMI waveform is shaped by multiple parameters, including the alpha, the optical feedback level factor (denoted as C) and other parameters related to the oscillation of the external target. In this work, we build a new equation based on the SMI model derived from the L-K theory, which can be used to calculate the alpha value. In the existing SMI based methods for measuring the alpha factor, the optical feedback level C is limited within a certain narrow range. The proposed method is able to relieve this limitation. The associated simulations and experiments are carried out for verifying the proposed method.