Degree of polarization of photoluminescence from facets of InP as a function of strain: some experimental evidence.

Previous work demonstrated a good fit to the degree of polarization (DOP) of luminescence measurements on {110} facets of InP using a simple dependence of DOP of luminescence on strain: ${-}{K_e} ({e_1} - {e_3})$, where ${K_e}$ is a positive calibration constant, and ${e_1}$ and ${e_3}$ are normal components of strain in the plane of the facet and along $\langle 1\bar 10\rangle$ and $\langle 001\rangle$ directions [Appl. Opt.43, 1811 (2004)APOPAI0003-693510.1364/AO.43.001811]. Recent analytic modeling, which by necessity to be analytic must make simplifying assumptions, has suggested that unless the measurements are along crystallographic axes, the dependence of the DOP of luminescence on strain is more complicated: ${-}{K_e} (1.315 {e_1} - 0.7987 {e_3})$ for measurements from an InP facet, with a similar "excess" ${e_1}$ for GaAs [Appl. Opt.59, 5506 (2020)APOPAI0003-693510.1364/AO.394624]. In this work, we fit finite element simulations (FEM) to DOP measurements of the photoluminescence from facets of InP bars with ${\{111\} _B}$ v-grooves that have been placed in a cylindrical bending moment. We find that the more complicated dependence of DOP on strain, as derived by the analytic model, fits the data better than the previously assumed simple dependence. This finding thus corroborates the analytical model and should have an impact on understanding the strain-dependent operation of optoelectronic devices.

Mapping of mechanical strain induced by thin and narrow dielectric stripes on InP surfaces.

We investigated deformation of InP that was introduced by thin, narrow, dielectric SiNx stripes on the (100) surface of InP substrates. Quantitative optical measurements were performed using two different techniques based on luminescence from the InP: first, by degree of polarization of photoluminescence; and second, by cathodoluminescence spectroscopy. The two techniques provide complementary information on deformation of the InP and thus together provide a means to evaluate approaches to simulation of the deformation owing to dielectric stripes. Ultimately, these deformations can be used to estimate changes in refractive index and gain that are a result of the stripes.

Rotation of principal axes and birefringence in III-V lasers owing to bonding strain.

Measurements of the degree of polarization (DOP) of photoluminescence from the facets of bonded III-V semiconductor diode laser chips show shear strain. The effect of shear strain on the refractive index is investigated. Finite element method (FEM) simulations are matched to the facet map of the DOP for a bonded GaAs chip and are used to extract estimates of the strain induced by die attach. Given estimates of the strains from the FEM simulations, changes in the refractive indices and rotations of the principal axes are calculated for the chip. This work has value in understanding operational changes owing to bonding-induced strain.

High-power diode laser bars and shear strain.

The emitters at the edges of high-power laser bars tend to produce less power than emitters that are near the center of the bar. We suggest that shear strain, which owes to strain induced by bonding, creates through a photoelastic effect a weak birefringence that rotates the plane of polarization of the light. A rotation of the plane of polarization reduces the net gain for the lasing modes and hence leads to a lower output power for the emitters at the edges of the bars, where the shear strain is dominant.

Analysis of broadly tunable coupled-cavity semiconductor lasers.

We report on the spectral properties for above-threshold operation of broadly tunable, asymmetric multiple quantum well (AMQW), coupled-cavity InGaAsP/InP semiconductor diode lasers. We developed a traveling wave model to understand the mode selection that the lasers exhibit. We find that a weak, short external cavity (SXC) can be used to obtain single frequency operation on each longitudinal mode over the ∼100 nm tuning range of the uncoated AMQW coupled-cavity lasers. We measured the spectral properties of AMQW coupled-cavity lasers with and without an SXC. In a synthesized optical coherent optical tomography application, the use of an SXC with an AMQW coupled-cavity laser reduces the coherence length and hence enhances the performance of the AMQW coupled-cavity laser for optical coherence tomography applications.

Gain flattening coatings for improved spectral performance of asymmetric multiple quantum well lasers.

A gain flattening coating was designed and fabricated to enhance the wavelength tuning for asymmetric multiple quantum well (AMQW) lasers. After coating, a nonlasing gap in the middle range of the lasing wavelengths, which might exist for AMQW lasers that are operated without an external cavity, was overcome and the total lasing range was increased. With the coating, the tuning range of an AMQW laser, as measured without an external cavity, was increased to 85 nm from 70 nm.

Widely tunable coupled-cavity semiconductor laser.

We describe a widely tunable coupled-cavity semiconductor laser with a nearly continuous tuning capability of approximately 100 nm. A below threshold model for coupled-cavity devices using a transfer matrix approach that takes into account the tilt of the facets forming the gap between the coupled sections was developed and is presented. Nonlinear fits of the below-threshold spectra to the model were used to extract device parameters. These fits and parameters were then used to understand the operation of the devices and the direction to take to improve the performance of the devices. It is observed that for facet angles > or = 7 degrees, a two-section coupled-cavity device works like an injection-locked laser, while for angles < or = 4 degrees, the sections work as a truly-coupled system.

Optical phase and intensity modulation from a rotating optical flat: effect on noise in degree of polarization measurements.

A spectral analysis of the electrical signal from the detector in degree of polarization (DOP) measurements that use a rotating polarizer shows base band frequencies that create a noise floor. The noise floor arises from phase and intensity modulation of the optical field owing to the varying thickness and transmission of the rotating optical polarizer in the DOP apparatus. A physical model is presented for the noise floor arising from the phase and intensity modulation, and a calibration procedure including configuration guidelines is provided to minimize the effects of the unwanted modulations.

Multichannel digital phase sensitive detection using a field programmable gate array development platform.

This paper describes the development and performance of digital phase sensitive detectors (PSDs) based on a field programmable gate array (FPGA) logic device. A FPGA development system combined with a custom analog data acquisition board provides a powerful platform for the development of digital signal processing systems. Conventional performance metrics for digital PSDs are determined by the analog front-end devices and do not represent improvements to the PSD itself. FPGAs offer a scalable platform on which digital PSDs can be implemented with enhanced capabilities including simultaneous demodulation of multiple independent analog signals at multiple frequencies with arbitrary demodulation functions. Circuits for providing dynamic frequency tracking and ultrafine (<0.001 degrees ) phase resolution are presented.

Strain mapping by measurement of the degree of polarization of photoluminescence.

A technique is described for the simultaneous measurement of the difference in the normal components of strain and of the shear strain in luminescent III-V material from the degree of polarization (DOP) of photoluminescence. This technique for the measurement of shear strain and of the difference in the normal components of strain in InP was calibrated by applying known external loads on the bars of InP with V grooves etched into the bars and by fitting the experimental results to two-dimensional finite-element simulations. Fits to the difference in the normal components of strain (as opposed to stress) yielded significantly smaller residues. On this basis we conclude that the DOP of luminescence is proportional to the difference in the normal components of strain.

Photoelastic effect from die bonding of diode lasers.

The effect of strain induced from die bonding on the optical guiding of a ridge-waveguide laser is examined. Measurements of the strain in a p-down-mounted InP-based diode laser with fits to polarization-resolved photoluminescence are used to compute the resulting photoelastic effect in a ridge waveguide. The strain is found to alter the refractive indices in the material sufficiently in the waveguide to change the mode and reduce the optical confinement. In the far field the beam is calculated to be made narrow and asymmetric by the die-bonding strain.