High-power diode lasers under external optical feedback

The chaos characteristics of a DFB semiconductor laser subjected to single and double external cavity optical feedback are analyzed experimentally. The results show that the correlation dimension, bandwidth of the chaos generated by one external cavity optical feedback can be expanded by using double external cavity optical feedback. When the laser is subjected to double external cavity optical feedback, the bandwidth with one external cavity feedback is increased from 5.5 GHz to 11 GHz. Correspondingly, the correlation dimension is increased from 2.8 to 3.6. The information of the external cavity length of the chaos generated by double external cavity optical feedback can be masked in the autocorrelation function.

External optical feedback can significantly alter the operating characteristics and output radiation properties of semiconductor diode lasers. A semiconductor injection laser operating in an external cavity is attractive for numerous applications requiring greater spectral purity, frequency stability, and smaller beam divergence than can be obtained from a solitary laser diode. A linear array of diode lasers was fabricated from a bar of five, 6-µm wide, 250-µm center spacing, single-stripe emitters. The external cavity was formed by a 0.75 pitch graded-index rod lens positioned ~1 µm from the front facet of the bar array. The rod lens transfers an image of the radiating bar facet directly from the back to the front of the rod. The external cavity is therefore formed by optical feedback from the front surface of the rod with the rod lens also performing the function of the intracavity lens element. This dual function of the graded-index rod lens enables the array laser to assume a very compact form in comparison to other external cavity configurations. It was observed that the wavelength of each diode element could be controlled through proper alignment of the external cavity. Once the diode elements were operating at the same wavelength, the array could be tuned over a 15-nm range (798-813 nm). A mutual coherence interferometer was constructed to determine qualitatively the existence of a phase locked condition. A very clear interference fringe pattern was produced. Frequency locked, mutually coherent, operation of five diode elements was achieved, with external wavelength control demonstrated.

The generation mechanism of self-sustained pulsation in vertical cavity surface emitting lasers is analyzed theoretically. The influence of self-focusing, diffraction loss as well as optical feedback is taken into consideration. It is shown that the condition of self-sustained pulsation can be modified significantly by optical feedback. In addition, it is possible to obtain self-sustained pulsation in vertical cavity surface emitting lasers by using external mirror.

An InGaAsP single-mode laser diode (LD) highly tolerant to optical feedback was realized utilizing a partially corrugated grating with a window-mirror structure. The length and the coupling coefficient of the grating were properly chosen to enable moderate output power, low feedback sensitivity, and a side mode suppression ratio of 40 dB simultaneously. Under -16 dB external optical feedback, the relative intensity noise (RIN) was improved by 10 dB compared with that of a conventional DFB LD. The RIN was maintained at less than -120 dB/Hz under strong external optical feedback as high as -10 dB. These results show that the fabricated lasers are potentially useful for isolator-free optical modules.

It is shown by computer simulation and in experiments that chaotic generation caused by the external optical feedback in a laser diode can be suppressed by means of a delayed optoelectronic feedback controlled by the injection current. The suppression of chaotic generation is realized for an AlGaAs/GaAs laser of the quantum wall structure. In the presence of the external optical feedback provided by the mirror placed at a distance of 1~mm the noise of optical intensity is of the oder of 10 μW, the single mode dominates in the spectrum, and its spectral width exceeds 600 MHz. The optoelectronic feedback controlled by the injection current reduces the noise by several times and narrows the generation line width down to 80 MHz.

The noise performance of the beat note generated by a dual-polarization fiber grating laser is very critical for sensing applications. To reduce the noise of the beat note, external optical feedback is employed with a fiber Bragg grating as a reflector. It then shows that a longer feedback time results in larger noise reduction. With a 50 m single mode fiber as the delay line, more than 20 dB phase noise reduction has been achieved for a dual-polarization fiber grating laser which shows a phase noise of -92 dBc/Hz @ 10 kHz offset with external optical feedback applied.

This work focuses on the effects of external feedback on the transient response of antiresonant reflecting optical waveguide (ARROW) vertical cavity surface emitting lasers (VCSELs). The study uses a self-consistent system, which is incorporated by the field transfer matrix method, laser rate equations and the reflective index change model, to simulate the dynamics of the VCSELs under optical feedback.

The residual phase noise at close-in to carrier offset frequency is studied for optical links using a laser diode with an external optical feedback. Since the measured FM noise degradation of the modulating signal was found to be insignificantly higher than the expected 20log (n) in dB, the residual phase noise of the laser diode was measured to quantify the expected carrier signal FM noise floor level. The measured residual phase noise of a InGaAsP laser diode at 1 KHz offset carrier signal of 5.08 GHz is measured to be -100 and -90 dBc/Hz with and without a 3-cm-long free-space external cavity, respectively. The close-in to carrier phase noise results of this laser at the external cavity resonance frequency of /spl ap/5 GHz is explained for the first time in terms of laser diode nonlinearity and FM noise theory of injection locked microwave oscillators. A good match between the predicted and measured results was observed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

InGaAs/AlGaAs based broad stripe LDs that have failed during the operation with external optical feedback has been investigated. Unusual failure which is accompanied by the extension of DLD from rear-facet to front has been confirmed. Failure mechanism of rear-facet DLD is discussed.

The effect of external optical feedback on a packaged monolithic quantum dot passively mode-locked laser is investigated. The radio-frequency (RF) linewidth narrows from 8 KHz in the free-running situation to a value as low as 170 Hz under relatively low feedback and temperature control. The RF linewidth characterization under resonant feedback at a multiple of the laser cavity length agrees well with the published theory. The narrow RF linewidth with external optical feedback makes the chip-scale quantum dot mode-locked laser an attractive source for ultra low noise applications such as optical clocking and optoelectronic oscillators.

We demonstrate the implementation of external cavity optical feedback to improve coherence and promote generation of harmonic states by a mid-infrared quantum cascade laser frequency comb. In particular, we present a Vernier-like scheme to realize harmonic comb states that increase the repetition rate of the comb by a factor of up to 6 and broaden spectral coverages from 46 cm-1 to 92 cm-1. Intermode beatnote and dual comb characterization indicate that the coherence of the comb has greatly improved for sub-optimal devices when the comb is operated in these harmonic states. This approach to control the generation of harmonic states and improve comb performance can be readily incorporated to various sensing systems and has great potential in spectroscopic measurements that require high repetition rates and/or broad optical bandwidth.

The effects of external optical feedback mainly introduced by Rayleigh backscattering (RB) in long lead fiber, reflection of fiber end facet, and the fusion splices on the stability of asymmetric distributed feedback fiber laser (DFB-FL) have been investigated. Theoretical analysis based on the compound cavity model is presented, and the critical length Lp,c of lead fiber is obtained to evaluate the tolerance of DFB-FL to RB. Analysis and experimental results show that RB is the main factor causing the instability. The critical length for the shorter end of our asymmetric DFB-FL is measured to be about 170 m. To isolate the laser from the effect of RB, we propose a method using isolator and WDMs, which is experimentally proved to be an efficient way.

Insensitivity to external optical feedback is experimentally demonstrated in a self-chaotic deformed square microcavity laser for the first time, to the best of our knowledge. Both the optical and radio frequency (RF) spectra of the microlaser remain unaffected for external optical feedback with feedback strength as high as 9.9 dB. In addition, the autocorrelation function curve exhibits no time-delayed peaks. The insensitivity makes the self-chaotic microcavity laser promising for applications in feedback-insensitive optical sources.

The effect of external optical feedback on the linewidth of a single-mode injection laser is considered theoretically. A set of three rate equations with Langevin noise sources is used to obtain the power spectrum. If the high-frequency structure in the power spectrum is ignored, the line shape is Lorentzian and exhibits broadening or narrowing depending on the external-cavity phase shift. Particular attention is paid to the line narrowing after including the effect of carrrier-induced index changes.

An analysis is made of how the emission amplitude and frequency of a semiconductor laser with external optical feedback change as the external reflector is moved. It is shown that the influence of the external optical feedback on the interference signal from vibrations of the reflector can be lessened not only by lowering the feedback level but also through the choice of the distance to the vibrating object.