External Cavity Mode Spacing Research Articles

Behavior of the relaxation oscillation frequency in vertical cavity surface-emitting laser with external feedback

The influence of weak optical feedback on the dynamics of a single longitudinal mode vertical-cavity surface-emitting laser (VCSEL) is studied experimentally under conditions when the feedback power is sufficiently low so as to avoid the onset of chaos. The VCSEL dynamics divides into two regimes depending on the ratio of the relaxation oscillation frequency to the external cavity mode spacing. In the long cavity regime, the modulation frequency response of the VCSEL exhibits resonances corresponding to the frequencies of the external cavity modes, while for the short cavity regime the modulation response of the VCSEL at the cavity resonances remains very low while the relaxation oscillation frequency is shifted and the modulation response at the shifted frequency exhibits a sharpened and enhanced resonance.

Enhancement in microwave modulation efficiency of vertical cavity surface-emitting laser by optical feedback

We investigate a versatile vertical cavity surface-emitting laser (VCSEL) with a slight optical feedback from a simple wedge positioned at a well determined distance from the VCSEL. We use such a laser in conjunction with a miniature Rb atomic cell to detect coherent population trapping (CPT) when the external cavity mode spacing frequency corresponds to half the hyperfine frequency of Rb. The feedback greatly enhances the rf modulation efficiency of the VCSEL and as a result we obtain high contrast CPT signals at an input of −16dBm of rf modulation power compared to +1.25dBm needed in the absence of the external cavity.

Experimental and Theoretical Study of Semiconductor Laser Dynamics Due to Filtered Optical Feedback

We report experimental results on the nonlinear dynamical response of a semiconductor laser subjected to time-delayed (>5 ns), frequency selective, optical feedback from a Fabry-Pe/spl acute/rot interferometer type of filter. Three regimes of interest, based on the relative value of the filter bandwidth with respect to the relevant laser parameters (relaxation oscillation frequency and external cavity mode spacing), are identified, viz. a wide filter case, an intermediate filter width case, and a narrow filter case. The dynamical response of the laser is shown to be quite different in each of these regimes. The principal results are 1) the laser's linewidth enhancement factor, coupled with the nonlinear response of the filter, can be exploited to induce nonlinear dynamics in the instantaneous optical frequency of the laser light on a time scale related to the time-delay of the feedback, 2) a mode mismatch effect which arises from a detuning between the filter center frequency and the nearest external cavity mode and manifests itself in a reduction of the maximum light available for feedback, and 3) a reduction in, or even disappearance of, relaxation oscillations in the laser dynamics when a filter of appropriate width is chosen. More generally, it is observed that certain dynamics that occur due to unfiltered optical feedback may be suppressed when the feedback light is spectrally filtered.

Design and performance of a frequency chirped external cavity diode laser

The design of a frequency chirped external cavity diode laser (CECDL) in the Littrow configuration with an intra-cavity electro-optic crystal is presented and its tuning and noise performance is studied. The CECDL has a center wavelength of 793 nm with a 20 nm tuning range by mechanical rotation of the feedback grating about a pivot point. Rapid tuning of the CECDL is achieved electronically by voltage control of the electro-optic crystal. The electro-optic tuning response of this laser is 2.01 MHz/V and linear frequency chirps of 800 MHz ranging in duration from 3 to 337 μs are demonstrated. The maximum electro-optic tuning is set by the external cavity mode spacing of 2.4 G Hz for this laser.

Measurement of diode laser characteristics affecting tunability with an external grating

We investigate theoretically the tuning characteristics of a diode with an external grating cavity for which one of the external-cavity modes and the peak of the grating reflection scan synchronously. We show that the tuning curve is hysteretic. Moreover, the theoretical analysis reveals that frequencies at turning points of the hysteretic tuning curve are related to the external cavity mode spacing, the reflectivity of the front facet, and the linewidth enhancement factor α of the laser diode. A method for achieving an overlap of the peak of the reflection profile of the grating with one of the external-cavity modes is described. Experimentally, by measuring the hysteretic tuning curve, we observed the predicted effect and found the reflectivity of the front facet and the α value for a Toshiba TOLD 9215 visible diode to be 2.9 ± 1.6% and 5.1 ∓ 1.3, respectively.

Frequency selection and transient dynamics in single-mode lasers with optical feedback

It is demonstrated that a closed delayed equation for the phase of the electric field can be used to describe accurately the transient switch on of a laser with external feedback when it can be described by field evolution equations. In contrast, it is shown that several more severe approximations to the dynamics, including an adiabatically evolving potential for one-dimensional relaxation dynamics (which had previously been used to accurately predict the laser linewidths for the steady-state solutions) fail to reliably describe the final state selection during transient switch on of the laser, the delay-induced oscillatory approach to the chosen state, and features of the amplitude and frequency spectra near multiples of the external cavity mode spacing which may be important for the stability and switching dynamics of the steady states that correspond to excitation of different external cavity resonances.

Low field operation of a pulsed InSb spin-flip Raman laser at 10.6 μm

The operation of an external cavity, CO 2 pumped InSb spin-flip Raman laser using an electromagnet is reported. Spin-flip operation has been achieved at fields down to 0.5 kG. Quantum oscillations have been observed in the spin-flip output power. At low fields the spin-flip output power shows a 100% modulation of period 0.02 cm -1, identical to the external cavity mode spacing, as the field is swept. The significance of these results to the future development of the spin-flip Raman laser as a tunable spectroscopic source is indicated.