R. O. Miles

Spectral characteristics of semiconductor lasers with optical feedback

Optical feedback-induced changes in the output spectra of several GaAlAs lasers operating at 0.83 μm are described. The feedback radiation obtained from a mirror 60 cm away from the laser is controlled in intensity and phase. Spectral line narrowing or broadening is observed in each laser depending on the feedback conditions. Minimum linewidths observed with feedback are less than 100 kHz. Improved wavelength stability is also obtained with optical feedback resulting in 15 dB less phase noise. An analytical model for the three-mirror cavity is developed to explain these observations.

Feedback‐induced line broadening in cw channel‐substrate planar laser diodes

The effect of optical feedback on the spectral characteristics of channel‐substrate planar single‐mode laser diodes operating at room temperature is reported. The impact on the performance of interferometric sensor systems using such sources is discussed. The linewidth for the free‐running laser at 10‐mW output power was determined to be less than 5 MHz at room temperature. Broadening on the order of 40 times the intrinsic linewidth was observed for 0.1% feedback and increased with increasing feedback. The presence of self‐oscillation modes was observed at 0.04% feedback. Satellite modes symmetrically located with respect to the primary mode appeared in the spectrum for feedback greater than 0.04%. These satellite modes are attributed to self‐oscillation in the laser induced by feedback. As the feedback was increased, the satellite mode spectrum began to overlap that of the primary mode, reducing the effective coherence length from 60 m (for the single‐mode linewidth) to less than a few centimeters.

An external cavity diode laser sensor

A simple compact optical sensor system consisting of a near-field external cavity coupled to a semiconductor laser is reported. In this device, a change of phase in the light fed back into the laser cavity by the perturbed external mirror modulates the output of the laser by effectively changing the laser facet reflectivity. Phase shifts of 9\cdot10^{-8} rad and 10-6rad were measured at 10 kHz and 100 Hz, respectively, using a 1-Hz bandwidth and a 98-percent reflector placed less than 10 μm from the laser facet. The sensitivity of these devices is shown to be limited by the intrinsic amplitude noise of the laser. The implementation of this sensor configuration is reported as an acoustic sensor, hydrophone, magnetic field, and current sensor as well as an accelerometer.

Laser noise in fiber‐optic interferometer systems

The amplitude stability of single‐mode channeled‐substrate planar diode lasers has been measured as a function of frequency (100 Hz–1 MHz) and driving current. The relative noise spectrum showed a decrease with increasing frequency and an overall reduction at all frequencies as the output power of the laser was increased. These results are compared with similar measurements of the most stable commercial single‐mode HeNe lasers. The problems of laser amplitude noise in fiber‐optic sensor systems is briefly discussed.

Single‐mode diode laser phase noise

Measurements have been made of the phase noise of a single‐mode diode laser in an unbalanced Michelson interferometer, as a function of optical path difference. The noise increased linearly with increasing optical path difference. The origin of the phase noise is discussed.

Low‐frequency noise characteristics of channel substrate planar GaAlAs laser diodes

The influence of external feedback on low‐frequency noise generation (100 Hz–10 kHz) in single longitudinal and transverse mode channel substrate planar laser diodes is reported. An increase in noise by as much as 60 dB was induced by external feedback as small as 0.04%. The induced intensity variations were observed to be sensitive to both phase and amplitude of the externally reflected light.

Phase sensitivity and linewidth narrowing in a Fox--Smith configured semiconductor laser

The behavior and spectral characteristics of a constricted double heterojunction injection laser in a regime in which the lasing characteristics are dominated by a cavity comprised of two external mirrors is reported. The Fox–Smith configuration is shown to limit the laser’s emission (previously multimode) to a single longitudinal mode with a linewidth of less than 200 kHz. It is also shown that this laser interferometer configuration is effective as an extended, external cavity laser diode sensor.

Polarization-resolved low-frequency noise in GaAlAs lasers

Low-frequency intensity noise displaying 1/ f dependency of the light polarized parallel and perpendicular to the junction plane and the correlation between these noise components is investigated in GaAlAs diode lasers. The absolute noise level for the component polarized parallel to the junction (lasing component) rises about 40 dB as the current passes through lasing threshold, while the noise for the orthogonal polarization drops by about 5 dB. The two noise components are well correlated slightly below threshold (coherence function ∼ 0.9), but the coherence function drops rapidly to near zero above threshold. An exception to this behavior occurs in the lasing mode when mode hopping takes place, when the two noise components are well correlated. A qualitative explanation for these phenomena is based on the relation between spontaneous and stimulated emission rates and fluctuations in the intracavity carrier density.

Optical gain measurements at 10.6 µm in an 80 µm hollow-core slab waveguide

N 2 , when externally excited in a gas discharge and mixed with CO 2 +He in a planar metallic waveguide with plate separation less than 80μm, has produced an active laser medium with gain in excess of 17 dB/m. These results demonstrate that an active medium capable of sustaining oscillation can be produced in a waveguide with core dimensions suitable for distributed feedback at 10.6μm. Results are reported for both longitudinal and transverse gas flow configurations.

External Cavity Diode Laser Sensor

A simple compact optical sensor system consisting of a near-field external cavity coupled to a semiconductor laser is reported. In this device, a change of phase in the light fed back into the laser cavity by the perturbed external mirror modulates the output of the laser by effectively changing the laser facet reflectivity. Phase shifts of9\cdot10^{-8}rad and 10-6rad were measured at 10 kHz and 100 Hz, respectively, using a 1-Hz bandwidth and a 98-percent reflector placed less than 10 μm from the laser facet. The sensitivity of these devices is shown to be limited by the intrinsic amplitude noise of the laser. The implementation of this sensor configuration is reported as an acoustic sensor, hydrophone, magnetic field, and current sensor as well as an accelerometer.